BACKGROUND OF THE INVENTION
Field of the Invention
[0001] The present invention relates to a method of driving a liquid crystal display device
and, in particular, to a method of driving an active-matrix liquid crystal display
device utilizing a two-terminal element as a switching element of a pixel.
Related Art
[0002] An active-matrix type of liquid crystal display device provides a higher contrast
than a conventional passive type of device, so they are becoming increasingly common
in various manufacturing fields that use displays. Two types of active elements are
used: two-terminal type and three-electrode type. The two-terminal type is considered
to be superior from the economical point of view.
[0003] Some of the two-terminal type of active elements that are used are metal-insulator-metal
(MIM) elements, ring diodes, and varistors.
[0004] In general, a two-terminal type of active element utilized in an active-matrix type
of liquid crystal display device has the I-V characteristic shown in FIG. 3. In other
words, it uses a switching function caused by a non-linear characteristic of current
with respect to applied voltage, to charge and discharge an effective electrical charge
applied to the picture element.
[0005] The configuration of an active-matrix liquid crystal display device using a two-terminal
type of active element is shown in FIG. 1. In this figure, reference number 101 denotes
a column drive circuit (X driver) that drives column electrodes of a liquid crystal
panel 115, and 102 denotes a row drive circuit (Y driver) that drives row electrodes
thereof.
[0006] In the X driver 101, reference number 104 denotes an AC video generation circuit
which accepts a video signal (P) input, and generates an AC video signal in synchronization
with an AC inversion signal FRX. Reference number 103 denotes a shift register that
is activated by a shift start signal DX, performs a shift operation in synchronism
with a shift clock signal XSCL, and sequentially generates a sampling signal Sm. Reference
number 105 denotes a first analog switch that samples the AC video signal generated
from the AC video generation circuit 104 by the sampling signal Sm and holds it in
a capacitor 106. The capacitor 106 is a first sample-and-hold capacitor. Reference
number 107 denotes a second analog switch that transfers the sampled video signal
held in the capacitor 106 to another capacitor 108 by a latch pulse LP. The capacitor
108 is a second sample-and-hold capacitor. Reference number 109 denotes a buffer amplifier
that drives a column electrode Xm on the basis of the video signal held in the capacitor
108.
[0007] Within the Y driver 102, reference number 110 denotes an inverter that uses Vp and
-Vp as power sources and generates a selection voltage signal Vs in synchronization
with an AC inversion signal FRY. Reference number 111 denotes a shift register that
is activated by a shift start signal DY, performs a shift operation in synchronism
with a shift clock signal YSCL, and generates a selection signal Cn. Reference number
112 denotes a power source selection switch for one cell of a drive circuit for a
row electrode Yn.
[0008] The internal configuration of the selection switch 112 is shown in FIG. 2. The AC
inversion signal FRY and selection signal Cn are input to shift register latches which
consists of NOR gates 201 and 202. An output from the NOR gate 201 and an inverted
signal obtained by an inverter 203 from the selection signal Cn are both input to
AND gates 204 and 205, and outputs therefrom are input to gate electrodes of analog
switches 207 and 208, respectively. The selection signal Cn is also input to a gate
electrode of an analog switch 206. The selection voltage signal Vs and power sources
-Va and Va are input to source electrodes of the analog switches 206 to 208, respectively,
drain electrodes of the analog switches 206 to 208 are connected in common, and a
signal Yn (a signal for driving the row electrode Yn) is output therefrom.
[0009] Reference number 115 denotes a liquid crystal panel. Column electrodes Xm and row
electrodes Yn are formed on the respective substrates of the liquid crystal panel
115, and at each intersection thereof a non-linear element 114 and a liquid crystal
layer 113 are arranged in series to form a pixel. In this case, voltages applied to
the liquid crystal layer 113 and the non-linear element 114 are Vm and Vl, with respect
to the row electrode.
[0010] The non-linear element 114 has the current-voltage characteristic shown in FIG. 3.
As can be seen from this figure, when the applied voltage is small, the current is
extremely small; when the applied voltage is large, the current characteristic increases
steeply.
[0011] The operation of the example of a prior art liquid crystal display device shown in
FIG. 1 and FIG. 2 will now be described with reference to the timing charts of FIG.
4 to FIG. 6.
[0012] As shown in FIG. 4, the video signal (P) is inverted in synchronization with the
AC inversion signal FRX (when FRX = 1, the phase of (P) is positive; when FRX = 0,
the phase of (P) is negative) to obtain an AC video signal 104. In this case, Va is
the 100% white level of the positive-phase video signal and the 0% white level (pedestal
level) of the negative-phase video signal, and -Va is the 0% white level (pedestal
level) of the positive-phase video signal and the 100% white level of the negative-phase
video signal. The shift start signal DY of the Y driver is sequentially transferred
by the shift clock signal YSCL to generate selection signals C1, C2, C3...Cn,....
The latch pulse LP and the shift start signal DX of the X driver are input every horizontal
scanning period.
[0013] An enlarged view of a specific horizontal scanning period is shown at the bottom
of FIG. 4. The latch pulse LP is positioned roughly at the synchronization portion
of the video signal, and it transfers the video signal that was sampled and held in
the capacitor 106 during the previous horizontal scanning period to the capacitor
108. The shift start signal DX is positioned roughly at the start of the video signal
portion in one horizontal scanning period, and it is transferred by the shift clock
signal XSCL to generate sampling signals S1, S2, S3...Sm,.... For example, the nth
video signal 104 sampled by sampling signal Sm (the sampling position marked by a
circle (o) in the FIG. 4) is output to the column electrode Xm at the timing of the
(n+1)th video signal after one horizontal scanning period.
[0014] FIG. 5 is a timing chart of the components shown in FIG. 2. According to logic combination
of the selection signal Cn and the AC inversion signal FRY (in this prior art example,
a common AC inversion signal is input to both the X driver and the Y driver -- in
other words,
), the outputs of the shift register latches 201 and 202 are made to repeatedly invert
between 1 (Cn = 1, FRY = 0) and 0 (Cn = 1, FRY = 1). When Cn = 1, Yn outputs the selection
voltage signal Vs (which is at -Vp when FRY = 1 or Vp when FRY = 0); when Cn = 0 (called
the non-selection period or hold period), Yn outputs a voltage corresponding to the
polarity at the immediately previous selection (when Cn = 1) -- i.e., it is Va after
a positive (Vp) selection or -Va after a negative (-Vp) selection.
[0015] FIG. 6 is a timing chart of the column electrode signal Xm and the row electrode
signal Yn, together with a difference signal Xm - Yn thereof. Video data which corresponds
to the mth column in the horizontal direction along the liquid crystal panel 115 is
sequentially sampled by the AC video signal, and is output as the line electrode signal
Xm. The row electrode signal Yn outputs the selection voltage signal Vs during a selection
period Ts, and a non-selection potential Va or -Va during a non-selection period Th.
The non-selection potential after a selection at positive potential Vp is Va, and
that after a selection at negative potential -Vp is -Va.
[0016] The difference signal Xm - Yn is shown as a solid line in the signal chart at the
bottom of FIG. 6. In this case, the broken-line track is that of the potential at
the connection between the liquid crystal layer 113 and the non-linear element 114.
Since a large voltage is applied to the non-linear element 114 during the selection
period Ts, the current flowing therein is large, as can be understood from the I-V
characteristic of FIG. 3, and the liquid crystal layer 113 is charged thereby. The
amount of this electric charge is controlled by the amplitude of Xm - Yn during the
selection period Ts or by the level of the column electrode signal Xm, i.e., by the
level of sampling by the AC video signal 104. As described above, by changing the
non-selection potential to match the polarity of the preceding selection potential,
the signal level of the difference signal Xm - Yn is made positive in the non-selection
period after a positive-polarity selection period, and negative in the non-selection
period after a negative-polarity selection period. Therefore, the voltage applied
to the non-linear element 114 in each non-selection period becomes small and thus
it becomes difficult for the charge on the liquid crystal layer 113 to leak through
the non-linear element 114 during the selection period. The effective voltage applied
to the liquid crystal layer 113 is proportional to the shaded area in FIG. 6, and
thus in effect depends on the level of the sampled video signal. The liquid crystal
layer 113 controls the amount of light that passes through it in correspondence with
the effective voltage applied to it, and thus a predetermined image is displayed on
the liquid crystal panel 115.
[0017] However, two-terminal non-linear elements, especially MIM and metal-insulator-semiconductor
(MIS) elements, experience a characteristic shift, as will be described below. As
shown in FIG. 7, I-V1 denotes the initial current-voltage characteristic of a two-terminal
non-linear element and I-V2 denotes the same characteristic that has shifted while
a voltage was continuously applied to the element (refer to: E. Mizobata et al., SID91
Digest, p. 226(1991)). In comparison with the I-V1 characteristic, the I-V2 characteristic
shows that resistance increases when the voltage is high, which means that a reduced
charge is written to the liquid crystal layer during the selection period. When the
voltage is low, there is very little difference in resistance, which means that there
is little difference in charge held in the liquid crystal layer during the non-selection
period. It is known that this I-V characteristic shift saturates with voltage applied
to the liquid crystal layer.
[0018] The effect of this I-V characteristic shift on the display will now be described
with reference to the display of a white window against a black background, as shown
in FIG. 8. Consider a pixel P1 that is displaying black at the intersection of column
electrode Xm1 and row electrode Yn, and a pixel P2 that is displaying white at the
intersection of column electrode Xm2 and row electrode Yn. When the entire screen
changes to an intermediate display, as shown in FIG. 9, the previous window display
remains as an afterimage. In other words, pixel P1 ends up lighter than pixel P2.
The reason for this will now be explained with reference to FIG. 10. Xm1 - Yn denotes
the signal applied to pixel P1 and Xm2 - Yn denotes the signal applied to pixel P2.
A voltage VmsW applied to the non-linear element of pixel P2 during the selection
period Ts of the white-display period is greater than a voltage VmsB applied to the
non-linear element of pixel P1 of the black-display period. Thus, the non-linear element
of pixel P2 has a greater I-V characteristic shift. Note that, since the voltages
applied to the non-linear elements of both of the pixels during the nonselection period
is smaller than that during the selection period, the I-V characteristic shift due
to the voltages applied to the non-linear element during the non-selection period
can be ignored. Therefore, when the display changes to an intermediate display, I-V
characteristic of the non - linear element of pixel P2 shifts so that the non-linear
element of pixel P2 develops a greater resistance when a large voltage is applied
than that of pixel P1. The effective voltage applied to the liquid crystal layer during
the selection period is proportional to the shaded area in FIG. 10. In the above case,
it is clear that S1 > S2 and, as a result, pixel P2 ends up darker than pixel P1 and
can be seen as an afterimage.
[0019] In this case, if a voltage applied to a non-linear element while it is displaying
black is VmB and a voltage applied to a non-linear element while it is displaying
white is VmW, a comparison of the effective voltages applied to each of the non-linear
elements in the above display gives:
where
. In view of the voltage applied during the non-selection period Th which has substantially
no effect on the I-V characteristic shift, the above Formula 1 can be rewritten as
follows:
Differences in the voltages applied to the non-linear elements during the selection
period Ts generate a difference in magnitude of the I-V characteristic shift in the
non-linear elements, and this leads to a difference in brightness between these two
pexels that ought to exhibit the same brightness.
[0020] The afterimage described above is also generated in the case that the drive circuitry
shown in FIG. 11 is used. Points at which the drive circuit of FIG. 11 differs from
that of FIG. 1 are described below.
[0021] In the X driver 101, reference number 120 denotes an A/D converter for digitizing
video signals which receives a video signal and generates n-bit digital data. Reference
number 121 denotes a shift register that performs a shift in synchronization with
the shift clock signal XSCL to sample the input digital signal. Reference number 122
denotes a latch circuit that latches and holds data that has been sampled by the shift
register 121. Reference number 123 denotes an Xm drive circuit which drives the column
electrode Xm by outputting a potential of either Va or -Va based on the AC inversion
signal FRX for the column and the data held in the latch circuit 122.
[0022] In the Y driver 102, reference number 125 denotes a liquid crystal power generation
circuit which inputs the Vp and -Vp voltages and generates the selection voltage signal
Vs multiplexed in synchronization with the AC inversion signal FRY for the rows. This
liquid crystal power generation circuit 125 is functionally the same as the inverter
110 of FIG. 1. The shift register 111 generates the selection signal Cn in the same
way as in the configuration of FIG. 1. Reference number 112 denotes, in the same way
as in FIG. 1, a power source selection switch for one cell of the drive circuitry
for the row electrode Yn which drives the row electrode Yn by outputting one of Vs,
Va, or -Va, based on the selection signal Cn. In this prior art example, the switching
of Cn and FRY is done simultaneously, so that, when the selection signal Cn = 1, an
output potential of
is selected when the row AC inversion signal FRY = 1, and
when FRY = 0. However, the timing at which the selection signal Cn and the row AC
inversion signal FRY switch over need not be simultaneous.
[0023] The operation of the prior art liquid crystal display device of FIG. 11 will now
be described with reference to the timing chart of FIG. 12.
[0024] FIG. 12 is a timing chart of the column electrode signal Xm and the row electrode
signal Yn, together with a difference signal Xm - Yn thereof. When FR (the polarity
inversion signal) = 0, Xm goes to -Va for an OFF level (Voff) and Va for an ON level
(Von); when FR = 1, Xm goes to Va for the OFF level (Voff) and -Va for the ON level
(Von). The ratio of Von to Voff varies with the level of the video signal, to enable
a display that includes intermediate displays obtained by pulse width modulation (PWM).
The row electrode signal Yn outputs the selection voltage signal Vs during a selection
period Ts, and a non-selection voltage Va or -Va during a non-selection period Th.
The non-selection potential after a selection at positive potential Vp is Va, and
that after a selection at negative potential -Vp is -Va.
[0025] The difference signal Xm - Yn is shown as a solid line in the signal chart at the
bottom of FIG. 12. In this case, the broken-line track is that of the potential at
the connection between the liquid crystal layer 113 and the non-linear element 114.
Since a large voltage is applied to the non-linear element 114 during the selection
period Ts, the current flowing therein is large, as can be seen from the I-V characteristic
of FIG. 3, and the liquid crystal layer 113 is charged thereby. The amount of this
charge is controlled by the amplitude of Xm - Yn during the selection period Ts, i.e.,
by the width of Von in the column electrode signal Xm. As described above, by changing
the non-selection period's potential to match the polarity of the preceding selection
period's potential, the signal level of the difference signal Xm - Yn is made positive
in the non-selection period after a positive-polarity selection period, and negative
in the non-selection period after a negative-polarity selection period. Therefore,
the voltage applied to the non-linear element 114 in each non-selection period becomes
small and thus it becomes difficult for the charge on the liquid crystal layer 113
to leak through the non-linear element 114 during the selection period.
[0026] The effect on the display of the I-V characteristic shift of FIG. 7 in the drive
circuits of FIG. 11 will now be described. In FIG. 13, Xm1 - Yn denotes the signal
applied to pixel P1 of FIG. 8 and FIG. 9 and Xm2 - Yn denotes the signal applied to
pixel P2 thereof. A voltage VmsW applied to the non-linear element of pixel P2 during
the selection period Ts of the white-display period is greater than a voltage VmsB
applied to the non-linear element of pixel P1 of the black-display period, in the
same manner as in FIG. 10. Thus, the non-linear element of pixel P2 has a greater
I-V characteristic shift. Therefore, when the display changes to an intermediate display,
I-V characteristic of the non-linear element of pixel P2 shifts to develop a greater
resistance when a large voltage is applied than that of pixel P1. The effective voltage
applied to the liquid crystal layer during the selection period is proportional to
the shaded area in FIG. 13. It is clear that S1 > S2 and, as a result, pixel P2 ends
up darker than pixel P1 and can be seen as an afterimage.
[0027] In both of the circuits shown in FIG. 1 and FIG. 11, if a voltage applied to a non-linear
element while it is displaying black is VmB and a voltage applied to a non-linear
element while it is displaying white is VmW, a comparison of the effective voltages
applied to the each of the non-linear elements in the above display gives the same
result as that of Formula 1. Since it is considered that the voltage applied during
the non-selection period Th has substantially no effect, Formula 1 can be rewritten
as the above Formula 2. Therefore, the circuitry of FIG. 11 suffers from the same
problem in that differences in the voltage applied to the non-linear elements during
the selection period Ts generate a difference in magnitude of the I-V characteristic
shift in the non-linear elements, and this leads to a difference in brightness between
non-linear elements that ought to exhibit the same brightness.
SUMMARY OF THE INVENTION
[0028] An objective of the present invention is to solve aforementioned technical problem,
i.e., the problem of afterimages in the active-matrix liquid crystal display device
utilizing the two-terminal element as the switching elemnt of the pixel.
[0029] In order to solve the above-described problem, the method of driving a liquid crystal
display device that comprises a plurality of row electrodes to which a scanning signal
is applied, a plurality of column electrodes to which a data signal is applied, and
a plurality of pixels formed at a plurality of intersections between the row and column
electrodes, each of the pixels comprising a liquid crystal layer and a two-terminal
element having non-linear resistance characteristics connected in series therewith,
the method of driving a liquid crystal display device comprising steps of: applying
the voltage of a difference signal between the scanning signal and the data signal
to each of the pixels; applying a write voltage to each of the pixels based on the
difference signal during a data write period TB in which each of the row electrodes
is selected and the liquid crystal layer of each of the pixels is charged with a data
charge voltage corresponding to the data signal; applying a hold voltage of an absolute
value smaller than the write voltage to each of the pixels based on the difference
signal during a data hold period after the data write period TB; and applying a compensatory
voltage to each of the pixels during a compensatory period TA before the data write
period TB, whereby a compensatory charge voltage of a polarity opposite to that of
the data charge voltage is charged into the liquid crystal layer of the pixel, the
compensatory charge voltage being set into a relationship with the data charge voltage
such that the compensatory charge voltage is small if the data charge voltage is large,
but large if the data charge voltage is small.
[0030] To difine the present invention from another aspect, a method of driving a liquid
crystal display device which comprises a plurality of column lines and row lines and
a plurality of pixels each pixel including a display element and a non-linear resistance
element connected in series between said column and row lines, said method of driving
the liquid crystal display device comprising the step of: applying a first higher
voltage between said column and row lines so that a display data is supplied to said
display element of the pixel, in a selection period of the pixel; applying a lower
voltage than said first higher voltage between said first and second lines, in a non-selection
period of the pixel after said selection period; applying a second higher voltage
than said lower voltage between said first and second lines, in a compensatory period
of the pixel before said selection period and after said non-selection period, wherein
said second higher voltage in said compensatory period has a polality oposite to that
of said first higher voltage in said selection period, a root-mean - square (RMS)
of said second higher voltage in said compensatory period and a RMS of said first
higher voltage in said selection period is in a relation that: the RMS of said second
higher voltage is large when the RMS of said first higher voltage is small, and is
small when the RMS of said first higher voltage is large.
[0031] In this case, the relationship between the compensatory charge voltage and the compensatory
charge voltage is and the compensatory charge voltage preferably in a complementary
relationship with the data charge voltage in view of a display gradation.
[0032] In accordance with the present invention, a compensatory voltage is applied to each
pixel immediately before the data write period so that the liquid crystal layer of
the pixel is charged with a compensatory charge voltage of a polarity opposite to
that of the data charge voltage in the data write period. Thus, a large voltage is
applied to the non-linear element immediately before the data write period and, moreover,
this compensatory voltage is preferably set to have a complementary relationship in
view of the display gradation with the voltage to be charged into the liquid crystal
layer of the pixel during the write. This means that the I-V characteristic shift
of the non-linear element can be kept substantially uniform, irrespective of what
the pixel is displaying, and that the occurrence of afterimages caused by the magnitude
of the I-V characteristic shift can thus be controlled.
[0033] The method of the present invention is suitable for driving a liquid crystal display
devices such as a TV, a display of personal computor, a projector, a head mounted
display or a printer having a liquid crystal shutter etc..
[0034] When the method of the present invention is carried out, the root-mean-square values
of the voltages applied to the two-terminal element could be set so that they are
substantially equal for each two-terminal element of the pixels. If the period of
time during which the compensatory voltage is applied is a compensatory period TA
and the period of time during which the data is written is a data write period TB,
the temporal ratio of the period TA to the period TB can be used to adjust the root-mean-square
values of the voltages to be applied to the two-terminal element. Alternatively, the
absolute values of the potentials of the scanning signal during the periods TA and
TB can be made different, and these scanning signal potentials can be used to adjust
the root-mean-square values of the voltages to be applied to the two-terminal element.
In a further alternative, the potentials of the data signal during the periods TA
and TB can be made different, and these data signal potentials can be used to adjust
the root-mean-square values of the voltages applied to the two-terminal element.
[0035] Noise can be counteracted by either making
, where (TA + TB) equals one horizontal scanning period, or by making sure that the
period TA is in a flyback period of the video signal along the time axis. In other
words, any noise that may be caused by the large voltage change at the boundary between
the compensatory voltage and the write voltage of different polarities can be superimposed
in the flyback period of the video signal, so that it does not appear as noise on
the liquid crystal panel.
[0036] When the present invention is carried out, the data signal is set to a voltage corresponding
to a display gradation in each horizontal scanning period and is also at the same
voltage level within one horizontal scanning period; and the scanning signal is set
to be such that the period TA and the data write period TB have different polarities
with respect to a middle potential of the data signal in the voltages each of which
is charged into the liquid crystal layer.
[0037] When the difference is obtained between the data signal which remains at the same
voltage level within one horizontal scanning period and the scanning signal which
gives different polarities to the voltages to be charged into the liquid crystal layer
in the periods TA and TB, the voltage applied to the pixel in the period TA and that
applied in the period TB are in a complementary relationship in view of the display
gradation regarding the voltage that is charged into the liquid crystal layer of the
pixel. Therefore, by simply improving the waveform of the scanning signal, it becomes
possible to control the afterimage phenomenon based on the principle of the method
according to the present invention.
[0038] In this case too, by setting the time ratio between the periods TA and TB, or by
setting the potentials of the scanning signal in the periods TA and TB, it is possible
to adjust the root-mean-square values of the voltages to be applied to the two-terminal
element to be substantially equal for each of the two-terminal elements of the pixels.
Therefore, the I-V characteristic shift of the non-linear element can be made always
uniform, irrespective of the type of display, and thus afterimages can be eliminated.
[0039] When the absolute values of the potential of the scanning signal in the periods TA
and TB with respect to a middle potential of the data signal are made equal, the relationship
could be such that TA > TB. Alternatively, if the times of the periods TA and TB are
substantially equal, and the voltage of the scanning signal with respect to a middle
potential of the data signal in the period TA is VTA and that in the period TB is
VTB, these voltages could be set such that |VTA| > |VTB|. In either case, the root-mean-square
values of the voltages to be applied to the two-terminal element can be made to approach
a situation in which they are substantially equal with respect to the two-terminal
element of each pixel.
[0040] If the voltage of the data signal is not made uniform in one horizontal scanning
period, but is set to a voltage corresponding to a display gradation in the period
TB, and to a voltage of an absolute value greater than that in the period TB in the
period TA, the root-mean-square values of the voltages to be applied to the two-terminal
element can be made to approach a situation in which they are substantially equal
for each two-terminal element of the pixels. In this way, the absolute value of the
scanning signal with respect to a middle potential of the data signal can be made
substantially equal in the periods TA and TB.
[0041] The method of the present invention can also be applied to a case in which the data
signal has a potential Von that is supplied a voltage of a large absolute value onto
each pixel or a potential Voff that is supplied a voltage of a small absolute value
thereto based on the polarity of the scanning signal in the data write period TB with
respect to a middle potential of the data signal, and works as a pulse-width modulation
signal that varies the pulse width of the potential Von in the period TB to correspond
with the voltage that is charged into the liquid crystal layer of each pixel. In this
case, the data signal has two potentials, Von or Voff in the period TB, and duty of
the pulse widths of the potential Von to the period TA is substantially equal to duty
of the pulse width of potential Von to the period TB.
[0042] In this way, the data signal is set to be such that the duties of the pulse widths
of the potential Von to the periods TA and TB are substantially equal and the scanning
signal is set to be such that the periods TA and TB have different polarities on the
voltages each of which is charged into the liquid crystal layer of the pixel, so that
the values of the difference signal between the data signal and the scanning signal
in the periods TA and TB are in a complementary relationship in view of the display
gradation, and thus afterimages can be controlled on the basis of the principle of
the present invention.
[0043] In this case, a difference signal between the data signal and the scanning signal
comprised a period ToffA which corresponds to the pulse width of the potential Von
of the data signal in the period TA, a period TonA which is is the rest of that period
TA (i.e.,
), a period TonB which corresponds to the pulse width of the potential Von of the
data signal in the period TB, and a period ToffB which is the rest of that period
TB (i.e.,
), and the ratios TonA/TA and TonB/TB are substantially in a complementary relationship.
[0044] By setting TonA, ToffA, TonB, and ToffB periods in each of the periods TA and TB
of the difference signal to be applied to the pixel, as described above, the difference
signals in the periods TA and TB can be made to be in a complementary relationship
in view of the display gradation, so afterimages can be controlled on the basis of
the principle of the present invention.
[0045] When the data signal is used as a pulse width modulation signal as described above,
the initial part of the period TB should be the period ToffB and the ending part thereof
should be the period TonB. However, the period TA can be set so that the period TonA
is either the initial part of the period TA or the ending part thereof. Setting the
period TonA to the initial part of the period TA is considered superior, from the
point of view that it is comparatively easy to generate the data signal in that case.
[0046] When the data signal is used as a pulse width modulation signal as described above,
the root-mean-square values of the voltages to be applied to the two-terminal element
can be adjusted so that they are substantially uniform for each of the two-terminal
elements of the pixels by setting the time ratio of the period TA to the period TB
or by making the potentials of the scanning signal different in the periods TA and
TB.
[0047] If the data signal has a potential VonB that supplies a voltage of a large absolute
value to each pixel or a potential VoffB that supplies a voltage of a small absolute
value thereto based on the scanning signal, and if the data signal works as pulse-width
modulation signal that varies the pulse width of the potential VonB in the period
TB to correspond with the voltage that is charged into the liquid crystal layer of
each pixel, the data signal can also have potential VonA or VoffA in the period TA
of absolute values greater than those of the corresponding potentials VonB or VoffB,
respectively, and the duty of the pulse widths of the potential VonA to the period
TA is substantially equal to duty of the pulse width of potential VonB to the period
TB.
[0048] The method of the present invention can be used with a two-terminal element that
has a metal-insulator-metal layer structure (an MIM element), a metal-insulator-semiconductor
layer structure (an MIS element), one type of MIM element comprises, the insulator
layer which is preferably an oxide film formed by anodic oxidization in an electrolytic
liquid including phosphorus in a form such as phosphoric acid or ammonium phosphate.
Other type of MIM element comprises the insulator layer whih is a silicon nitride.
[0049] It has been confirmed experimentally that the above-described configurations can
greatly reduce the I-V characteristic shift in comparison with the driving method
shown by Fig. 6.
[0050] The insulator layer is also preferably formed by anodic oxidization of tantalum.
If an MIM element is used as the non-linear element, one of the metal layers thereof
can be made a transparent conductive layer, so that it can also function as a transparent
electrode of the liquid crystal panel.
BRIEF DESCRIPTION OF THE DRAWINGS
[0052] FIG. 1 is a structural diagram of a liquid crystal display device.
[0053] FIG. 2 is a structural diagram of a voltage selection switch within the row drive
circuit of the device of FIG. 1.
[0054] FIG. 3 shows the I-V characteristic of a non-linear element.
[0055] FIG. 4 is a timing chart in accordance with the prior art example of components shown
in FIG. 1.
[0056] FIG. 5 is a timing chart in accordance with the prior art example of the row drive
circuit.
[0057] FIG. 6 is a drive waveform chart in accordance with the prior art example of the
liquid crystal panel.
[0058] FIG. 7 is a diagram illustrating the I-V characteristic shift of a non-linear element.
[0059] FIG. 8 shows a window being displayed on a liquid crystal panel.
[0060] FIG. 9 shows an intermediate display on a liquid crystal panel.
[0061] FIG. 10 is a drive waveform chart in accordance with the prior art example, illustrating
the generation of an afterimage due to the I-V characteristic shift of non-linear
elements.
[0062] FIG. 11 is a structural diagram of another example of a liquid crystal display device.
[0063] FIG. 12 is a drive waveform chart of the prior art liquid crystal panel used in the
device of FIG. 11.
[0064] FIG. 13 is a drive waveform chart in accordance with the prior art example, illustrating
the generation of an afterimage due to the I-V characteristic shift of non-linear
elements.
[0065] FIG. 14 is a timing chart of driving in the row direction, in accordance with a first
embodiment of the present invention.
[0066] FIG. 15 is a drive waveform diagram of the liquid crystal panel in accordance with
the first embodiment of the present invention.
[0067] FIG. 16 illustrates the principle of afterimage reduction in accordance with the
first embodiment of the present invention.
[0068] FIG. 17 is a drive waveform diagram used to reduce afterimages in accordance with
the first embodiment of the present invention.
[0069] FIG. 18 is a timing chart of driving in the row direction, in accordance with a second
embodiment of the present invention.
[0070] FIG. 19 is a drive waveform diagram of the liquid crystal panel in accordance with
the second embodiment of the present invention.
[0071] FIG. 20 illustrates the principle of afterimage reduction in accordance with the
second embodiment of the present invention.
[0072] FIG. 21 is a drive waveform diagram used to illustrate that the non-linear element
in accordance with the second embodiment of the present invention can reduce afterimages.
[0073] FIG. 22 is a timing chart of driving in the row direction, in accordance with a third
embodiment of the present invention.
[0074] FIG. 23 illustrates the principle of afterimage reduction in accordance with the
third embodiment of the present invention.
[0075] FIG. 24 is a drive waveform diagram using to illustrate that the non-linear element
in accordance with the third embodiment of the present invention can reduce afterimages.
[0076] FIG. 25 is a drive waveform diagram of a liquid crystal panel in accordance with
the fourth embodiment of the present invention.
[0077] FIG. 26 is a circuit diagram of an X driver using the fourth embodiment of the present
invention.
[0078] FIG. 27 is a characteristic graph used to illustrate the signal changes in the various
components of FIG. 26.
[0079] FIG. 28 is a timing chart of the operation of the grayscale signal generation circuit
of FIG. 26.
[0080] FIG. 29 is divided into FIG. 29A to FIG. 29D, each used to illustrate the drive circuits
for generating the column electrode signal Xm, or the operation thereof.
[0081] FIG. 30 is a drive waveform diagram used to illustrate a fifth embodiment of the
present invention.
[0082] FIG. 31 is a drive waveform diagram of a liquid crystal panel in accordance with
a sixth embodiment of the present invention.
[0083] FIG. 32 is another drive waveform diagram of the liquid crystal panel in accordance
with the sixth embodiment of the present invention.
[0084] FIG. 33 is a diagram used to illustrate how the X driver of FIG. 31 and FIG. 32 generates
the column electrode signal.
[0085] FIG. 34 is a plan view of a non-linear element of a seventh embodiment of the present
invention.
[0086] FIG. 35 is a cross-sectional view of the non-linear element of the seventh embodiment
of the present invention.
[0087] FIG. 36 is a graph of measurements taken of afterimage levels in the seventh embodiment
of the present invention and an example of the prior art.
[0088] FIG. 37 is a circuit diagram of a variation of the X driver using in the fourth embodiment
of the present invention.
DETAILD DESCRIPTION OF THE PREFERRED ENBODIMENTS
First Embodiment
[0089] A first embodiment of the present invention is an improvement to the drive method
used by the liquid crystal display device of FIG. 1, and its internal configuration
and functions are those of the device of FIG. 1 that has already been described. Note
that the non-linear elements 114 of FIG. 1 and FIG. 11 that are used in the method
of driving a liquid crystal display device in accordance with the present invention,
as described as embodiments 1 to 6 herein, could be MIM elements, MIS elements, varistors,
ring diodes, or back-to-back diodes. Note also that an MIM element could be configured
with one metal layer thereof being a transparent conductive layer which can be made
to serve also as a transparent electrode of the liquid-crystal panel. Further, the
insulator layer of the MIM element could be a silicon nitride. In these embodiments,
scanning signals are supplied to row electrodes, and data signals are supplied to
column electrodes as shown in Fig. 11. Alternatively, it is also possible to supply
scanning signals to column electrodes, and to supply data signals to row electrodes.
[0090] The timing chart of the first embodiment is illustrated in FIG. 14 and FIG. 15. FIG.
14 is a timing chart of the Y driver which is necessary for carrying out the first
embodiment of the present invention. Note that the DY, YSCL, FRY, C1, and Cn signals
are the same as those that have already been described with reference to FIG. 5. Y
driver outputs the selection voltage signal Vs as the output Vn when the selection
signal Cn is 1 in the same way, but in order to carry out this first embodiment, the
phase relationship between the selection signal Cn and the AC inversion signal FRY
is changed. In the prior art drive (see FIG. 5), the timing at which Cn and FRY switch
is the same, but in this first embodiment their phase relationship is shifted so that
the phase of FRY is changed in the time period when Cn = 1. FIG. 14 illustrates this
change in detail. The left-hand side of FIG. 14 shows that when Cn is 1 and the selection
voltage signal Vs is being output as the output Vn, FRY switches from 1 to 0. Since
the potential of the selection voltage signal Vs switches by switching of FRY (when
FRY = 1, Vs = Vp; when FRY = 0, Vs = -Vp), the potential -Vp changes into Vp after
switching of FRY. Similarly, the right-hand side of FIG. 14 shows the opposite case
for Yn when Cn = 1, where FRY switches from 0 to 1. In this case, the potential Vp
changes into -Vp after the switch in FRY. In this embodiment, the switch in FRY is
set to occur in the ending part of the period during which Cn = 1, so the duty ratio
is increased in this ending part, but this change in the phase relationship between
Cn and FRY enables this duty ratio to be freely selected.
[0091] The actual drive created by combining the operation of the above-described Y driver
and the X driver will now be described with reference to FIG. 15. This figure shows
the waveforms of signals applied to the pixel that is the mth pixel in the horizontal
direction and the nth pixel in the vertical direction, in the liquid crystal panel
115 of FIG. 1, in the same manner as already described for FIG. 6. The column electrode
signal Xm sequentially samples the AC video signal 104, holds the data for one horizontal
scanning period, then outputs the data, in the same way as in the prior art, so that
the output data itself represents the data for the previous horizontal scanning period.
The row electrode signal Yn is the same as the signal in FIG. 11, which was described
above. In this case, one horizontal scanning period is the period obtained by multiplying
one field period by the number of data lines (the number of column electrodes). If
a rest period is set for a certain column electrode, only column electrodes that do
not have a rest period set for them are included in the calculation.
[0092] Now look at the difference signal Xm - Yn between the column electrode signal Xm
and the row electrode signal Yn, both of which are actually applied to the non-linear
element 114 and liquid crystal layer 113. The column electrode signal Xm outputs the
same level within one horizontal scanning period, but the polarity of the row electrode
signal Yn inverts greatly during one horizontal scanning period. Therefore, the polarity
of the difference signal also changes greatly between positive and negative, as shown
at the bottom of FIG. 15. On the left-hand side of FIG. 15, large potential of positive
polarity is applied to the pixel, then the potential swings over greatly to the negative
polarity side. On the right-hand side of FIG. 15, after application of large negative
potential to the pixel, the potential changes greatly to positive polarity. It can
be seen from FIG. 15 that the pixel is selected twice, i.e., in two selection periods,
TA and TB, within one horizontal scanning period. When the non-linear element 114
is selected twice, the second selection determines the gradation value. Therefore,
as shown in FIG. 15, it is the selection of the ending period TB that determines the
display gradation (in the example on the right-hand side of the figure, this is a
write at positive polarity side). This means that, with the drive of FIG. 15, a display
gradation (or rather, the root-mean-square value actually applied to the liquid crystal
layer 113) has the same effect as that of FIG. 6.
[0093] A point that should be noted is the potential of the difference signal Xm - Yn in
each of the initial period TA and the ending period TB of one horizontal scanning
period. As described above, the display gradation is determined by the ending selection.
Regarding the initial selection, the selection data for the initial period TA is the
complement of the data of the ending period TB in this first embodiment. Note that
in this case, "complement" is defined here as either of two numbers which together
achieve a level of 100%. For example, 0 is the complement of 1, and 0.9 is the complement
of 0.1. The state of the column electrode signal described below shows the reason
for this complementary relationship.
[0094] In the following description, the pixels show a white display when a voltage equal
to or more than the threshold voltage is applied to the liquid crystal layer and show
a black display when a voltage less than the threshold voltage is applied to the liquid
crystal layer. This mode of display is defined here as a negative display. In the
case of applying a voltage of positive polarity to the liquid crystal layer, the potential
of the column electrode signal is Va when the pixels show a white display and -Va
when the pixels show a black display. Conversely, in the case of applying a voltage
of negaive polarity to the liquid crystal layer, the potential of the column electrode
signal is -Va when the pixels show a white display and Va when the pixels show a black
display. When the data to be displayed is white and a voltage to be applied is positive
polarity, the potential of the column electrode signal is Va. When the data to be
displayed is black and a voltage to be applied is negative polarity, the potential
of the column electrode signal is also Va. In other words the complement relationship
defined above for the column electrode signal is valid when viewed from the viewpoint
of positive and negative polarity. It should be clear from FIG. 15 that an intermediate
display would function in exactly the same manner. In such a case, with the drive
of FIG. 15, to correspond with the selection that will determine the display gradation
in the ending period TB (hereinafter referred to as the "actual selection"), data
that is the complement thereof is written in the initial period TA (hereinafter referred
to as the "compensatory selection"). This operation enables a reduction in afterimages
caused by I-V characteristic shift in the non-linear elements.
[0095] The principle by which the first embodiment can reduce the afterimages in the non-linear
elements caused by I-V characteristic shift will now be described in detail with reference
to FIG. 16. The description first concerns a white display, shown in the left-hand
side of FIG. 16. In order to display white, the write potential for the actual selection
in the ending period TB is at a maximum at |Vp + Va|. The compensatory selection in
the initial period TA writes data that is the complement thereof, and so is at the
minimum potential of |Vp - Va|. Conversely, when black is displayed (corresponding
to the right-hand side of FIG. 16), the write potential for the actual selection in
the ending period TB is at a minimum at |Vp - Va|, and the compensatory selection
in the initial period TA is at a maximum of |Vp + Va|. When white is selected, a large
potential is applied to the non-linear element during the actual selection, but only
a small potential is applied to the non-linear element in the preceding compensatory
selection. Conversely, when black is selected, the potential to be applied in the
actual selection is small, but a large potential is applied to the non-linear element
during the compensatory selection. This means that the total potential applied to
the non-linear element can be made the same, regardless of whether black or white
is selected.
[0096] To be precise, if the effective voltage VmW for white display and the effective voltage
VmB for black display which are applied to the non-linear element are made the same,
the I-V characteristic shift of the non-linear element can be made uniform, and the
phenomenon of afterimages caused by I-V characteristic shift can be eliminated. Writing
the conditions of FIG. 16 as an equation gives:
Therefore, it is best to ensure that the values in the initial period TA (the compensatory
selection) and the ending period TB (the actual selection) satisfy the above equation.
In general, in the beginning of the compensatory selection period, the potential applied
to the non-linear element changes to the compensatory selection potential from that
applied in the non-selection period. On the left-hand side of FIG. 16, this potential
goes from -2Va to -(Vp - Va); on the right-hand side, it goes from 0 to -(Vp + Va).
In the beginning of the actual selection, however, the potential applied to the non-linear
element changes much more, from the compensatory selection potential to the actual
selection potential. On the left-hand side of FIG. 16, this potential goes from -(Vp
- Va) to (Vp + Va); on the right-hand side, it goes from -(Vp + Va) to (Vp - Va).
Therefore, a larger potential is applied to the non-linear element for the actual
selection, and consequently TA > TB.
[0097] The enabling of a dramatic reduction in afterimages due to I-V characteristic shift
of the non-linear element will now be described with reference to the example of FIG.
17. In the same way as described with reference to FIG. 10, the signal applied to
pixel P1 (in other words, Xm1 - Yn) and the signal applied to pixel P2 (in other words,
Xm2 - Yn) are shown in FIG. 17. Pixel P1 shows a black display and pixel P2 shows
a white display. When the display is switched to an intermediate display at this point,
the principle described above dictates that the effective voltage VmsB applied to
pixel P1 and the effective voltage VmsW applied to pixel P2 are equal, so the magnitude
of I-V characteristic shift in the non-linear elements thereof is similarly equal.
Therefore, the effective voltage S1 applied to pixel P1 and the effective voltage
S2 applied to pixel P2 during intermediate display are also equal, and thus the difference
in brightness described with reference to FIG. 10 can be eliminated.
[0098] Note that, for the purpose of facilitating understanding of this embodiment of the
present invention, only the cases of black and white display were described, but the
present invention is also capable of equalizing the I-V characteristic shift of non-linear
elements under all display conditions. This means that the phenomenon of afterimages
can be reduced, irrespective of the type of display.
Second Embodiment
[0099] FIG. 18 is a timing chart of the Y driver which is necessary for carrying out a second
embodiment of the present invention. Note that the DY, YSCL, FRY, C1, and Cn signals
are the same as those that have already been described with reference to FIG. 5. The
duty ratio of YSCL is 50% and it is used as a switching signal to switch the potential
of the signal Vs from ±Vr to ±Vp. In other words, Vs is output in accordance with
the following relationships:
When YSCL (switching signal) = 0 with FRY = 0:
When YSCL (switching signal) = 1 with FRY = 0:
When YSCL (switching signal) = 0 with FRY = 1:
When YSCL (switching signal) = 1 with FRY = 1:
Y driver outputs the selection voltage signal Vs as the output Vn when the selection
signal Cn is 1 but, in order to carry out this second embodiment, the phase relationship
between the selection signal Cn and the AC inversion signal FRY is changed to make
sure that |Vr| > |Vp|. In the prior art drive, the timing at which Cn and FRY switch
is the same, but in this second embodiment, their phase relationship is shifted so
that the phase of FRY is changed in the time period when Cn = 1. FIG. 18 illustrates
this change in detail. The left-hand side of FIG. 18 shows the selection voltage signal
Vs output as Yn when Cn is 1. At this point, the above relationships dictate that
Vs is -Vr and ouptputs relatively larger voltage than -Vp. Next, in the period in
which Cn = 1, FRY switches from 1 to 0. At the same timing, YSCL (the Vr and Vp switching
signal) switches from 0 to 1, so that Vp is output as the selection voltage signal
Vs for Yn. In other words, after the Yn output is -Vr, the switch in FRY changes it
to Vp. The right-hand side of FIG. 18 shows the opposite case for Yn when Cn = 1;
where FRY switches from 0 to 1 and YSCL also switches from 0 to 1. In this case, Yn
outputs -Vp by switching in FRY after Yn outputs Vr. In this embodiment, the switch
in FRY is set to occur in the middle of the period when Cn = 1, in such a manner that
this period is divided evenly, but this duty ratio can be freely selected.
[0100] The actual drive created by combining the operation of the above-described Y driver
and the X driver will now be described with reference to FIG. 19. FIG. 19 shows the
waveforms of signals applied to the pixel that is the mth pixel in the horizontal
direction and the nth pixel in the vertical direction, in the liquid crystal panel
115 of FIG. 1, in the same manner as already described for FIG. 6. The column electrode
signal Xm sequentially samples the AC video signal 104, holds the data for one horizontal
scanning period, then outputs the data, in the same way as in the prior art, so that
the output data itself represents the data for the previous horizontal scanning period.
The row electrode signal Yn is the same as the signal in FIG. 18, which was described
above.
[0101] Now look at the difference signal Xm - Yn between the column electrode signal Xm
and the row electrode signal Yn, both of which are actually applied to the non-linear
element 114 and liquid crystal layer 113. The column electrode signal Xm outputs the
same level within one horizontal scanning period but the polarity of the row electrode
signal Yn inverts greatly during one horizontal scanning period. Therefore, the polarity
of the difference signal also changes greatly between positive and negative, as shown
at the bottom of FIG. 19. Therefore, in the same manner as in the first embodiment,
there are two selection periods, TA and TB, within one horizontal scanning period.
When the non-linear element 114 is selected twice, the second selection determine
the gradation value. Therefore, as shown in FIG. 19, it is the selection of the ending
period TB that determines the display gradation (in the example on the right-hand
side of the figure, this is a write at positive polarity side). This means that, with
the drive of FIG. 19, a display gradation (or rather, the root-mean-square value actually
applied to the liquid crystal layer 113) has the same effect as that of FIG. 6. The
selection data for the initial period TA is the complement of the data of the ending
period TB, in the same manner as in the first embodiment. The reason for this is the
same as that given for the first embodiment. Therefore, with the drive of FIG. 19,
corresponding to the actual selection for determining the display gradation in the
ending period TB, the complementary data of the actual selection is written in the
compensatory selection in the initial period TA. This operation enables a reduction
in afterimages caused by I-V characteristic shift in the non-linear elements.
[0102] The principle by which the second embodiment can reduce the afterimages in the non-linear
elements caused by I-V characteristic shift will now be described in detail with reference
to FIG. 20. The description first concerns a white display, shown in the left-hand
side of FIG. 20. In order to show a white display by the pixels, the write potential
for the actual selection in the ending period TB is at a maximum at |Vp + Va|. The
compensatory selection in the initial period TA writes data that is the complement
thereof, and so is at a minimum potential within the compensatory voltage |Vr - Va|.
Conversely, when the pixels show a black display (corresponding to the right-hand
side of FIG. 20), the write potential for the actual selection in the ending period
TB is at a minimum at |Vp - Va|, and the compensatory selection in the initial period
TA is at a maximum of |Vp + Va|. When white is selected, a large potential is applied
to the non-linear element during the actual selection, but only a small potential
is applied to the non-linear element in the preceding compensatory selection. Conversely,
when black is selected, the potential to be applied for the actual selection is small,
but a large potential is applied to the non-linear element during the compensatory
selection. In other words, this means that the total potential applied to the non-linear
element can be made the same, regardless of whether black or white is selected.
[0103] To be precise, if the effective voltage VmW for white display and the effective voltage
VmB for black display that are applied to the non-linear element are made the same,
the I-V characteristic shift of the non-linear element can be made uniform, and the
phenomenon of afterimages caused by I-V characteristic shift can be eliminated. Writing
the conditions of FIG. 20 as an equation gives Equation 3.
[0104] Therefore, it is best to determine a selection voltage |Vr| for the compensatory
selection that satisfies Equation 3. In general, in the beginning of the compensatory
selection period, the potential applied to the non-linear element changes to the compensatory
selection potential from that applied in the non-selection period. On the left-hand
side of FIG. 20, this potential goes from -2Va to -(Vr - Va); on the right-hand side,
it goes from 0 to -(Vr + Va). In the beginning of the actual selection, however, the
potential applied to the non-linear element changes much more, from the compensatory
selection potential to the actual selection potential. On the left-hand side of FIG.
20, this potential goes from -(Vp - Va) to (Vp + Va); on the right-hand side, it goes
from -(Vr + Va) to (Vp - Va). Therefore, a larger potential is applied to the non-linear
element for the actual selection, and consequently Vr > Vp.
[0105] The enabling of a dramatic decrease in afterimages due to I-V characteristic shift
of the non-linear element will now be described with reference to the example of FIG.
21. In the same way as described with reference to FIG. 10, the signal applied to
pixel P1 (in other words, Xm1 - Yn) and the signal applied to pixel P2 (in other words,
Xm2 - Yn) are shown in FIG. 21. Pixel P1 shows a black display and pixel P2 shows
a white display. When the display is switched to an intermediate display at this point,
the principle described above dictates that the effective voltage VmsB applied to
pixel P1 and the effective voltage VmsW applied to pixel P2 are equal, so the magnitude
of I-V characteristic shift in these non-linear elements is similarly equal. Therefore,
the effective voltage S1 applied to pixel P1 and the effective voltage S2 applied
to pixel P2 during intermediate display are also equal, and thus the difference in
brightness described with reference to FIG. 10 can be eliminated.
[0106] Note that, for the purpose of facilitating understanding of this second embodiment
of the present invention, only the cases of black and white display were described,
but the present invention is also capable of equalizing the I-V characteristic shift
of non-linear elements under all display conditions. This means that the phenomenon
of afterimages can be reduced, irrespective of the type of display.
Third Embodiment
[0107] A third embodiment of the present invention is an improvement to the drive method
used by the liquid crystal display device of FIG. 11.
[0108] FIG. 22 is a timing chart of the Y driver which is necessary for carrying out a third
embodiment of the present invention.
[0109] Note that the DY, YSCL, FRY, C1, and Cn signals are the same as those that have already
been described with reference to FIG. 5. Y driver outputs the selection voltge signal
Vs as the output Vn when the selection signal Cn is 1 in the same way, but in order
to carry out this third embodiment, the phase relationship between the selection signal
Cn and the AC inversion signal FRY is changed.
[0110] The example shown in FIG. 22 differs from that of the first embodiment shown in FIG.
14 in that the switching of FRY is set to be at the center of the period during which
Cn is 1, so the duty ratio of Yn is uniform. Note that this duty ratio can be freely
selected by changing the phases of Cn and FRY.
[0111] The actual drive created by combining the operation of the above-described Y driver
and the X driver will now be described with reference to FIG. 23. FIG. 23 shows the
waveforms of signals applied to the pixel that is the mth pixel in the horizontal
direction and the nth pixel in the vertical direction, in the liquid crystal panel
115 of FIG. 11, in the same manner as already described for FIG. 12. The column electrode
signal Xm sequentially samples the video signal in the A/D converter 120 and outputs
Von and Voff periods based on that data. In this third embodiment, one horizontal
scanning period is divided into two parts, TA and TB, with the selection data for
the initial period TA being the complement of the data of the ending period TB. Since
the AC inversion signal FRX switches within one horizontal scanning period, the data
of the initial part of the actual column electrode signal is symmetrical with the
ending data about the axis of this switch in FRX. Therefore, the potential Von of
the column electrode signal Xm in the write period TB is Va in the waveform shown
in FIG. 23 so that a relatively large voltage is applied to the pixel in connection
with the polarity of the scanning signal in the period TB, but the column electrode
signal Xm has a potential Von of a substantially equal pulse width in each of the
periods TA and TB. The description of this embodiment deals with the case in which
(and thus
horizontal scanning period), but the drive of the third embodiment of the present
invention can equally well be applied to the case in which TA ≠ TB. The row electrode
signal Yn is the signal described with reference to FIG. 22.
[0112] Since the polarity of the row electrode signal Yn inverts hugely in one horizontal
scanning period, the polarity of the difference signal Xm - Yn between the column
electrode signal Xm and the row electrode signal Yn, both of which are actually applied
to the non-linear element 114 and liquid crystal layer 113 also changes greatly between
positive and negative, as shown at the bottom of FIG. 23. It can be seen from this
figure that the pixel is selected twice, i.e., in two selection periods, TA and TB,
within one horizontal scanning period. In FIG. 23 too, it is the selection of the
ending period TB that determines the display gradation (in the example on the right-hand
side of FIG. 23, this is a write at positive polarity side). This means that, with
the drive of FIG. 23, a display gradation (or rather, the root-mean-square value actually
applied to the liquid crystal layer 113) has the same effect as that of FIG. 12.
[0113] In the drive example in accordance with the third embodiment of the present invention
as well, the compensatory selection in the initial period TA is the complement of
the actual selection in the ending period TB. When the mode of display is the negative
display, the left-hand side of FIG. 23 shows an example where the pixel shows an intermediate
display close to white. In the selection of the ending period TB, the ratio of Von
to Voff is comparatively large and thus a large root-mean-square value is applied
to the liquid crystal layer 113 of FIG. 11. However, since the complementary data
described above is selected in the selection in the initial period TA, a wide pulse
of the same potential as the potential Von in the ending period TB functions as the
Voff potential when it is applied to the pixel, so that in effect the selection is
such that the ratio with Von is small. Conversely, in the example shown on the right-hand
side of FIG. 23, where the pixels' showing is close to black, the ratio of Von to
Voff in the actual selection in the ending part is small and thus a comparatively
small root-mean-square value is applied to the liquid crystal layer 113. However,
for the same reason as described above, the selection in the compensatory selection
of the ending part is in effect at a ratio of Von to Voff that is large.
[0114] To summarize the above description: in the selection for an intermediate display
close to white where a large root-mean-square value is required for the liquid crystal
layer, the root-mean-square value of the compensatory selection of the initial part
is small. Conversely, for a selection for an intermediate display close to black,
the root-mean-square value of the compensatory selection of the initial part is large.
In other words, in order to compensate in the compensatory selection of the initial
part for the actual selection in the ending part that is necessary for the actual
display, the root-mean-square values applied to the non-linear element 114 that is
changing from white to black can be made to be substantially uniform. This operation
enables a reduction in afterimages caused by I-V characteristic shift in the non-linear
elements.
[0115] To be precise, if the effective voltage VmW for white display and the effective voltage
VmB for black display that are applied to the non-linear element are made the same,
the I-V characteristic shift of the non-linear element can be made uniform, and the
phenomenon of afterimages caused by I-V characteristic shift can be eliminated. Writing
the conditions of FIG. 23 as an equation gives Equation 3, in the same way as with
the first and second embodiments.
[0116] Therefore, it is best to ensure that the values of the periods TA and TB of the compensatory
selection and the actual selection satisfy this equation. Note that if
and
and the ratio TonA/ToffA is substantially the reciprocal of TonB/ToffB, the data
of the compensatory selection is the complement of the data of the actual selection.
In general, in the beginning of the compensatory selection period, the potential applied
to the non-linear element changes to the compensatory selection potential from that
applied in the non-selection period. On the left-hand side of FIG. 23, this potential
goes from -2Va to -(Vp - Va); on the right-hand side, it goes from 0 to -(Vp + Va).
In the beginning of the actual selection, however, the potential applied to the non-linear
element changes much more, from the compensatory selection potential to the actual
selection potential. On the left-hand side of FIG. 23, this potential goes from -(Vp
- Va) to (Vp + Va); on the right-hand side, it goes from -(Vp + Va) to (Vp - Va).
Therefore, a larger potential is applied to the non-linear element for the actual
selection, and consequently TA > TB.
[0117] The enabling of a dramatic decrease in afterimages due to I-V characteristic shift
of the non-linear element will now be described with reference to the example of FIG.
24. In the same way as described with reference to FIG. 13, the signal applied to
pixel P1 (in other words, Xm1 - Yn) and the signal applied to pixel P2 (in other words,
Xm2 - Yn) are shown in FIG. 24. Pixel P1 shows a black display and pixel P2 shows
a white display. When the display is switched to an intermediate display at this point,
the principle described above dictates that the effective voltage VmsB applied to
pixel P1 and the effective voltage VmsW applied to pixel P2 are equal, so the magnitude
of I-V characteristic shift in these non-linear elements is similarly equal. Therefore,
the effective voltage S1 applied to pixel P1 and the effective voltage S2 applied
to pixel P2 during intermediate display are also equal, and thus the difference in
brightness described with reference to FIG. 13 can be eliminated.
[0118] Note that, for the purpose of facilitating understanding of this third embodiment
of the present invention, only the cases of black and white display were described,
but the third embodiment of the present invention is also capable of equalizing the
I-V characteristic shift of non-linear elements under all display conditions. This
means that the phenomenon of afterimages can be reduced, irrespective of the type
of display.
Fourth Embodiment
[0119] A fourth embodiment of the present invention concerns a case where the signal waveform
of the difference signal Xm - Yn is changed as shown in FIG. 25 by changing the waveform
of the column electrode signal Xm in FIG. 23, which is the timing chart of the third
embodiment. In the third embodiment of the present invention, the data of the initial
period TA which is the compensatory selection is symmetrical with the data of the
ending period TB which is the actual selection, about the axis of the switch in FRX,
as shown in the column electrode signal Xm of FIG. 23. In this fourth embodiment of
the present invention, the same data is temporally compressed into, for example, half
the horizontal scanning period and is output twice in succession within one horizontal
scanning period, as shown by the column electrode signal Xm of FIG. 25. As a result,
in contrast to the difference signal Xm - Yn of the third embodiment shown in FIG.
23, in which the period TonA that governs Von of the compensatory selection is on
the right of the period TA of the compensatory selection, that of the fourth embodiment
shown in FIG. 25 is such that the period TonA that governs Von of the compensatory
selection is on the left of the period TA.
[0120] A configuration example of the X driver that outputs the column electrode signal
Xm shown in FIG. 25 is shown in FIG. 26. This figure shows a drive circuit that supplies
an M-bit video data signal to each of N data lines. In this figure, reference number
300 denotes a data bus that supplies the M-bit video data signal which inverts in
N cycles within one field, as shown by 421 in FIG. 27. The M-bit video data signal
is written to first memories 311 to 315 corresponding to addresses determined in accordance
with the outputs of N-stage shift registers 301 to 305. Reference numbers 351 to 355
in FIG. 26 denote output signals from the shift registers 301 to 305, respectively.
The output signals 351 to 355 are usually at 0 but become 1 once only within one field
to write the contents of the data bus 300 to the corresponding first memories 311
to 315.
[0121] The timing chart of FIG. 27 illustrates this state, where the output signals of the
shift registers 301 to 305 are shown as reference numbers 401 to 405, respectively.
The contents of the data stored in each of the first memories 311 to 315 are shown
as reference numbers 411 to 415, respectively. Note that the shading in FIG. 27 denotes
an undetermined status.
[0122] In FIG. 26 and FIG. 27, the video data signal on the data bus 300 is written to memory
311 at a timing T1, to memory 312 at a timing T2, and to memory 313 at a timing T3.
Thereafter, video data is written to each memory in turn until data is written to
the final-stage memory 315 at a timing Tn, which completes the operation of writing
video data to memory in this field. This one field of video data is equivalent to
the video data for one scan line.
[0123] In FIG. 26, a latch pulse (LP) 360 is input to second memories 321 to 325. This latch
pulse 360 acts to transfer the data that is in the first memories 311 to 315 to the
second memories 321 to 325. The signal waveform of the latch pulse 360 is shown as
reference number 422 in FIG. 27. During the time that this latch pulse 422 is high
level, the data in the first memories 311 to 315 is written in a batch to the second
memories 321 to 325. During the time it is low level, the data in the second memories
321 to 323 is held stable, as shown by data 423 in FIG. 27.
[0124] The operation that generates the column electrode signal Xm shown in FIG. 25, based
on the data in the second memories 321 to 325, will now be described with reference
to the timing chart of FIG. 28.
[0125] The second memories 321 to 325 each output M-bit data signals 371 to 375, as shown
in FIG. 26. These M-bit data signals 371 to 375 and a basic pulse train 361 (which
form constituent elements of a grayscale signal) are combined by grayscale signal
generation circuits 331 to 335 to generate individual-stage grayscale signals 381
to 385 (in other words, the column electrode signal Xm).
[0126] In this case, the basic pulse train 361 comprises a reset signal RES and a GCP signal
formed of, for example, 2
M pulses of a different pulse width, as shown in FIG. 28. The reset signal RES has
a pulse that goes high at the initial-period position and final-period position of
the horizontal scanning period, in the same manner as the latch pulse 422, as shown
in FIG. 28, and it also has a pulse at the central position of the horizontal scanning
period. The GCP signal are output in succession in one horizontal scanning period
as two identical signals, each compressed into the time of half the horizontal scanning
period. The reset signal RES and the GCP signal compound the outputs from the second
memories 321 to 325, as shown in FIG. 28, the signal 381 to 385 can be generated in
the form of a uniform waveform which reflects the grayscale data is output sequentially
within the horizontal scanning period so that the signals 381 to 385 can be generated.
Note that these grayscale signal generation circuits 331 to 335 can invert the polarity
of the data 381 to 385 within the horizontal scanning period, based on the polarity
inversion signal (FR).
[0127] The outputs 381 to 385 of the grayscale signal generation circuits 331 to 335 are
input to liquid crystal drive circuits 341 to 345. These liquid crystal drive circuits
341 to 345 generate liquid crystal drive signals 391 to 395 (i.e., the column electrode
signal Xm) of levels that are shifted from the grayscale signals 381 to 385 in accordance
with a voltage level 362 (i.e., Va or -Va) that turns the liquid crystal on and a
voltage level 363 (i.e., -Va or Va) that turns the liquid crystal off.
[0128] The X driver that outputs the column electrode signal Xm shown in FIG. 25 can be
configured as shown in FIG. 37. In this figure, a bidirectional shift register 601,
a first latch 602, a second latch 604, a decoder 605, a level shifter 607, and an
LCD driver 608 correspond to the shift registers 301 to 305, the first memories 311
to 315, the second memories 321 to 325, the grayscale signal generation circuits 331
to 335, and the liquid crystal drive circuits 341 to 345, respectively, of FIG. 26.
In FIG. 37, an enable controller 600 controls the bidirectional shift register 601
and a data controller 604 at the timing of a signal such as the latch pulse LP, to
implement the data transfer of FIG. 27.
[0129] The circuitry of FIG. 37 differs from that of FIG. 26 in that GCP is not a signal
having a 2
M pulse width, it is a signal having a pulse at a 2
Mth position in accordance with grayscale value, and in that the decoder 605 that inputs
the GCP signal through a grayscale controller 606 detects data 371 to 375 output from
the second latch 604 together with the GCP signal to generate the grayscale signals
381 to 385 shown in FIG. 28. Except for these points, the operation of the circuit
of FIG. 37 is the same as that shown in the timing chart of FIG. 28.
[0130] In this way, the fourth embodiment of the present invention can easily generate the
column electrode signal Xm that is configured by outputting a waveform which reflects
the grayscale data twice in succession within one horizontal scanning period, using
the relatively simple drive circuit shown in either FIG. 26 or FIG. 37. It can also
shape the difference signal Xm - Yn of the column electrode signal Xm and the row
electrode signal Yn (shown in FIG. 25) in such a manner that the period TonA for Von
in the compensatory period TA is on the left of the compensatory period TA. Note that
in this fourth embodiment too, the difference signal Xm - Yn shown in FIG. 25 divides
the horizontal scanning period into two periods, TA and TB, and the initial-part and
ending-part data is in a complementary relationship. This enables a reduction in afterimages
caused by I-V characteristic shift in the non-linear elements, in the same way as
in the third embodiment.
[0131] The drive circuit shown in FIG. 26 and used in this fourth embodiment is simple to
construct, as can easily be understood by a comparison with the drive circuit of the
third embodiment for generating the column electrode signal Xm shown in FIG. 23. The
waveform of the column electrode signal Xm used in the third embodiment is shown in
FIG. 29A. It reflects the grayscale data, but it has also been made symmetrical about
the boundary between the periods TA and TB. An example of the configuration of the
drive circuit for generating the waveform of FIG. 29A is shown in FIG. 29B. In this
case, a comparatively large memory such as a first-in, first-out (FIFO) memory 501
is necessary. The video signal that is the input signal to the FIFO memory 501 is
shown in FIG. 29C, and the output signal from the FIFO memory 501 is shown in FIG.
29D. The read clock of the FIFO memory 501 is set to be at twice the speed of the
write clock thereof. As a result, the output signal from the FIFO memory 501 has a
timing that is delayed by half the horizontal scanning period with respect to the
input signal shown in FIG. 29C, and the video signal of FIG. 29C is compressed to
half along the time axis so that the same signal waveform is output twice in succession.
The output from the FIFO memory 501 is input to an X driver 503 via a signal processing
circuit 502 that processes signal on the basis of an AC inversion signal FRX, so that
the symmetrical pulse waveform shown in FIG. 29A can be output as the column electrode
signal Xm.
Fifth Embodiment
[0132] A fifth embodiment of the present invention concerns an improvement which ensures
that any noise caused by the difference signal Xm - Yn of the first to fourth embodiments
(or rather, by the huge change in potential that occurs at the boundary between the
periods TA and TB) does not appear on the liquid crystal panel 115.
[0133] At the boundary between the periods TA and TB in the difference signal Xm - Yn, the
voltage experiences a huge shift from a negative potential to a positive one, as shown
in FIG. 30. It is feared that this huge potential difference generated in the liquid
crystal display device will be superimposed as noise in the video circuitry of the
previous stage, and also in the tuner circuitry and antenna of earlier stages, and
will eventually appear as noise on the liquid crystal screen. It is possible to remove
this noise by providing an exclusion circuit in the video circuitry, for example,
but this would make the structure complicated. The fifth embodiment of the present
invention solves this problem by placing the compensatory selection period TA of the
difference signal Xm - Yn so that it is included within the flyback period of the
video signal. In terms of the relationship between the compensatory selection period
TA and the horizontal scanning period (TA + TB), this can be expressed by:
Even more preferably, it is best if TA is no more than 15% of (TA + TB), to ensure
that the width of the period TA substantially matches the width of the flyback signal.
[0134] As shown in FIG. 30, the video signal and the difference signal Xm - Yn are synchronized.
Therefore, by making sure that the above equation is satisfied, the compensatory selection
pulse (i.e., the compensatory selection period TA) within the difference signal can
be set to substantially correspond to the flyback signal of the video signal along
the time axis. Then, even if noise caused by the huge potential difference within
the difference signal Xm - Yn is superimposed on the video signal, it is superimposed
on the signal during the flyback period. The signal within this flyback period is
not displayed on the liquid crystal screen, and thus it does not appear as noise on
the liquid crystal screen.
Sixth Embodiment
[0135] A sixth embodiment of the present invention concerns an improvement to the drive
waveform of the column electrode signal Xm, whereby the voltages applied to the two
ends of the non-linear element and the liquid crystal layer are made even larger during
the compensatory selection than during the actual selection.
[0136] An improved version of the column electrode signal Xm of FIG. 25, which shows the
waveforms of the fourth embodiment, is shown in FIG. 31. In this figure, the reference
potentials Vb and -Vb during the compensatory selection are set with respect to reference
potentials Va and -Va of the column electrode signal Xm during the actual selection
such that |Vb| > |Va|. In the example shown in FIG. 31, even when the amplitude of
the row electrode signal Yn changes by 2 x Vp, the potential of the difference signal
Xm - Yn during the compensatory selection is made relatively greater than that during
the actual selection.
[0137] Similarly, improved versions of the column electrode signal Xm and row electrode
signal Yn of FIG. 19, which shows the waveforms of the second embodiment, are shown
in FIG. 32. In this figure too, the reference potentials Vb and -Vb during the compensatory
selection are set with respect to reference potentials Va and -Va of the column electrode
signal Xm during the actual selection such that |Vb| > |Va|. The description based
on FIG. 32 refers to white display (corresponding to the left-hand side of the figure).
In order to show a white display by the pixels, the write potential is at a maximum
of |Vp + Va| during the actual selection. In this case, the potential of the compensating
voltage is |Vr - Vb| which is the minimum potential, because it is necessary to write
the complementary data to the selection data at the compensatory selection. Conversely,
when the pixels show a black display (corresponding to the right-hand side of the
figure), the write potential for the actual selection is at a minimum at |Vp - Va|
and the compensatory selection is at a maximum potential of |Vr + Vb|. Therefore,
when white is selected, a large potential is applied to the non-linear element during
the actual selection, but the non-linear element is subjected to only a small potential
in the preceding compensatory selection. Conversely, when black is selected, the potential
in the actual selection is small, but a large potential is applied to the non-linear
element during the compensatory selection. In other words, this means that the total
potential applied to the non-linear element can be made the same, regardless of whether
black or white is selected.
[0138] An example of the configuration of the drive circuit on the row electrode side is
shown in FIG. 33. In this figure, the drive circuit is provided with a first power
supply 701 for setting the reference voltage Va during the actual selection as the
power voltage for the X driver 700, and a second power supply 702 for setting the
reference voltage Vb during the compensatory selection. The configuration is such
that power is supplied to the X driver 700 by the switching of the two supplies by
a switch 703 that operates based on the AC inversion signal FR. In this case, the
configuration could be such that relatively low voltages are supplied, such as about
5 V by the first power supply 701 and about 6 V by the second power supply 702.
[0139] In contrast, when the potential during the compensatory selection of the row electrode
signal Yn is set to be greater than that during the actual selection, as in the second
embodiment of the present invention, the power supply voltage connected to the Y driver
must be large. This is particularly necessary when an MIM element is used as the non-linear
element, because an MIM element needs a much larger voltage such as 30 to 40 V, and
setting an even larger voltage on top of that will increase the load. It is possible
to enable a liquid crystal drive circuit that uses an MIM element to obtain such a
large voltage as 30 to 40 V by swinging a lower voltage to generate a large voltage,
but it is difficult to accurately generate the potential Vr for the complementary
selection by a voltage swing with respect to the potential Vp necessary during the
actual selection shown in FIG. 19. Therefore, an X driver that can set a difference
signal which has a larger potential in the compensatory selection than that in the
actual selection can be formed simply by adding a power source of a relatively low
voltage, and thus is it possible to accurately set a high voltage, with a drive circuit
of a simple configuration.
[0140] Note that the first to sixth embodiments of the present invention have been described
as having compensatory selection data in a period TA and actual selection data in
a period TB, obtained by a difference signal Xm - Yn, which are in a mutually complementary
relationship, but the present invention is not limited thereto. For instance, the
complementary relationship need not hold, so long as the relationship is such that,
when the data write charge voltage that charges the liquid crystal layer 113 during
the actual selection is large, the compensatory charge voltage applied to the liquid
crystal layer 113 during the compensatory selection is small and, conversely, when
the data write charge voltage is small, the compensatory charge voltage is large.
So long as this relationship between the actual selection data and the compensatory
selection data holds, the afterimage phenomenon can be controlled.
[0141] In addition, the first to sixth embodiments of the present invention can also be
carried out with polarity inversion techniques at every line that drive the device
in such a manner that the polarity of the voltage charged into the liquid crystal
layers of pixels in odd-numbered frames, for instance, is different from the polarity
of the voltage that charges the liquid crystal layers of pixels in even-numbered frames.
Seventh Embodiment
[0142] A plan view of the non-linear element (denoted by reference number 114 in FIG. 1
and FIG. 11) of one pixel in a liquid crystal panel that uses a seventh embodiment
of the present invention is shown in FIG. 34. In this figure, an MIM element using
tantalum-tantalum oxide-chromium as materials is used as the non-linear element, and
reference number 801 denotes a tantalum covered with tantalum oxide that also serves
as a column electrode. Reference number 802 denotes chromium patterns. An MIM element
is formed at an intersection between chromium patterns 802 and tantalum 801. Reference
number 803 denotes a transparent ITO pattern that forms a pixel electrode.
[0143] FIG. 35 is a cross-sectional view through the structure of the MIM element, taken
along the dot-dash line 804 in FIG. 34. Reference number 901 denotes a transparent
substrate, reference number 902 denotes a tantalum portion, and reference number 903
denotes a tantalum oxide portion.
[0144] The method of fabricating the MIM element of this configuration will now be described.
[0145] A tantalum layer is formed on the transparent substrate by sputtering, and this layer
is patterned to form a tantalum electrode that will be the column electrode. This
tantalum electrode is subjected to anodic oxidization to cover the surface thereof
with tantalum oxide. A diluted aqueous solution of phosphoric acid is used as the
electrolyte for the anodic oxidization.
[0146] An oxide layer of a uniform thickness is obtained by initially controlling the current,
then by applying a constant voltage. Subsequently, a chromium layer is sputtered and
patterned, then ITO that is to form a transparent pixel electrode is also sputtered
and patterned thereon. This enables the formation of an electrode substrate having
MIM element. A liquid crystal panel can be formed by pasting this substrate together
with a substrate formed of a resistance electrode, with a liquid crystal inserted
into a gap therebetween.
[0147] FIG. 36 is a graph of measurements done on the magnitude of afterimages in the present
embodiment. These measurements were done after the window display of FIG. 8 had been
left on the liquid crystal panel for a fixed time then the entire panel was switched
to the uniform display shown in FIG. 9. This graph shows variations in brightness
ratio for the points PI and P2 in FIG. 9.
[0148] Elapsed time after the display is switched to the uniform display is shown along
the X-axis on a logarithmic scale, and the Y-axis shows RΔT when the brightnesses
of the points P1 and P2 are TP1 and TP2, respectively, where RΔT expresses the degree
of brightness ratio and is defined as:
If a prior art drive method is defined as a, the drive method described as the
third embodiment of the present invention is A, a device using a liquid crystal panel
formed by a prior art anodic oxidization method is b, and a device using a liquid
crystal panel formed by the anodic oxidization method of the seventh embodiment is
B, the change in brightness ratio obtained by a combination of a and b is shown as
line 1, that for a and B is line 2, that for A and b is line 3, and that for A and
B (completely in accordance with the present invention) is line 4. Results obtained
by comparison with functional tests show that, if the brightness ratio RΔT is in general
initially 8% or less, afterimages cannot be discerned. Actual tests with display devices
in accordance with embodiments of the present invention in comparison with prior art
devices have proved that, provided fixed patterns were not held on display for excessively
long times, afterimages could not be discerned.
[0149] By forming the insulator layer that will become a non-linear resistance element by
anodic oxidization using an electrolytic liquid that includes phosphorus, it is possible
to greatly reduce the magnitude of the I-V characteristic shift in comparison with
a prior art non-linear element. This phenomenon has been confirmed by experimental
results, but the mechanism thereof has not yet been clarified. It is assumed that
perhaps the inclusion of phosphorus in an oxide film stabilizes the ranking of impurities
existing within the film, and thus the current flowing therethrough is also stabilized
by the Poole-Frenkel effect or Schottky effect. Note that the insulator layer described
above would have the same effect if it is the insulator layer of an MIS element. Note
also that if one of the metal layers of an MIM element is formed as transparent electrode
layer, it can also be used as a transparent electrode of the liquid crystal panel.
1. In a method of driving a liquid crystal display device that comprises a plurality
of first electrodes to which a scanning signal is applied, a plurality of second electrodes
to which a data signal is applied, and a plurality of pixels formed at a plurality
of intersections between said first and second electrodes, each of said pixels comprising
a liquid crystal layer and a two-terminal element having non-linear resistance characteristics
connected in series therewith, wherein:
(a) applying the voltage of a difference signal between said scanning signal and said
data signal to each of said pixels;
(b) applying a write voltage to each of said pixels based on said difference signal
during a data write period TB in which each of said first electrodes is selected and
said liquid crystal layer of each of said pixels is charged with a data charge voltage
corresponding to said data signal;
(c) applying a hold voltage of an absolute value smaller than said write voltage to
each of said pixels based on said difference signal during a data hold period after
said data write period TB; and
a method of driving a liquid crystal display device characterized in that:
(d) applying a compensatory voltage to each of said pixels during a compensatory period
TA based on said difference signal before said data write period TB, whereby a compensatory
charge voltage of a polarity opposite to that of said data charge voltage is charged
into said liquid crystal layer of each pixel said compensatory charge voltage being
set into a relationship with said data charge voltage such that said compensatory
charge voltage is small if said data charge voltage is large, but large if said data
charge voltage is small.
2. A method of driving a liquid crystal display device in accordance with claim 1, wherein
said compensatory charge voltage is substantially in a complementary relationship
with said data charge voltage.
3. A method of driving a liquid crystal display device in accordance with claim 1, wherein
a time ratio of said compensatory period TA to said data write period TB is set to
be such that the root-mean-square values of the voltages applied to said two-terminal
element are made substantially equal for each of said two-terminal elements of said
pixels.
4. A method of driving a liquid crystal display device in accordance with claim 1, wherein
a potential of said scanning signal in said compensatory period TA is different from
that in and said data write period TB, each of said potentials being set in such a
manner that the root-mean-square values of the voltages applied to said two-terminal
element are made substantially equal for each of said two-terminal elements of said
pixels.
5. A method of driving a liquid crystal display device in accordance with claim 1, wherein
a potential of said data signal in said compensatory period TA is different from that
in said data write period TB, each of said potentials being set in such a manner that
the root-mean-square values of the voltages applied to said two-terminal element are
made substantially equal for each of said two-terminal elements of said pixels.
6. A method of driving a liquid crystal display device in accordance with claim 3, wherein
said compensatory period TA and said data write period TB represent a first and second
parts of one horizontal scanning period that is divided into two, respectively, satisfying
following formula:
7. A method of driving a liquid crystal display device in accordance with claim 3, wherein
said period TA includes a flyback period of a video signal used for generating said
scanning signal and said data signal on a time axis.
8. A method of driving a liquid crystal display device in accordance with claim 1, wherein
said data signal is set to a voltage corresponding to a display gradation and is also
at the same voltage level within said period TA and said period TB immediately after
said period TA; and
said scanning signal in set to be such that said period TA and said period TB have
different polarities on the voltages each of which is charged into said liquid crystal
layer.
9. A method of driving a liquid crystal display device in accordance with claim 8, wherein
a time ratio of said period TA to said period TB is set to be such that the root-mean-square
values of the voltages applied to said two-terminal element are made substantially
equal for each of said two-terminal elements of said pixels.
10. A method of driving a liquid crystal display device in accordance with claim 9, wherein
said scanning signal is set to have a potential such that the absolute values thereof
in said periods TA and TB with respect to a middle potential of said data signal are
substantially equal, and the period TA > the period TB.
11. A method of driving a liquid crystal display device in accordance with claim 8, wherein
the absolute value of the potential of said scanning signal with respect to a middle
potential of said data signal is set to be different in each of said periods TA and
TB, and each of said potentials is set in such a manner that the root-mean-square
values of the voltages applied to said two-terminal element are made substantially
equal for each of said two-terminal elements of said pixels.
12. A method of driving a liquid crystal display device in accordance with claim 11, wherein
the time widths of said periods TA and TB are substantially equal, and if the voltage
in said period TA with respect to said middle potential of the data signal is VTA
and the voltage in said period TB with respect to said middle potential is VTB, said
voltages VTA and VTB are set such that:
|VTA| > |VTB|
13. A method of driving a liquid crystal display device in accordance with claim 1, wherein
said scanning signal is set to be such that said period TA and said period TB have
different polarities on the voltages each of which is charged into said liquid crystal
layer of the pixel;
in said period TB, said data signal is set to a voltage corresponding to a display
gradation, and in said period TA, said data signal is set to a voltage of an absolute
value that is greater than said voltage with respect to a middle potential of said
data signal in said period TB; and
the root-mean-square values of the voltages applied to said two-terminal element
are made substantially equal for each of said two-terminal elements of said pixels.
14. A method of driving a liquid crystal display device in accordance with claim 13, wherein
the absolute values of said scanning signal with respect to said middle potential
in said periods TA and TB are set to be equal.
15. A method of driving a liquid crystal display device in accordance with claim 1, wherein
said scanning signal is set to be such that said period TA and said period TB have
different polarities on the voltages each of which is changed into said liquid crystal
layer;
said data signal has a potential Von that supplies a voltage of a large absolute
value to each of said pixels or a potential Voff that supplies a voltage of a small
absolute value thereto on the relation to a potential of said scanning signal in said
data write period TB, said data signal being supplied as a pulse-width modulation
signal that varies the pulse width of said potential Von in said data write period
TB to correspond with said voltage that is charged into said liquid crystal layer
of each of said pixels, said data signal also having said potential Von or said potential
Voff in said period TA; and
duty of the pulse widths of said potential Von to said period TA is substantially
equal to duty of the pulse width of said potential Von to said period TB.
16. A method of driving a liquid crystal display device in accordance with claim 15, wherein
said difference signal has a period Toff A which corresponds to the pulse width of
said potential Von of said data signal in said period TA, a period TonA which is the
rest of said period TA (i.e.,
), a period TonB which corresponds to the pulse width of said potential Von of said
data signal in said period TB, and a period ToffB which is the rest of said period
TB (i.e.,
), the ratios TonA/TA and TonB/TB being substantially in a complementary relationship.
17. A method of driving a liquid crystal display device in accordance with claim 16, wherein
a time ratio of said period TA to said period TB is set to be such that the root-mean-square
values of the voltages applied to said two-terminal element are made substantially
equal for each of said two-terminal elements of said pixels.
18. A method of driving a liquid crystal display device in accordance with claim 16, wherein
the absolute value of the potential of said scanning signal with respect to a middle
potential between said potential Von and Voff is set to be different in each of said
periods TA and TB, and each of said potentials of said scanning signal is set in such
a manner that the root-mean-square values of the voltages applied to said two-terminal
element are made substantially equal for each of said two-terminal elements of said
pixels.
19. A method of driving a liquid crystal display device in accordance with any one of
claims 16 to 18, wherein an initial period of said period TA is said period ToffA
and an ending period thereof is said period TonA, and an initial period of said period
TB is said period ToffB and an ending period thereof is said period TonB.
20. A method of driving a liquid crystal display device in accordance with any one of
claims 16 to 18, wherein an initial period of said period TA is said period TonA and
an ending period thereof is said period ToffA, and said initial period of said period
TB is said period ToffB and an ending period thereof is said period TonB.
21. A method of driving a liquid crystal display device in accordance with claim 1, wherein
said scanning signal is set to be such that said period TA and said period TB have
different polarities on the voltages each of which is charged into the liquid crystal
layer of the pixel;
said data signal has a potential VonB that supplies a voltage of a large absolute
value to each of said pixels or a potential VoffB that supplies a voltage of a small
absolute value thereto on the relation to a potential of said scanning signal, said
data signal being supplied as a pulse-width modulation signal that varies the pulse
width of said potential VonB in said period TB to correspond with said voltage that
is charged into said liquid crystal layer of each of said pixels, said data signal
also having potential VonA or potential VoffA in said period TA of absolute values
greater than those of the corresponding potentials VonB or VoffB, respectively; and
the duty of the pulse widths of potential VonA to said period TA is substantially
equal to duty of the pulse width of potential VonB to said perid TB.
22. A method of driving a liquid crystal display device in accordance with claim 21, wherein
said difference signal has a period ToffA which corresponds to the pulse width of
said potential VonA of said data signal in said period TA, a period TonA which is
the rest of said period TA (i.e.,
), a period TonB corresponds to the pulse width of said potential VonB of said data
signal in said period TB, and a period ToffB which is the rest of said period TB (i.e.,
), the ratios TonA/TA and TonB/TB being substantially in a complementary relationship.
23. A method of driving a liquid crystal display device in accordance with claim 22, wherein
a time ratio of said period TA to said period TB is set to be such that the root-mean-square
values of the voltages applied to said two-terminal element are made substantially
equal for each of said two-terminal elements of said pixels.
24. A method of driving a liquid crystal display device in accordance with claim 22 or
23, wherein an initial period of said period TA is said period ToffA and an ending
period thereof is said period TonA, and an initial period of said period TB is said
period ToffB and an ending period thereof is said period TonB.
25. A method of driving a liquid crystal display device in accordance with claim 22 or
23, wherein an initial period of said period TA is said period TonA and an ending
period thereof is said period ToffA, and said initial period of said period TB is
said period ToffB and an ending period thereof is said period TonB.
26. A method of driving a liquid crystal display device in accordance with any one of
claims 1 to 25, wherein said two-terminal element has a metal-insulator-metal layer
structure (an MIM element) or a metal-insulator-semiconductor layer structure (an
MIS element).
27. A method of driving a liquid crystal display device in accordance with claim 26, wherein
an oxide film formed by anodic oxidization in an electrolytic liquid including phosphorus
in a form such as phosphoric acid or ammonium phosphate is used as said insulator
layer of said two-terminal element.
28. A method of driving a liquid crystal display device in accordance with claim 27, wherein
said insulator layer is formed by anodic oxidization of tantalum.
29. A method of driving a liquid crystal display device in accordance with any one of
claims 26 to 28, wherein one of the metal layers of said MIM element is a transparent
conductive layer.
30. A method of driving a liquid crystal display device in accordance with claim 26, wherein
said the insulator layer of the MIM element comprises a silicon-nitride.
31. A method of driving a liquid crystal display device which comprises a plurality of
column lines and row lines and a plurality of pixels, each pixel including a display
element and a non-linear resistance element connected in series between said column
and row lines, wherein:
applying a first higher voltage between said column and row lines so that a display
data is supplied to said display element of the pixel, in a selection period of the
pixel;
applying a lower voltage than said first higher voltage between said first and
second lines, in a non-selection period of the pixel after said selection period;
a method of driving a liquid crystal display device characterized in that:
applying a second higher voltage than said lower voltage between said first and
second lines, in a compensatory period of the pixel before said selection period and
after said non-selection period, wherein said second higher voltage in said compensatory
period has a polality oposite to that of said first higher voltage in said selection
period, a root-mean - square (RMS) of said second higher voltage in said compensatory
period and a RMS of said first higher voltage in said selection period is in a relation
that:
the RMS of said second higher voltage is large when the RMS of said first higher
voltage is small, and is small when the RMS of said first higher voltage is large.
32. A method of driving a liquid crystal display device in accordance with claim 31, wherein
the RMS of said first higher voltage in said selection period and the RMS of said
second higher voltage in said compensatory period are substantially in a complementary
relationship with each other.
33. A method of driving a liquid crystal display device in accordance with claim 32, wherein
a sam of the RMS of said first and second higher voltages in said selection and compensatory
periods is set to be such that the RMS of a voltage applied to said the non-linear
resistance element of the pixel in said selection and compensatory periods is substantially
made equal for each of said non-linear resistance element of said pixels.