The present invention relates to a video display device comprising a plurality of light-emitting elements arranged in a matrix having lines and columns, and to a method for operating such a display device.

Display devices of this type require driver circuits for controlling the luminosity of each of its light-emitting elements. In passive matrix technology, one driver circuit is associated to a plurality of light-emitting elements and supplies these with operating current pulses according to a time division scheme, i.e. at a given instant, only one of the light-emitting elements associated to a given driver circuit receives from it an operating current and emits light, whereas the others receive no current and remain dark. Passive matrix technology involves simple and inexpensive circuitry, but in order to achieve a reasonable overall luminosity of the display device, the intensity of the pulses supplied to the individual light-emitting elements must be high, causing problems such as premature aging of the light-emitting elements and a low reliability of the display device.

In active matrix technology, each light-emitting element has a current modulator associated to it, which is programmable to supply its light-emitting element with a continuous operating current, the intensity of which is updated when a new image is to be displayed.

EP 1 622 120 A1 discloses a display device of this type. Each of the light-emitting elements of this display device has a first current modulator associated to it, for drawing a feed current of programmable intensity for the associated light-emitting element from a circuit node associated to each of the matrix columns. Further, each column has a current generator associated to it, which is controlled by video data representative of desired luminosities of the light-emitting elements of said column for supplying a first current to said circuit node, the intensity of which is representative of a total desired luminosity of all light-emitting elements of said column. I. e. the first current from the current generator is distributed and supplied to the light-emitting elements of the column in proportion to their intended luminosities as defined by the video data received by the current source.

When displaying a variable image such as a TV image, the luminosities of the light emitting elements must be updated continuously, element by element. Putting things simply, one might say that the luminosity of one element is updated by having the current generator outputting to said circuit node a first current proportional to the sum of the updated luminosity of said one element and the present luminosities of the other elements, and by programming the first current modulator associated to the element to be updated so that it will absorb from the circuit node that portion of the first current which is not absorbed by the first current modulators of all the other elements. It is readily apparent that for such a scheme to work, the current source must be able to control the first current intensity at a resolution of n*m, if n is the number of light-emitting elements in a column and m is the number of luminosity levels which these elements shall be able to display. In a practical embodiment, in which n has a value of approximately 1000 typical for a TV image and m is e.g. 256, the required resolution is approximately 256,000. Such a resolution requires sophisticated and expensive circuitry for the current source.

The object of the present invention is to provide a video display device in which the required resolution of the voltage source is considerably reduced, so that simple and inexpensive circuitry may be used.

The invention is as defined in the claims.

The invention will be more easily understood and further features and advantages will become apparent from the subsequent description of embodiments thereof referring to the appended drawings.

Fig. 1

is a schematic circuit diagram of an exemplary portion of the display device according to an embodiment of the invention;

Fig. 2

is a waveform diagram illustrating currents in the display device of Fig. 1 during the build-up of a first display image; and

Fig. 3

is a waveform diagram illustrating currents during the build-up of a subsequent image.

The display device of the invention comprises a large number n*1 of light-emitting elements such as OLEDs (Organic Light-Emitting Diodes) arranged on a substrate in a matrix of n lines and 1 columns. Since the columns are identical in design and operation, Fig. 1 illustrates just one of these columns. The column comprises OLEDs 1-1, 2-2,..., 1-n serially connected to an associated current modulator 2-1, 2-2,..., 2-n. The OLEDs and the current modulators are connected in parallel between a circuit node 3 and a negative supply potential V-.

The current modulators 2-1, 2-2, ..., 2-n may each be formed by a FET having two current electrodes, one connected to the circuit node 3 and the other to the OLED 1-1, 1-2, ... or 1-n, respectively, and a control electrode connected to first sides of a switch 4-1, 4-2, ..., 4-n and of a storage capacitor 5-1, 5-2,..., 5-n. Incidentally, the storage capacitors have their second sides connected to ground, but they might as well be connected to said negative supply voltage V-, to a positive supply voltage V+ or to any other appropriate constant potential. The switches 4-1, 4-2, 4-n have their second sides connected to an output of an operational amplifier 6, of which a non-inverting input is connected to circuit node 3 and an inverting input is connected to ground.

An operational amplifier 7 has its non-inverting input connected to ground, its inverting input connected to circuit node 3 and its output connected to a first side of a switch 8, a second side of which is connected to a storage capacitor 9 and to a control terminal of a current modulator 10 which may be of the same type as current modulators 2-1, 2-2, ..., 2-n. Current modulator 10 has its current terminals connected to positive supply voltage V+ and to circuit node 3.

An exemplary current generator 11 comprises a control block 12, a transistor 13 and a resistor 14. Transistor 13 and resistor 14 are connected in series between the positive supply voltage V+ and the circuit node 3. Control block 12 has an input 15 for receiving digital data representative of desired luminosities of OLEDs 1-1, 1-2, ..., 1-n, inputs 16 for detecting a voltage drop across resistor 14 and an output connected to a control electrode of transistor 13. Transistor 13 may be a bipolar or MOS-FET transistor.

For explaining the operation of the circuitry of Fig. 1, let us assume that the display device is just starting operation, and that initially all current modulators 2-1, 2-2, ..., 2-n, 10 and the current generator 11 are in a blocking state, so that all OLEDs are dark. Further, for the sake of convenience, it will be assumed that the first digital luminosity value received at input 15 is a value D1 corresponding to OLED 1-1. Reference is made to Figs 2 and 3, which illustrate waveforms of output currents I_DATA of current generator 11 and I₁₀ of current modulator 10.

The control block reacts to the luminosity value D₁ being input by closing switch 4-1 and making transistor 13 conductive, so that at a time t_1c (cf. Fig. 2) a positive current I_DATA begins to flow from current generator 11 to circuit node 3. The potential of circuit node 3 thus becomes positive. This causes operational amplifier 6 to output a positive voltage which charges storage capacitor 5-1 and causes current modulator 2-1 to become conductive, enabling a continuous flow of current through resistor 14 and circuit node 3. The control block 12 continuously adapts the voltage applied by it to the control electrode of transistor 13 until the voltage drop detected at inputs 16 is in a predetermined relation to the input luminosity value D1, indicating that a current I_DATA=I_D1=C*D1 having the necessary intensity for generating the desired luminosity D1 is flowing from current generator 11 through current modulator 2-1 and OLED 1-1.

When this happens, it is likely that the circuit node 3 will at first have a positive potential. This positive potential causes operational amplifier 6 to output a current which continues to charge storage capacitor 2-1, thus gradually increasing the potential at the control electrode of current modulator 2-1 and increasing its conductivity. Control block 12 continuously adjusts the control voltage applied to transistor 13, so that the current through circuit node 3 is kept constant at I_D1. Soon, a steady state is reached in which circuit node 3 has ground potential. In this state, control block 12 reopens switch 4-1.

In a next step, at a time t_1d, control block 12 blocks transistor 13, so that current generator 11 becomes non-conductive (I_DATA=0), and closes switch 8. Since current modulator 2-1 stays conductive, the potential of circuit node 3 decreases, which causes operational amplifier 7 to output a positive current to storage capacitor 9 and to the control electrode of current modulator 10. Again, a steady state is reached as soon as circuit node 3 has returned to ground potential. When this happens, the current through OLED 1-1 is exactly equal to I_D1, but the current is supplied not by current generator 11 any more, but by current modulator 10.

In a subsequent step, from time t_2a to t_2c, the control block 12 carries out a reset procedure which, for better understanding, will be explained later on.

By the time t_2c, the control block 12 has received a second digital data specifying a desired luminosity D2 of OLED 1-2. At t_2c, it closes switch 4-2 and begins to control transistor 13 so as to have a current I_DATA=I_D2 corresponding to said desired luminosity D2 flowing through current generator 11. Again, the potential of circuit node 3 becomes slightly positive, this time causing amplifier 6 to charge capacitor 5-2, and to make current modulator 2-2 conductive. A steady state is reached in which circuit node 3 is at ground potential, and the current I_DATA=I_D2 from current generator 11 is absorbed by OLED 2-2, whereas the current I₁₀ from current modulator 10 flows through OLED 1-1. Control block 12 then opens switch 4-2, and at the time t_2d, it blocks transistor 13 and closes switch 8 again. A potential decrease at circuit node 3 causes amplifier 7 to continue to charge capacitor 9, until the current I₁₀ through current modulator 10 becomes equal to I_D1+I_D2.

The procedure is repeated for all remaining OLEDs of the column, and at the end of each repetition, the current from current modulator 10 is increased by the desired intensity I_Di, i=3,..., n, for each of the OLEDs, finally reaching I_Σ=I_D1+I_D2+...+I_Dn. At this stage, an entire image is visible on the display device.

The next digital data received by control block 12 is data specifying a desired luminosity D1' of OLED 1-1 in a subsequent picture. In order to adapt the luminosity of OLED 1-1 to this new value, at a time t_1a, (see Fig. 3), control block 12 begins a reset procedure by closing switch 4-1, whereby storage capacitor 5-1 is discharged and current modulator 2-1 becomes non-conductive. Then, switch 4-1 is reopened. The potential at circuit node 3 has become slightly positive. By closing switch 8 at time t_1b', current modulator 10 is caused to adapt to this new situation: its current decreases to I_Σ-I_D1. This procedure of resetting OLED 1-1 enables the control block 12 to set the new luminosity D1' of this OLED in exactly the same way as described before referring to Fig. 2: At a time t_1c', it causes current generator 11 to output I_DATA=I_D1, and closes switch 4-1, so that current modulator 2-1 will draw precisely the current I_D1' from circuit node 3 when the latter is at ground potential. Switch 4-2 is reopened, and at time t_1d', current generator 11 blocks, and switch 8 is closed, so that the current I₁₀ supplied by modulator 10 increases to I_Σ-I_D1+I_D1'. The procedure is continued in a similar manner for all other OLEDs 1-2, ...1-n.

Since the control block 12 is used to program the luminosities of the OLEDs one by one, the resolution of the current generator 11 need not be higher than that of a single luminosity data received by the control block 12, regardless of the number of OLEDs in a column.

Referring to the teachings of EP 1 621 20 A1 it will be readily apparent to a skilled person that the operating procedure of the circuitry of Fig. 1 might be modified as follows: at first, the control block consecutively programs the luminosities of a small number of OLEDs, e. g. OLEDs 1-1, 1-2, as described in the cited document. At the end of this programming, the current I_DATA output by current generator 11 amounts to I_D1+I_D2, if it is assumed that I_D1, I_D2 are the current intensities corresponding to the desired luminosities D1, D2 of OLEDs 1-1, 1-2. Then, control block 12 makes the current generator 11 non-conductive, as described above referring to Fig. 2 or 3, and closes switch 8, so that the current I_D1+I_D2 previously supplied by current generator 11 is "copied" to current modulator 10. It is readily apparent that the number of copying steps required for the build-up of a complete image is the smaller, the larger the number of OLEDs consecutively programmed between two copying steps is. On the other hand, the required resolution of the current generator 11 increases in proportion to the number of consecutively programmed OLEDs.

According to another embodiment, image build-up speed may be increased by not resetting the current modulators 2 prior to programming them. It is easily understood that when the display has just been activated and a first image is formed, the reset step is not necessary. When forming the second image, the luminosity of e. g. OLED 1-1 is programmed by having current generator 11 output a current I_DATA=I_D1'-I_D1, wherein I_D1' is the current intensity corresponding to the desired luminosity D1' of OLED 1-1 in the second image. Since in this embodiment I_DATA may be negative, the current generator 11 must be adapted to generate negative currents, e. g by means of a second transistor, not shown, connected in series between transistor 13 and V- and controlled by control block 12.

In this embodiment, any inaccuracy of I_DATA may cause the luminosities of the OLEDs to drift. In order to limit such drifts, it is conceivable to apply a reset to each OLED when it has been reprogrammed without reset a predetermined number of times.