Field
[0002] This relates generally to electronic devices and, more particularly, to electronic
devices with displays.
Background
[0003] Electronic devices often include displays. For example, cellular telephones and portable
computers include displays for presenting information to users.
[0004] Displays such as organic light-emitting diode displays have an array of display pixels
based on light-emitting diodes. In this type of display, each display pixel includes
a light-emitting diode and thin-film transistors for controlling application of a
signal to the light-emitting diode to produce light.
[0005] For instance, a display pixel often includes a drive thin-film transistor that controls
the amount of current flowing through the light-emitting diode and a switching transistor
connected to the gate terminal of the drive thin-film transistor. The switching transistor
may be implemented as a semiconducting-oxide transistor, which typically exhibits
low leakage when the switching transistor is turned off. This low-leakage property
of the semiconducting-oxide switching transistor helps to keep the voltage at the
gate terminal of the drive thin-film transistor relatively constant during a given
emission period of the display pixel when the drive thin-film transistor passes current
to the light-emitting diode to produce light.
[0006] The semiconducting-oxide switching transistor, however, exhibits reliability issues
over the lifetime of the display. In particular, the semiconducting-oxide transistor
has a threshold voltage that drifts over time as the semiconducting-oxide transistor
is repeatedly turned on and off. As the threshold voltage of the semiconducting-oxide
transistor changes, the voltage at the gate terminal of the drive thin-film transistor
immediately prior to emission will also be affected. This directly impacts the amount
of current flowing through the light-emitting diode, which controls the amount of
light or luminance produced by the display pixel. This sensitivity of the light-emitting
diode current to the threshold voltage of the semiconducting-oxide switching transistor
increases the risk of non-ideal display behaviors such as luminance drop over the
lifetime of the display, undesired color shifts over the lifetime of the display (e.g.,
resulting in a cyan/greenish tint on the display), etc.
Summary
[0007] An electronic device may include a display having an array of display pixels. The
display pixels may be organic light-emitting diode display pixels.
[0008] Each display pixel may include a light-emitting diode, a drive transistor coupled
in series with the light-emitting diode, a transistor of a first semiconductor type
(e.g., a semiconducting-oxide thin-film transistor) coupled between the drain terminal
and the gate terminal of the drive transistor, a first emission transistor coupled
in series with the drive transistor and the light-emitting diode, a second emission
transistor coupled in series with the drive transistor and a power line, an initialization
transistor coupled to the light-emitting diode, and a data loading transistor coupled
to the source terminal of the drive transistor. In particular, the semiconducting-oxide
transistor may be configured to reduce leakage at the gate terminal of the drive transistor.
[0009] The scan control signal that controls the semiconducting-oxide transistor may be
adapted to changes in the threshold voltage of the semiconducting oxide transistor
to compensate for any luminance drop in the display. In one compensation scheme, a
prediction of the change in threshold voltage of the semiconducting oxide transistor
may be used to change the high voltage level of the scan control signal according
to a predetermined profile. The changes in the high voltage level of the scan control
signal may track the changes in the threshold voltage of the semiconducting oxide
transistor, preventing luminance drop in the display.
[0010] In another suitable arrangement, current sensing circuitry may be coupled to the
display to measure a display current while a calibration image is displayed. The expected
display current associated with the calibration image may be known. The actual display
current (obtained by the current sensing circuitry) may be compared to the expected
display current. Differences between the actual and expected display current may occur
due to changes of the threshold voltage of the semiconducting oxide transistor. If
a difference is detected, the high voltage level of the scan control signal of the
semiconducting oxide transistor may be compensated accordingly. In one example, the
high voltage level may be set based on a lookup table that includes information associated
with the expected display current, actual display current, and temperature. In another
example, the high voltage level may be adjusted incrementally until the actual display
current matches the expected display current.
Brief Description of the Drawings
[0011]
FIG. 1 is a diagram of an illustrative electronic device having a display in accordance
with an embodiment.
FIG. 2 is a diagram of an illustrative organic light-emitting diode display having
an array of organic light-emitting diode (OLED) display pixels in accordance with
an embodiment.
FIG. 3 is a diagram of a low refresh rate display driving scheme in accordance with
an embodiment.
FIG. 4 is a circuit diagram of an organic light-emitting diode display pixel configured
to produce an emission current in accordance with an embodiment.
FIG. 5 is a timing diagram that illustrates the operation of the organic light-emitting
diode display pixel shown in FIG. 4.
FIG. 6 is a diagram illustrating how the threshold voltage of a semiconducting-oxide
transistor and how the threshold voltage of a silicon transistor vary over time in
accordance with an embodiment.
FIG. 7 is a diagram illustrating the sensitivity of OLED emission current to the threshold
voltage of the semiconducting-oxide transistor in the organic light-emitting diode
display pixel shown in FIG. 4 in accordance with an embodiment.
FIG. 8 is a diagram of a scan control signal that may be provided to the gate of a
semiconducting-oxide transistor such as T3 in FIG. 4 in accordance with an embodiment.
FIG. 9 is a diagram illustrating how the high voltage level of the scan control signal
of a semiconducting oxide transistor in a pixel may be predictively decreased to track
expected drift in the threshold voltage of the transistor in accordance with an embodiment.
FIG. 10 is a schematic diagram of an illustrative display that includes sensing circuitry
for actively monitoring display current and accordingly compensating the high voltage
level of the scan control signal of a semiconducting oxide transistor in accordance
with an embodiment.
FIG. 11 is a schematic diagram of an illustrative display that includes a sensing
resistor and analog-to-digital converter for actively monitoring display current in
accordance with an embodiment.
FIG. 12 is a flowchart of an illustrative method of compensating the high voltage
level of the scan control signal of a semiconducting oxide transistor based on a measured
display current and using a lookup table in accordance with an embodiment.
FIG. 13 is a flowchart of an illustrative method of compensating the high voltage
level of the scan control signal of a semiconducting oxide transistor by incrementally
changing the high voltage level in accordance with an embodiment.
Detailed Description
[0012] An illustrative electronic device of the type that may be provided with an organic
light-emitting diode display is shown in FIG. 1. Electronic device 10 may be a computing
device such as a laptop computer, a computer monitor containing an embedded computer,
a tablet computer, a cellular telephone, a media player, or other handheld or portable
electronic device, a smaller device such as a wrist-watch device, a pendant device,
a headphone or earpiece device, a device embedded in eyeglasses or other equipment
worn on a user's head, or other wearable or miniature device, a display, a computer
display that contains an embedded computer, a computer display that does not contain
an embedded computer, a gaming device, a navigation device, an embedded system such
as a system in which electronic equipment with a display is mounted in a kiosk or
automobile, or other electronic equipment.
[0013] Device 10 may include control circuitry 15. Control circuitry 15 may include storage
and processing circuitry for supporting the operation of device 10. The storage and
processing circuitry may include storage such as nonvolatile memory (e.g., flash memory
or other electrically-programmable-read-only memory configured to form a solid state
drive), volatile memory (e.g., static or dynamic random-access-memory), etc. Processing
circuitry in control circuitry 15 may be used to gather input from sensors and other
input devices and may be used to control output devices. The processing circuitry
may be based on one or more microprocessors, microcontrollers, digital signal processors,
baseband processors and other wireless communications circuits, power management units,
audio chips, application specific integrated circuits, etc.
[0014] To support communications between device 10 and external equipment, control circuitry
15 may communicate using communications circuitry 21. Circuitry 21 may include antennas,
radio-frequency transceiver circuitry, and other wireless communications circuitry
and/or wired communications circuitry. Circuitry 21, which may sometimes be referred
to as control circuitry and/or control and communications circuitry, may support bidirectional
wireless communications between device 10 and external equipment over a wireless link
(e.g., circuitry 21 may include radio-frequency transceiver circuitry such as wireless
local area network transceiver circuitry configured to support communications over
a wireless local area network link, near-field communications transceiver circuitry
configured to support communications over a near-field communications link, cellular
telephone transceiver circuitry configured to support communications over a cellular
telephone link, or transceiver circuitry configured to support communications over
any other suitable wired or wireless communications link). Wireless communications
may, for example, be supported over a Bluetooth® link, a WiFi® link, a 60 GHz link
or other millimeter wave link, a cellular telephone link, or other wireless communications
link. Device 10 may, if desired, include power circuits for transmitting and/or receiving
wired and/or wireless power and may include batteries or other energy storage devices.
For example, device 10 may include a coil and rectifier to receive wireless power
that is provided to circuitry in device 10.
[0015] Device 10 may include input-output devices such as devices 12. Devices 12 may be
mounted in a housing of the electronic device (e.g., an electronic device housing
formed from metal and/or glass that forms exterior surfaces of the electronic device).
In cases where electronic device 10 is a wristwatch device, a wrist strap may be attached
to the electronic device housing. For example, the wrist strap may be attached to
first and second opposing sides of the electronic device housing, with display 14
interposed between the first and second opposing sides. Input-output devices 12 may
be used in gathering user input, in gathering information on the environment surrounding
the user, and/or in providing a user with output. Devices 12 may include one or more
displays such as display(s) 14. Display 14 may be an organic light-emitting diode
display, a liquid crystal display, an electrophoretic display, an electrowetting display,
a plasma display, a microelectromechanical systems display, a display having a pixel
array formed from crystalline semiconductor light-emitting diode dies (sometimes referred
to as microLEDs), and/or other display. Display 14 may have an array of pixels configured
to display images for a user. The display pixels may be formed on a substrate such
as a flexible substrate (e.g., display 14 may be formed from a flexible display panel).
Conductive electrodes for a capacitive touch sensor in display 14 and/or an array
of indium tin oxide electrodes or other transparent conductive electrodes overlapping
display 14 may be used to form a two-dimensional capacitive touch sensor for display
14 (e.g., display 14 may be a touch sensitive display).
[0016] Sensors 17 in input-output devices 12 may include force sensors (e.g., strain gauges,
capacitive force sensors, resistive force sensors, etc.), audio sensors such as microphones,
touch and/or proximity sensors such as capacitive sensors (e.g., a two-dimensional
capacitive touch sensor integrated into display 14, a two-dimensional capacitive touch
sensor overlapping display 14, and/or a touch sensor that forms a button, trackpad,
or other input device not associated with a display), and other sensors. If desired,
sensors 17 may include optical sensors such as optical sensors that emit and detect
light, ultrasonic sensors, optical touch sensors, optical proximity sensors, and/or
other touch sensors and/or proximity sensors, monochromatic and color ambient light
sensors, image sensors, fingerprint sensors, temperature sensors, sensors for measuring
three-dimensional non-contact gestures ("air gestures"), pressure sensors, sensors
for detecting position, orientation, and/or motion (e.g., accelerometers, magnetic
sensors such as compass sensors, gyroscopes, and/or inertial measurement units that
contain some or all of these sensors), health sensors, radio-frequency sensors, depth
sensors (e.g., structured light sensors and/or depth sensors based on stereo imaging
devices), optical sensors such as self-mixing sensors and light detection and ranging
(lidar) sensors that gather time-of-flight measurements, humidity sensors, moisture
sensors, gaze tracking sensors, and/or other sensors. In some arrangements, device
10 may use sensors 17 and/or other input-output devices to gather user input (e.g.,
buttons may be used to gather button press input, touch sensors overlapping displays
can be used for gathering user touch screen input, touch pads may be used in gathering
touch input, microphones may be used for gathering audio input, accelerometers may
be used in monitoring when a finger contacts an input surface and may therefore be
used to gather finger press input, etc.).
[0017] If desired, electronic device 10 may include additional components (see, e.g., other
devices 19 in input-output devices 12). The additional components may include haptic
output devices, audio output devices such as speakers, light-emitting diodes for status
indicators, light sources such as light-emitting diodes that illuminate portions of
a housing and/or display structure, other optical output devices, and/or other circuitry
for gathering input and/or providing output. Device 10 may also include a battery
or other energy storage device, connector ports for supporting wired communication
with ancillary equipment and for receiving wired power, and other circuitry.
[0018] A display in an electronic device may be provided with driver circuitry for displaying
images on an array of display pixels. An illustrative display is shown in FIG. 2.
As shown in FIG. 2, display 14 may have one or more layers such as substrate 24. Layers
such as substrate 24 may be formed from planar rectangular layers of material such
as planar glass layers. Display 14 may have an array 27 of display pixels 22 for displaying
images for a user. The array of display pixels 22 may be formed from rows and columns
of display pixel structures on substrate 24. These structures may include thin-film
transistors such as polysilicon thin-film transistors (e.g., thin-film transistors
having an active region formed from polysilicon), semiconducting oxide thin-film transistors
(e.g., thin-film transistors having an active region formed from semiconducting oxide),
etc. There may be any suitable number of rows and columns in the array of display
pixels 22 (e.g., ten or more, one hundred or more, or one thousand or more).
[0019] Display driver circuitry such as display driver integrated circuit 16 may be coupled
to conductive paths such as metal traces on substrate 24 using solder or conductive
adhesive. Display driver integrated circuit 16 (sometimes referred to as a timing
controller chip) may contain communications circuitry for communicating with system
control circuitry over path 25. Path 25 may be formed from traces on a flexible printed
circuit or other cable. The system control circuitry may be located on a main logic
board in an electronic device such as a cellular telephone, computer, computer tablet,
television, set-top box, media player, wrist watch, portable electronic device, or
other electronic equipment in which display 14 is being used. During operation, the
system control circuitry may supply display driver integrated circuit 16 with information
on images to be displayed on display 14 via path 25. To display the images on display
pixels 22, display driver integrated circuit 16 may supply clock signals and other
control signals to display driver circuitry such as row driver circuitry 18 and column
driver circuitry 20. Row driver circuitry 18 and/or column driver circuitry 20 may
be formed from one or more integrated circuits and/or one or more thin-film transistor
circuits on substrate 24.
[0020] Row driver circuitry 18 may be located on the left and right edges of display 14,
on only a single edge of display 14, or elsewhere in display 14. During operation,
row driver circuitry 18 may provide row control signals on horizontal lines 28 (sometimes
referred to as row lines or "scan" lines). Row driver circuitry 18 may therefore sometimes
be referred to as scan line driver circuitry. Row driver circuitry 18 may also be
used to provide other row control signals such as emission control lines, if desired.
[0021] Column driver circuitry 20 may be used to provide data signals D from display driver
integrated circuit 16 onto a plurality of corresponding vertical lines 26. Column
driver circuitry 20 may sometimes be referred to as data line driver circuitry or
source driver circuitry. Vertical lines 26 are sometimes referred to as data lines.
During compensation operations, column driver circuitry 20 may use paths such as vertical
lines 26 to supply a reference voltage. During programming operations, display data
is loaded into display pixels 22 using lines 26.
[0022] Each data line 26 is associated with a respective column of display pixels 22. Sets
of horizontal signal lines 28 run horizontally through display 14. Power supply paths
and other lines may also supply signals to pixels 22. Each set of horizontal signal
lines 28 is associated with a respective row of display pixels 22. The number of horizontal
signal lines in each row may be determined by the number of transistors in the display
pixels 22 that are being controlled independently by the horizontal signal lines.
Display pixels of different configurations may be operated by different numbers of
control lines, data lines, power supply lines, etc.
[0023] Row driver circuitry 18 may assert control signals on the row lines 28 in display
14. For example, driver circuitry 18 may receive clock signals and other control signals
from display driver integrated circuit 16 and may, in response to the received signals,
assert control signals in each row of display pixels 22. Rows of display pixels 22
may be processed in sequence, with processing for each frame of image data starting
at the top of the array of display pixels and ending at the bottom of the array (as
an example). While the scan lines in a row are being asserted, the control signals
and data signals that are provided to column driver circuitry 20 by circuitry 16 direct
circuitry 20 to demultiplex and drive associated data signals D onto data lines 26
so that the display pixels in the row will be programmed with the display data appearing
on the data lines D. The display pixels can then display the loaded display data.
[0024] In an organic light-emitting diode (OLED) display such as display 14, each display
pixel contains a respective organic light-emitting diode for emitting light. A drive
transistor controls the amount of light output from the organic light-emitting diode.
Control circuitry in the display pixel is configured to perform threshold voltage
compensation operations so that the strength of the output signal from the organic
light-emitting diode is proportional to the size of the data signal loaded into the
display pixel while being independent of the threshold voltage of the drive transistor.
[0025] Display 14 may be configured to support low refresh rate operation. Operating display
14 using a relatively low refresh rate (e.g., a refresh rate of 1 Hz, 2 Hz, 1-10 Hz,
less than 100 Hz, less than 60 Hz, less than 30 Hz, less than 10 Hz, less than 5 Hz,
less than 1 Hz, or other suitably low rate) may be suitable for applications outputting
content that is static or nearly static and/or for applications that require minimal
power consumption. FIG. 3 is a diagram of a low refresh rate display driving scheme
in accordance with an embodiment. As shown in FIG. 3, display 14 may alternate between
a short data refresh phase (as indicated by period T_refresh) and an extended blanking
period T_blank. During period T refresh, the data value in each display pixel may
be refreshed, "repainted," or updated.
[0026] As an example, each data refresh period T_refresh may be approximately 16.67 milliseconds
(ms) in accordance with a 60 Hz data refresh operation, whereas each period T_blank
may be approximately 1 second so that the overall refresh rate of display 14 is lowered
to 1 Hz (as an example of a low refresh rate display operation). Configured as such,
the duration of T_blank can be adjusted to tune the overall refresh rate of display
14. For example, if the duration of T_blank is tuned to half a second, the overall
refresh rate would be increased to 2 Hz. As another example, if the duration of T_blank
was tuned to a quarter of a second, the overall refresh rate would be increased to
4 Hz. In the embodiments described herein, the blanking interval T_blank may be at
least two times the duration of T refresh, at least 10 times the duration of T_refresh,
at least 20 times the duration of T_refresh, at least 30 times the duration of T refresh,
at least 60 times the duration of T_refresh, 2-100 times the duration of T_refresh,
more than 100 times the duration of T_refresh, etc.
[0027] A schematic diagram of an illustrative organic light-emitting diode display pixel
22 in display 14 that can be used to support low refresh rate operation is shown in
FIG. 4. As shown in FIG. 4, display pixel 22 may include a storage capacitor Cst and
transistors such as n-type (i.e., n-channel) transistors T1, T2, T3, T4, T5, and T6.
The transistors of pixel 22 may be thin-film transistors formed from a semiconductor
such as silicon (e.g., polysilicon deposited using a low temperature process, sometimes
referred to as LTPS or low-temperature polysilicon), semiconducting oxide (e.g., indium
gallium zinc oxide (IGZO)), or other suitable semiconductor material. In other words,
the active region and/or the channel region of these thin-film transistors may be
formed from polysilicon or semi-conducting oxide material.
[0028] Display pixel 22 may include light-emitting diode 304. A positive power supply voltage
ELVDD (e.g., 1 V, 2 V, more than 1 V, 0.5 to 5 V, 1 to 10 V, or other suitable positive
voltage) may be supplied to positive power supply terminal 300 and a ground power
supply voltage ELVSS (e.g., 0 V, -1 V, -2 V, or other suitable negative voltage) may
be supplied to ground power supply terminal 302. The power supply voltages ELVDD and
ELVSS may be provided to terminals 300 and 302 from respective power supply traces.
For example, a conductive layer may serve as a ground power supply voltage trace that
provides the ground power supply voltage ELVSS to all of the pixels within the display.
The state of transistor T2 controls the amount of current flowing from terminal 300
to terminal 302 through diode 304 and therefore controls the amount of emitted light
306 from display pixel 22. Transistor T2 is therefore sometimes referred to as the
"drive transistor." Diode 304 may have an associated parasitic capacitance C
OLED (not shown).
[0029] Terminal 308 is used to supply an initialization voltage Vini (e.g., a positive voltage
such as 1 V, 2 V, less than 1 V, 1 to 5 V, or other suitable voltage) to assist in
turning off diode 304 when diode 304 is not in use. Control signals from display driver
circuitry such as row driver circuitry 18 of FIG. 2 are supplied to control terminals
such as terminals 312, 313, 314, and 315. Terminals 312 and 313 may serve respectively
as first and second scan control terminals, whereas terminals 314 and 315 may serve
respectively as first and second emission control terminals. Scan control signals
Scan1 and Scan2 may be applied to scan terminals 312 and 313, respectively. Emission
control signals EM1 and EM2 may be supplied to terminals 314 and 315, respectively.
A data input terminal such as data signal terminal 310 is coupled to a respective
data line 26 of FIG. 2 for receiving image data for display pixel 22.
[0030] Transistors T4, T2, T5, and diode 304 may be coupled in series between power supply
terminals 300 and 302. In particular, transistor T4 has a drain terminal that is coupled
to positive power supply terminal 300, a gate terminal that receives emission control
signal EM2, and a source terminal (labeled as node N1) coupled to transistors T2 and
T3. The terms "source" and "drain" terminals of a transistor can sometimes be used
interchangeably. Drive transistor T2 has a drain terminal that is coupled to node
N1, a gate terminal coupled to node N2, and a source terminal coupled to node N3.
Transistor T5 has a drain terminal that is coupled to node N3, a gate terminal that
receives emission control signal EM1, and a source terminal coupled to node N4. Node
N4 is coupled to ground power supply terminal 302 via organic light-emitting diode
304.
[0031] Transistor T3, capacitor Cst, and transistor T6 are coupled in series between node
N1 and terminal 308. In particular, transistor T3 has a drain terminal that is coupled
to node N1, a gate terminal that receives scan control signal Scan1 from scan line
312, and a source terminal that is coupled to node N2. Storage capacitor Cst has a
first terminal that is coupled to node N2 and a second terminal that is coupled to
node N4. Transistor T6 has a drain terminal that is coupled to node N4, a gate terminal
that receives scan control signal Scan1 via scan line 312, and a source terminal that
receives initialization voltage Vini via terminal 308.
[0032] Transistor T1 has a drain terminal that receives a data signal via data line 310,
a gate terminal that receives scan control signal Scan2 via scan line 313, and a source
terminal that is coupled to node N3. Connected in this way, emission control signal
EM2 may be asserted to enable transistor T4 (e.g., signal EM2 may be driven to a high
voltage level to turn on transistor T4); emission control signal EM1 may be asserted
to activate transistor T5; scan control signal Scan2 may be asserted to turn on transistor
T1; and scan control signal Scan1 may be asserted to simultaneously switch on transistors
T3 and T6. Transistors T4 and T5 may sometimes be referred to as emission transistors.
Transistor T6 may sometimes be referred to as an initialization transistor. Transistor
T1 may sometimes be referred to as a data loading transistor.
[0033] In one suitable arrangement, transistor T3 may be implemented as a semiconducting-oxide
transistor while remaining transistors T1, T2, and T4-T6 are silicon transistors.
Semiconducting-oxide transistors exhibit relatively lower leakage than silicon transistors,
so implementing transistor T3 as a semiconducting-oxide transistor will help reduce
flicker at low refresh rates (e.g., by preventing current from leaking through T3
when signal Scan1 is deasserted or driven low).
[0034] FIG. 5 is a timing diagram that illustrates the operation of organic light-emitting
diode display pixel 22 shown in FIG. 4. Prior to time t1, signals Scan1 and Scan2
are deasserted (e.g., the scan control signals are both at low voltage levels), whereas
signals EM1 and EM2 are asserted (e.g., the emission control signals are both at high
voltage levels). When both emission control signals EM1 and EM2 are high, an emission
current will flow through drive transistor T2 into the corresponding organic light-emitting
diode 304 to produce light 306 (see FIG. 4). The emission current is sometimes referred
to as the OLED current or OLED emission current, and the period during which the OLED
current is actively producing light at diode 304 is referred to as the emission phase.
[0035] At time t1, emission control signal EM1 is deasserted (i.e., driven low) to temporarily
suspend the emission phase, which begins a data refresh or data programming phase.
At time t2, signal Scan1 may be pulsed high to activate transistors T3 and T6, which
initializes the voltage across capacitor Cst to a predetermined voltage difference
(e.g., ELVDD minus Vini).
[0036] At time t3, scan control signal Scan1 is pulsed high while signal Scan2 is asserted
and while signals EM1 and EM2 are both deasserted to load a desired data signal from
data line 310 into display pixel 22. At time t4, scan control signal Scan1 is deasserted
(e.g., driven low), which signifies the end of the data programming phase. The emission
phase then commences at t5 when emission control signals EM1 and EM2 are reasserted.
[0037] Although implementing transistor T3 as a semiconducting-oxide transistor helps minimize
leakage current at the gate terminal of drive transistor T2, semiconducting-oxide
transistor T3 may suffer from reliability issues. During data programming operations
of display pixel 22, scan clock signal Scan1 may be pulled up to a high voltage level
VSH (e.g., 10.5 V, more than 10 V, 1-10 V, more than 5 V, 1-5 V, 7-11 V, 10-15 V,
20 V, more than 20 V, or other suitable positive/elevated voltage level) and also
pulled down to a low voltage level VGL (e.g., - 5 V, -1 V, 0 to -5 V, -5 to -10 V,
less than 0 V, less than -1 V, less than -4 V, less than -5 V, less than -10 V, or
other suitable negative/depressed voltage level). In particular, the application of
negative voltage VGL at the gate terminal of semiconducting-oxide transistor T3 during
the emission phase places a negative gate-to-source voltage stress across transistor
T3, which can lead to oxide degradation (sometimes referred to as aging effects) and
will cause the threshold voltage of semiconducting oxide transistor T3 (sometimes
referred to as Vth_ox) to drift over time.
[0038] FIG. 6 is a diagram illustrating how the threshold voltage of semiconducting-oxide
transistor T3 varies over time. Trace 550 represents the threshold voltage of a silicon
transistor over the lifetime of display 14. As shown, the threshold voltage may remain
relatively steady throughout the lifetime of the display. In contrast, trace 500 represents
the threshold voltage of semiconducting-oxide transistor T3 over the lifetime of display
14. As illustrated by trace 500, Vth_ox will change over time (e.g., over 1-4 weeks
of normal display operation, over 1-12 months of normal display operation, over at
least one year of display operation, over 1-5 years of display operation, over 1-10
years of display operation, etc.). In particular, Vth ox may decrease over time as
the display operates. This may be in a problem in cases where display 14 is expected
to be used much of the day (e.g., a display in a wearable device such as a wristwatch
device, sometimes referred to as an 'always on' display), as Vth_ox may degrade more
quickly (due to frequent use) than in displays that are not 'always on.'
[0039] FIG. 7 plots the percentage change of the OLED emission current I
OLED as a function of the amount of voltage change in Vth_ox. As shown by trace 552, the
OLED emission current I
OLED is not very sensitive to changes in the threshold voltage of silicon transistors
within the pixel. Trace 502 illustrates the sensitivity of I
OLED to threshold voltage Vth ox of transistor T3 in organic light-emitting diode display
pixel 22 of FIG. 4. As shown by trace 502 in FIG. 7, current I
OLED may increase by approximately 50% if Vth_ox deviates from the nominal threshold voltage
amount by 1.5 V and may decrease by approximately 40% if Vth_ox deviates from the
nominal threshold voltage amount by -1.5 V. This relatively high sensitivity of the
OLED current to changes in Vth_ox as represented by trace 502 can cause non-ideal
behaviors such as luminance drop and undesired color shifts in the display as Vth_ox
drifts over time.
[0040] FIG. 8 is a diagram of a scan control signal (e.g., Scan1) that is used to control
T3 during operation of pixel 22. As shown, the control signal may rise to a positive
voltage level VSH to assert transistor T3. To turn off T3 (among other transistors),
signal Scan 1 may be driven from the positive voltage level VSH (sometimes referred
to as an active voltage level, on voltage level, or high voltage level) to a negative
voltage level VGL (sometimes referred to as an inactive voltage level, off voltage
level, or low voltage level).
[0041] When T3 is deasserted (by driving scan control signal Scan1 from VSH to VGL), the
voltage on N2 may change by an amount that is proportional to the difference between
VSH and Vth_ox (e.g., ΔV α VSH - Vth_ox). As the voltage on N2 controls the amount
of current through diode 304 and therefore the luminance of the pixel, precise control
of the N2 voltage is important for optimal operation of the display. The Vth_ox drift
experienced by T3 therefore causes non-ideal behaviors (because it causes the ΔV on
N2 to vary over time). To prevent non-ideal behaviors caused by the Vth ox drift,
VSH may be changed to track the changes in Vth_ox. If VSH is changed by a similar
amount to Vth_ox, then ΔV will remain constant over time (despite the Vth_ox drift).
[0042] There are numerous schemes that may be used to update VSH to compensate for Vth_ox
drift. In one example, VSH may drop at a predetermined rate based on the expected
Vth_ox drop over time. FIG. 9 is a diagram showing how VSH of scan control signal
Scan1 can be adjusted to adapt to the changes in Vth_ox and thereby mitigate display
luminance drop in accordance with an embodiment. At time T
0 (i.e., when the display is still relatively new), VSH may be biased at a nominal
positive power supply level VSH
0. After some period of time and at time T1, the luminance of display 14 might have
dropped by some amount due to the threshold voltage drift of oxide transistor T3.
The amount of time between T0 and T1 might be at least 50 hours, at least 100 hours,
100 to 500 hours, more than 500 hours, or other suitable time period of operation
during which display 14 might have suffered from undesirable changes in luminance.
To mitigate the luminance drop, VSH might be reduced to a new positive power supply
level VSH
1. After an additional period of time and at time T
2, the luminance of display 14 might have dropped by some amount due to the threshold
voltage drift of oxide transistor T3 between t
1 and t
2. The amount of time between T1 and T2 might be at least 50 hours, at least 100 hours,
100 to 500 hours, more than 500 hours, or other suitable time period of operation
during which display 14 might have suffered from undesirable changes in luminance.
To mitigate the luminance drop, VSH might be reduced to a new positive power supply
level VSH
2.
[0043] This process may continue indefinitely until the end of the life cycle of display
14, lasting for least 2 years of normal operational use, 2-5 years or normal operational,
5-10 years of normal operational use, or more than 10 years of normal operational
usage.
[0044] VSH may be adjusted by display driver integrated circuit 16. Alternatively, additional
control and processing circuitry (e.g., control circuitry 15 in FIG. 1) may send instructions
to display driver integrated circuit 16 to update VSH. To know when and by how much
to adjust VSH, testing may be done to determine an average drift curve for Vth_ox.
For example, a given number of devices (e.g., one hundred devices, five hundred devices,
more than one hundred devices, more than one thousand devices, less than one thousand
devices, etc.) may have their Vth_ox monitored over time. The average Vth ox value
of the devices may be obtained at various times. The average Vth ox values may then
be used to create an average Vth_ox drift curve. The average Vth_ox drift curve may
inform the VSH updates. VSH may be updated in a stepwise fashion that approximates
the Vth_ox drift curve, if desired.
[0045] During use of an electronic device, the actual Vth ox drift may deviate from the
average Vth_ox drift curve. However, updating VSH according to the average Vth_ox
drift curve may still improve display performance relative to displays where VSH is
never updated. To further improve display performance, current sensing may be used
to actively monitor display performance and periodically update VSH to maintain display
brightness. VSH is therefore updated based on a real-time data instead of a predicted
Vth_ox change.
[0046] A schematic diagram of an illustrative display that includes sensing circuitry for
active VSH compensation is shown in FIG. 10. As shown in FIG. 10, display 14 includes
a pixel array 27 with a plurality of pixels 22. Each pixel may be coupled to the ground
power supply trace ELVSS. In one illustrative example, ground power supply trace ELVSS
may be a blanket metal layer that is formed over the entire pixel array, with each
pixel having a corresponding diode terminal electrically connected to the blanket
metal layer.
[0047] To actively compensate VSH, an image (e.g., a calibration image) may first be displayed
on the pixel array using a starting VSH value. The VSH value may be provided to the
pixel array from display driver integrated circuit 16. Ground power supply trace ELVSS
(sometimes referred to as ground power supply terminal ELVSS) may be electrically
connected to sensing circuitry 102. Sensing circuitry 102 may include circuit components
that allow the detection of the current at ground power supply terminal ELVSS while
the calibration image is displayed by the pixel array. Sensing circuitry 102 may output
the sensed current value to processing circuitry 104. Processing circuitry 104 may
use the sensed current value to determine an updated VSH value that is then provided
to the display driver integrated circuit.
[0048] To allow processing circuitry 104 to update VSH based on the sensed current value
while the calibration image is displayed, expected display current values may be needed.
For example, testing may be performed before manufacturing of the device is completed
to determine that, when the calibration image is displayed at a first brightness value,
a first display current value should be sensed by sensing circuitry 102. Then, during
operation of the device, the calibration image may be displayed at the first brightness
value. If the sensed display current value matches the expected, first display current
value, processing circuitry 104 may determine that the display is operating correctly
and no changes to VSH are required. However, if the sensed display current value is
different (e.g., lower) than the first display current value, processing circuitry
may determine that VSH needs to be updated to correct for Vth_ox drift. Calibration
at numerous brightness levels and temperatures may be performed to determine the expected
display current value in a variety of conditions. Then, when operating the electronic
device, the actual display current value may be compared to the expected display current
value for the present conditions. The processing circuitry may then take remedial
action (e.g., updating VSH) based on the comparison.
[0049] Any desired circuitry may be used to form the sensing circuitry of FIG. 10. FIG.
11 is a schematic diagram of an illustrative embodiment for sensing circuitry 102
in which a sensing resistor is used to sense the display current. As shown in FIG.
102, ground power supply terminal ELVSS may be coupled to sensing resistor 106. By
sinking the display current into sensing resistor 106, a voltage proportional to the
current may be present at node 108, which may optionally be coupled to an amplifier
110. Amplifier 110 may amplify the voltage from node 108 to allow for higher resolution
current sensing (and therefore more accurate sensing at low current levels). Amplifier
110 may optionally be omitted if desired. The voltage from node 108 (either amplifier
or not amplified) may be provided to analog-to-digital converter (ADC) 112. The sensed
current value output from ADC 112 may be a digital value representative of the display
current.
[0050] Processing circuitry 104 may receive the sensed current value and determine an updated
VSH value based on the sensed current value. Processing circuitry 104 may factor in
other information when determining an updated VSH value. For example, the display
current may be dependent on the ambient temperature in which the electronic device
operates. Processing circuitry 104 may use a lookup table 114 to determine an updated
VSH. The lookup table may output an updated VSH value based on the temperature (e.g.,
temperature data received from a temperature sensor), the actual display current,
the display brightness, and/or the expected display current. Once the updated VSH
value is determined, the updated VSH value may be provided to display driver IC 16
for subsequent operation of the display.
[0051] The examples of sensing circuitry 102 and processing circuitry 104 shown in FIG.
11 are merely illustrative. In general, sensing circuitry 102 may include any desired
circuit components. Additionally, in FIGS. 10 and 11 the sensing circuitry is depicted
as being formed separately from the display driver integrated circuit 16 (e.g., sensing
circuitry may be formed as discrete components outside of the display driver IC).
This example is merely illustrative. Sensing circuitry 102 may instead be incorporated
into display driver IC 16 if desired. Similarly, in FIGS. 10 and 11 processing circuitry
104 is depicted as being formed separately from the display driver integrated circuit
16. For example, processing circuitry 104 may be formed as part of control circuitry
15 in FIG. 1, may be formed in the main processor for the electronic device, etc.
These examples are merely illustrative, and processing circuitry 104 may be incorporated
into display driver IC 16 if desired.
[0052] In the embodiments of FIGS. 10 and 11, the display senses current for the entire
display globally (e.g., the current at ground power supply terminal ELVSS that is
electrically connected to all of the pixels in the pixel array). It should be understood
that, in other embodiments, each pixel (or groups of pixels with more than one pixel
but less than all of the pixels in the array) may have its current measured and the
VSH for that pixel may be updated locally. In other words, VSH may instead be updated
on a per-pixel basis instead of globally. However, herein the example of VSH being
updated globally will be discussed.
[0053] FIG. 12 is a flowchart of illustrative method steps that may be used to operate a
display with sensing circuitry for dynamic VSH compensation (e.g., the display of
FIG. 11). As shown in FIG. 12, at step 202 a calibration image may be displayed on
the display. The calibration image may be any image for which the expected display
current is known (due to testing of the display before the display operates in the
field). The calibration image must be displayed any time VSH updating is performed
so that there is a known standard with which to compare the actual display current.
[0054] It may be desirable for the calibration to be undetectable to the user of the electronic
device. Therefore, an image that is displayed during routine use of the display may
be selected as the calibration image. For example, an image with a keypad for unlocking
the electronic (sometimes referred to as a lock screen), an image of a logo for the
electronic device, a startup screen that is displayed during startup of the electronic
device, or an image of a battery charge status symbol (sometimes referred to as a
battery charge status screen) may be selected as the calibration image. Consider the
battery charge status symbol as one example. To operate an electronic device, the
user may routinely charge the device, thus guaranteeing that the battery charge status
screen is displayed on a regular basis. This provides regular opportunities for VSH
compensation without being detectable to the user. If desired, calibration data may
be obtained for more than one calibration image (e.g., a battery charge status screen
and lock screen). This may increase the flexibility of times in which VSH compensation
may be performed without detection by the user.
[0055] Another factor that may be taken into consideration when choosing the calibration
image is the magnitude of the display current generated by that image. To ensure sufficiently
accurate testing of the display current, it may be desirable for the display current
to be above a given magnitude. Increasing the display brightness may increase the
magnitude of the display current. Similarly, increasing the percentage of pixels within
the array that are active may increase the magnitude of the display current. For example,
if a calibration image primarily includes active pixels (e.g., a primarily white image),
the requisite display brightness for VSH calibration may be lower than if the calibration
image includes a low percentage of active pixels (e.g., a primarily black image).
At step 202, the calibration image may be displayed at a brightness level that is
sufficiently high to enable accurate current sensing.
[0056] Next, at step 204, the actual display current value may be determined using sensing
circuitry (e.g., sensing circuitry 102 in FIGS. 10 and 11). The display current value
may be determined by coupling the ground power voltage supply terminal ELVSS to a
sensing resistor and sensing a corresponding voltage that is proportional to the display
current. At step 206, the actual display current value may be used to determine a
new VSH current value (VSH
NEW). For example, a lookup table (e.g., lookup table 114 in FIG. 11) or algorithm may
be used to generate VSH
NEW based on the actual display current value, expected display current value, temperature,
etc. Finally, at step 208, VSH
NEW may be used as VSH for subsequent display operations. In general, VSH may be adjusted
between any desired voltage values. For example, VSH may be adjusted between 7 V and
11 V, between 7 V and 10.5 V, or between any other desired voltages.
[0057] Negative voltage level VGL of Scan1 may optionally be adjusted at step 210. In general,
the lower the voltage of VGL, the faster T3 will shut off during operation of the
display. However, a lower VGL may result in a bigger bias on the transistor, ultimately
causing deterioration of pixel performance over time. If VGL is too high, however,
T3 may not completely shut off when Scan1 is driven to VGL as desired. VGL may be
updated in step 210 based on a number of factors such as VSH, the age of the display,
etc. In one illustrative example, VGL may track VSH. This means that the difference
between VGL and VSH may remain constant (even as VSH changes). For example, if VSH
drops from 10.5 V to 9.5 V, VGL may have a corresponding drop from -5 V to -6 V. The
difference between VGL and VSH may instead be allowed to change if desired (e.g.,
VGL may remain at -5 V even if VSH drops from 10.5 V to 9.5 V). In general, processing
circuitry within the electronic device (e.g., processing circuitry 104 in FIG. 10)
may update VGL to be any desired voltage.
[0058] The method shown in FIG. 12 may be intermittently repeated to ensure VSH continuously
tracks Vth_ox drift. The VSH compensation process (e.g., the method of FIG. 12) may
be performed once every day, once every week, once every month, more than once a day,
more than once a week, less than once a day, etc. The VSH compensation process may
be performed based on the elapsing of real-time (e.g., calendar days, calendar weeks,
etc.) or may be performed based on the elapsing of time in which the display is on.
In general, VSH compensation may be performed at any desired time with any desired
frequency.
[0059] In the example of FIG. 12, an updated VSH value is generated based on the sensed
current value. However, this example for updating VSH is merely illustrative. Instead
of using calibration data to select an updated VSH value (e.g., based on the lookup
table), VSH may be incrementally adjusted until the actual display current matches
(or is within a given percentage of matching) the expected display current. A VSH
compensation method of this type is shown in the flowchart of FIG. 13.
[0060] As shown in FIG. 13, at step 212 a calibration image may be displayed on the display.
As previously discussed, the calibration image may be any image for which the expected
display current is known (e.g., a battery charge status screen). The calibration image
may be displayed at a brightness level that is sufficiently high to enable accurate
current sensing. A high scan voltage VSH may be used for scanning signal Scan1 when
the image is displayed.
[0061] Next, at step 214, the actual display current value may be determined using sensing
circuitry (e.g., sensing circuitry 102 in FIGS. 10 and 11). The display current value
may be determined by coupling the ground power voltage supply terminal ELVSS to a
sensing resistor and sensing a corresponding voltage that is proportional to the display
current. At step 206, the actual display current value (I
ACTUAL) may be compared to the expected display current value (I
EXPECTED) associated with the calibration image. If the actual display current value does
not match the expected display current value, the method may proceed to step 218.
[0062] At step 218, VSH may be incrementally adjusted by adding (or subtracting) a predetermined
adjustment factor to VSH (e.g., VSH
NEW = VSH +/- ADJUSTMENT). The adjustment factor may be 10 mV, 10-50 mV, 30-70 mV, 50-100
mV, less than 1 mV, between m1V and 10 mV, less than 10 mV, less than 50 mV, less
than 100 mV, less than 1V, greater than 10mV, greater than 50 mV, greater than 100
mV, greater than 1 V, or other suitable adjustment factor. After adjusting VSH, the
method may loop back to step 212 per feedback loop 222. Before step 212 occurs again,
VSH is set to VSH
NEW from step 218. The process may repeat as long as I
ACTUAL does not equal I
EXPECTED. Once I
ACTUAL equals I
EXPECTED, the method may proceed to step 220 where the VSH compensation is determined to be
complete and normal display operations continue using VSH.
[0063] For example, at the beginning of the lifetime of the display, VSH may be equal to
10.5 V. If, during testing, it is determined that the I
ACTUAL is not equal to I
EXPECTED, VSH may be reduced by 0.1 V to 10.4 V. The test will be repeated to obtain a new
I
ACTUAL value. The updated I
ACTUAL will again be compared to I
EXPECTED. In this example, it is determined that the I
ACTUAL is still not equal to I
EXPECTED. VSH may be reduced again by 0.1 V to 10.3 V. The test will be repeated to obtain
a new I
ACTUAL value. In this example, I
ACTUAL now equals I
EXPECTED. Accordingly, the VSH compensation may be complete and the new value of VSH (10.3
V) may be used moving forward in operating the display.
[0064] It should be understood that the determination of whether I
ACTUAL equals I
EXPECTED may include a tolerance. For example, I
ACTUAL may be deemed to equal (match) I
EXPECTED if I
ACTUAL is within 1% of I
EXPECTED, within 2% of I
EXPECTED, within 3% of I
EXPECTED, within 5% of I
EXPECTED, within 10% of I
EXPECTED, etc.
[0065] In the aforementioned examples, oxide transistor T3 is controlled by an active-high
scan control signal (e.g., Scan1 is high when T3 is asserted and Scan1 is low when
T3 is deasserted). However, this example is merely illustrative and is not intended
to limit the scope of the present embodiments. One of ordinary skill can appreciate
that T3 may instead be a p-channel thin-film transistor that is controlled by an active-low
scan control signal (i.e., scan control signal Scan1 is driven low to turn on transistor
T3 and driven high to turn off transistor T3). In this type of embodiment, the principles
described above (of modifying VSH to track Vth_ox) may still be applied. However,
instead of lowering VSH to compensate for Vth_ox decreasing, VGL may be raised when
T3 is a p-channel thin-film transistor. The same techniques for adjusting VSH as described
above may be applied to adjusting VGL when T3 is a p-channel thin-film transistor.
[0066] In accordance with an embodiment, a display is provided that includes an array of
pixels, each pixel in the array of pixels includes a light-emitting diode, a drive
transistor and a switching transistor, gate driver circuitry configured to provide
a control signal to the switching transistor at a first voltage level to turn on the
switching transistor, sensing circuitry coupled to the array of pixels and processing
circuitry configured to change the first voltage level based on information from the
sensing circuitry.
[0067] In accordance with another embodiment, the switching transistor has a channel formed
in semiconducting oxide.
[0068] In accordance with another embodiment, the drive transistor has a channel formed
in silicon.
[0069] In accordance with another embodiment, the light-emitting diode of each pixel is
coupled between a first power supply terminal and a second power supply terminal.
[0070] In accordance with another embodiment, the sensing circuitry includes a current sensing
resistor coupled to the second power supply terminal and an analog-to-digital converter
coupled to the sensing resistor.
[0071] In accordance with another embodiment, the sensing circuitry includes an amplifier
interposed between the current sensing resistor and analog-to-digital converter.
[0072] In accordance with another embodiment, an output terminal of the analog-to-digital
converter is coupled to the processing circuitry.
[0073] In accordance with another embodiment, the processing circuitry is configured to
change the first voltage level based on a comparison between an actual current sensed
by the current sensing resistor and an expected current.
[0074] In accordance with another embodiment, the processing circuitry is configured to
incrementally change the first voltage level until the actual current sensed by the
current sensing resistor matches the expected current.
[0075] In accordance with another embodiment, the gate driver circuitry is configured to
provide the control signal to the switching transistor at a second voltage level to
turn off the switching transistor and the processing circuitry is configured to change
the second voltage level
[0076] In accordance with another embodiment, the processing circuitry is configured to
lower the first voltage level by a first amount and the second voltage level by the
first amount in response to determining that the actual current is less than the expected
current.
[0077] In accordance with another embodiment, the processing circuitry is configured to
use a lookup table to determine an updated first voltage level based at least on the
actual current sensed by the sensing resistor and a temperature.
[0078] In accordance with an embodiment, a method of operating a display with an array of
pixels, sensing circuitry coupled to the array of pixels, and processing circuitry
coupled to the sensing circuitry is provided that includes displaying an image using
the array of pixels, while displaying the image using the array of pixels, using the
sensing circuitry to obtain a display current value, and based at least on the display
current value and an expected display current value, updating an active voltage level
of a scan control signal that is provided to the array of pixels.
[0079] In accordance with another embodiment, each pixel in the array of pixels includes
a light-emitting diode, a drive transistor connected in series with the light-emitting
diode and a switching transistor that has a gate terminal that receives the scan control
signal and updating the active voltage level of the scan control signal includes lowering
the active voltage level of the scan control signal in response to determining that
the display current value is less than the expected display current value.
[0080] In accordance with another embodiment, the switching transistor has a channel formed
in semiconducting oxide.
[0081] In accordance with another embodiment, updating the active voltage level of the scan
control signal includes using a lookup table to determine an updated active voltage
level based at least on the display current value, the expected display current value,
and a temperature.
[0082] In accordance with another embodiment, updating the active voltage level of the scan
control signal includes repeatedly displaying the image using the active voltage level
of the scan control signal, using the sensing circuitry to obtain the display current
value, and incrementally adjusting the active voltage level of the scan control signal
until the display current value matches the expected display current value.
[0083] In accordance with another embodiment, displaying the image using the array of pixels
includes displaying a battery charge status screen using the array of pixels.
[0084] In accordance with an embodiment, an electronic device that includes a display is
provided that includes an array of pixels, each pixel in the array of pixels includes
a light-emitting diode, a drive transistor coupled in series with the light-emitting
diode, the drive transistor has a drain terminal and a gate terminal and a switching
transistor coupled between the drain terminal and the gate terminal of the drive transistor,
the switching transistor has a gate terminal that receives a scan control signal that
switches between a high voltage level and a low voltage level a ground power supply
voltage trace, the light-emitting diode of each pixel in the array of pixels is coupled
to the ground power supply voltage trace, sensing circuitry coupled to the ground
power supply voltage trace that is configured to determine a display current and processing
circuitry configured to change a magnitude of the high voltage level of the scan control
signal for each pixel based on the display current determined by the sensing circuitry.
[0085] In accordance with another embodiment, the drive transistor has a channel formed
in silicon and the switching transistor has a channel formed in semiconducting oxide.
[0086] In accordance with another embodiment, the electronic device includes a housing,
the display is mounted in the housing between first and second opposing sides of the
housing and a wrist strap attached to the first and second sides of the housing, the
switching transistor is a first switching transistor and each pixel in the array of
pixels further includes a first emission transistor coupled between the drive transistor
and a positive power supply voltage trace, a second emission transistor coupled between
the drive transistor and the light-emitting diode, a second switching transistor coupled
between a data line and a node that is interposed between the drive transistor and
the second emission transistor, a capacitor coupled between the gate terminal of the
drive transistor and a node that is interposed between the second emission transistor
and the light-emitting diode and a third switching transistor coupled between an initialization
voltage terminal and the capacitor.
[0087] The foregoing is merely illustrative and various modifications can be made to the
described embodiments. The foregoing embodiments may be implemented individually or
in any combination.
1. A display, comprising:
an array of pixels, wherein each pixel in the array of pixels comprises a light-emitting
diode, a drive transistor, and a switching transistor;
gate driver circuitry configured to provide a control signal to the switching transistor
at a first voltage level to turn on the switching transistor;
sensing circuitry coupled to the array of pixels; and
processing circuitry configured to change the first voltage level based on information
from the sensing circuitry.
2. The display defined in claim 1, wherein the switching transistor has a channel formed
in semiconducting oxide.
3. The display defined in claim 2, wherein the drive transistor has a channel formed
in silicon.
4. The display defined in claim 1, wherein the light-emitting diode of each pixel is
coupled between a first power supply terminal and a second power supply terminal.
5. The display defined in claim 4, wherein the sensing circuitry comprises a current
sensing resistor coupled to the second power supply terminal and an analog-to-digital
converter coupled to the sensing resistor.
6. The display defined in claim 5, wherein the sensing circuitry comprises an amplifier
interposed between the current sensing resistor and analog-to-digital converter.
7. The display defined in claim 5, wherein an output terminal of the analog-to-digital
converter is coupled to the processing circuitry.
8. The display defined in claim 7, wherein the processing circuitry is configured to
change the first voltage level based on a comparison between an actual current sensed
by the current sensing resistor and an expected current, wherein the processing circuitry
is configured to incrementally change the first voltage level until the actual current
sensed by the current sensing resistor matches the expected current, wherein the gate
driver circuitry is configured to provide the control signal to the switching transistor
at a second voltage level to turn off the switching transistor, wherein the processing
circuitry is configured to change the second voltage level, and wherein the processing
circuitry is configured to lower the first voltage level by a first amount and the
second voltage level by the first amount in response to determining that the actual
current is less than the expected current.
9. The display defined in claim 7, wherein the processing circuitry is configured to
use a lookup table to determine an updated first voltage level based at least on the
actual current sensed by the sensing resistor and a temperature.
10. A method of operating a display with an array of pixels, sensing circuitry coupled
to the array of pixels, and processing circuitry coupled to the sensing circuitry:
displaying an image using the array of pixels;
while displaying the image using the array of pixels, using the sensing circuitry
to obtain a display current value; and
based at least on the display current value and an expected display current value,
updating an active voltage level of a scan control signal that is provided to the
array of pixels.
11. The method defined in claim 10, wherein each pixel in the array of pixels includes
a light-emitting diode, a drive transistor connected in series with the light-emitting
diode, and a switching transistor that has a gate terminal that receives the scan
control signal and wherein updating the active voltage level of the scan control signal
comprises lowering the active voltage level of the scan control signal in response
to determining that the display current value is less than the expected display current
value.
12. The method defined in claim 11, wherein the switching transistor has a channel formed
in semiconducting oxide and wherein updating the active voltage level of the scan
control signal comprises repeatedly displaying the image using the active voltage
level of the scan control signal, using the sensing circuitry to obtain the display
current value, and incrementally adjusting the active voltage level of the scan control
signal until the display current value matches the expected display current value.
13. An electronic device that includes a display, the display comprising:
an array of pixels, wherein each pixel in the array of pixels comprises:
a light-emitting diode;
a drive transistor coupled in series with the light-emitting diode, wherein the drive
transistor has a drain terminal and a gate terminal; and
a switching transistor coupled between the drain terminal and the gate terminal of
the drive transistor, wherein the switching transistor has a gate terminal that receives
a scan control signal that switches between a high voltage level and a low voltage
level;
a ground power supply voltage trace, wherein the light-emitting diode of each pixel
in the array of pixels is coupled to the ground power supply voltage trace;
sensing circuitry coupled to the ground power supply voltage trace that is configured
to determine a display current; and
processing circuitry configured to change a magnitude of the high voltage level of
the scan control signal for each pixel based on the display current determined by
the sensing circuitry.
14. The electronic device defined in claim 13, wherein the drive transistor has a channel
formed in silicon and wherein the switching transistor has a channel formed in semiconducting
oxide.
15. The electronic device defined in claim 13, further comprising:
a housing, wherein the display is mounted in the housing between first and second
opposing sides of the housing; and
a wrist strap attached to the first and second sides of the housing, wherein the switching
transistor is a first switching transistor and each pixel in the array of pixels further
comprises:
a first emission transistor coupled between the drive transistor and a positive power
supply voltage trace;
a second emission transistor coupled between the drive transistor and the light-emitting
diode;
a second switching transistor coupled between a data line and a node that is interposed
between the drive transistor and the second emission transistor;
a capacitor coupled between the gate terminal of the drive transistor and a node that
is interposed between the second emission transistor and the light-emitting diode;
and
a third switching transistor coupled between an initialization voltage terminal and
the capacitor.