Patent Application
20210335232
The present invention relates to display technology, more particularly, to a method for driving a pixel circuit with feedback compensation, a circuit for driving a light-emitting device, and a display apparatus having the same.
Organic Light Emitting Diode (OLED) display has many advantages including wider view angles, greater brightness, higher contrast, lower power consumption, thinner physical thickness over the many conventional display technologies. Low Temperature Poly Silicon (LIPS) substrate with its fast electron mobility characteristics has become a popular substrate for making thin-film transistors based pixel driving circuit for driving light emission of each OLED associated with each sub-pixel in the display panel. In real OLED display apparatus, every thin-film transistor formed in the display panel may not have uniform characteristics in threshold voltage, carrier mobility, or resistor series, leading to non-uniform variations in image display across the display panel. In addition, OLED as a sub-pixel light source is a diode. The V-I characteristics of the diode may be drifted due to changes of environment or working hours, also leading to unwanted variation in image display.
In an aspect, the present disclosure provides a method for driving a pixel circuit with feedback compensation in consecutive cycles. The method includes initializing a voltage setting in the pixel circuit including at least a driving transistor coupled to a light-emitting device and obtaining a first threshold voltage of the driving transistor. The method further includes inputting a first data voltage from a data voltage terminal to the pixel circuit to generate a first driving current independent of the first threshold voltage, to drive light emission of the light-emitting device for displaying a pixel image in a current cycle. Additionally, the method includes generating a compensation voltage via a feedback sub-circuit coupled between the data voltage terminal and the light-emitting device based on a change of the first driving current due to a change of a second threshold voltage of the light-emitting device. Furthermore, the method includes inputting a second data voltage from the data voltage terminal combined with the compensation voltage as a negative feedback to generate a second driving current to drive light emission of the light-emitting device for displaying a pixel image in a next cycle.
Optionally, each of the current cycle and the next cycle is one of two consecutive durations for the light-emitting device to emit light for producing two consecutive frames of pixel images under a progressive scanning scheme, each duration comprising consecutively a first period, a second period, a third period, and a fourth period.
Optionally, the initializing the pixel circuit includes releasing charges in a source electrode of the driving transistor in the first period of the current cycle. The source electrode is coupled to an anode of the light-emitting device.
Optionally, the obtaining a first threshold voltage of the driving transistor includes setting a voltage level at a first electrode of a first capacitor in the pixel circuit to a first reference voltage in the second period of the current cycle and storing the first threshold voltage as a voltage difference between the first electrode and a second electrode of the first capacitor. The first electrode of the first capacitor is coupled to a gate electrode of the driving transistor.
Optionally, the inputting a first data voltage includes transferring the first data voltage to the second electrode of the first capacitor in the third period of the current cycle and resetting the voltage level at the first electrode of the first capacitor to a sum of the first data voltage and the first threshold voltage.
Optionally, the resetting the voltage level at the first electrode of the first capacitor includes making the voltage level at the source electrode of the driving transistor to at least a second threshold voltage in the fourth period of the current cycle and generating the first driving current through the driving transistor.
Optionally, the generating a compensation voltage includes using a first resistor to obtain a first sampling voltage equal to the first driving current multiplying a resistance of the first resistor in the fourth period of the current cycle. Additionally, the generating the compensation voltage includes inputting the first sampling voltage to a first positive input terminal of a first voltage-difference comparator in the feedback sub-circuit. Furthermore, the generating the compensation voltage includes coupling the first data voltage to a fust negative input terminal of the first voltage-difference comparator to output a first voltage difference between the first sampling voltage and the first data voltage.
Optionally, the method further includes inputting the first voltage difference to a second negative input terminal of a second voltage-difference comparator in the feedback sub-circuit. Additionally, the method includes coupling a second reference voltage to a second positive input terminal of the second voltage-difference comparator. Moreover, the method includes outputting a second voltage difference between the second reference voltage and the first voltage difference. The second voltage difference is proportional to the change of the first driving current due to the change of the second threshold voltage of the light-emitting device.
Optionally, the method further includes coupling the second voltage difference as the compensation voltage to the data voltage terminal to be added with the second data voltage.
Optionally, the compensation voltage is zero when the second threshold voltage remains substantially unchanged. The compensation voltage is a negative value to compensate an increasing driving current when the second threshold voltage decreases. The compensation voltage is a positive value to compensate a decreasing driving current when the second threshold voltage increases.
In another aspect, the present disclosure provides a circuit for driving a light emitting device in a series of cycles of displaying frames of pixel images. The circuit includes a driving transistor having a gate coupled to a first node, a source coupled to a second node connected to an anode of the light emitting device, and a drain connected to a first voltage terminal. The circuit further includes an initialization sub-circuit coupled to a second voltage terminal and the first node and configured to initialize potentials at the first node and the second node under control of a first control signal from a first control terminal. Additionally, the circuit includes a data-input and compensation sub-circuit coupled to the second voltage terminal, a data voltage terminal, the first node, and the second node, and configured to receive a data voltage and change potentials at the first node and the second node under control of the first control signal and a second control signal from a second control terminal. Furthermore, the circuit includes a feedback sub-circuit coupled to a cathode of the light emitting device and the data voltage terminal, being configured to receive the data voltage and compensate a threshold voltage difference of the light-emitting device.
Optionally, the feedback sub-circuit includes a first voltage-difference comparator having a first positive input port coupled to the cathode of the light-emitting device connected to a first constant voltage terminal via a first resistor, a first negative input port and a first output port. The feedback sub-circuit also includes a second voltage-difference comparator having a second negative input port coupled to the first output port, a second positive input port coupled to a second constant voltage terminal via a second resistor, and a second output port being coupled to the second positive input port via a third resistor. Additionally, the feedback sub-circuit includes a third capacitor having one terminal coupled to the data voltage terminal and the other one terminal coupled to the first negative input port of the first voltage-difference comparator and the second output port of the second voltage-difference comparator.
Optionally, the initialization sub-circuit includes a second transistor having a gate coupled to a first control terminal, a source coupled to the first node, and a drain coupled to a second voltage terminal.
Optionally, the data-input and compensation sub-circuit includes a third transistor having a gate coupled to the second control terminal, a source coupled to a third node, and a drain coupled to the data voltage terminal; a fourth transistor having a gate coupled to the first control terminal, a source coupled to the second node, and a drain coupled to the third node; a first capacitor having one terminal coupled to the first node and the other one terminal coupled to the third node; and a second capacitor having one terminal coupled to the second voltage terminal and the other one terminal coupled to the third node.
Optionally, each cycle of the series of cycles includes consecutively a first period, a second period, a third period, and a fourth period. The initialization sub-circuit is configured, in the first period of a current cycle of the series of cycles, to set a voltage level at the first node to a first reference voltage and a voltage level at the second node to zero under a condition that the first voltage terminal is provided at 0V. The second voltage terminal is provided with a first reference voltage at a turn-on voltage level. The first control terminal is provided with a first control signal at the turn-on voltage level to turn the second transistor on to pass the first reference voltage to the first node. The second control terminal is provided with a second control signal at a turn-off voltage level.
Optionally, the initialization sub-circuit and the data-input and compensation sub-circuit are configured in the second period of the current cycle to keep the voltage level at the first node unchanged, to increase the voltage level at the second node to the first reference voltage minus a first threshold voltage of the driving transistor, and to set a voltage level at the third node equal to the voltage level at the second node to store the first threshold voltage to the first capacitor under a condition that the first voltage terminal is provided with a turn-on voltage level. The second voltage terminal is kept at the first reference voltage. The first control signal is kept at the turn-on voltage level. The second control signal is kept at the turn-off voltage level.
Optionally, the data-input and compensation sub-circuit is configured in the third period of the current cycle to input a first data voltage from the data voltage terminal to set the voltage level at the second node unchanged, to change the voltage level at the third node to the first data voltage, and to change the voltage level at the first node to the first data voltage plus the first threshold voltage under a condition that the first voltage terminal and the second voltage terminal are provided at 0V. The first control signal is changed to a turn-off voltage level. The second control signal is changed to a turn-on voltage level.
Optionally, the data-input and compensation sub-circuit and the driving transistor are configured in the fourth period of the current cycle to generate a first driving current flowing through the driving transistor under a condition that the first voltage terminal is changed to the turn-on voltage level. The second voltage terminal is kept at 0V. The first control signal remains to be the turn-off voltage level. The second control signal is changed to the turn-off voltage level. The first driving current is independent of the first threshold voltage yet depended on a second threshold voltage of the light-emitting device.
Optionally, the feedback sub-circuit is operated to output a first voltage difference of the first data voltage at the first positive input port minus a sampling voltage at the first negative input port. The sampling voltage equals to a product of the first driving current and a resistance of a first resistor coupled to the cathode of the light-emitting device.
Optionally, the feedback sub-circuit is operated to output a second voltage difference of a second reference voltage deduced from the second positive input port minus the first voltage difference at the second negative input port.
Optionally, the second voltage difference is feed back to the data voltage terminal via the third capacitor as a compensation voltage to combine with a second data voltage to be inputted into the pixel circuit in the third period of a next cycle.
In yet another aspect, the present disclosure provides a display apparatus including a display panel and a circuit described herein.
Optionally, the display panel is an organic light-emitting diode display panel.
The following drawings are merely examples for illustrative purposes according to various disclosed embodiments and are not intended to limit the scope of the present invention.
The disclosure will now be described more specifically with reference to the following embodiments. It is to be noted that the following descriptions of some embodiments are presented herein for purpose of illustration and description only. It is not intended to be exhaustive or to be limited to the precise form disclosed.
Many existing OLED display apparatuses have adopted various compensation approaches in designing different pixel circuit in order to solve problems of display abnormity due to the thin-film transistor threshold voltage variation, turn-on voltage difference, driving current difference, and capacitor charging time difference. Nearly all these compensation approaches focus on internal compensations of the pixel circuit such as compensation for a first threshold voltage of a driving transistor, IR drop for connecting the driving transistor, etc. However, the existing compensation approach is rarely focused on external devices such as light-emitting diode which also may cause display variation due to characteristics drift due to environmental change and prolonged working hours. In an example, when the OLED display apparatus is in use for performing a panel self refresh (PSR) operation to save power, a conventional OLED driving scheme used a fixed driving current generated by a timing control internal shift register which is not responsive to variation of the OLED device characteristics such as a second threshold voltage of OLED itself. This may cause unstable image display and result in false image luminance off a target luminance when entering or existing the PSR operation.
Accordingly, the present disclosure provides, inter alia, a method for providing a compensation on pixel voltage with negative feedback in real time, a circuit for implementing the negative feedback, and a display apparatus having the same that substantially obviate one or more of the problems due to limitations and disadvantages of the related art. In one aspect, the present disclosure provides a method for driving a pixel circuit.
Referring to
The method further includes obtaining a first threshold voltage of the driving transistor. Through circuit design and controls of several switch transistors coupled with the driving transistor, a threshold voltage of the driving transistor is effectively deduced and stored in a capacitor in a second period of the cycle. The second period is also named as a threshold-voltage retrieve period.
Referring to
Referring to
In the embodiment, the method includes inputting a second data voltage from the data voltage terminal combined with the compensation voltage as a negative feedback to generate a second driving current to drive light emission of the light-emitting device for displaying a pixel image in a next cycle. The second voltage-difference comparator is coupled back to the data voltage terminal via a coupling capacitor to couple the second voltage difference as a compensation voltage back to the inputting data voltage terminal. Optionally, there is no drift in the second threshold voltage, then no change in the first driving current, leading to the first voltage difference to be zero and subsequently the second voltage difference or the compensation voltage to be zero. Optionally, there is a change of the first driving current in the current cycle due to the drift in the second threshold voltage, then the first voltage difference is a non-zero value and subsequently the second voltage difference is a non-zero value, leading a non-zero value in the compensation voltage that is added into a second data voltage inputted in the next cycle (a cycle that is subsequently to the current cycle of driving the pixel circuit for displaying a frame of pixel image after a current frame.
Referring to
Referring to
Referring to
As the voltage level VB at the second node B increases, the turning-on voltage of the driving transistor T1 Vth_t1=VA−VB decreases until it reaches a threshold voltage Vth to have the driving transistor T1 to be turned off. At the turn-off point, the voltage level at the second node B is VB=VA−Vth=Vref−Vth. At the same time, the fourth transistor T4 is on as the first control terminal EN remains to be a high voltage level. The source and the drain of the fourth transistor T4 have a same voltage level. Thus, a voltage level at the third node C is charged to VC=VB=Vref−Vth. Since the first node A and the third node C are two terminals of the first capacitor Cs1, the voltage difference ΔVCs1 between them is stored in Cs1:
ΔVCs1=VA−VC=Vref−(Vref−Vth)=Vth (1)
In other words, the first threshold voltage Vth of the driving transistor T1 is deduced and stored in the first capacitor Cs1 as a voltage difference ΔVCs1.
Referring to
V
A
=Vref ΔVC=Vref+Vdata−(Vref−Vth)=Vdata+Vth (2)
In this period t3, the first voltage terminal VCC is changed to a low voltage level while the second voltage terminal Vref is changed to a low voltage level, the driving transistor T1 is turned off with a reversed bias.
In the next period, t4, or a display period, the data-input and compensation sub-circuit and the driving transistor of the pixel circuit are operated to generate a driving current, or a first driving current of this cycle, through the driving transistor T1. The first voltage terminal VCC now is changed again to the high voltage level to make the driving transistor T1 working in a saturation state to yield a first driving current Idata=(½)×(W/L)μC1[Vgs−Vth]2 flowing through the driving transistor T1. The gate-to-source voltage Vgs=VA−VB. The voltage level at the gate of the driving transistor T1 is just the voltage level at the first node A: VA=Vdata+Vth. The voltage level at the source of the driving transistor T1 is just the voltage of the second node B: VB, which is also a voltage applied to the anode of the OLED. The voltage of OLED must at least be equal to or greater than a second threshold voltage Vth_oled, which is a minimum driving voltage that initializes the OLED to allow the first driving current Idata to be a driving current Ioled flowing through the OLED to induce light emission:
V
B
=Vth_oled (3)
I
oled=(½)×(W/L)μC1[Vdata−Vth_oled]2 (4)
Here W/L is a ratio of channel width over length of the driving transistor T1, μ is an electron mobility of the driving transistor T1, and C1 is intrinsic capacitance of the driving transistor T1.
As seen in Formula (4), the driving current Ioled is independent of the first threshold voltage Vth of the driving transistor while dependent upon the second threshold voltage Vth_oled of the light-emitting diode OLED. The drift of Vth_oled shall cause the change of Ioled, resulting in an offset of real pixel luminance away from a target pixel luminance. In order to eliminate the affection of the drift of the second threshold voltage Vth_oled, a compensation voltage is required to adjust the input data voltage to follow the drift of Vth_oled.
Referring back to
U
1+
=I
oled
×R1
U
1−
=Vdata
Then the first voltage-difference comparator U1 outputs a first voltage difference U1_out at a first output port:
U
1_out
=U
1+
−U
1−
=I
oled
×R1−Vdata
Referring to
U
2−
=U
1_out
=I
oled
×R1−Vdata.
Referring to
U
2+
=F(R2,Rf)×Vdd.
Here F(R2, Rf) is amplification coefficient associated with the second voltage-difference comparator U2 which acts as a close-loop amplifier and Vdd is a fixed voltage provided to the second constant voltage terminal VDD. Optionally, Vdd≥0V.
When there is no drift of the second threshold voltage Vth_oled in the current cycle, no feedback is needed to be fed to a next cycle. Thus, the second output port U2_out should output 0V, i.e., U2+=U2−. In case there is a change in Vth_oled in the current cycle, the first driving current will also change, e.g., to I′oled. Then, the input voltages respectively at the first positive input port and the first negative input port, the first voltage difference at the first output port are,
U′
1+
=I′
oled
×R1
U′
1−
=Vdata
U′
1_out
=I′
oled
×R1−Vdata
The second negative input port U2− of the second voltage-difference comparator U2 is receiving the first voltage difference U′1_out outputted in the current cycle. The output voltage at the second output port U2_out of the second voltage-difference comparator U2 is given by
Referring to
V′data[n+1]=Vdata[n+1]+U2_out[n] (6)
Here [n] represents a current cycle, and [n+1] a subsequent next cycle.
From the Formula (5), when there is no change of driving current ΔLoled induced by a drift in the Vth_oled, then the output voltage at the second output port U2_out of the second voltage-difference comparator U2 will be 0V. In this case, referring to
In yet another aspect, the present disclosure provides a display apparatus including a display panel and a circuit of
The foregoing description of the embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form or to exemplary embodiments disclosed. Accordingly, the foregoing description should be regarded as illustrative rather than restrictive. Obviously, many modifications and variations will be apparent to practitioners skilled in this art. The embodiments are chosen and described in order to explain the principles of the invention and its best mode practical application, thereby to enable persons skilled in the art to understand the invention for various embodiments and with various modifications as are suited to the particular use or implementation contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents in which all terms are meant in their broadest reasonable sense unless otherwise indicated. Therefore, the term “the invention”, “the present invention” or the like does not necessarily limit the claim scope to a specific embodiment, and the reference to exemplary embodiments of the invention does not imply a limitation on the invention, and no such limitation is to be inferred. The invention is limited only by the spirit and scope of the appended claims. Moreover, these claims may refer to use “first”, “second”, etc. following with noun or element. Such terms should be understood as a nomenclature and should not be construed as giving the limitation on the number of the elements modified by such nomenclature unless specific number has been given. Any advantages and benefits described may not apply to all embodiments of the invention. It should be appreciated that variations may be made in the embodiments described by persons skilled in the art without departing from the scope of the present invention as defined by the following claims. Moreover, no element and component in the present disclosure is intended to be dedicated to the public regardless of whether the element or component is explicitly recited in the following claims.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/CN2018/100803 | 8/16/2018 | WO | 00 |
