Patent Grant
10504436
This application is a Section 371 National Stage Application of PCT/CN2016/088294, filed on Jul. 4, 2016, which has not yet published and claims priority to Chinese Application No. 201610005060.1, filed on Jan. 4, 2016 and entitled “PIXEL DRIVING CIRCUITS, PIXEL DRIVING METHODS AND DISPLAY DEVICES,” which are incorporated herein by reference in their entirety.
The present disclosure relates to the field of display technology, and more particularly, to pixel driving circuits, pixel driving methods and display devices capable of improving display quality by compensating a threshold voltage of a driving circuit of a light emitting element.
Active Matrix Organic Light Emitting Diode (AMOLED) displays are hot in current flat panel display research. As compared with a liquid crystal display (LCD), an organic light emitting diode (OLED) has advantages such as low power consumption, low production costs, self-luminous, wide viewing angle, short response time and so on. At present, in the display field, such as mobile phones, PDAs, digital cameras and the like, the OLED display screens are taking places of traditional LCD display screens. Among others, pixel driving is core technical content for the AMOLED display, and has important research value.
Unlike thin film transistor-Liquid Crystal Displays (TFT-LCD), which use a stable voltage to control brightness, OLEDs are driven by current and require a constant current to control light emission. As shown in
An AMOLED is driven by a current generated in a saturated state of the driven thin film transistors (DTFT), so that it is capable of emitting light. Difference of threshold voltages may exist for driving thin film transistors at different locations, due to process non-uniformity, regardless of a low-temperature polysilicon (LTPS) process or an oxide process. This difference is fatal for the uniformity of the current-driven devices since different threshold voltages generate different driving currents when the same drive voltages are applied, resulting in inconsistency of the currents flowing through the OLED leading to non-uniform display brightness, and thus affecting displaying effect of the display panel.
Therefore, there is a need for a method which can improve the uniformity of the driving current of the driving transistor and thereby improve the display quality.
The present disclosure provides a pixel driving circuit, a pixel driving method, and a display device capable of improving display quality by compensating a threshold voltage of a driving unit of a light emitting element.
According to an aspect of the present disclosure, a pixel driving circuit for driving a light emitting element is provided. The pixel driving circuit comprises: a scanning line for supplying a scanning signal; a power supply line for supplying a voltage to the pixel driving circuit; a data line for supplying a data signal Vdata; a reference signal line for supplying a reference signal Vref; a first control signal line for supplying a first control signal; a driving unit having an input terminal connected to the first node, a control terminal connected to the third node and an output terminal connected to one terminal of the light emitting element; a first light emitting control unit having an input terminal connected to the power supply line, a control terminal connected to the first control signal line and an output terminal connected to the first node; a storage unit having a first terminal connected to the first node and a second terminal connected to the second node; a second emitting control unit having an input terminal connected to the second node, a control terminal connected to the first control signal line and an output terminal connected to the third node; a first charging control unit having a first input terminal connected to the data line, a second input terminal connected to the reference signal line, a control terminal connected to the scan line, a first output terminal connected to the second node and a second output terminal connected to the third node; a second charging control unit having an input terminal connected to the third node, a control terminal connected to the scan line and an output terminal connected to the output terminal of the drive unit.
The pixel driving circuit is configured so that, under control of the first control signal and the scan signal: during an initialization stage of the pixel driving circuit, the first light emitting control unit and the second light emitting control unit are turned on, the first charging control unit and the second charging control unit are turned off, thereby initializing the pixel driving unit; during a compensation stage of the pixel driving circuit, the first light emitting control unit and the second light emitting control unit are turned off, the first charging control unit and the second charging control unit are turned on, and the storage cell is charged until a voltage across the storage unit is equal to a value of Vdata−Vref+Vth, where Vth is a threshold voltage of the driving unit; and during a light emitting holding stage of the pixel driving circuit, the first light emitting control unit and the second light emitting control unit are turned on, and the first charging control unit and the second charging control unit are turned off, thereby the voltage across the storage unit remains unchanged so that a driving current supplied from the driving unit to the light emitting element is irrespective of the threshold voltage of the driving unit.
In one exemplary embodiment, the pixel driving circuit further comprises: a second control signal line for supplying a second control signal; a third emitting control unit having an input terminal connected to the output terminal of the driving unit, a control terminal connected to the second control signal line and an output terminal connected to one terminal of the light-emitting element. The pixel driving circuit is configured so that, under control of the second control signal, during the initialization stage, the third light emitting control unit is turned off, and during the compensation stage and the light-emitting holding stage, the third emitting control unit is turned on.
In one exemplary embodiment, the driving unit includes a driving transistor, the first emitting control unit includes a second transistor, the second emitting control unit includes a third transistor, the first charging control unit includes a fourth transistor and a fifth transistor, gates of which are connected together, the second charging control unit includes a sixth transistor, and the third light emitting control unit includes a seventh transistor.
In one exemplary embodiment, the storage unit includes a storage capacitor.
In one exemplary embodiment, during the initialization stage, the scan signal is at a high level and the first control signal is at a low level; during the compensation stage, the scan signal is at a low level and the first control signal is at a high level; and during the light-emitting holding stage, the scanning signal is at a high level and the first control signal is at a low level.
In one exemplary embodiment, during the initialization stage, the scan signal is at a high level, the first control signal is at a low level, and the second control signal is at a high level; during the compensation stage, the scan signal is at a low level, the first control signal is at a high level, and the second control signal is at a low level; and during the light-emitting holding stage, the scan signal is at a high level, the first control signal is at a low level, and the second control signal is at a low level.
According to another aspect of the present disclosure, a pixel driving method applied to a pixel driving circuit is provided. The pixel driving circuit comprising a scanning line for supplying a scanning signal; a power supply line for supplying a voltage to the pixel driving circuit; a data line for supplying a data signal Vdata; a reference signal line for supplying a reference signal Vref; a first control signal line for supplying a first control signal; a driving unit having an input terminal connected to the first node, a control terminal connected to the third node and an output terminal connected to one terminal of the light emitting element; a first light emitting control unit having an input terminal connected to the power supply line, a control terminal connected to the first control signal line and an output terminal connected to the first node; a storage unit having a first terminal connected to the first node and a second terminal connected to the second node; a second emitting control unit having an input terminal connected to the second node, a control terminal connected to the first control signal line and an output terminal connected to the third node; a first charging control unit having a first input terminal connected to the data line, a second input terminal connected to the reference signal line, a control terminal connected to the scan line, a first output terminal connected to the second node and a second output terminal connected to the third node; a second charging control unit having an input terminal connected to the third node, a control terminal connected to the scan line and an output terminal connected to the output terminal of the drive unit.
The pixel driving method comprises: during an initialization stage of the pixel driving circuit, controlling the first light emitting control unit and the second light emitting control unit to be turned on and the first charging control unit and the second charging control unit to be turned off, thereby initializing the pixel driving unit; during a compensation stage of the pixel driving circuit, controlling the first light emitting control unit and the second light emitting control unit to be turned off, and controlling the first charging control unit and the second charging control unit to be turned on, so that the storage cell is charged until a voltage across the storage unit is equal to a value of Vdata−Vref+Vth, where Vth is a threshold voltage of the driving unit; and during a light emitting holding stage of the pixel driving circuit, controlling the first light emitting control unit and the second light emitting control unit to be turned on, and controlling the first charging control unit and the second charging control unit to be turned off, thereby the voltage across the storage unit remains unchanged so that a driving current supplied from the driving unit to the light emitting element is irrespective of the threshold voltage of the driving unit.
According to another aspect of the present application, there is also provided a display device including the pixel driving circuit as described above.
The above and other objects, features and advantages of the present disclosure will become more apparent from the following description of preferred embodiments of the present disclosure in conjunction with the accompanying drawings in which:
Exemplary embodiments of the present disclosure are described in detail below with reference to the accompanying drawings. In the following description, certain specific embodiments are provided for the purpose of description and should not be construed as limiting the present disclosure, but are merely exemplary examples of the present disclosure. In the event of confusion that may result in an understanding of the present disclosure, conventional structures or constructions will be omitted.
It will be understood by those skilled in the art that both of the switching transistor and the driving transistor employed in all embodiments of the present application may be thin film transistors or field effect transistors or other devices having the same characteristics. Preferably, the thin film transistor used in the embodiments of the present disclosure may be an oxide semiconductor transistor. A term of “control terminal” as used herein refers to a gate of a transistor, a term of “input terminal” refers to one of the source and drain of the transistor and a term of “output terminal” refer to the other one of the source and the drain of the transistor. Since the source and drain of the switching transistor as used here are symmetrical, the source and the drain of the switching transistor are interchangeable. In the embodiment of the present disclosure, in order to distinguish between the two electrodes of the transistor except for the gate, one of the electrodes is referred to as a source and the other one is referred to as a drain.
As shown in
As shown in
As shown in
During the initializing stage of the pixel driving circuit 300, the equivalent circuit configuration of the pixel driving circuit 300 is shown in
During the compensation stage of the pixel driving circuit 300, an equivalent circuit configuration of the pixel driving circuit 300 is shown in
During the light-emitting holding stage of the pixel driving circuit 300, the equivalent circuit configuration of the pixel driving circuit 300 is shown in
As compared with
As shown in
The driving transistor T1, the second transistor T2, the third transistor T3, the fourth transistor T4, the fifth transistor T5 and the sixth transistor T6 as shown in
The seventh transistor T7 as shown in
First, during the first time period t1, the scanning signal Vscan is at a high level, and the first control signal Vem1 is at a low level. Therefore, the transistors T2 and T3 are turned on, and the transistors T4, T5 and T6 are turned off. At this time, since the second transistor T2 is turned on, the level Vdd supplied from the second power supply line Ldd is written to the first node N1, that is, Vn1=Vdd. The voltages Vn2 and Vn3 at the nodes N2 and N3 are the data voltage of the previous frame or arbitrary voltage Vx after starting up, i.e. Vn2=Vn3=Vx. Therefore, the voltage Vc across both ends of the capacitor C at this time is shown as: Vc=Vn2−Vn1=Vx−Vdd. Since Vn1=Vdd, transistor T1 is initialized. Such a stage may be referred to as an initialization stage.
During the second time period t2, the scanning signal Vscan is at a low level, the first control signal Vem1 is at a high level, and the data signal Vdata supplied from the data line Data is at a high level. Therefore, the transistors T4, T5 and T6 are turned on, and the transistors T2 and T3 are turned off. At this time, the data signal Vdata is written into the second node N2, and the reference voltage Vref supplied from the reference signal line Ref is written into the third node N3, so that Vn2=Vdata and Vn3=Vref. Under control of the reference voltage Vref, the gate voltage of the driving transistor T1 is Vref and the level of the source voltage Vn1 falls from the high level Vdd to Vref−Vth, where Vth is the threshold voltage of the driving transistor T1. Thus, the source voltage Vs of the driving transistor compensates Vth such that Vs=Vref−Vth. At this time, the gate-source voltage Vgs of the driving transistor T1 is Vgs=Vn3−Vn1=Vref−(Vref−Vth)=Vth. The driving transistor T1 is in a saturated state and outputs a current to the light emitting element 3000 so that the light emitting element 3000 starts to emit light. The voltage Vc across the capacitor C is shown as Vc=Vn2−Vn1=Vdata−Vref+Vth. Since the source voltage of the driving transistor T1 at this time is equal to Vref−Vth, which is irrespective of Vdd, the influence of the IR drop in Vdd is eliminated. In addition, since the sixth transistor T6 is turned on, the voltage Vn4 of the fourth node N4 is Vref, so that the anode voltage of the previous frame of the OLED 3000 can be cleared. Such a stage may be referred to as the compensation stage.
During the third time period t3, the scanning signal Vscan is at the high level, and the first control signal Vem1 is at the low level. Accordingly, the transistors T2 and T3 are turned on, and the transistors T4, T5 and T6 are turned off. At this time, the both ends of the capacitor C are connected to the gate and the source of the driving transistor T1, respectively, and the end of the capacitor C which is connected to the gate of the driving transistor T1 (the third node N3) is floated. Therefore, any voltage change at the first node N1 is fed back to the third node N3, that is, the voltage difference across the capacitor C (i.e., Vgs) does not change, Vgs=Vdata−(Vref−Vth). Such a stage may be referred to as a light-emitting holding stage. At this time, Vgs≤Vds+Vth, so the driving transistor T1 is in a stable saturated state, and the current flowing through the OLED 3000 is:
Ioled=K(Vgs−Vth)2=K[Vdata−(Vref−Vth)−Vth]2=K(Vdata−Vref)2,
where K is a constant related to the process parameters and geometric dimensions of the driving transistor T1.
As can be seen from the above equation, the light emitting current loled for driving the OLED is only related to the reference voltage Vref and the data voltage Vdata, irrespective of the threshold voltage Vth of the driving transistor. Since there is not a path for the capacitor C to be charged or discharged, even if the voltage Vdd changes during the light-emitting stage, the charge in the capacitor C and the voltage across the capacitor both remain unchanged according to the principle of charge conservation since there is not a loop for consuming the charges. Thus, the current flowing through the OLED remains I=K(Vdata−Vref)2, and the OLED maintains this light emitting state. It is possible to improve uniformity of the current and achieve uniformity of the luminance. The reference voltage Vref may be set to a voltage such as Vss or 0V.
During the subsequent time periods, the respective control signals are the same as those of the stage t3, so that the light emitting state of the OLED is maintained until the low valid level of the scanning signal Vscan comes again.
Firstly, during the first time period t1′, the scanning signal Vscan is at a high level, the first control signal Vem1 is at a low level, and the second control signal Vem2 is at a high level. Therefore, the transistors T2 and T3 are turned on, and the transistors T4, T5, T6 and T7 are turned off. Since the second control signal Vem2 is at a high level, the transistor T7 is turned off and there is no current flowing through the driving transistor T1 and the light emitting element, so that the initialization of the transistor T1 can be better realized. The other operations of the circuit at such a stage are the same as those of the circuit at the initialization stage according to the first embodiment.
During the second time period t2′, the scanning signal Vscan is at a low level, the first control signal Vem1 is at a high level, and the second control signal Vem2 is at a low level. Therefore, the transistors T4, T5, T6 and T7 are turned on, and the transistors T2 and T3 are turned off. It can be seen that this is substantially the same as the equivalent circuit in the compensation stage of the pixel driving circuit according to the first embodiment, and thus the operation of the circuit is also the same and will not be described here for brevity.
During the third time period t3′, the scanning signal Vscan is at a high level, the first control signal Vem1 is at a low level, and the second control signal Vem2 is at a low level. Accordingly, the transistors T2, T3 and T7 are turned on, and the transistors T4, T5 and T6 are turned off. It can be seen that this is substantially the same as the equivalent circuit in the light-emitting holding stage of the pixel driving circuit according to the first embodiment. Thus, the operation of the circuit is also the same and will not be described here for brevity.
At a step of S910, an initialization stage of the pixel driving circuit is implemented, in which the first light emitting control unit and the second light emitting control unit are controlled to be turned on and the first charging control unit and the second charging control unit are controlled to be turned off, thereby initializing the pixel driving unit;
At a step of S920, a compensation stage of the pixel driving circuit is implemented, in which the first light emitting control unit and the second light emitting control unit are controlled to be turned off, and the first charging control unit and the second charging control unit are controlled to be turned on, so that the storage cell is charged until the voltage across the storage unit is equal to Vdata−Vref+Vth, where Vth is the threshold voltage of the driving unit;
At a step of S930, a light emitting holding stage of the pixel driving circuit is implemented, in which the first light emitting control unit and the second light emitting control unit are controlled to be turned on, and the first charging control unit and the second charging control unit are controlled to be turned off, thereby the voltage across the storage unit remains unchanged so that the driving current supplied from the driving unit to the light emitting element is irrespective of the threshold voltage of the driving unit.
At a step of S1010, an initialization stage of the pixel driving circuit is implemented, in which the first light emitting control unit and the second light emitting control unit are controlled to be turned on and the first charging control unit, the second charging control unit and the third light emitting control unit are controlled to be turned off, thereby initializing the pixel driving unit;
At a step of S1020, a compensation stage of the pixel driving circuit is implemented, in which the first light emitting control unit and the second light emitting control unit are controlled to be turned off, and the first charging control unit, the second charging control unit and the third light emitting control unit are controlled to be turned on, so that the storage cell is charged until the voltage across the storage unit is equal to Vdata−Vref+Vth, where Vth is the threshold voltage of the driving unit;
At a step of S1030, a light emitting holding stage of the pixel driving circuit is implemented, in which the first light emitting control unit, the second light emitting control unit and the third light emitting control are controlled to be turned on, and the first charging control unit and the second charging control unit are controlled to be turned off, thereby the voltage across the storage unit remains unchanged so that the driving current supplied from the driving unit to the light emitting element is irrespective of the threshold voltage of the driving unit.
The pixel driving circuit provided by the present disclosure has been described in detail above. In addition, the present disclosure provides a display device including the above pixel driving circuit.
The disclosure has been described in connection with a preferred embodiment. It will be understood by those skilled in the art that various other changes, substitutions and additions may be made thereto without departing from spirit and scope of the present disclosure. Accordingly, the scope of the present disclosure is not limited to the specific embodiments described above, but should be defined by the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2016 1 0005060 | Jan 2016 | CN | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/CN2016/088294 | 7/4/2016 | WO | 00 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2017/117938 | 7/13/2017 | WO | A |
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| Number | Date | Country | |
|---|---|---|---|
| 20180108298 A1 | Apr 2018 | US |
