TWI881827B - Driving method of pixel circuit - Google Patents

Abstract

A driving method of a pixel circuit includes: providing an emission signal to the pixel circuit, such that a light-emitting element of the pixel circuit emits light according to the emission signal; modulating the emission signal according to an initial instantaneous brightness waveform of the light-emitting element; and providing the modulated emission signal to the pixel circuit, such that the light-emitting element emits light according to the modulated emission signal, and an instantaneous brightness waveform of the light-emitting element presents the following characteristics in each display frame. The bright-state brightness of the updated frame is greater than the bright-state brightness of the last skipped frame. The dark-state brightness of the updated frame is approximately the same as the dark-state brightness of the last skipped frame. The brightness of the first skipped frame to the brightness of the penultimate skipped frame changes gradually. The bright-state brightness of the first skipped frame is approximately the same as the bright state brightness of the last skipped frame.

Description

Pixel circuit driving method

The present invention relates to a pixel circuit driving method, and more particularly to a pixel circuit driving method for improving the flickering phenomenon of a display screen at a low screen refresh rate.

In order to reduce the power consumption of wearable organic light-emitting diode (OLED) displays and extend the time that users use the device at a time, the pixel circuit will operate at a low screen refresh rate (e.g., 5Hz). However, the current of the light-emitting element of the pixel circuit (i.e., I _OLED ) will change due to the leakage and hysteresis effect of the transistors in the pixel circuit, causing the brightness of the display panel to gradually change over time, thereby causing the display screen to flicker and reduce the display screen quality.

At least one embodiment of the present invention provides a method for driving a pixel circuit, comprising: providing a light-emitting control signal to the pixel circuit, wherein the pixel circuit operates in a plurality of display frames, wherein each display frame includes an update frame and a plurality of skip frames located after the update frame; when a light-emitting element of the pixel circuit emits light according to the light-emitting control signal, measuring the light-emitting element to obtain an initial instantaneous brightness waveform of the light-emitting element; modulating the light-emitting control signal according to the initial instantaneous brightness waveform; and providing the pixel circuit with the modulated light-emitting control signal so that when the light-emitting element emits light according to the modulated light-emitting control signal, the light-emitting element is The instantaneous brightness waveform is presented in each display frame: the bright state brightness of the update frame is greater than the bright state brightness of the last one of the multiple skip frames; the dark state brightness of the update frame is approximately the same as the dark state brightness of the last one of the multiple skip frames; the bright state brightness of the first one of the multiple skip frames to the bright state brightness of the second to last one of the multiple skip frames presents a gradual change, and the dark state brightness of the first one of the multiple skip frames to the dark state brightness of the second to last one of the multiple skip frames presents a gradual change; and the bright state brightness of the first one of the multiple skip frames is approximately the same as the bright state brightness of the last one of the multiple skip frames.

In at least one embodiment of the present invention, the above-mentioned gradual change is a decreasing change.

In at least one embodiment of the present invention, the gradual change is incremental.

In at least one embodiment of the present invention, the pixel circuit operates at a low screen refresh rate, and the light-emitting element is a current-driven element.

In at least one embodiment of the present invention, when the light-emitting element emits light according to the modulated light-emitting control signal, the flicker of the instantaneous brightness waveform of the light-emitting element is less than -50 dB.

In at least one embodiment of the present invention, the light control signal includes a pulse signal in each update frame of the display frame, wherein modulating the light control signal includes: increasing the number of pulse signals included in the light control signal in each update frame of the display frame from one to two.

In at least one embodiment of the present invention, the above-mentioned light control signal includes a pulse signal in each skipped frame of each display frame, wherein modulating the light control signal includes: increasing the number of pulse signals included in each skipped frame of the light control signal in each display frame from one to at least two.

In at least one embodiment of the present invention, the above-mentioned luminous control signal includes multiple pulse signals in each display frame, wherein modulating the luminous control signal includes: increasing the pulse width of one of the multiple pulse signals to reduce the bright state brightness and dark state brightness of the initial instantaneous brightness waveform at a time point, wherein the time point corresponds to one of the multiple pulse signals.

In at least one embodiment of the present invention, the above-mentioned luminance control signal includes multiple pulse signals in each display frame, wherein modulating the luminance control signal includes: reducing the pulse width of one of the multiple pulse signals to increase the bright state brightness and dark state brightness of the initial instantaneous brightness waveform at a time point, wherein the time point corresponds to one of the multiple pulse signals.

In at least one embodiment of the present invention, the number of the skipped frames included in each display frame is 8.

In order to make the above features and advantages of the present invention more clearly understood, embodiments are specifically cited below and described in detail with reference to the accompanying drawings.

The following is a detailed discussion of embodiments of the present invention. However, it is understood that the embodiments provide many applicable concepts that can be implemented in a variety of specific contexts. The embodiments discussed and disclosed are for illustration only and are not intended to limit the scope of the present invention. The terms "first", "second", etc. used herein do not specifically refer to order or sequence, but are only used to distinguish between components or operations described with the same technical terms.

FIG. 1 is a schematic diagram of a pixel circuit according to an embodiment of the present invention. The pixel circuit shown in FIG. 1 includes transistors T1, T2, T31, T32, T4, T5, T6, T7, a capacitor _CST and a light-emitting element OLED. The light-emitting element OLED is a current-driven element, such as an organic light-emitting diode. Specifically, the pixel circuit shown in FIG. 1 is applicable to an organic light-emitting diode display.

The transistors T4, T5 and the light-emitting element OLED constitute a light-emitting circuit of the pixel circuit. The transistor T4 has a gate terminal for receiving the data signal V _DATA , and the transistor T5 has a gate terminal for receiving the light-emitting control signal EM[N]. The light-emitting element OLED and the transistors T4 and T5 are connected in series and coupled between the system voltage terminals OVDD and OVSS to form a current path.

Among them, transistors T1, T2, T31, T32, T6, T7 and capacitor C _ST constitute a control and compensation circuit of the pixel circuit. Transistor T1 has a gate terminal for receiving a scanning signal S1[N], transistor T2 has a gate terminal for receiving a light emission control signal EM[N], transistors T31, T32 and T6 have gate terminals for receiving a scanning signal S2[N], and transistor T7 has a gate terminal for receiving a scanning signal S1[N+1].

During the reset period of the pixel circuit, the scanning signal S1[N] is controlled to turn on the transistor T1, so that one end of the transistor T1 is reset by the reference voltage V _REF received by the other end of the transistor T1. On the other hand, the scanning signal S1[N+1] is controlled to turn on the transistor T7 to reset the voltage of the anode end of the light-emitting element OLED.

During the compensation period of the pixel circuit, the scanning signal S1[N] is controlled to turn off the transistor T1. On the other hand, the scanning signal S2[N] is controlled to turn on the transistors T31, T32, and T6, so that the end of the capacitor _CST coupled to the transistor T6 receives the data voltage Vdata, and the transistors T31 and T32 form a charging path, so that the end of the capacitor _CST coupled to the transistor T31 is charged to reach the difference between the system voltage OVDD and the critical voltage of the transistor T4. In this way, the capacitor _CST will store the critical voltage in the transistor T4. In other words, during the compensation period of the pixel circuit, compensation can be performed for the critical voltage in the transistor T4.

During the luminescence period of the pixel circuit, the luminescence control signal EM[N] is controlled to turn on the transistors T2 and T5, so that the end of the capacitor C _ST coupled to the transistor T2 changes from the data voltage Vdata to the reference voltage V _REF , and the above voltage change is coupled by the capacitor C _ST to the other end of the capacitor C _ST coupled to the transistor T4. On the other hand, since both the transistors T4 and T5 are turned on, the luminescence circuit of the pixel circuit can generate a conduction current flowing through the luminescence element OLED to make it emit light.

FIG. 2 is a flow chart of a driving method of a pixel circuit according to an embodiment of the present invention. The driving method shown in FIG. 2 is applicable to the pixel circuit shown in FIG. 1 . However, it is worth mentioning that the circuit state of the pixel circuit shown in FIG. 1 is only an example, and the present invention is not limited thereto. Other known pixel circuits applicable to organic light-emitting diode displays or other known pixel circuits including light-emitting elements of current-driven elements can also be applicable to the driving method shown in FIG. 2 .

As shown in FIG2 , in step S1, a light-emitting control signal (as the light-emitting control signal EM[N] of the pixel circuit shown in FIG1 ) is provided to the pixel circuit, so that the light-emitting element of the pixel circuit (such as the light-emitting element OLED shown in FIG1 ) emits light according to the light-emitting control signal. In an embodiment of the present invention, the light-emitting control signal includes an update frame and multiple skip frames in each display frame, and the light-emitting control signal includes a pulse signal in the update frame of each display frame and includes a pulse signal in each skip frame of each display frame. In an embodiment of the present invention, the pixel circuit operates at a low frame update frequency, that is, the light-emitting control signal corresponds to the low frame update frequency, such as 5 Hz, 10 Hz, 15 Hz, etc. It is worth mentioning that the low screen refresh rate corresponding to the light control signal of the present invention and the pulse width of the pulse signal will depend on the requirements of the display product.

FIG3 is an illustrative schematic diagram of a light control signal according to an embodiment of the present invention. The light control signal EM shown in FIG3 includes an update frame RF and 8 skip frames SF in each display frame DF, and the light control signal EM shown in FIG3 includes a pulse signal PS in an update frame RF included in each display frame DF, and the light control signal EM shown in FIG3 includes a pulse signal PS in each of the 8 skip frames SF included in each display frame DF. Specifically, the luminous control signal EM shown in FIG3 corresponds to a low picture refresh rate and the frame rate of each display frame DF is 5 Hz, and the frame rate of one update frame RF included in each display frame DF is 45 Hz, and the frame rate of each of the eight skip frames SF included in each display frame DF is 45 Hz. However, it should be noted that the number of skip frames, the frame rate of the display frame, and the frame rates of the update frame and the skip frame shown in FIG3 are only examples, and the present invention is not limited thereto.

In addition, FIG. 3 also shows a TE (Tearing Effect) signal TE, which is a trigger signal carried by the display panel control circuit, and it shows that the frame rate of each display frame DF is 5 Hz.

Please return to FIG. 2. In step S2, when the light-emitting element emits light according to the light-emitting control signal, the light-emitting element is measured to obtain the initial instantaneous brightness waveform of the light-emitting element. In the embodiment of the present invention, the initial instantaneous brightness waveform of the step S2 is obtained by measuring through a display color analyzer. It is worth mentioning that in the embodiment of the present invention, the initial instantaneous brightness waveform of the light-emitting element refers to the brightness waveform obtained by measuring the light-emitting element through a display color analyzer when the light-emitting element emits light according to the preset light-emitting control signal.

FIG4 is an exemplary schematic diagram of the initial instantaneous brightness waveform of the light-emitting element of the pixel circuit according to the embodiment of the present invention according to the light-emitting control signal. As shown in FIG4, the pixel circuit will operate at a low screen refresh rate (for example, the frame rate of the light-emitting control signal EM in each display frame DF shown in FIG3 is 5 Hz), so that the current of the light-emitting element of the pixel circuit will change due to the leakage and hysteresis effect of the transistor of the pixel circuit, causing the brightness of the display panel to gradually change over time (for example, the oblique arrow in FIG4 shows that the brightness of the display panel decreases over time), thereby causing the display screen to produce a flickering phenomenon to reduce the display screen quality. Furthermore, the flicker of the initial instantaneous brightness waveform shown in FIG4 is -40.59 dB, and the resulting image will have flickering at a low image refresh rate, resulting in reduced image quality. Accordingly, the present invention proposes a pixel circuit driving method to improve the flickering phenomenon of a display image operating at a low image refresh rate, thereby improving the display image quality.

Please return to FIG. 2 , in step S3 , the luminance control signal is modulated according to the initial instantaneous brightness waveform. In the embodiment of the present invention, modulating the luminance control signal is to modulate the number of multiple pulse signals included in each display frame of the luminance control signal and modulate the pulse width of each pulse signal included in the luminance control signal.

Specifically, the modulation method of the light control signal in step S3 first increases the number of pulse signals included in one update frame included in each display frame from one to two, and increases the number of pulse signals included in each of the multiple skip frames included in each display frame from one to at least two, that is, increasing the number of pulse signals included in one update frame and multiple skip frames included in each display frame. The frequency of skipping frames is then changed according to the brightness of the initial instantaneous brightness waveform obtained in step S2 at the corresponding time point. If the brightness is relatively low, the pulse width of the pulse signal at the corresponding time point is narrowed to increase the brightness. If the brightness is relatively high, the pulse width of the pulse signal at the corresponding time point is widened to reduce the brightness.

Specifically, modulating the pulse width of each pulse signal included in the luminescence control signal includes: increasing the pulse width of one of the plurality of pulse signals to reduce the bright state brightness and dark state brightness of the initial instantaneous brightness waveform at a time point, wherein the time point corresponds to one of the plurality of pulse signals. Specifically, modulating the pulse width of each pulse signal included in the luminescence control signal includes: reducing the pulse width of one of the plurality of pulse signals to increase the bright state brightness and dark state brightness of the initial instantaneous brightness waveform at a time point, wherein the time point corresponds to one of the plurality of pulse signals.

FIG5 is an exemplary schematic diagram of a modulated light control signal according to an embodiment of the present invention. The modulated light control signal EM shown in FIG5 includes an update frame RF and eight skip frames SF in each display frame DF, and the modulated light control signal EM shown in FIG5 includes two pulse signals PS in one update frame RF included in each display frame DF, and the modulated light control signal EM shown in FIG5 includes at least two pulse signals PS in each of the eight skip frames SF included in each display frame DF. Specifically, the modulated luminance control signal EM shown in FIG5 corresponds to a low screen refresh rate and the frame rate of each display frame DF is 5 Hz, and the frame rate of each of the one refresh frame RF and the eight skip frames SF included in each display frame DF is 45 Hz. However, it should be noted that the number of skip frames, the frame rate of the display frame, and the frame rates of the refresh frame and the skip frame shown in FIG5 are only examples, and the present invention is not limited thereto.

The number of pulse signals PS included in each of the eight skip frames SF of each display frame DF of the modulated luminescence control signal EM shown in FIG5 is not exactly the same. As shown in FIG5, the number of pulse signals PS included in the first skip frame SF is three, and the number of pulse signals PS included in each of the remaining skip frames SF is two. However, it should be noted that the number of pulse signals included in each of the multiple skip frames shown in FIG5 is only an example, and the present invention is not limited thereto. In addition, in other embodiments of the present invention, the number of pulse signals included in each of the eight skip frames of each display frame of the modulated luminescence control signal is the same as each other, for example, the number is two.

The pulse widths of the multiple pulse signals PS included in one update frame RF of each display frame DF and each of the eight skip frames SF of the modulated luminance control signal EM shown in Fig. 5 are identical to each other. In addition, in other embodiments of the present invention, the pulse widths of the multiple pulse signals included in one update frame of each display frame and each of the eight skip frames of the modulated luminance control signal are not identical to each other.

The pulse widths of the modulated luminance control signal EM in one update frame RF of each display frame DF and the multiple pulse signals PS included in eight skip frames SF are not completely the same. In addition, in other embodiments of the present invention, the pulse widths of the modulated luminance control signal in one update frame of each display frame and the multiple pulse signals PS included in eight skip frames are the same.

Please return to FIG. 2. In step S4, a modulated luminescence control signal (as the luminescence control signal EM[N] of the pixel circuit shown in FIG. 1) is provided to the pixel circuit, so that the luminescence element (such as the luminescence element OLED of the pixel circuit shown in FIG. 1) emits light according to the modulated luminescence control signal. When the luminescence element emits light according to the modulated luminescence control signal, the instantaneous brightness waveform of the luminescence element exhibits the following four characteristics in each display frame: ：(1) the bright state brightness of the update frame is greater than the bright state brightness of the last one of the multiple skip frames;(2) the dark state brightness of the update frame is substantially the same as the dark state brightness of the last one of the multiple skip frames;(3) the bright state brightness of the first one of the multiple skip frames to the bright state brightness of the second to last one of the multiple skip frames presents a gradual change;(4) the bright state brightness of the first one of the multiple skip frames is substantially the same as the bright state brightness of the last one of the multiple skip frames. It is worth mentioning that in the embodiment of the present invention, the instantaneous brightness waveform of the light-emitting element refers to the brightness waveform obtained by measuring the light-emitting element through a display color analyzer when the light-emitting element emits light according to the modulated light-emitting control signal.

FIG6 is an exemplary schematic diagram of the instantaneous brightness waveform of the light-emitting element of the pixel circuit according to the embodiment of the present invention according to the modulated light-emitting control signal. As shown in FIG6 , the instantaneous brightness waveform of the light-emitting element will present the following four characteristics in each display frame: (1) The bright state brightness of the update frame RF is greater than the bright state brightness of the last one of the eight skip frames SF (as indicated by “(1)” in FIG6 ); (2) The dark state brightness of the update frame RF is substantially the same as the dark state brightness of the last one of the eight skip frames SF. (as indicated by "(2)" in FIG6); (3) the brightness of the bright state of the first of the 8 skip frames SF to the brightness of the second to last of the 8 skip frames SF presents a gradual change (as indicated by "(3)" and a diagonal thick arrow in FIG6), wherein the above-mentioned gradual change is shown as decreasing in FIG6, but it should be noted that the above-mentioned gradual change can also be increasing; (4) the brightness of the bright state of the first of the 8 skip frames SF is approximately the same as the brightness of the bright state of the last of the 8 skip frames SF.

Specifically, it is found through actual measurement that the instantaneous brightness waveform of the light-emitting element is presented in each display frame and meets the above four characteristics, and the resulting picture is stable and has no obvious flicker at a low picture update frequency. Specifically, in the embodiment of the present invention, when the light-emitting element emits light according to the modulated light control signal, the flicker of the instantaneous brightness waveform of the light-emitting element is less than -50 dB. Accordingly, the pixel circuit driving method proposed in the present invention can indeed improve the flicker phenomenon of the display screen operating at a low picture update frequency, thereby improving the display screen quality.

In summary, the present invention proposes a pixel circuit driving method, which modulates the frequency of the light-emitting control signal in one update frame and multiple skip frames of each display frame and the pulse width of the pulse signal, so that the instantaneous brightness waveform of the light-emitting element will present the above four characteristics in each display frame, which can improve the flickering phenomenon of the display screen operating at a low frame refresh frequency, thereby improving the display screen quality.

The above summarizes the features of several embodiments, so that those skilled in the art can better understand the present invention. Those skilled in the art should understand that they can easily use the present invention as a basis to design or modify other processes and structures to achieve the same goals and/or achieve the same advantages as the embodiments introduced herein. Those skilled in the art should also understand that these equivalent constructions do not deviate from the spirit and scope of the present invention, and they can make various changes, substitutions and modifications without departing from the spirit and scope of the present invention.

C _ST : Capacitor DF: Display frame EM: Light control signal/modulated light control signal EM[N]: Light control signal OLED: Light emitting element OVDD: System voltage terminal/system voltage OVSS: System voltage terminal PS: Pulse signal RF: Update frame S1, S2, S3, S4: Step S1[N], S1[N+1], S2[N]: Scan signal SF: Skip frame T1, T2, T31, T32, T4, T5, T6, T7: Transistor TE: TE signal V _DATA : Data signal V _REF : Reference voltage

The following detailed description in conjunction with the attached drawings will provide a better understanding of the present invention. It should be noted that, in accordance with standard industry practice, the features are not drawn to scale. In fact, the size of each feature may be increased or decreased arbitrarily to make the discussion clearer. [FIG. 1] is a schematic diagram of a pixel circuit according to an embodiment of the present invention. [FIG. 2] is a flow chart of a driving method of a pixel circuit according to an embodiment of the present invention. [FIG. 3] is an example schematic diagram of a light control signal according to an embodiment of the present invention. [FIG. 4] is an example schematic diagram of an initial instantaneous brightness waveform of a light-emitting element of a pixel circuit according to an embodiment of the present invention emitting light in accordance with a light control signal. [Figure 5] is an example schematic diagram of a modulated luminescence control signal according to an embodiment of the present invention. [Figure 6] is an example schematic diagram of an instantaneous brightness waveform of a luminescence element of a pixel circuit according to an embodiment of the present invention emitting light in accordance with a modulated luminescence control signal.

S1, S2, S3, S4: Steps

Claims (10)

A method for driving a pixel circuit, comprising: Providing a light-emitting control signal to a pixel circuit, wherein the pixel circuit operates in a plurality of display frames, wherein each of the display frames includes an update frame and a plurality of skip frames located after the update frame; When a light-emitting element of the pixel circuit emits light according to the light-emitting control signal, measuring the light-emitting element to obtain an initial instantaneous brightness waveform of the light-emitting element; Modulating the light-emitting control signal according to the initial instantaneous brightness waveform; and Providing the modulated light-emitting control signal to the pixel circuit, so that when the light-emitting element emits light according to the modulated light-emitting control signal, an instantaneous brightness waveform of the light-emitting element is presented in each of the display frames: A bright state brightness of the update frame is greater than a bright state brightness of the last one of the skipped frames; A dark state brightness of the update frame is substantially the same as a dark state brightness of the last one of the skipped frames; A bright state brightness of the first one of the skipped frames to a bright state brightness of the second to last one of the skipped frames presents a gradual change, and a dark state brightness of the first one of the skipped frames to a dark state brightness of the second to last one of the skipped frames presents the gradual change; and The bright state brightness of the first one of the skipped frames is substantially the same as the bright state brightness of the last one of the skipped frames. A method for driving a pixel circuit as described in claim 1, wherein the gradual change is decreasing. A method for driving a pixel circuit as described in claim 1, wherein the gradual change is incremental. A method for driving a pixel circuit as described in claim 1, wherein the pixel circuit operates at a low screen refresh rate, and wherein the light-emitting element is a current-driven element. A method for driving a pixel circuit as described in claim 1, wherein when the light-emitting element emits light according to the modulated light-emitting control signal, the flicker of the instantaneous brightness waveform of the light-emitting element is less than -50 dB. A method for driving a pixel circuit as described in claim 1, wherein the light control signal includes a pulse signal in the update frame of each of the display frames, and wherein modulating the light control signal includes: Increasing the number of pulse signals included in the update frame of each of the display frames from one to two. A method for driving a pixel circuit as described in claim 1, wherein the light control signal includes a pulse signal in each of the skipped frames in each of the display frames, wherein modulating the light control signal includes: Increasing the number of pulse signals included in each of the skipped frames in each of the display frames from one to at least two. A method for driving a pixel circuit as described in claim 1, wherein the luminous control signal includes a plurality of pulse signals in each of the display frames, wherein modulating the luminous control signal includes: Increasing the pulse width of one of the pulse signals to reduce a bright state brightness and a dark state brightness of the initial instantaneous brightness waveform at a time point, wherein the time point corresponds to one of the pulse signals. A method for driving a pixel circuit as described in claim 1, wherein the luminous control signal includes a plurality of pulse signals in each of the display frames, wherein modulating the luminous control signal includes: Reducing the pulse width of one of the pulse signals to increase a bright state brightness and a dark state brightness of the initial instantaneous brightness waveform at a time point, wherein the time point corresponds to one of the pulse signals. A method for driving a pixel circuit as described in claim 1, wherein the number of skipped frames included in each of the display frames is 8.