Pixel circuit

Abstract

A pixel circuit is configured to control the emission of a light-emitting diode (LED). The pixel circuit includes a constant current circuit, a pulse width modulation (PWM) circuit, a current-controlled switching transistor located in the path of the lighting current supplied from the constant current circuit and flowing through the LED, and a bypass switching transistor connected to the anode of the LED. The bypass switching transistor is configured to be controlled synchronously with the current-controlled switching transistor. The current-controlled switching transistor is turned on and then off by a first pulse signal based on a pulse control signal from the PWM circuit, and the bypass switching transistor is turned off and then on by a second pulse signal based on the PWM circuit, thereby controlling the emission period of the LED.

Description

Technical Field

[0001] This disclosure relates to a pixel circuit. Background Technology

[0002] Display devices utilizing miniature light-emitting diodes (micro-LEDs) employ pulse modulation (PWM) driving, which modulates the emission period to display halftone colors. Among various PWM driving methods, analog PWM driving has become standardized in recent years.

[0003] The pixel circuit driven by analog PWM includes a constant current generation (CCG) unit, a PWM unit, and a switch. The CCG unit generates a constant current. The PWM unit compares the grayscale data voltage representing the grayscale data with a ramp voltage and converts the grayscale data voltage into a pulse signal. The switch turns the current generated by the CCG unit on / off according to the pulse signal from the PWM unit.

[0004] Analog PWM driving requires rectangular pulses to obtain the ideal drive current; however, the current in a real circuit does not drop immediately, and its finite fall time (switching time) limits the range of low grayscale displays. Reducing this fall time is a major issue. Summary of the Invention

[0005] The PWM drive for LEDs requires a technique that gives the pulse waveform of the drive current a steep falling edge.

[0006] One aspect of this disclosure is a pixel circuit configured to control the emission of a light-emitting diode (LED). The pixel circuit includes a constant current circuit, a pulse width modulation (PWM) circuit, a current-controlled switching transistor located in the path of the lighting current supplied from the constant current circuit and flowing through the LED, and a bypass switching transistor connected to the anode of the LED. The bypass switching transistor is configured to be controlled synchronously with the current-controlled switching transistor. The current-controlled switching transistor is turned on and then off by a first pulse signal based on a pulse control signal from the PWM circuit, and the bypass switching transistor is turned off and then on by a second pulse signal based on the PWM circuit, thereby controlling the emission period of the LED.

[0007] Another aspect of this disclosure is a display device including a display area and control circuitry. The display area includes a plurality of light-emitting diodes (LEDs) and a plurality of pixel circuits configured to control the illumination of the LEDs. The control circuitry is located outside the display area and configured to control the pixel circuits. Each pixel circuit includes a constant current circuit, a pulse width modulation (PWM) circuit, a current-controlled switching transistor located in the path of the lighting current supplied from the constant current circuit and flowing through the LED, and a bypass switching transistor connected to the anode of the LED. The bypass switching transistor is configured to be controlled synchronously with the current-controlled switching transistor. The current-controlled switching transistor is turned on and then off by a first pulse signal based on a pulse control signal from the PWM circuit, and the bypass switching transistor is turned off and then on by a second pulse signal based on the PWM circuit, thereby controlling the illumination period of the LED.

[0008] One aspect of this disclosure is to sharpen the smooth falling edge of the pulse waveform of the driving current of the light-emitting element.

[0009] It should be understood that the foregoing general description and the following detailed description are exemplary and explanatory, and do not limit the content of this disclosure. Attached Figure Description

[0010] Figure 1 The configuration of the pixel circuitry in an embodiment of this disclosure is illustrated schematically.

[0011] Figure 2 It is a timing diagram used to illustrate the driving control (light emission control) of a micro LED during a time period of one frame.

[0012] Figure 3 Control signals are shown VSWEEP slope voltage and grayscale data voltage The relationship between them.

[0013] Figure 4 schematically shown Figure 1 The waveforms of the lighting current in the pixel circuit and the lighting current in the circuit configured by removing the bypass switch from the aforementioned pixel circuit are shown.

[0014] Figure 5 A detailed configuration example of the pixel circuit is shown.

[0015] Figure 6A It is a sequence diagram showing the time variation of a frame of signal in a pixel circuit.

[0016] Figure 6B Instructions Figure 6ADetails of the signal waveforms in the two graphs.

[0017] Figure 7 It is a graph showing waveform examples of the lighting current at different gray levels.

[0018] Figure 8 It is a graph used to explain the waveform differences of the lighting current between related technologies and embodiments of this disclosure.

[0019] Figure 9 This is a timing diagram used to illustrate the driving control (light emission control) of the micro LEDs in a frame time period in Embodiment 2.

[0020] Figure 10 The implementation of embodiments of this disclosure is shown. Figure 9 Example of a timing diagram pixel circuit configuration.

[0021] Figure 11 This is a schematic diagram illustrating the change in the waveform of the lighting current caused by a change in the threshold voltage Vth of the bypass switch of an n-type thin-film transistor.

[0022] Figure 12A It is an instruction Figure 11 The graph shows the relationship between the threshold voltage Vth of the bypass switch and the time delay (T2-T1).

[0023] Figure 12B It is a graph showing the relationship between the control voltage VBG of the bypass switch and the threshold voltage Vth of the bypass switch.

[0024] Figure 13 An example of the circuit configuration of the pixel circuit in Embodiment 3 is shown.

[0025] Figure 14 The temporal variations of some signals in the pixel circuit of Example 3 are provided.

[0026] Figure 15 This is a plan view showing an example configuration of a micro LED display device.

[0027] Figure 16 This is a perspective view schematically showing the display area of ​​a micro LED display device. Detailed Implementation

[0028] One aspect of this disclosure describes a pixel circuit for controlling the emission of a micro light-emitting diode (microLED). The pixel circuit illuminates the microLED during an emission period of length based on grayscale data and keeps the microLED off during other periods within a frame. A longer emission period means higher brightness of light from the microLED.

[0029] Embodiments of this disclosure control the emission period (brightness) of a micro-LED using pulse width modulation (PWM) based on grayscale data. The method of driving a micro-LED using PWM control (PWM driving) provides the micro-LED with a pulsed drive current (also referred to as the lighting current or LED current) having a pulse width based on the grayscale data to illuminate the micro-LED. The pulse width is the length between the median rise and fall of the drive current pulse; a longer pulse width implies a longer emission period or higher brightness. The drive current for the low grayscale range does not reach the maximum value for the high grayscale range; its waveform can consist of a steep rising edge and a gentle falling edge.

[0030] Analog PWM driving requires a rectangular waveform as the ideal drive current. However, the current driven by a real circuit does not drop immediately; there is a finite fall time (transition region) during which the drive current value gradually decreases. During the fall time, the drive current gradually decreases.

[0031] The emission wavelength of a micro-LED shifts to shorter wavelengths with increasing drive current density, and then shifts to longer wavelengths with further increases. When the drive current density is low, the external quantum efficiency (EQE) of the micro-LED decreases significantly. This is particularly detrimental to the luminescence of the micro-LED when the drive current supply period for low grayscale levels consists solely of the fall time. Therefore, reducing this fall time is a major issue in the PWM driving of micro-LEDs.

[0032] One aspect of the pixel circuit disclosed herein includes a constant current circuit, a PWM circuit, a current control switch, and a bypass switch. The constant current circuit generates a constant current. The PWM circuit generates a pulse signal based on grayscale data. For example, the PWM circuit compares the grayscale data voltage representing the grayscale data with a ramp voltage to generate the pulse signal. The current control switch turns the current flowing from the constant current circuit to the micro-LED on / off according to the pulse signal from the PWM circuit.

[0033] A bypass switch is used to turn the microLED on / off in the bypass path between the anode and cathode. When the bypass switch is on, the anode and cathode of the microLED are electrically connected externally. When the bypass switch is off, the anode and cathode are electrically disconnected externally.

[0034] The pixel circuit utilizes the control signal output from the PWM circuit ( The bypass switch and current control switch are controlled to turn on / off. Because the transition from on to off of the current control switch is gradual, the lighting current decreases gradually. One reason for this is the long transition time of the PWM_CNTL signal. The bypass switch can bypass the current path from the constant current circuit, and by smoothly rotating the current control switch, it can quickly cut off the lighting current flowing in the miniature LED.

[0035] Example 1

[0036] Figure 1 The configuration of pixel circuits in an embodiment of this disclosure is illustrated schematically. The display area of ​​the display device includes a plurality of pixel circuits 10 arranged in a predetermined layout (e.g., in a matrix). Each pixel circuit 10 includes a micro LED ( 11. Constant current circuit 14. PWM circuit 12. Current control switch 16. And bypass switch 18. All micro-LEDs 11 in the pixel circuit 10 can be used for the same color of light, or the display area can include micro-LEDs (pixels) 11 for different colors of light (e.g., red, blue and green). Pixels used for different colors of light can also be called sub-pixels.

[0037] The miniature LED 11 includes an anode 111 and a cathode 112. The cathode 112 of the miniature LED 11 is supplied with a constant power supply voltage PVEE. The constant current circuit 14 and the PWM circuit 12 can have any internal configuration.

[0038] The constant current circuit 14 generates a constant current. A current-controlled switch 16 is positioned between the constant current circuit 14 and the micro-LED 11. The current-controlled switch 16 is a thin-film transistor, and... Figure 1 In the configuration example, it is a p-type thin-film transistor. The active layer of a p-type thin-film transistor can be made of, for example, low-temperature polycrystalline silicon.

[0039] exist Figure 1 In the configuration example, the source of the current-controlled switch 16 is connected to the terminal of the constant current circuit 14, and the drain is connected to the anode 111 of the micro LED 11. The current-controlled switch 16 is positioned on the current path from the constant current circuit 14 through the micro LED 11 to the power line used to provide the power supply voltage PVEE, and turns this path on / off.

[0040] A current-controlled switch 16 can be positioned between the miniature LED 11 and the power line used to provide the power supply voltage PVEE. The current-controlled switch 16 can be an n-type thin-film transistor.

[0041] A bypass switch 18 is disposed on the bypass path between the anode 111 and the cathode 112 of the miniature LED 11. Figure 1In the configuration example, bypass switch 18 is an n-type thin-film transistor; its drain is connected to the anode 111 of the micro-LED 11, and its source is connected to the cathode 112 of the micro-LED 11. The active layer of the n-type thin-film transistor can be made of oxide semiconductor or low-temperature polycrystalline silicon. The source of bypass switch 18 can be connected to a given negative power supply other than the cathode 112.

[0042] Bypass switch 18 is switched on / off in the bypass path between the anode 111 and cathode 112 of the micro LED 11. When bypass switch 18 is on, the anode 111 and cathode 112 of the micro LED 11 are electrically connected externally to the micro LED 11. When bypass switch 18 is off, the anode 111 and cathode 112 are disconnected externally to the micro LED 11.

[0043] The constant current circuit 14 has an input terminal for the power supply voltage PVDD and current value data for indicating the output current value. The input terminal is [not specified]. The power supply voltage PVDD is higher than the power supply voltage PVEE. The current output of the constant current circuit 14 is turned on / off by the current control switch 16.

[0044] PWM circuit 12 based on grayscale data voltage Generate control signals And output it. PWM circuit 12 has grayscale data signals. The input terminals are for the slope voltage VSWEEP and the input terminal for the grayscale data. The slope voltage VSWEEP is a voltage (signal) that increases or decreases linearly with time, representing the grayscale data voltage. It is based on the voltage of the grayscale level of the pixels in the video frame.

[0045] PWM circuit 12 also has an input terminal for power supply voltage VH2 and an input terminal for power supply voltage VH2. The input terminal. Power supply voltage VH2 and These correspond to the control signals from PWM circuit 12. The H and L levels. PWM circuit 12 will display the grayscale data voltage representing the grayscale data. The control signal is compared with the ramp voltage VSWEEP to generate a pulse signal. .

[0046] The current control switch 16 and the bypass switch 18 are controlled by the control signal from the PWM circuit 12. And to turn on / off. Figure 1In the configuration example, the path from PWM circuit 12 to the gate of current control switch 16 consists only of wiring; there are no resistive elements, capacitive elements, or thin-film transistors. Similarly, the path from PWM circuit 12 to the gate of bypass switch 18 consists only of wiring; there are no resistive elements, capacitive elements, or thin-film transistors.

[0047] Circuit elements (other than lines) can be provided between the output of the PWM circuit 12 and the gate of the current control switch 16 and / or the bypass switch 18. For example, the current control switch 16 and the bypass switch 18 can be thin-film transistors with opposite polarities (p-type or n-type) and are derived from the control signal from the PWM circuit 12. The gate of either switch can be supplied via an inverter. In another configuration example, a delay circuit can be provided between the PWM circuit 12 and the current control switch 16 or the bypass switch 18.

[0048] Figure 2 This is a timing diagram illustrating the driving control (light emission control) of the micro LED 11 within a frame period. From the perspective of the control pixel circuit 10, a frame period PF is divided into three segments. A frame period is a segment in which one frame of image from external video data is displayed.

[0049] In the first time period P1, the data voltage is written to the pixel circuit 10, and in this case, the data voltage is adjusted to meet the threshold voltage Vth of the thin-film transistor. Specifically, Vth compensates for the grayscale data voltage. The data is written into the PWM circuit 12, and the Vth compensation current value is also written into it. It is written into constant current circuit 14.

[0050] The second time period, P2, following the first time period P1, is the light-emitting period. During this period, a starting current is supplied to the micro-LED 11, and the micro-LED 11 emits light. The third time period, P3, following the second time period, is the non-light-emitting period. During this period, the micro-LED 11 does not emit light. Time period P1 is also a non-light-emitting period, and the emission of light from the micro-LED 11 stops. When the light-emitting period is longer, the brightness of the micro-LED 11 becomes higher.

[0051] The lighting current IE flowing from the constant current circuit 14 into the miniature LED 11 has a predetermined maximum current value Imax and a high-level pulse waveform that is low relative to a reference level. The maximum current value Imax is determined by the current value data. specified.

[0052] like Figure 2As shown, the pulse shape of the lighting current IE has a steep rising edge (leading edge) and a gentle falling edge (trailing edge) that changes more gradually than the rising edge. The time of the rising edge (at its specific point) corresponds to the start time of the light-emitting period P2, and the time when the lighting current IE becomes almost zero corresponds to the end time of the light-emitting period P2. The gradient of the edge can be defined by the gradient at the inflection point.

[0053] Reference Figure 1 In the pixel circuit 10, the lighting current IE is controlled by a current control switch 16 and a bypass switch 18. The current control switch 16 and the bypass switch 18 are controlled by a control signal from the PWM circuit 12. Controls on / off switching. Figure 1 and Figure 2 In the configuration example shown, the current control switch 16 and the bypass switch 18 are controlled synchronously.

[0054] Furthermore, in this configuration example, the current control switch 16 and the bypass switch 18 are supplied with the same control signal. They are controlled to be switched on / off mutually exclusively (opposite to each other). In other words, when one of the current control switch 16 or the bypass switch 18 is on, the other switch is off; when one is off, the other is on. These switches switch between on and off simultaneously. The switching time of the current control switch 16 and the bypass switch 18 coincides almost exactly with the start and end times of the light-emitting period.

[0055] Figure 2 Control signals in The example has a low-level pulse waveform that is high relative to a reference level. Control signal The pulse shape has a steep falling edge (leading edge) and a gently rising edge (falling edge) that varies more gradually than the falling edge. Control signal The trailing edge can be smoother than the trailing edge of the lighting current IE.

[0056] It describes the use of gray level data voltage corresponding to gray levels. To control the control signal The principle of pulse width. Figure 3 Control signals are shown 31. Ramp voltage VSWEEP 32 and grayscale data voltage The relationship between 33. Figure 3 In the graph, the horizontal axis represents time, and the vertical axis represents voltage. Although Figure 3 Control signals in 31 has an ideal waveform, but the actual waveform has the following characteristics: Figure 2 The gently rising edge is shown.

[0057] The ramp voltage VSWEEP 32 decreases linearly from the emission period P2. In another example, the ramp voltage can increase linearly. For grayscale data voltage... 33. Predefined voltages are assigned to each grayscale level, and each voltage assigned to a grayscale level is a constant voltage. Lower grayscale level data voltages... 33 corresponds to a higher grayscale level. Ramp voltage VSWEEP 32 and grayscale data voltage. The intersection of 33 and the control signal The rising edges of 31 coincide. In this way, the PWM circuit 12 uses the input ramp voltage VSWEEP according to the grayscale data voltage. Generate control signals .

[0058] For reference Figure 1 and Figure 2 In addition to the current control switch 16, the pixel circuit 10 also includes a bypass switch 18. The bypass switch 18 is used to sharpen the falling edge of the lighting current IE.

[0059] Figure 4 schematically shown Figure 1 The waveforms of the lighting current IE in the pixel circuit 10 and the lighting current in the circuit configured by removing the bypass switch 18 from the pixel circuit 10 are shown. Figure 4 In the graph, the horizontal axis represents time, and the vertical axis represents the current value. Curve 41 is the waveform of the lighting current in the circuit configured by removing the bypass switch 18 from the pixel circuit 10. Curve 42 is... Figure 1 The waveform of the lighting current IE in the middle pixel circuit 10.

[0060] As described above, bypass switch 18 is connected between the anode and cathode of miniature LED 11, and bypass switch 18 and current control switch 16 are controlled by the output of PWM circuit 12. The control is on / off. Furthermore, the on / off of the bypass switch 18 and the current control switch 16 are mutually exclusive.

[0061] One reason for the gradual decrease in lighting current shown in curve 41 is the slow transition of the current-controlled switch 16 from the on state to the off state. This is due to the control signal from the PWM circuit 12. The transition time is relatively long, as shown in reference Figure 2 As described.

[0062] from Figure 4 A comparison of waveforms 42 and 41 shows that the bypass switch 18 accelerates the cutting off of the lighting current flowing in the micro LED 11 by bypassing the gradually changing current from the current control switch 16.

[0063] Figure 5 A detailed configuration example of pixel circuit 10 is shown. PWM circuit 12 consists of seven thin-film transistors (also simply transistors) and two capacitors. Constant current circuit 14 consists of five thin-film transistors and one capacitor. Both circuits 12 and 14 can include any number of transistors and capacitors; the numbers between the two circuits 12 and 14 can be the same or different. These circuits can include other types of circuit elements such as resistors.

[0064] Figure 5 The PWM circuit 12 includes transistors M11 to M17 and capacitors C11 and C12. Transistors M11 to M17 are p-type thin-film transistors and they are switching transistors.

[0065] The source of transistor M11 is supplied with a power supply voltage VH2, and its drain is connected to the source of transistor M13 and one of the source / drain terminals of transistor M12. The source and drain terminals of the transistors are interchanged according to the direction of the current. The term "source / drain" refers to either the source or the drain. The gate of transistor M11 is supplied with a control signal. .

[0066] The gate of transistor M12 is provided with a scan signal. One source / drain of transistor M12 is connected to the drain of transistor M11 and the source of transistor M13; the other source / drain of transistor M12 is supplied with grayscale data voltage. .

[0067] The gate of transistor M13 is connected to one end of capacitor C11, the source / drain of transistor M14, and the source / drain of transistor M15. The other end of capacitor C11 is supplied with a ramp voltage VSWEEP. The source of transistor M13 is connected to the drain of transistor M11 and one source / drain of transistor M12. The drain of transistor M13 is connected to the other source / drain of transistor M14 and the source of transistor M16.

[0068] The gate of transistor M14 is provided with a scan signal. One source / drain of transistor M14 is connected to the drain of transistor M13 and the source of transistor M16, and the other source / drain of transistor M14 is connected to the gate of transistor M13 and the source / drain of transistor M15.

[0069] The gate of transistor M15 is provided with a scan signal. One source / drain of transistor M15 is connected to the gate of transistor M13 and the source / drain of transistor M14. The other source / drain of transistor M15 is supplied with a constant reference voltage. .

[0070] The gate of transistor M16 is provided with a control signal. The drain of transistor M16 is connected to the source of transistor M17. Control signal. Output from node N1 between transistors M16 and M17.

[0071] The gate of transistor M17 is provided with a control signal. The drain of transistor M17 is supplied with a constant negative potential VSE. Capacitor C12 is connected between the source and drain of transistor M17. (The last sentence appears to be incomplete and possibly refers to a different data voltage setting.) Control signals before being written The voltage level drops to low, thus turning on transistor M17 and setting the potential of node N1 to VSE. Capacitor C12 maintains the potential of node N1.

[0072] The grayscale data voltage PWM_DATA is written to capacitor C11 via transistors M12, M13, and M14. Subsequently, a falling ramp voltage VSWEEP is supplied to capacitor C11 until the gate voltage of transistor M13 becomes equal to the voltage. Transistor M13 remains off. During this period, the potential at node N1 is maintained at VSE through capacitor C12. When the gate voltage of transistor M13 becomes lower than VSE... When this occurs, transistor M13 is turned on to output VH2 to node N1, thereby outputting a pulse control signal. The principle behind this operation is the same as that already referenced. Figure 3 The control signals described The principle is the same, this control signal Based on grayscale data voltage The difference between the slope voltage VSWEEP and the voltage varies.

[0073] The constant current circuit 14 includes transistors M21 to M25 and capacitor C21. Transistors M21 to M25 are p-type transistors, and all transistors except transistor M23 are switching transistors.

[0074] The gate of transistor M21 is provided with a control signal. The source of transistor M21 is supplied with a constant power supply voltage PVDD, and its drain is connected to the source / drain of transistor M22 and the source of transistor M23.

[0075] The gate of transistor M22 is provided with a scan signal. The other source / drain of transistor M22 is provided with current value data. .

[0076] The gate of transistor M23 is connected to capacitor C21, the source / drain of transistor M24, and the source / drain of transistor M25. The drain of transistor M23 is connected to the output node N2 of constant current circuit 14.

[0077] The gate of transistor M24 is provided with a scan signal. The source / drain of transistor M24 is connected to the output node N2 of constant current circuit 14, and the other source / drain is connected to the gate of transistor M23, capacitor C21, and the source / drain of transistor M25.

[0078] The gate of transistor M25 is provided with a scan signal. The source / drain of transistor M25 is supplied with a constant reference voltage. Furthermore, another source / drain is connected to the gate of transistor M23, capacitor C21, and the source / drain of transistor M24.

[0079] Current value data The current is written to capacitor C21 via transistors M22, M23, and M24. Transistor M23 outputs current to output node N2 based on the voltage of capacitor C21.

[0080] The current-controlled switch 16 is a p-type transistor; its source is connected to the output node N2 of the constant current circuit 14, and its drain is connected to the source of transistor M31. The gate of the current-controlled switch 16 is supplied with a control signal from the PWM circuit 12. .

[0081] A p-type transistor M31 is connected between the anode of the miniature LED 11 and the current control switch 16. Transistor M31 acts as a switch, and its gate is supplied with a control signal. The source is connected to the drain of the current control switch 16, and the drain is connected to the anode of the micro LED 11.

[0082] Transistor M32 is a switch; its gate is supplied with a scan signal. Transistor M32 is a p-type transistor; the source of transistor M32 is connected to the anode of miniature LED 11, and the drain is supplied with a constant power supply potential PVEE.

[0083] The bypass switch 18 is an n-type transistor; its source and drain are connected to the cathode and anode of the miniature LED 11, respectively. The gate of the bypass switch 18 is supplied with a control signal from the PWM circuit 12. .

[0084] The current control switch 16 is composed of a p-type transistor, and the bypass switch 18 is composed of an n-type transistor. Therefore, the current control switch 16 and the bypass switch 18 are controlled solely by a control signal. On / Off. Both the bypass switch 18 and transistor M32 have the function of resetting the anode of the miniature LED 11. Therefore, transistor M32 can be excluded.

[0085] Figure 6A This is a sequence diagram showing the time variation of a frame of signal in pixel circuit 10. The horizontal axis of graphs 51 to 56 represents time, and the vertical axis represents voltage or current. Graph 51 indicates the time variation of the control signal (CC) of constant current circuit 14. Graph 52 indicates the time variation of the control signal (PWM) of PWM circuit 12. Figure 6A In the graphs 51 and 52, each schematically illustrates the time variations of multiple control signals. Figure 6B The diagram provides details of the signals during the time period enclosed by dashed line 510 in graph 51 and by dashed line 520 in graph 52.

[0086] Graph 53 indicates the time variation of the ramp voltage VSWEEP input to PWM circuit 12. Graph 54 indicates the control signal output from PWM circuit 12. The time variation of the anode voltage of the micro LED 11 is shown in Figure 55. The time variation of the driving current (lighting current) of the micro LED 11 is shown in Figure 56.

[0087] Figure 6B The time variation of the signal is shown in graph 51 within the time period enclosed by dashed line 510 and in graph 52 within the time period enclosed by dashed line 520. The horizontal axis of graphs 510 and 520 represents time, and the vertical axis represents the voltage of the signal.

[0088] Graph 510 indicates the time variation of the control signal (CC) of the constant current circuit 14. In graph 510, line 511 indicates the signal... Time variation; Line 512 indicator signal Time variation; and line 513 indicates the signal. The changes over time.

[0089] Signal (Line 511) is a pulse signal that changes from high to low at time t1 and then from low to high at time t2. (Line 512) is a pulse signal that changes from high to low at time t2 and returns from low to high at time t3. (Line 513) is a pulse signal that transitions from high to low at time t9 and from low to high at a predefined time (not shown in the frame period). In one example, the signal... and The pulse width is a horizontal time interval.

[0090] Graph 520 indicates the time variation of the control signal (PWM) of the PWM circuit 12. In graph 520, line 521 indicates the signal... Time variation; Line 522 indicator signal Time variation; Line 523 indicator signal Time variation; and line 524 indicates the signal. The changes over time.

[0091] Signal (Line 521) is a pulse signal that changes from high to low at time t4 and then from low to high at time t5. (Line 522) is a pulse signal that changes from high to low at time t5 and returns from low to high at time t6. (Line 523) is a pulse signal that changes from high to low at time t6 and returns from low to high at time t7. (Line 524) is a pulse signal that transitions from high to low at time t8 and from low to high at a predefined time (not shown in the frame period). In one example, the signal... , and The pulse width is a horizontal time interval.

[0092] Figure 7 This is a graph providing waveform examples of the lighting current at different gray levels. In this graph, the horizontal axis represents time, and the vertical axis represents the amount of lighting current. The rising edge 601 is common to the waveforms of the lighting current for different gray levels. Curve 602 is the waveform for the maximum gray level 255. Line 610 represents the maximum value of the lighting current for all gray levels.

[0093] As the grayscale level increases, the time from the rising edge 601 to the falling edge becomes longer. The amount of charge supplied in a frame increases with the grayscale level. The maximum lighting current for some low grayscale levels may be less than this maximum value 610. In other words, the waveform of the lighting current for these levels can only include the falling edge. For example, the lighting current for low grayscale levels rises to a value less than the maximum value 610 and immediately begins to decrease little by little.

[0094] Figure 8 This is a graph used to explain the waveform differences in lighting current between related technologies and embodiments of this disclosure. In this graph, the horizontal axis represents grayscale levels, and the vertical axis represents the highest value of the lighting current. The solid line 651 indicates the inclusion of... Figure 1 The diagram illustrates the relationship between the grayscale level and the highest value of the illumination current in the pixel circuit 10 with the bypass switch 18 shown. The dashed line 652 indicates the relationship between the grayscale level and the highest value of the illumination current in the pixel circuit configured by removing the bypass switch 18 from the pixel circuit 10.

[0095] A comparison of the two waveforms 651 and 652 shows that, compared to pixel circuits of related technologies, the pixel circuit 10 in the embodiments of this disclosure can maintain the maximum current value decreasing at a lower gray level. The lighting current at the low gray level rises to a value lower than the highest value at the high gray level, and then immediately begins to decrease. The lighting current at the high gray level rises to a predefined maximum value, maintains that maximum value for a predefined time period, and then begins to decrease along the falling edge.

[0096] As described above, the pixel circuit 10 in the embodiments of this disclosure utilizes the same control signal as the current control switch 16. The bypass switch 18 is controlled and turned on at different times when the light stops illuminating at each gray level. The bypass switch 18 causes the lighting current to drop sharply, thereby keeping the maximum current value at a lower gray level.

[0097] Example 1 is based on the control signal from PWM circuit 12 The current control switch and bypass switch are synchronously controlled using control signals. Example 1 uses the same control signal. The current control switch 16 and the bypass switch 18 are controlled. The on and off states of the current control switch 16 and the bypass switch 18 are mutually exclusive, and the states change simultaneously.

[0098] Example 2

[0099] For example, as referenced Figure 2 As described above, the foregoing embodiments provide the same control signal to the gates of the current control switch 16 and the bypass switch 18. The switching on / off states are mutually exclusive. In other words, the aforementioned embodiment switches the current control switch 16 on while simultaneously disconnecting the bypass switch 18, and vice versa.

[0100] Another embodiment of this disclosure, described below, turns on the bypass switch at a time different from the time the current control switch is turned off. Specifically, this embodiment turns on the bypass switch before the current control switch is turned off. This configuration results in a steeper drop in the lighting current. However, this disclosure does not preclude designs that turn on the bypass switch after the current control switch is turned off.

[0101] Figure 9 This is a timing diagram used to illustrate the driving control (light emission control) of the miniature LED 11 within a single frame. The main description is related to... Figure 2 The timing diagrams differ from those of the previous ones. Unless otherwise stated, regarding... Figure 2 The description is applicable.

[0102] exist Figure 2 In the timing diagram, the time for turning on the bypass switch 18 is the same as the time for turning off the current control switch 16. Figure 9 In the timing diagram, the time T1 for turning on the bypass switch 28 is different from the time T2 for turning off the current control switch 16.

[0103] Specifically, the time T1 for turning on the bypass switch 28 is before the time T2 for turning off the current control switch 16. This control increases the ratio of the bypass current. As a result, the lighting current drops more sharply, making its waveform closer to the ideal rectangular waveform. Incidentally, Figure 9 Control signals in the timing diagram The falling edge is essentially vertical, therefore, the time for turning on the current control switch 16 and the time for turning off the bypass switch 28 are the same time T0 (essentially).

[0104] Figure 9 The control timing of the medium current control switch 16 and the bypass switch 28 can be improved by increasing the current drive efficiency of the bypass switch 28. The current drive efficiency of the bypass switch 28 can be improved by reducing the absolute value of the threshold voltage Vth or by increasing the channel width of the thin-film transistor used as the bypass switch 28.

[0105] The following describes how the control timing is adjusted by regulating the threshold voltage Vth of the bypass switch 28. Figure 10 The implementation of embodiments of this disclosure is shown. Figure 9 The timing diagram shows an example of the circuit configuration for pixel circuit 20. The following mainly describes the circuit configuration with... Figure 5 The differences in pixel circuit 10. Unless otherwise stated, regarding Figure 5 The description is applicable.

[0106] Pixel circuit 20 includes replacing Figure 5 The bypass switch 18 in the circuit has a bypass switch 28 with a dual-gate structure. The dual-gate structure includes a top gate and a bottom gate sandwiching the channel region in the stacking direction. One end of the charge storage capacitor C31 is connected to the back gate 281 of the bypass switch 28. The back gate 281 is either the top gate or the bottom gate. The other end of the charge storage capacitor C31 is connected to the cathode of the miniature LED 11.

[0107] The control voltage VBG is written into the charge storage capacitor C31, and the charge storage capacitor C31 maintains this voltage. The n-type transistor switch M35 can be turned on during the period when the control voltage VBG is written into the charge storage capacitor C31, and can be turned off at other times. Switch M35 is turned on / off by the control signal SBG, and the control voltage VGB is written into the charge storage capacitor C31 via the on state of switch M35.

[0108] The control voltage VBG is the back gate bias voltage of the bypass switch 28. The threshold voltage Vth of the bypass switch 28 shifts in response to the back gate bias voltage. For example, when the back gate bias voltage of the n-type thin-film transistor is increasing, the threshold voltage Vth decreases. The display device including the pixel circuit 20 achieves the desired falling edge in the waveform of the lighting current by writing a predefined optimal control voltage VBG into the charge storage capacitor C31.

[0109] In manufacturing display devices, the back gate bias voltage of each pixel can be adjusted individually to precisely adjust the emission period using a two-dimensional luminance measurement camera, thereby reducing display non-uniformity in the display area. Although pixel circuit 20 does not include transistor M32, pixel circuit 20 may include transistor M32.

[0110] Figure 11 This is a schematic diagram illustrating the waveform change of the lighting current caused by the change in the threshold voltage Vth of the bypass switch 28 of the n-type thin-film transistor. Figure 11 The data writing period immediately preceding the light-emitting period P2 is omitted. Time T2 is the time to turn off the current control switch 16, and time T1 is the time to turn on the bypass switch 28.

[0111] Graph 71 indicates the time variation of the lighting current in pixel circuit 20 with its bypass switch 28 having different threshold voltages Vth. Graph 71 also includes the waveform of the lighting current in pixel circuit configured by removing the bypass switch 28 from pixel circuit 20.

[0112] In graph 71, the horizontal axis represents time, and the vertical axis represents the value of the lighting current. Curve 711 is obtained by... Figure 10The waveform of the lighting current in the pixel circuit 20 configured with the bypass switch 28 removed is shown. Other curves, including curve 712, are the waveforms of the lighting current in the pixel circuit 20 with different threshold voltages Vth for their bypass switches 28. Curve 712 is in response to... Figure 11 The waveform of the lighting current due to the state changes of the current control switch 16 and the bypass switch 28 in the circuit.

[0113] Figure 71 illustrates the change in the lighting current waveform as the threshold voltage Vth of bypass switch 28 changes from 4.5 V to 0.5 V. As the threshold voltage Vth decreases, time T1 arrives earlier. That is, the time lag between time T2 and time T1 increases. Consequently, the lighting current begins to decrease earlier as the threshold voltage Vth decreases. Furthermore, the gradient of the falling edge of the lighting current becomes steeper as the threshold voltage Vth decreases.

[0114] Figure 12A It means Figure 11 The curve in Figure 71 shows the relationship between the threshold voltage Vth of the bypass switch 28 and the time delay (T2-T1). Figure 12B This is a graph showing the relationship between the control voltage VBG (back gate bias) of bypass switch 28 and the threshold voltage Vth of bypass switch 28. (Example) Figure 12A As indicated, the time delay (T2-T1) increases as the threshold voltage Vth of the bypass switch 28 decreases. In other words, time T1 becomes earlier. Figure 12B As indicated, the threshold voltage Vth decreases with increasing back gate bias. Note that the relationships between the threshold voltage Vth and the time lag (T2-T1) and between the back gate bias and the threshold voltage Vth in p-type thin-film transistors are the opposite of those in n-type thin-film transistors.

[0115] The synchronous control of the current control switch and the bypass switch is common to both Embodiment 1 and Embodiment 2. Specifically, the time delay between the time the bypass switch is turned on and the time the current control switch is turned off is fixed (including 0), and the time delay between the time the bypass switch is turned off and the time the current control switch is turned on is also fixed (including 0). In both Embodiment 1 and Embodiment 2, the bypass switch is turned on before (or simultaneously with) the current control switch is turned off.

[0116] The current-controlled switch can have a dual-gate structure instead of a bypass switch. The time to turn off the current-controlled switch can be adjusted by the threshold voltage of the current-controlled switch. The timing of changing the state of the current-controlled switch and / or bypass switch can also be adjusted by circuit configuration, rather than by the threshold voltage or channel width of the thin-film transistor.

[0117] For example, a delay circuit can be inserted between the output of the PWM circuit 12 and the current control switch 16. This delay circuit delays the change (falling or rising) of the control signal input to the current control switch relative to the change of control information input to the bypass switch. The signal from this delay circuit is based on the control signal... The control signal. (Refer to...) Figure 11 In the example, after the bypass switch is turned off, the current control switch is turned on to begin the light-emitting period P2. Subsequently, after the bypass switch is turned on, the current control switch is turned off.

[0118] As described above, Embodiment 2 is based on the control signal from PWM circuit 12. The current control switch 16 and the bypass switch 28 are synchronously controlled using control signals. In Example 2, the synchronous control turns on the bypass switch 28 and turns off the current control switch 16 at different times to end the light-emitting period.

[0119] More specifically, in embodiment 2, the current control switch 16 is turned off after the bypass switch 28 is turned on. For example, the current control switch 16 and the bypass switch 28 can be provided with the same control signal. Furthermore, their control timing can be adjusted using the threshold voltage of the bypass switch 28.

[0120] Example 3

[0121] Example 3 arranges all thin-film transistors in a pixel circuit with thin-film transistors of the same conductivity type. This configuration simplifies the manufacturing process. In the pixel circuit configuration example described below, all thin-film transistors are p-type thin-film transistors. All thin-film transistors can also be n-type thin-film transistors.

[0122] Figure 13 An example circuit configuration of the pixel circuit 30 in an embodiment of this disclosure is shown. The following mainly describes the... Figure 5 The differences in pixel circuit 10 shown. Unless otherwise stated, regarding Figure 5 The description is applicable.

[0123] In pixel circuit 30, bypass switch 38 is a p-type thin-film transistor. Pixel circuit 30 includes features for generating control signals for bypass switch 38. The inverter 40. The inverter 40 operates according to the control signal from the PWM circuit 12. Generate control signals Control signals The control signal is supplied to the gate of bypass switch 38. Its polarity is relative to the control signal. A reversal signal.

[0124] Inverter 40 includes switching transistors M41, M42, and M43. These are p-type thin-film transistors. Inverter 40 also includes capacitor C41. Control signals. The power supply is provided to the gate of transistor M41. The source of transistor M41 is supplied with a constant power supply voltage VDD2, and its drain is connected to output node N5. Output node N5 is connected to the gate of bypass switch 38 to transmit the control signal. The gate of the bypass switch 38 is provided.

[0125] The drain of transistor M42, connected to the diode, is provided with a constant potential (negative potential) VSE, and its source is connected to the gate of transistor M43. The drain of transistor M43 is provided with a constant potential VSE, and its source is connected to the output node N5. One end of capacitor C41 is connected to the gate of transistor M43, and the other end is connected to the output node N5. The potential (voltage) VSE can be the same as the potential VSE of PWM circuit 12. The power supply voltage VDD2 can be the same as the potential VH2 used for PWM circuit 12 or the potential PVDD used for constant current circuit 14, but conveniently, the potential VDD2 is a different power supply voltage to allow independent adjustment for reasons described later.

[0126] Although pixel circuit 30 does not include transistor M32, pixel circuit 30 may include transistor M32. The configuration of PWM circuit 12 and constant current circuit 14 is similar to... Figure 5 The configuration is the same as that in pixel circuit 10. Current control switch 16 is also a p-type thin-film transistor.

[0127] Figure 14 The time variations of certain signals in pixel circuit 30 are provided. Graph 81 indicates the time variation of the ramp voltage VSWEEP. The horizontal axis represents time, and the vertical axis represents voltage. Graph 82 indicates the control signals. and The time variation is shown. The horizontal axis represents time, and the vertical axis represents voltage. Curve 821 is the control signal. The waveform. The curve enclosed by the dashed line 822 represents the control signal under different power supply voltages VDD2. The waveform is shown in Figure 83. The curve indicates the time variation of the lighting current. The horizontal axis represents time, and the vertical axis represents current. Figure 83 provides the waveforms of the lighting current under different power supply voltages VDD2.

[0128] Time T1 is the time during which the bypass switch 38 is turned on at a specific power supply voltage VDD2. Time T2 is the time during which the current control switch 16 is turned off. Similar to Embodiment 2, the current control switch 16 is turned off after the bypass switch 38 is turned on. As in Embodiment 1, times T1 and T2 can be the same.

[0129] The time delay between time T1 and T2 can be adjusted by changing the value of the voltage VDD2 of inverter 40 to select the optimal operating conditions. For example, by adjusting the power supply voltage VDD2 of inverter 40, the current control switch 16 and the bypass switch 38 can be controlled to satisfy the condition that time T2 > time T1.

[0130] The curve enclosed by the dashed line 822 in graph 82 indicates the control signal when the power supply voltage VDD2 of inverter 40 changes from 3.0 V to -1.0 V. The waveform changes. As the power supply voltage VDD2 decreases, the signal... The highest potential is reduced to shorten the pulse width. To this end, the time T1 for turning on the p-type bypass switch 38 is made earlier to reduce the pulse width of the lighting current and increase the gradient of its falling edge, as shown in Figure 83.

[0131] As described above, Embodiment 3 is based on the control signal from PWM circuit 12. The current control switch and the bypass switch are synchronously controlled using a control signal. In embodiment 3, the gate of the current control switch 16 is provided with a control signal. Furthermore, a control signal is provided to the gate of the bypass switch 38. The opposite signal.

[0132] In other words, based on control signals The control signal controls the current control switch 16 and the bypass switch 38. Based on the control signal... The control signal is from the control signal The generated signals, and these can be control signals. Control signals that are themselves or different from them (such as inverted signals and delayed signals).

[0133] Example 3 is based on control signals from PWM circuit 12 The current control switch 16 and the bypass switch 38 are synchronously controlled using control signals. In Example 3, the synchronous control turns on the bypass switch 38 and turns off the current control switch 16 at different times to end the light-emitting period.

[0134] More specifically, in embodiment 3, the current control switch 16 is turned off after the bypass switch 38 is turned on. The control timing of these switches can be adjusted using the power supply voltage VDD2 of the inverter circuit, which is based on the control signal. Generates control signals for bypass switch 38.

[0135] Example 4

[0136] A configuration example of a micro-LED display device that may include the pixel circuitry described in the foregoing embodiments is described. Figure 15 This is a plan view illustrating an example configuration of a micro-LED display device. The micro-LED display device includes a display area and control circuitry for controlling pixel circuitry 95. The display area includes an array of pixel circuitry 95 and micro-LEDs 951. The control circuitry includes signal circuitry 91 and scanning circuitry 92. Signal circuitry 91 and scanning circuitry 92 provide a power supply voltage (constant voltage) and control signals for controlling pixel circuitry 95. Signal circuitry 91 and scanning circuitry 92 are controlled by video processing circuitry (not shown). The video processing circuitry is used to process video data input from an external source of the micro-LED display device.

[0137] Pixel circuit 95 controls miniature LED 951. In Figure 16 In this design, the components of the pixel circuit 95 are fabricated on a thin-film transistor (TFT) substrate. A micro-LED 951 is connected to connection pads 947 and 948 on the TFT substrate to be electrically connected to the pixel circuit 95 via the connection pads 947 and 948. For example, the anode and cathode of the micro-LED 951 are physically and electrically connected to the pads 947 and 948 by soldering.

[0138] Figure 16 This is a schematic perspective view of the display area of ​​a micro LED display device. Red LED chip 901R, green LED chip 901G, and blue LED chip 901B are arranged in a matrix on the TFT substrate 905. Figure 16 It also includes data or power lines 911 and transmission lines 912. For illustration, pads 947 and 948 exposed when the LED chips are removed are shown. The area between the LED chips 901R, 901G, and 901B mounted on the TFT substrate 905 is filled with a separator material 903. This separator material 903 is a black material such as black resin to reduce surface reflectivity.

[0139] As described above, embodiments of the present disclosure have been presented; however, the present disclosure is not limited to the foregoing embodiments. Those skilled in the art can readily modify, add to, or transform each element in the foregoing embodiments within the scope of this disclosure. A portion of the configuration of one embodiment may be replaced by the configuration of another embodiment, or the configuration of one embodiment may be incorporated into the configuration of another embodiment.

Claims

1. A pixel circuit configured to control the emission of a light-emitting diode, the pixel circuit comprising: Constant current circuit; Pulse width modulation circuit; A current-controlled switching transistor is located on the path of the lighting current supplied from the constant current circuit and flowing through the light-emitting diode; as well as A bypass switching transistor is connected to the anode of the light-emitting diode, and the bypass switching transistor is configured to be controlled synchronously with the current-controlled switching transistor. The current-controlled switching transistor is turned on and then turned off by a first pulse signal based on a pulse control signal from the pulse width modulation circuit, and the bypass switching transistor is turned off and then turned on by a second pulse signal based on a pulse control signal from the pulse width modulation circuit, in order to control the light-emitting period of the light-emitting diode.

2. The pixel circuit according to claim 1, wherein, Both the first pulse signal and the second pulse signal are pulse control signals from the pulse width modulation circuit.

3. The pixel circuit according to claim 1, wherein, The bypass switch transistor is turned off while the current control switch transistor is turned on, and turned on while the current control switch transistor is turned off.

4. The pixel circuit according to claim 1, wherein, The bypass switch transistor is turned on before the current control switch transistor is turned off.

5. The pixel circuit according to claim 4, in, The bypass switching transistor has a dual-gate structure, and The back gate of the bypass switch transistor is provided with a voltage to adjust the threshold voltage of the bypass switch transistor.

6. The pixel circuit according to claim 1, wherein, The current-controlled switching transistor and the bypass switching transistor have different conductivity types.

7. The pixel circuit according to claim 1, in, The current-controlled switching transistor and the bypass switching transistor have the same conductivity type. Wherein, one of the first pulse signal and the second pulse signal is a pulse control signal from the pulse width modulation circuit, and Among them, the other pulse signal in the first pulse signal and the second pulse signal is the inverse signal of the pulse control signal from the pulse width modulation circuit.

8. The pixel circuit according to claim 7, further comprising: An inverter circuit configured to output another of a first pulse signal and a second pulse signal generated according to a pulse control signal from the pulse width modulation circuit. The time delay between the time the current control switch transistor is turned off and the time the bypass switch transistor is turned on can be controlled by adjusting the power supply voltage to be supplied to the inverter circuit.

9. A display device, comprising: The display area includes a plurality of light-emitting diodes and a plurality of pixel circuits, the plurality of pixel circuits being configured to control the light emission of the plurality of light-emitting diodes; as well as A control circuit, disposed outside the display area, is configured to control the plurality of pixel circuits. Each of the plurality of pixel circuits includes: Constant current circuit; Pulse width modulation circuit; A current-controlled switching transistor, located on the path of the lighting current supplied from the constant current circuit and flowing through the light-emitting diode; and A bypass switch transistor is connected to the anode of the light-emitting diode, and the bypass switch transistor is configured to be controlled synchronously with the current-controlled switch transistor. The current-controlled switching transistor is turned on and then turned off by a first pulse signal based on a pulse control signal from the pulse width modulation circuit, and the bypass switching transistor is turned off and then turned on by a second pulse signal based on a pulse control signal from the pulse width modulation circuit, in order to control the light-emitting period of the light-emitting diode.

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