Indication device
The display device addresses the sluggish falling edge of the drive current in micro LEDs by using different slope ramp signals for high and low gradation ranges, enhancing image quality and consistency in PWM-driven micro LED displays.
Patent Information
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Current Assignee / Owner
- SHANGHAI AVIC OPTO ELECTRONICS CO LTD
- Filing Date
- 2024-12-02
- Publication Date
- 2026-06-12
Smart Images
Figure 2026096101000001_ABST
Abstract
Description
Technical Field
[0001] This disclosure relates to a display device.
Background Art
[0002] In a display device using micro LEDs (Light Emitting Diodes), a PWM (Pulse Width Modulation) driving method for modulating the emission time to display intermediate tones is used. Among PWM driving methods, an analog PWM driving method that analogously changes the emission pulse width according to gradation data has been becoming common in recent years.
[0003] A pixel circuit for analog PWM driving includes a CCG (Constant Current generation) unit, a PWM unit, and a switch. The CCG unit generates a constant current, the PWM unit compares a gradation data voltage representing gradation data with a ramp signal and converts it into a pulse signal, and the switch turns on and off the current generated by the CCG unit according to the pulse signal from the PWM unit.
[0004] In analog PWM driving, the driving current is ideally required to be a rectangular pulse, but in a real circuit, the fall of the current is sluggish, and due to the finite fall transition time, the image quality is poor in the low gradation display area. The length of this fall transition time is one of the problems, and it is a major problem especially for achieving good gradation representation in a particularly low gradation range.
Prior Art Documents
Patent Documents
[0005]
Patent Document 1
Patent Document 2
Patent Document 3
[0006] In PWM driving of light-emitting elements, there is a need for a technology that can improve the falling edge sluggishness in the pulse waveform of the drive current. [Means for solving the problem]
[0007] A display device according to one aspect of the present disclosure includes a plurality of pixel circuits and a control circuit for controlling the plurality of pixel circuits, each of the plurality of pixel circuits including a control transistor for the drive current of the pixel and a pulse width modulation circuit that supplies a control signal to the control transistor, the pulse width modulation circuit includes a drive transistor that outputs the control signal, the pulse width modulation circuit controls the light emission time of the pixel in one frame period by switching the control transistor ON / OFF by the control signal, the pulse width modulation circuit controls the ON / OFF of the drive transistor using a gradation data voltage from the control circuit and a first ramp signal and a second ramp signal that is steeper than the first ramp signal, the second ramp signal is used for control in a low gradation range including the minimum gradation level and is excluded from control in a high gradation range consisting of a higher gradation level than the low gradation range, and the first ramp signal is used for control in at least the high gradation range. [Effects of the Invention]
[0008] According to one aspect of this disclosure, the falling edge sluggishness in the pulse waveform of the drive current can be improved when driving a light-emitting element with PWM. [Brief explanation of the drawing]
[0009] [Figure 1] A schematic diagram shows the configuration of a pixel circuit according to one embodiment of this disclosure. [Figure 2] This shows the time variation of the input signal voltages VRAMP and VDATA of the PWM circuit, the control signal voltage VOUT, and the drive current ILED of the micro LED. [Figure 3A] Figure 2 shows the state of the pixel circuit at time T1. [Figure 3B] Figure 2 shows the state of the pixel circuit at time T2. [Figure 3C] Figure 2 shows the state of the pixel circuit at time T3. [Figure 3D] Figure 2 shows the state of the pixel circuit at time T4. [Figure 3E] Figure 2 shows the state of the pixel circuit at time T5. [Figure 4] This example shows a PWM circuit constructed using thin-film transistors and capacitors. [Figure 5] The waveforms of the lamp signal VRAMP, the gate voltage Vg of the driving thin-film transistor of the PWM circuit, the control signal voltage VOUT from the PWM circuit, and the driving current ILED of the micro LED are schematically shown. [Figure 6] The simulation results of the input / output characteristics (static characteristics) of the PWM circuit are shown. [Figure 7] This shows the results of a simulation verifying the relationship between the slope of the lamp signal VRAMP and the fall time of the LED drive current ILED. [Figure 8] The waveforms of the lamp signal VRAMP and the LED drive current ILED, which are based on different grayscale data voltages, are schematically shown. [Figure 9] This shows a timing chart of the control signals for a pixel circuit according to one embodiment of the present disclosure. [Figure 10] The simulation results for LED drive current at different grayscale levels are shown. [Figure 11] The simulation results showing the relationship between grayscale level and LED drive current amplitude are presented. [Figure 12] This shows the simulation results of the relationship between the error in the average current due to variations in the threshold voltage of the driving thin-film transistor in a PWM circuit and the slope of the ramp signal VRAMP. [Figure 13] This shows the simulation results of the relationship between the slope of the lamp signal and the LED drive current. [Figure 14]FIG. 0 shows a conceptual diagram of waveforms of two lamp signals in one frame period. [Figure 15] FIG. 3 is a plan view showing a configuration example of a micro LED display device. [Figure 16] FIG. 6 shows a configuration example of a tone data voltage generation unit of the signal circuit 31. [Figure 17] FIG. 9 shows an example of a gamma LUT used in a configuration using a single lamp signal in one frame. [Figure 18A] FIG. 12 shows a configuration example of a gamma LUT for a first partial period using a gentle lamp signal in Embodiment 1. [Figure 18B] FIG. 15 shows a configuration example of a gamma LUT for a second partial period using a steep lamp signal in Embodiment 1. [Figure 19A] FIG. 18 shows a configuration example of a gamma LUT for a first partial period using a gentle lamp signal in Embodiment 2. [Figure 19B] FIG. 21 shows a configuration example of a gamma LUT for a second partial period using a steep lamp signal in Embodiment 2. [Figure 20] FIG. 24 shows simulation results of LED drive currents at different tone levels in Embodiment 2. [Figure 21] FIG. 27 shows the result of simulating the average current of Embodiment 1 while maintaining the tone of areas other than the central area of the display area and changing the tone only in the central area. [Figure 22] FIG. 30 shows the result of simulating the average current of Embodiment 1 while maintaining the tone of areas other than the central area of the display area and changing the tone only in the central area. [Figure 23] FIG. 33 shows the result of simulating the average current of Embodiment 2 while maintaining the tone of areas other than the central area of the display area and changing the tone only in the central area. [Figure 24] FIG. 36 shows a timing chart of pixel control signals in sequential emission driving in Embodiment 3. [Figure 25] FIG. 39 shows simulation results of the time change of the lamp signal and the time change of the LED drive current at different tone levels within one frame period in Embodiment 4. [Figure 26] The timing chart for the pixel control signal in Embodiment 4 is shown. [Figure 27] Embodiment 5 shows an example configuration in which each pixel is divided into two subpixels. [Figure 28] The timing chart for the pixel control signal in Embodiment 5 is shown. [Modes for carrying out the invention]
[0010] One aspect of this disclosure describes the control of light emission from a micro-LED (Light Emitting Diode). The pixel circuit that controls the light emission of the micro-LED emits light from the micro-LED for a duration corresponding to the grayscale data within one frame period, and then stops the light emission from the micro-LED. A longer emission period indicates higher brightness.
[0011] One embodiment of this disclosure controls the light emission period (brightness) of a microLED by PWM (Pulse Width Modulation) according to grayscale data. The PWM-controlled method for driving a microLED (PWM driving) involves supplying a pulse drive current (also called light emission current or LED current) having a pulse width according to grayscale data to the microLED, thereby causing the microLED to emit light.
[0012] The pulse width is the period between the midpoint of the rising and falling edges of the pulse drive current; a longer pulse width means a longer emission period, i.e., higher brightness. In the low-gradation range, the drive current may consist of a steep rising waveform and a gentler falling waveform without reaching its maximum value at higher gradations.
[0013] In analog PWM driving, the waveform of the drive current is ideally required to be rectangular. However, in real circuits, there is a slowdown in the falling edge of the current, and a finite falling transition period (transition region) exists during which the drive current value gradually decreases. During this falling transition period, the drive current gradually decreases.
[0014] The emission wavelength of a micro-LED shifts to shorter wavelengths as the drive current density increases, and then shifts to longer wavelengths as the drive current density increases further. Furthermore, the external quantum efficiency (EQE) of a micro-LED decreases significantly at low drive current densities. This negative impact on micro-LED emission is particularly pronounced when the drive current supply period consists solely of the falling transition period at low grayscale levels. Therefore, the length of this falling transition period is one of the important challenges in PWM driving of micro-LEDs.
[0015] A pixel circuit according to one aspect of this disclosure includes a constant current circuit, a PWM circuit, and a current control switch. The constant current circuit generates a constant current. The PWM circuit generates a control signal from grayscale data voltages and ramp signals with different slopes. The current control switch turns the current flowing from the constant current circuit to the micro-LED ON / OFF in response to the control signal from the PWM circuit. <Embodiment 1>
[0016] Figure 1 schematically shows the configuration of a pixel circuit according to one embodiment of the present disclosure. The display area of the display device includes micro(μ)LEDs 11 arranged in a predetermined manner, for example, in a matrix. The display device includes a pixel circuit 10 that controls each of the microLEDs 11. The pixel circuit 10 includes a constant current circuit 14, a PWM circuit 12, and a current control switch 16. The current control switch 16 is a control transistor for the drive current of the microLEDs. The color of all microLEDs 11 may be the same, and the display area may include microLEDs 11 of different colors, for example, red, blue, and green. Here, one microLED 11 constitutes one light-emitting area and corresponds to a pixel circuit. Note that the features of the present disclosure may also be applied to light-emitting elements other than microLEDs.
[0017] The micro LED 11 includes an anode and a cathode. A constant power supply voltage PVEE is applied to the cathode of the micro LED 11. The internal configuration of the constant current circuit 14 and the PWM circuit 12 is arbitrary, and Figure 1 shows an example of the PWM circuit 12.
[0018] The constant current circuit 14 generates a constant current. A current control switch 16 is located between the micro LED 11 and the constant current circuit 14. The current control switch 16 is a thin-film transistor (also simply called a transistor), and in the configuration example shown in Figure 1, it is a P-type thin-film transistor. The active layer of the P-type thin-film transistor can be formed using, for example, low-temperature polysilicon. Here, instead of the current control switch 16, a driving thin-film transistor included in the constant current circuit 14 may be controlled by the PWM circuit 12. This driving thin-film transistor is a control transistor for the LED driving current. In this case, the gate voltage of the driving thin-film transistor in the constant current circuit 14 is switched on and off by the change in the output voltage of the PWM circuit 12, while controlling the magnitude of the LED driving current.
[0019] In the configuration example shown in Figure 1, the source of the current control switch 16 is connected to the terminal of the constant current circuit 14, and the drain is connected to the anode of the micro LED 11. The current control switch 16 is positioned on the path of the current flowing from the constant current circuit 14 through the micro LED 11 to the power supply line that provides the power supply voltage PVEE, and switches that path ON / OFF.
[0020] The current control switch 16 may be placed between the micro LED 11 and the power line supplying the power supply voltage PVEE. The current control switch 16 may also be an N-type thin-film transistor. The active layer of the N-type thin-film transistor can be formed using, for example, an oxide semiconductor or low-temperature polysilicon.
[0021] The constant current circuit 14 receives the power supply voltage PVDD and generates and outputs a constant current. The power supply voltage PVDD is higher than the power supply voltage PVEE. The current output from the constant current circuit 14 is turned ON / OFF by the current control switch 16.
[0022] The PWM circuit 12 includes a comparator 121, a switch 122, capacitors 123 and 124, and a switch 125. One end of each of switches 122 and capacitors 123 is connected to the inverting input of comparator 121. A constant voltage VH2 is input to the non-inverting input of comparator 121. The comparator 121 is further input to a constant voltage VH2 as the power supply voltage.
[0023] The comparator 121 compares the input signal voltage VIN to the inverting input terminal with the reference voltage VH2 to the non-inverting input terminal and outputs an output signal voltage VOUT representing the comparison result. The comparator's output signal voltage VOUT is applied to the gate of the current control switch 16 and is a control signal voltage that controls its ON / OFF state.
[0024] Switch 122 switches the path between the transmission line of the grayscale data voltage VDATA and the inverting input of comparator 121 ON / OFF. The other end of capacitor 123 is input to a ramp signal VRAMP. The ramp signal VRAMP is a voltage (signal) that increases or decreases linearly over time, and the grayscale data voltage VDATA is a voltage value corresponding to the grayscale of the pixels in the video frame. Below, we will mainly describe an example of a ramp signal whose voltage decreases, but it is also possible to use a ramp signal whose voltage increases.
[0025] Capacitor 124 is configured between the gate of the current control switch 16 and the wiring that provides a constant voltage VSET. The constant voltage VSET is lower than the constant voltage VH2. One end of capacitor 124 is connected to the node between the gate of the current control switch 16 and the output of the comparator 121, and the other end is connected to the wiring that provides the constant voltage VSET.
[0026] Switch 125 switches the path between the gate of the current control switch 16 and the wiring that provides the constant voltage VSET ON / OFF. One end of switch 125 is connected to the node between the gate of the current control switch 16 and the output of the comparator 121, and the other end is connected to the wiring that provides the constant voltage VSET.
[0027] The PWM circuit 12 generates and outputs a control signal voltage VOUT from the grayscale data voltage VDATA. The signal voltage input to the PWM circuit 12 includes the grayscale data voltage VDATA and the change in the ramp signal ΔVRAMP. The PWM circuit 12 compares the grayscale data voltage VDATA, which represents the grayscale data, with the change in the ramp signal ΔVRAMP, and outputs a control signal voltage VOUT, which is a pulse signal.
[0028] The PWM circuit 12 shown in Figure 1 uses a comparator 121 to compare the sum of the grayscale data voltage VDATA and the change in the ramp signal ΔVRAMP with a constant voltage VH2, and outputs a control signal voltage VOUT according to the relative magnitudes of the two values. This corresponds to the comparison between the grayscale data voltage VDATA and the change in the ramp signal VRAMP ΔVRAMP. The PWM circuit 12 outputs a high (H) level voltage VH2 using the comparator 121, and by turning off the switch 16, it stops the supply of current to the micro LED 11.
[0029] Figure 2 shows the time variation of the input signal voltages VRAMP and VDATA and the control signal voltage VOUT of the PWM circuit 12, as well as the drive current ILED of the micro LED 11. The input signal voltage VIN to the inverting input terminal of the comparator 121 is the sum of the grayscale data voltage VDATA and the change in the ramp signal ΔVRAMP.
[0030] Figures 3A-3E show the state of the pixel circuit 10 at times T1-T5, as shown in Figure 2. The circuit operation of the pixel circuit 10 will be explained below with reference to Figures 2 and 3A-3E.
[0031] Referring to Figure 3A, at time T1, the micro LED 11 is not emitting light. Time T1 is included in the non-emitting period. Switches 122 and 125 are OFF. Referring to Figure 2, the input voltage to comparator 121 at time T1 is the reference minimum voltage. The control signal voltage VOUT of the PWM circuit 12 is VH2, which is at an H level and is output from comparator 121. Therefore, the current control switch 16 is OFF, and the drive current ILED for the micro LED 11 is interrupted.
[0032] Referring to Figure 3B, at time T2, switches 122 and 125 are turned ON. Referring to Figure 2, at time T2, the grayscale data voltage VDATA corresponding to the grayscale level of the video frame data is written to the PWM circuit 12. From time T2 to T3 is the writing period for the grayscale data voltage. Since switch 125 is ON, the control signal voltage VOUT of the PWM circuit 12 is a low level VSET. Therefore, the current control switch 16 is ON, the drive current ILED is supplied to the micro LED 11, and the micro LED 11 lights up. In Figure 3B, during the period from time T2 to T3, switches 122 and 125 are simultaneously ON. In other examples, the timing at which they are turned ON may differ, as shown in S1 and S2 in Figure 4.
[0033] Referring to Figure 3C, at time T3, switches 122 and 125 are turned OFF. Referring to Figure 2, at time T3, the input of the ramp signal VRAMP is started. The sum of the grayscale data voltage VDATA and the change in the ramp signal ΔVRAMP is higher than the voltage VH2. The control signal voltage VOUT of the PWM circuit 12 is maintained at the L level VSET. The current control switch 16 remains ON, and the micro LED 11 continues to emit light.
[0034] Referring to Figure 3D, at time T4, switches 122 and 125 remain OFF. Referring to Figure 2, the sum of the grayscale data voltage VDATA and the change in the ramp signal ΔVRAMP is higher than the voltage VH2. The control signal voltage VOUT of the PWM circuit 12 is maintained at the L level VSET. The current control switch 16 remains ON, and the micro LED 11 continues to emit light.
[0035] Referring to Figure 3E, at time T5, switches 122 and 125 remain OFF. Referring to Figure 2, the sum of the grayscale data voltage VDATA and the change in the ramp signal ΔVRAMP has decreased to voltage VH2. The control signal voltage VOUT of the PWM circuit 12 changes from a low level VSET to a high level VH2. In response to the change in the control signal voltage VOUT, the current control switch 16 is turned OFF, and the light emission of the micro LED 11 stops.
[0036] As described above, the pulse width of the drive current for the micro-LED 11 depends on the grayscale data voltage VDATA. In other words, the illumination period of the micro-LED 11 is controlled by the grayscale data voltage VDATA.
[0037] Figure 4 shows an example in which the PWM circuit 12 is constructed using thin-film transistors and capacitors. Switches 122 and 125, and comparator 121, are each made up of P-type thin-film transistors. A selection signal (scan signal) S1 is input to the gate of switch 125, and a selection signal (scan signal) S2 is input to the gate of switch 122. Switches 122 and 125 are controlled by the selection signals S1 and S2 as described with reference to Figures 3A-3E.
[0038] In the pixel circuit shown in Figure 4, a P-type switch thin-film transistor 131 is positioned between the constant current circuit 14 and the source of the current control switch 16. The thin-film transistor 131 is controlled by the control signal EM. The thin-film transistor 131 may be positioned at other locations on the path of the LED drive current, and its conductivity type is also arbitrary.
[0039] The gate of thin-film transistor 121 corresponds to the inverting input of the comparator, and the input signal voltage VIN is applied to it. A constant voltage VH2 is applied to the source of thin-film transistor 121. The drain of thin-film transistor 121 is connected to the gate of the current control switch 16. Thin-film transistor 121 outputs a control signal voltage VOUT that controls the ON / OFF state of the current control switch 16. For this reason, thin-film transistor 121 is also called the driving thin-film transistor of the PWM circuit 12.
[0040] Figure 4 shows that all thin-film transistors constituting the pixel circuit 10 are P-type thin-film transistors, but some or all of the thin-film transistors may be N-type thin-film transistors. Furthermore, the pixel circuit 10 may include other components such as thin-film transistors and capacitors in addition to the components shown in Figure 4, and some components may be omitted. The same applies to the control signals of the pixel circuit 10; other types of control signals may be added or some signals may be omitted.
[0041] In Figure 2, the drive current ILED of the micro LED 11 falls sharply at time T5. This waveform is an ideal waveform; in reality, the fall of the drive current ILED is very gradual. Note that the rise of the drive current ILED, unlike the fall, has a steep slope close to the ideal. This is because a switch thin-film transistor 131 is present in the path of the LED drive current, and the voltage at its gate changes sharply from high to low in sub-microseconds, similar to the EM (light emission control signal).
[0042] The drive current ILED gradually decreases from its maximum value over time, eventually becoming zero. Thus, in constant-current PWM driving, there is a period during which the drive current ILED is not instantaneously cut off and does not remain constant, meaning that ideal constant-current PWM driving is not achieved.
[0043] Figure 5 schematically shows the waveforms of the lamp signal VRAMP, the gate voltage Vg of the driving thin-film transistor 121 of the PWM circuit 12, the control signal voltage VOUT from the PWM circuit 12, and the driving current ILED of the micro LED 11.
[0044] As the lamp signal VRAMP gradually decreases, the gate voltage Vg of the driving thin-film transistor 121 of the PWM circuit 12 also decreases. When the gate voltage Vg reaches the threshold voltage Vth, the driving thin-film transistor 121 changes from OFF to ON. However, the PWM circuit 12 gradually increases from VSET to VH2. This rise time is the response time of the voltage output VOUT of the PWM circuit 12. In accordance with the gradual increase of the output voltage VOUT of the PWM circuit 12, the driving current ILED of the micro LED 11 gradually decreases.
[0045] In PWM of the micro LED 11, if the fall time of the drive current ILED is long, the variation in luminous efficiency and chromaticity will increase, and the image quality may deteriorate. This is because the LED drive current density is low during the fall period. As mentioned above, the fall time of the drive current ILED is due to the response time (rise time) of the control signal voltage VOUT output from the PWM circuit 12.
[0046] Figure 6 shows the simulation results of the input / output characteristics (static characteristics) of the PWM circuit 12. The dashed line shows the ideal characteristics, and the solid line shows the characteristics of the actual circuit. The input / output characteristics reflect the steepness of the Id-Vg characteristics of the drive thin-film transistor 121. It can be seen that a gate voltage change of 0.61V is necessary for a sufficient drain current Id to flow after the drive thin-film transistor 121 turns ON.
[0047] The time required for the gate voltage to change by 0.61V is the response time of the control signal voltage VOUT output from the PWM circuit 12. Also, the time required for the potential of the lamp signal VRAMP to gradually decrease and for the gate voltage to change by 0.61V corresponds to the fall time of the LED drive current. In the case of grayscale display using a single lamp signal, the slope of the lamp signal becomes gradual depending on the pulse width of the output VOUT of the PWM circuit 12 when outputting peak brightness, which is the maximum grayscale. In order to maximize the light emission duty cycle at the maximum grayscale, a longer pulse width is better, and the lamp signal's potential decreases gradually over a long period of slightly less than one frame. For this reason, the input voltage of 0.61V required for the response of the control signal voltage VOUT is a magnitude that cannot be ignored in the light emission control of the micro LED 11.
[0048] In their research on constant-current PWM driving of micro-LEDs, the inventors found that the response of the PWM circuit 12, i.e., the fall time of the LED driving current ILED, is correlated with the slope of the RAMP voltage and the steepness of the Id-Vg characteristic of the thin-film transistor. Specifically, they found that increasing the slope of the RAMP voltage shortens the response time of the output voltage VOUT of the PWM circuit 12, and as a result, shortens the fall time of the driving current ILED. Furthermore, they found that there is a more appropriate range for the slope of the RAMP voltage, as will be described later.
[0049] Figure 7 shows the results of a simulation verifying the relationship between the slope of the lamp signal VRAMP and the fall time of the LED drive current ILED. The horizontal axis of the graph represents the slope of the lamp signal VRAMP, and the vertical axis represents the fall time of the LED drive current ILED. As can be seen from the graph in Figure 7, as the slope of the lamp signal VRAMP increases, the fall time of the output current from the current control switch 16 decreases.
[0050] The importance of the fall time of the LED drive current ILED is higher in the low-gradation range where the illumination time is short. Figure 8 schematically shows the waveforms of the lamp signal VRAMP and the LED drive current ILED for different gradation data voltages.
[0051] Waveform 201 shows the drive current waveform for high grayscale levels, waveform 202 shows the drive current waveform for intermediate grayscale levels, and waveform 203 shows the drive current waveform for low grayscale levels. For example, the maximum grayscale level is 255 and the minimum grayscale level is 0.
[0052] Waveforms 201 and 202, representing high and intermediate gray levels, have pulse widths longer than their fall time, and share the same peak value (maximum current value). Waveforms 201 and 202 have periods where the current value is constant (maximum value). In waveforms 201 and 202, the drive current rises, reaches its maximum value, maintains that value, and then falls. Here, the pulse width is the time width (half-power width) at the midpoint of the waveform (half of the maximum value) during the rising and falling phases. The rising phase can be considered virtually vertical.
[0053] The drive current waveform 203 at low grayscale levels has a pulse width shorter than its fall time, and its peak value (maximum current value) is smaller than that of the other drive current waveforms 201 and 202. Furthermore, the drive current waveform 203 begins to decrease immediately from its maximum value and does not have a consistent period of decrease. Thus, when the pulse width is shorter than the fall time of the drive current, the peak value of the drive current decreases. In other words, in the low grayscale range, the current density flowing through the LED is low, making luminance and chromaticity prone to variation. Therefore, a long fall time of the LED drive current waveform has a greater impact on light emission in the low grayscale range.
[0054] One embodiment of this disclosure applies different ramp signals to the high-gradation and low-gradation ranges in generating the LED drive current. More specifically, a ramp signal (slope) with a gentler slope is applied to the high-gradation range, and a ramp signal (slope) with a steeper slope is applied to the low-gradation range. This configuration shortens the fall time in the low-gradation range, where the long fall time has a significant impact, while enabling high brightness in the high-gradation range. Here, in the high-gradation range, the upper limit of the average current, i.e., the light emission duty cycle, is increased by lengthening the width of the ramp signal in order to make the slope of the ramp signal gentler. It is also possible to change the slope of the ramp signal by changing the voltage amplitude of the ramp signal, but this is finite, and increasing it increases power consumption. Shortening the fall time in the low-gradation range reduces variations in brightness and chromaticity based on the characteristics of micro-LEDs (improves uniformity), and as described later, it can also reduce variations in the average current due to variations in thin-film transistors (improves uniformity).
[0055] The following describes an example of applying two ramp signals with different slopes to different grayscale ranges. Three or more ramp signals with different slopes may be applied to different grayscale ranges. A ramp signal with a steeper slope is applied to a smaller grayscale range. The slope of a ramp signal can be rising (positive) or falling (negative) depending on the circuit design. A steeper slope means a larger absolute value.
[0056] Figure 9 shows a timing chart of the control signals of a pixel circuit 10 according to one embodiment of the present disclosure. The control signals are supplied from a drive circuit (not shown in Figure 9). Figure 9 shows the changes over time within one frame period of the grayscale data voltage VDATA, the scan signal (selection signal) SCAN[k], the light emission control signal EM, and the lamp signal VRAMP. The scan signal SCAN[k] is a scan signal that selects the k-th pixel row in N pixel rows. The scan signal SCAN is given for each pixel row, shifted by one horizontal period. N and k are natural numbers. The light emission control signal EM is a control signal for the switch thin-film transistor 131 on the path of the LED drive current. Here, the relationship with S1 and S2 described in Figure 4 is explained in more detail. S2 is delayed by one horizontal period relative to S1, and the relationships SCAN[K-1]=S1 and SCAN[K]=S2 exist. For the pixel circuit of the K-th row, the scan signals of the (K-1)th row and the K-th row are used. In other words, by utilizing signals from adjacent rows, the increase in the number of units in the scanning circuit is suppressed.
[0057] The pixel circuit 10 of this disclosure divides one frame period in video data into two partial periods. In one partial period, the high-gradation pixel circuit 10 illuminates the micro-LED 11, while the low-gradation pixel circuit 10 maintains the micro-LED 11 in a non-emitting state. In the other partial period, the low-gradation pixel circuit 10 illuminates the micro-LED 11, while the high-gradation pixel circuit 10 maintains the micro-LED 11 in a non-emitting state.
[0058] In the configuration example shown in Figure 9, during the first partial period, the high-gradation pixel circuit 10 writes a gradation data voltage corresponding to the gradation level shown by the video frame and causes the micro LED 11 to light up. The low-gradation pixel circuit 10 writes a non-emitting (zero current level) gradation data voltage and maintains the micro LED 11 in a non-emitting state.
[0059] In the second period following the first period, the pixel circuit 10 that displays high-gradation range writes a non-emitting (zero current level) gradation data voltage and maintains the micro LED 11 in a non-emitting state. The pixel circuit 10 that displays low-gradation range writes a gradation data voltage corresponding to the gradation level indicated by the video frame and causes the micro LED 11 to emit light. In this way, each pixel circuit 10 is selected in both the first and second periods by the scanning signal SCAN[k] and has a gradation data voltage written to it.
[0060] The pixel circuit 10 is provided with a ramp signal VRAMP with a gentle slope during the first partial period for the high-gradation region, and with a ramp signal VRAMP with a gentle slope during the second partial period for the low-gradation region. For example, the pixel circuit 10 uses a substantially steep ramp signal VRAMP for the low-gradation region of 64 gradations or less, and a gentle ramp signal VRAMP for the high-gradation region of 65 gradations or more. Alternatively, light emission in the low-gradation region may be performed during the first partial period, and light emission in the high-gradation region may be performed during the second partial period.
[0061] In the first phase, the drive circuit of the display device writes gradation data voltage VDATA to all pixel circuits 10, and then illuminates the micro LED 11 that displays the high gradation range. After that, all pixel circuits 10 stop emitting light from the micro LED 11. In the second phase, the drive circuit writes gradation data voltage VDATA to all pixel circuits 10, and then illuminates the micro LED 11 that displays the low gradation range. After that, all pixel circuits 10 stop emitting light from the micro LED 11.
[0062] Figure 10 shows the simulation results of LED drive current at different grayscale levels. Graph 221 shows the waveform of the ramp signal VRAMP during one frame period. Graphs 222 to 225 show the waveforms of the LED drive current during one frame period at grayscale levels 255, 127, 65, and 64, respectively. The pixel circuit 10 uses a gentle ramp signal 227 for high grayscale levels above 65 and a steep ramp signal 228 for low grayscale levels below 64. For example, the maximum value of the grayscale level is 255 and the minimum value is 0. As shown in graphs 222 to 225, the pulse width of the LED drive current decreases as the grayscale level decreases.
[0063] Figure 11 shows the simulation results of the relationship between grayscale levels and the peak value of the LED drive current. In the graph in Figure 11, the horizontal line represents the grayscale level, and the vertical axis represents the peak value of the LED drive current. The dashed line represents the LED drive current with a single VRAMP lamp signal waveform, and the solid line represents the LED drive current with two VRAMP lamp signal waveforms with different slopes.
[0064] As explained with reference to Figure 8, the peak value of the LED drive current is the same (constant) in the high-gradation range, but decreases in the low-gradation range as the gradation level decreases. Referring to the graph in Figure 11, both lines decrease in the low-gradation range as the gradation level decreases. However, the LED drive current from two lamp signal waveforms with different slopes is greater than the LED drive current from a single lamp signal waveform at all gradation levels.
[0065] Furthermore, with an LED drive current driven by a single lamp signal (voltage waveform), the lower limit at which the maximum value across the entire grayscale range can be maintained is 26 grayscale levels. On the other hand, with an LED drive current driven by two lamp signals, the lower limit at which the maximum value across the entire grayscale range can be maintained is 15 grayscale levels. In other words, light emission control using two lamp signals can maintain the maximum value of the LED drive current down to lower grayscale levels.
[0066] Figure 12 shows the simulation results of the relationship between the error in the average current associated with the threshold voltage shift of the driving thin-film transistor 121 of the PWM circuit 12 and the slope of the ramp signal VRAMP. Here, the average current is calculated by integrating the current over an interval of integer multiples of one frame and dividing by that interval. There is a strong correlation between the average current and brightness. In the graph shown in Figure 12, the horizontal axis represents the slope (absolute value) of the ramp signal, and the vertical axis represents the error in the average current associated with the threshold voltage shift of the thin-film transistor 121. The threshold voltage shift was set to -0.5V, and the PWM driving thin-film transistor 121 was assumed to perform threshold voltage compensation.
[0067] In the graph shown in Figure 12, for example, the slope of a single ramp signal is 0.9 [V / ms] (assuming a frame rate of 120 Hz, the width of the ramp signal corresponds to the length of its one-frame duration), while the steeper slope of the two ramp signals is 29.1 [V / ms]. As can be seen from the graph in Figure 12, by making the slope of the ramp signal steeper, the error in the average current due to the threshold voltage shift of the thin-film transistor 121 can be greatly reduced. For example, the error in the average current is reduced to 1 / 13 between a slope of 0.9 [V / ms] and a slope of 29.1 [V / ms].
[0068] Figure 13 shows the simulation results of the relationship between the slope (absolute value) of the ramp signal and the error in the average current. The horizontal axis represents the slope (absolute value) of the ramp signal, and the vertical axis represents the error in the average current when the threshold voltage of the driving thin-film transistor 121 of the PWM circuit 12 is changed by -0.5V. Line 251 shows the relationship in a pixel circuit without threshold voltage compensation, and line 252 shows the relationship in a pixel circuit with threshold voltage compensation. Regardless of whether threshold voltage compensation is performed or not, the error in the average current decreases as the slope of the ramp signal increases.
[0069] To simplify this principle, we will explain it without threshold voltage compensation. The driving thin-film transistor 121 of the PWM circuit 12 is OFF at the start of light emission and turns ON when the gate-source voltage Vgs exceeds the threshold voltage Vth. This causes the LED driving current to fall.
[0070] Therefore, as the threshold voltage Vth increases (negative direction in the case of P-type semiconductors), the pulse width of the LED drive current increases. It can be seen that there is a strong correlation between the current pulse width and the threshold voltage. However, even if the threshold voltage Vth is different, if the gate voltage Vgs changes in a very short time, the difference in current pulse width decreases. That is, as the slope of the ramp signal increases, the gate voltage of the drive thin-film transistor 121 changes in a very short time, so the effect of the difference in threshold voltage is reduced. Although the error is reduced by threshold voltage compensation, the tendency for the error to decrease with increasing slope of the ramp signal remains the same. In other words, even with threshold voltage compensation, some of the error before compensation is carried over. Therefore, although the drive thin-film transistor is usually compensated for by the threshold voltage, the effect of the Vth shift is reduced by increasing the slope of the ramp signal.
[0071] As described above, a configuration using ramp signals with different slopes can improve the characteristics of the pixel circuit that controls the light emission of the micro-LED 11 in various ways. According to the inventors' research, it has been found that there is a more appropriate range for the slope of the ramp signal. In particular, there is a more appropriate range for the slope of the ramp signal with the steepest slope. It should be noted that the steepest slope among multiple ramp signals (waveforms) may fall outside the range described below.
[0072] First, let's explain the lower limit of the slope (absolute value) of the ramp signal. Figure 14 shows a conceptual diagram of the waveforms of two ramp signals 261 and 262 during one frame period. Here, it is assumed that the slope of the second ramp signal 262 is steeper than the slope of the first ramp signal 261. Also, the amplitudes (absolute value of the difference between the start voltage and the end voltage) of ramp signals 261 and 262 are the same.
[0073] Let b be the amplitude of the second ramp signal 262 and a be its slope. In the example in Figure 14, the slope a is negative. Let f be the frame frequency. The duration of one frame is 1 / f. For the slope of the second ramp signal 262 to be steeper than the slope of the first ramp signal 261 during one frame duration, the following conditions must be met. a < -2bf
[0074] Therefore, the maximum value of the slope a, i.e., its absolute value |a|, must satisfy the following conditions. |a|>2bf The lower limit of the absolute value |a| of the slope of ramp signal 262 is 2bf. When the ramp voltage amplitude b=6V and the frame rate f=120Hz, the 2bf that defines the lower limit is 1.44V / ms.
[0075] Furthermore, when three or more ramp signals are used, the following conditions apply to the absolute value of the slope of the steepest ramp signal. |a|>nbf n is an integer greater than or equal to 3. The following describes a configuration using two ramp signals with different slopes.
[0076] Here, as shown in Figure 7, as the absolute value |a| of the slope of the lamp signal increases from 0, the fall time tf of the LED drive current decreases. However, the fall time tf reaches a saturation value at a specific value of the absolute value of the slope |a|, and remains substantially constant in the range above that specific value. This is because the potential of the lamp signal drops to its lowest point (the potential saturates), and the Vgs and Id of the PWM drive thin-film transistor 121 also saturate. For this reason, even if the slope of the lamp signal is increased beyond the specific value, the fall time tf does not change.
[0077] Within the range in which the fall time tf decreases before reaching the saturation value, the fall time tf can be logically expressed by the following formula. tf = -0.2s / a (Equation 1) s is the S value of the driving thin-film transistor 121 of the PWM circuit 12.
[0078] Next, we will explain the upper limit of the absolute value |a| of the slope of the lamp signal 262. As mentioned above, the fall time tf reaches a saturation value at a specific value of the absolute value of the slope |a|, and is substantially constant above that value. This is because the amplitude of the lamp signal is finite and fixed. The amplitude of the lamp signal matches the data voltage range and is generally around 6V. Also, increasing the data voltage range increases power consumption in order to rewrite the voltage of the data line. Referring to the simulation results of the fall time of the LED current and the absolute value |a| of the slope of the lamp signal 262 in Figure 7, the upper limit at which |a| begins to saturate is |a|=100. Furthermore, the saturation value of the fall time can be logically expressed by the following formula. Tf = 8.7 × C / β (VDATA - VH2) 2 (Equation 2) β is the gain coefficient of the driving thin-film transistor 121 of the PWM circuit 12, C is the capacitance of the capacitor 124 coupled to the output node of the driving thin-film transistor 121, VH2 is the positive power supply voltage value to the PWM driving thin-film transistor 121, and VDATA is the grayscale data voltage value.
[0079] From the above equations 1 and 2, the slope a at which the fall time tf reaches the saturation value can be expressed by the following equation. a = (-0.023s × β) × (VDATA - VH2) 2 / C As the slope of the lamp signal increases, the width of the lamp signal decreases, thus reducing the upper limit of the current pulse width. In other words, we should avoid unnecessarily shortening the width of the lamp signal. That is, even if we make |a| larger than a certain point, we cannot shorten the fall time of the LED current, but we will lower the upper limit of the light emission duty cycle.
[0080] From the above explanation, a more appropriate range for the absolute value |a| of the slope of the ramp signal 262 can be expressed by the following formula. 2×b×f<|a|<(0.023s×β)×(VDATA-VH2) 2 / C
[0081] Here are some numerical examples. The lamp signal amplitude b = 6V, frame frequency f = 120Hz, capacitance value C = 307fF, voltage value VH2 = 9V, grayscale data voltage value VDATA = 6.5V, and gain coefficient β = 5.9 × 10^(-7). In addition, in the above formula that defines the range of the absolute value of the slope |a|, an example value that defines the lower limit of the absolute value of the slope |a| is 1.44V / ms, and an example value that defines the upper limit of the absolute value of the slope |a| is 100V / ms.
[0082] The generation of grayscale data voltage will be described below. Figure 15 is a plan view showing an example configuration of a microLED display device. The microLED display device includes a display area composed of a pixel circuit 10 and an array of microLEDs 11, a signal circuit 31 and a scanning circuit 32.
[0083] Each of the signal circuit 31 and the scanning circuit 32, and their combinations, are drive circuits (sometimes called control circuits) that drive and control the pixel circuit 10. The signal circuit 31 and the scanning circuit 32 supply control signals and power supply voltages for controlling the pixel circuit 10. Figure 15 shows the types of output signals (including control signals and power supply voltages) from the signal circuit 31 and the scanning circuit 32, using the pixel circuit 10 shown in Figure 4 as an example. The types of output signals from the drive circuits depend on the configuration of the pixel circuit.
[0084] The pixel circuit 10 controls the micro LED 11. The components of the pixel circuit 10 are formed on a TFT (thin-film transistor) substrate. The micro LED 11 is connected to connection pads 111 and 112 on the TFT substrate and is electrically connected to the pixel circuit 10 via the connection pads 111 and 112.
[0085] Figure 16 shows an example configuration of the grayscale data voltage generation unit 310 of the signal circuit 31. The grayscale data voltage generation unit 310 includes a DAC (digital-to-analog converter) 311, a gamma voltage generation unit 313, and a memory 315. The memory 315 stores gamma LUTs (lookup tables) 317A and 317B. Gamma LUTs 317A and 317B are tables for two ramp signals with different slopes.
[0086] The gradation data voltage generation unit 310 receives RGB image data extracted from the video data and converts it from a digital signal to an analog signal using a DAC (digital-to-analog converter) 311. The analog signal is written as gradation data voltage to the retaining capacitor of the pixel circuit 10. The retaining capacitor in Figure 16 corresponds to the capacitor 123 in Figure 4. The DAC 311 generates gradation data voltage from the gradation level by referencing multiple gamma voltages. The gamma voltage defines the relationship between the gradation level and the gradation data voltage.
[0087] The gamma voltage generation unit 313 reads and uses either the gamma LUT 317A or 317B stored in the memory 315. The gamma voltage generation unit 313 generates and outputs gamma voltages corresponding to each grayscale level from the digital data indicated by the gamma LUT using a DAC. For example, the gamma voltage generation unit 313 uses gamma LUT 317A for gradual ramp signals and gamma LUT 317B for steep ramp signals. In a single frame period, the gamma voltage generation unit 313 swaps the gamma LUTs before writing the grayscale data voltages between the first and second partial periods. <Embodiment 2>
[0088] Embodiment 2 has technical features in the relationship between the grayscale level and the grayscale data voltage in the gamma LUT. On the other hand, in conventional examples, in a configuration that uses a single ramp signal in one frame, the pulse width of the LED drive current increases as the grayscale level increases. The number of gamma LUTs used is one.
[0089] Figure 17 shows an example of a gamma LUT used in a configuration that uses a single ramp signal in one frame. In the graph shown in Figure 17, the horizontal axis represents the pixel gradation level obtained from the video data, and the vertical axis represents the gradation data voltage. As the gradation level increases, the gradation data voltage increases. It is a desirable condition in the design of the gradation data voltage generation unit 310 that the gradation data voltage does not decrease with increasing gradation levels, that is, that it increases or remains constant. This is because, in its simplest example, the gradation data voltage generation unit 310 applies a voltage across a string of resistors connected in series and takes the voltage from the midpoint between the series-connected resistors.
[0090] Embodiment 1 generates an LED drive current based on a steep ramp signal for 64 gradations or less, and generates an LED drive current based on a gentle ramp signal for 65 gradations or more. Embodiment 1 also uses a gentle ramp signal in a first partial period and a steep ramp signal in a subsequent second partial period.
[0091] Figure 18A shows an example configuration of the gamma LUT 317A for a first partial period using a gradual ramp signal in Embodiment 1. Figure 18B shows an example configuration of the gamma LUT 317A for a second partial period using a steep ramp signal in Embodiment 1. In each graph, the horizontal axis represents the grayscale level, and the vertical axis represents the grayscale data voltage.
[0092] Referring to Figure 18A, the gradation data voltage maintains a constant minimum value at 64 gradations or less, and then monotonically increases with increasing gradation levels. This satisfies the desired conditions in the design of the gradation data voltage generation unit. On the other hand, there are pixel circuits in which the data voltage monotonically decreases as the gradation level increases. In other words, it is desirable for the data voltage to change monotonically or remain constant as the gradation level increases.
[0093] Referring to Figure 18B, the gradation data voltage increases monotonically with increasing gradation levels up to 64 gradations, then decreases to its lowest value at 65 gradations, and remains at that lowest value. In other words, the gradation voltage increases with increasing gradation levels, and then begins to decrease. This does not satisfy the desired conditions in the design of the gradation data voltage generation unit described above.
[0094] This embodiment creates a gamma LUT within the above conditions. Figure 19A shows an example configuration of gamma LUT 327A for a first partial period using a gradual ramp signal according to Embodiment 2. Gamma LUT 327A is for high gradation and is the first table. Figure 19B shows an example configuration of gamma LUT 327B for a second partial period using a steep ramp signal. In each graph, the horizontal axis represents the gradation level and the vertical axis represents the gradation data voltage. Gamma LUT 327B is for low gradation and is the second table.
[0095] Referring to Figure 19A, the gamma LUT327A maintains the gradation data voltage at a constant minimum value for 64 gradations and below, and then monotonically increases the gradation data as the gradation level increases. Referring to Figure 19B, the gamma LUT327A monotonically increases the gradation data voltage as the gradation level increases from 0 to 64 gradations, and then maintains it at a constant maximum value thereafter. The gradation data voltages for 64 gradations and 65 gradations are the same.
[0096] In this embodiment, the gradation data voltage generation unit 310, during the second partial period using a steep ramp signal, increases the gradation data voltage as the gradation increases in the low gradation range (e.g., 64 gradations or less), similar to Embodiment 1. In the high gradation range (e.g., 65 gradations or more), it maintains the gradation data voltage constant. In a small portion of the high gradation range, the gradation data voltage may increase in accordance with the increase in gradation.
[0097] In other words, the grayscale data voltage generation unit 310 outputs an LED drive current using a steep ramp signal even at 65 grayscale levels or higher. Accordingly, the grayscale data voltage generation unit 310 reduces the LED drive current in the high-grayscale range, which corresponds to the gradual ramp signal in the first partial period, compared to Embodiment 1.
[0098] Figure 20 shows the simulation results of LED drive current at different grayscale levels. Graph 281 shows the waveform of the ramp signal VRAMP over one frame period. Graphs 282 to 285 show the waveforms of the LED drive current over one frame period at grayscale levels 255, 127, 65, and 64, respectively. The maximum grayscale level is 255, and the minimum is 0.
[0099] The gradation data voltage generation unit 310 uses gentle ramp signals 287 and 288 for high gradation ranges of 65 gradations or more, and uses only the steep ramp signal 288 for low gradation ranges of 64 gradations or less. In other words, for high gradation ranges, it outputs current pulses for ramp signals 287 and 288, respectively. The brightness of the micro LED 11 depends on the average value (average current value) of the LED drive current over one frame period or multiple frame periods. The average current value is the value obtained by dividing the time integral of the LED drive current by time. Therefore, compared to Embodiment 1, the pulse width of the LED drive current in the high gradation range during the first period is shortened.
[0100] Referring to Graph 284, at 65 levels, a waveform with low current peak values in the first partial period is produced, but because the steep waveform in the second partial period is dominant, the quality of the averaged current waveform is superior to that of Embodiment 1. Furthermore, Embodiment 2 can reduce video false contours, which are a type of video noise, more effectively than Embodiment 1.
[0101] The video quality was evaluated as follows. In video, the boundary between light and dark shifts, and the human eye follows it, but the brightness is calculated by integrating the time for each position. Because there is a time delay of 1 frame in updating the brightness, to the human eye, the light and dark on the left and right of the boundary are mixed, resulting in a wide light and dark gradient that appears blurred. In Embodiment 1, the light is turned on in one of the first and second partial periods and off in the other. When displayed in a gradient while changing the gradation, at the boundary between 64th and 65th gradation, the light emitted in both partial and second partial periods is added together, and the brightness increases. On the other hand, in this embodiment, since light is always emitted in both periods at 65th gradation and above, the continuity of brightness at the gradation boundary is improved and false contours can be reduced.
[0102] This embodiment can further suppress the degradation of image quality due to ir-drop. IR-drop is a phenomenon where the LED drive current flows through the positive power line, causing a voltage drop due to the wiring resistance. The layout of the positive power line includes, for example, a thick, frame-like line outside the display area and thin, straight lines for each pixel row. Therefore, the drop in positive power voltage is greatest in the center of the display area in the pixel row direction (vertical direction).
[0103] Embodiment 1 illuminates the display during one of the first and second partial periods, and turns it off during the other, depending on the grayscale level. Therefore, for example, when displaying an image with high grayscale across the entire display area, ir-drop is large during the first partial period, but no ir-drop occurs during the second partial period.
[0104] Figures 21 and 22 show the results of simulating the average current of Embodiment 1 while maintaining the gradation of areas other than the central area of the display region and changing the gradation only in the central area. Figure 21 shows the simulation results when the areas other than the central area are at the maximum gradation of 255. Figure 22 shows the simulation results when the areas other than the central area are at 50 gradations within the low gradation range. The horizontal axis of each graph shows the gradation level and the vertical axis shows the average current. In Figures 21 and 22, the balance of ir-drop is disrupted at the gradation transitions when using different lamp signals, resulting in a discontinuity in the relationship between brightness and gradation.
[0105] Figure 23 shows the results of simulating the average current of Embodiment 2 while maintaining the gradation in areas other than the central area of the display region and changing the gradation only in the central area. Embodiment 2 causes the micro LED 11 to emit light in the first and second partial periods in the high-gradation range. As a result, ir-drop occurs, but the balance is maintained and the continuity of brightness and gradation is improved.
[0106] The method for creating the gamma LUT in this embodiment is described below. The gamma LUT is created by distributing the total average current I_sum over one frame period to IH and IL. IH is the average current due to a gentle ramp signal, and IL is the average current due to a steep ramp signal. The relationship I_sum = IH + IL holds true. In the creation of the gamma LUT, IL is fixed to its maximum value in the high-gradation range, and only IL is supplied in the low-gradation range. In the low-gradation range, IL is adjusted by the gradation data voltage.
[0107] To create a gamma LUT327B for a steep ramp signal, the gradation data voltage is monotonically increased as the gradation level increases when the average current is less than or equal to the maximum value ILmax, and the gradation data voltage DATA is kept constant when the average current is greater than or equal to the maximum value ILmax. To create a gamma LUT327A for a gentle ramp signal, the gradation data voltage is kept constant when the average current is less than or equal to the maximum value ILmax, and the gradation data voltage is monotonically increased when it is greater than or equal to ILmax. <Embodiment 3>
[0108] The above embodiment describes simultaneous light emission driving. Simultaneous light emission driving writes gradation data voltage to each pixel row, and after writing to all pixel rows is complete, all pixels are made to light up. Simultaneous light emission driving does not write gradation data voltage to any pixel row during the light emission period.
[0109] The drive circuit of this embodiment controls the display area by progressive driving. In progressive driving, after writing the grayscale data voltage to each pixel row, illumination starts immediately without waiting for the grayscale data voltage to be written to other pixel rows. When estimating the duration of one frame for each drive, progressive driving can reduce the duration by about 34% compared to simultaneous illumination, enabling a higher frame rate. In addition, since the number of micro-LEDs that emit light simultaneously is reduced, display unevenness due to ir-drop can be reduced.
[0110] Figure 24 shows the timing chart of pixel control signals in sequential line emission drive. The main point of explanation is the difference from the timing chart for simultaneous emission drive shown in Figure 9. The emission control signal EM[k] represents the emission control signal for the k-th pixel row. Also, VRAMP[k] represents the ramp signal for the k-th pixel row.
[0111] The timing for controlling the gate of the drive transistor 121 in the k-th pixel row and the timing for controlling the gate of the drive transistor 121 in the k+1-th pixel row are shifted by one horizontal period. The light emission control signal EM and the lamp signal VRAMP are also applied to each pixel row with a shift of one horizontal period, similar to the scan signal SCAN.
[0112] The pixel circuit configuration can be the same as for simultaneous light emission drive. Compared to the drive circuit for simultaneous light emission drive, a scanning circuit for the light emission control signal EM and the lamp signal VRAMP is implemented. In the drive circuit shown in Figure 15, the signal circuit 31 outputs the lamp signal VRAM, but in line sequential light emission drive, the lamp signal VRAMP is output sequentially to the pixel row from the additional scanning circuit. <Embodiment 4>
[0113] In this embodiment, the slope of the lamp signal is continuously changed. Figure 25 shows the simulation results of the time variation of the lamp signal and the time variation of the LED drive current at different grayscale levels within one frame period.
[0114] Graph 291 shows the waveform of the ramp signal VRAMP over one frame period. Graph 292 shows the waveform of the LED drive current over one frame period at low grayscale levels. Graph 293 shows the waveform of the LED drive current over one frame period at high grayscale levels.
[0115] The ramp signal waveform in Graph 291 is composed of consecutive ramp signals with different slopes. In this example, the slope of the ramp signal changes from a steep slope to a gentle slope. That is, the ramp signal includes a ramp signal 295 with a steep slope and a subsequent ramp signal 296 with a gentle slope. By keeping the slope of the gentle ramp signal 296 constant, it is possible to avoid the slope becoming too small at the maximum grayscale level.
[0116] In Figure 25, lamp signals 295 and 296 are bent in a broken line shape as segment lines, but they may also be connected smoothly by gradually changing the slope. This improves brightness uniformity when using the area near the boundary between lamp signals 295 and 296 for grayscale display. In Figure 25, segment lines are used, but the slope may also be gradually reduced as a nonlinear method.
[0117] In this embodiment, a ramp signal with a continuously changing slope is input to the pixel circuit, and the pulse width of the LED drive current is controlled by the gradation data voltage. A ramp signal with a large slope can shorten the fall time of short pulse widths. This is because when the LED current pulse is short, the light emission duty cycle is low and the gradation is low, and when the current pulse is long, the light emission duty cycle is high and the gradation is high. To prioritize improving the image quality of low gradations, the first half of the ramp signal, ramp signal 295, is made steep, while the second half, ramp signal 296, is made gentler, thereby making the width of the ramp signal sufficiently long and increasing the upper limit of the light emission duty cycle, i.e., the peak brightness.
[0118] Figure 26 shows the timing chart of the pixel control signals in this embodiment. Compared with the timing chart in Figure 24, the selection for writing the grayscale data voltage by the scan signal SCAN[k] occurs once per frame period. Also, the waveform of the ramp signal VRAMP[k] has consecutive different slopes. The first slope is steep, and the subsequent slopes are gentle. Although Figure 26 shows an example of line-sequential driving, simultaneous light emission driving may also be used.
[0119] In this embodiment, since the data writing period for each frame is uniform, a higher frame rate is possible compared to other embodiments. Furthermore, consecutive ramp signals with different inclinations can reduce video false contours, which are a type of video noise, more effectively than in other embodiments.
[0120] This is because the light emission pulse in this embodiment is single. Also, in Graph 292, the segment-shaped lamp signal is shown as a single unit per frame, but multiple units may be used. To support sequential light emission drive, lamp signals delayed by one horizontal period are required, but as shown in Figure 26, if there is a single lamp signal per frame, it is necessary to generate lamp signals for the number of scan lines N (VRAMP[1]~VRAMP[N]). The segment-shaped lamp signals 295 and 296 are set as one set, with a width W of one set and a horizontal period of H. The number of lamp signals = W / H, and as W decreases, the number of lamp signals decreases. In other words, to support sequential light emission drive, the width of the lamp signals can be shortened and the sets can be repeated to significantly reduce the number of lamp signals. <Embodiment 5>
[0121] Embodiment 5 divides a pixel (light-emitting region) into multiple sub-pixels and provides each with a ramp signal of a different angle. Figure 27 shows an example configuration in which each pixel is divided into two sub-pixels. A pair of sub-pixel circuits 10HR and 10LR constitutes the entire pixel circuit of one red pixel. A pair of sub-pixel circuits 10HG and 10LG constitutes the entire pixel circuit of one green pixel. A pair of sub-pixel circuits 10HB and 10LB constitutes the entire pixel circuit of one blue pixel. Two micro-LEDs are placed on each sub-pixel circuit and connected to the sub-pixel circuit. Each micro-LED corresponds to a sub-pixel, and one light-emitting region of a pixel is composed of two micro-LEDs.
[0122] In the configuration example shown in Figure 27, each sub-pixel circuit controls the illumination of a corresponding micro-LED 11. Sub-pixel circuits 10HR, 10HG, and 10HB control the illumination period of the micro-LED 11 with a gradual ramp signal. Sub-pixel circuits 10LR, 10LG, and 10LB control the illumination period of the micro-LED 11 with a steep ramp signal.
[0123] Dedicated lamp signal transmission lines are wired within the display area for each of the two types of lamp signals. Lamp transmission line 401H, which transmits the gentle lamp signal VRAMP[k]_H, passes through the area of the sub-pixel circuit row including sub-pixel circuits 10HR, 10HG, and 10HB. Lamp transmission line 401L, which transmits the steep lamp signal VRAMP[k]_L, passes through the area of the sub-pixel circuit row including sub-pixel circuits 10LR, 10LG, and 10LB. Lamp transmission line 401H is wired to pass through each sub-pixel circuit row that uses the gentle lamp signal. In addition, lamp transmission line 401L is wired to pass through each sub-pixel circuit row that uses the steep lamp signal.
[0124] Figure 28 shows the timing chart of the pixel control signals in this embodiment. Signal group 411H is a control signal group for a sub-pixel circuit that uses a gentle ramp signal VRAMP_H. Signal group 411L is a control signal group for a sub-pixel circuit that uses a steep ramp signal VRAMP_L.
[0125] The signal group 411H sequentially selects sub-pixel circuit rows that use the gentle ramp signal VRAMP_H based on the selection signal SCAN_H[k], and writes the grayscale data voltage. A typical gamma LUT as shown in Figure 17 is used.
[0126] The signal group 411L sequentially selects sub-pixel circuit rows that use the steep ramp signal VRAMP_L based on the selection signal SCAN_L[k] and writes the gradation data voltage to them. There is only one period during which the gradation data voltage is written to each sub-pixel circuit. Furthermore, the period during which the signal group 411H writes the gradation data voltage to all sub-pixel circuit rows is the same as the period during which the signal group 411L writes the gradation data voltage to all sub-pixel circuit rows.
[0127] After the grayscale data voltage is written to each sub-pixel circuit row, all sub-pixel circuits begin to emit light. The light emission control signal EM_H controls the emission of micro-LEDs by sub-pixel circuits using a gentle ramp signal VRAMP_H. The light emission control signal EM_L controls the emission of micro-LEDs by sub-pixel circuits using a steep ramp signal VRAMP_H. These waveforms are identical.
[0128] In this embodiment, the writing period for the gradation data voltage to each sub-pixel circuit is single within a single frame period. During the light emission period, lamp signals with different slopes are input simultaneously. Embodiments 1, 2, and 3 require time division to change the slope of the lamp signals. Embodiment 5 eliminates the need for this time division, and by reducing the data writing period to a single operation, it is possible to achieve both a high frame rate and improved image quality.
[0129] While embodiments of the present disclosure have been described above, the present disclosure is not limited to the embodiments described above. Those skilled in the art can easily modify, add to, and transform each element of the above embodiments within the scope of the present disclosure. It is possible to replace parts of the configuration of one embodiment with the configuration of another embodiment, and to add the configuration of another embodiment to the configuration of one embodiment. [Explanation of Symbols]
[0130] 10 Pixel Circuit 11 Micro LEDs 12 PWM circuit 14 Constant current circuit 31 Signal circuit 32 Scanning Circuit 121 PWM circuit driving thin-film transistor 227, 228, 261, 262 Lamp signals 310-level data voltage generation unit 317A, 317B, 327A, 327B Gamma LUT
Claims
1. A display device, Multiple pixel circuits, A control circuit for controlling the plurality of pixel circuits, Includes, Each of the multiple pixel circuits is A control transistor for the pixel drive current, A pulse width modulation circuit that supplies a control signal to the control transistor, Includes, The pulse width modulation circuit includes a drive transistor that outputs the control signal, The pulse width modulation circuit controls the light emission time of the pixel during one frame period by switching the control transistor ON / OFF according to the control signal. The pulse width modulation circuit controls the ON / OFF state of the drive transistor using the gradation data voltage from the control circuit, as well as a first ramp signal and a second ramp signal that is steeper than the first ramp signal. The second ramp signal is used for control in the low-gradation range, including the minimum gradation level, and is excluded from control in the high-gradation range, which consists of gradation levels higher than the low-gradation range. The first lamp signal is used for control in at least the high-gradation range. Display device.
2. A display device according to claim 1, The aforementioned control circuit is It includes a first table and a second table that define the relationship between grayscale level and grayscale data voltage, Using the first table, the gradation data voltage of the high gradation range is determined and output. Using the second table, the grayscale data voltage for the low grayscale range is determined and output. The first and second tables define voltages that change monotonically or maintain a constant value as the grayscale level increases. Display device.
3. A display device according to claim 2, In the second table, the grayscale data voltage increases with increasing grayscale level and then maintains a maximum value. In the first table, the grayscale data voltage increases as the grayscale level increases, after maintaining a minimum value. Display device.
4. A display device according to claim 1, The timing for controlling the gate of the drive transistor in the k-th pixel row and the timing for controlling the gate of the drive transistor in the k+1-th pixel row are shifted by one horizontal period. Display device.
5. A display device according to claim 1, The slope of the second lamp signal and the slope of the first lamp signal are continuous, with the slope of the second lamp signal preceding the slope of the first lamp signal. Display device.
6. A display device according to claim 1, The aforementioned pixel includes a first light-emitting diode and a second light-emitting diode, The pixel circuit includes a first sub-pixel circuit that controls a first light-emitting diode and a second sub-pixel circuit that controls a second light-emitting diode. The first sub-pixel circuit uses the first lamp signal, The second sub-pixel circuit uses the second lamp signal. Display device.
7. A display device according to claim 1 or 5, The absolute value |a| of the slope of the second lamp signal satisfies 1.44 V / ms < |a| < 100 V / ms. Display device.