Display device and method for driving same

Abstract

The present invention suppresses a decrease in luminance in a low gradation region and occurrence of color shift accompanying the decrease in luminance in a display device that adjusts the brightness of a screen by changing a light emission duty ratio.　The light emission duty ratio is controlled to have a higher value as a brightness level value becomes higher and to have a lower value as the brightness level value becomes lower. On that premise, an initialization voltage control unit controls the voltage value of a second initialization voltage such that the difference between the voltage value of the second initialization voltage and the voltage value of a high-level power supply voltage becomes lesser as the brightness indicated by the brightness level value darkens. Specifically, the voltage value of the second initialization voltage is controlled to be a lower value as the brightness level value becomes higher and to be a greater value as the brightness level value becomes lower.

Description

Display device and its driving method

[0001] The following disclosure relates to a display device having a pixel circuit including a display element driven by current and having a function of adjusting the brightness of a screen, and a driving method thereof.

[0002] In recent years, an organic EL display device including a pixel circuit including an organic EL element has been put into practical use. The organic EL element is also called an OLED (Organic Light-Emitting Diode), and is a self-emitting display element that emits light with a luminance corresponding to the current flowing through it. Since the organic EL element is a self-emitting display element in this way, the organic EL display device can be easily made thinner, lower in power consumption, and higher in luminance than a liquid crystal display device that requires a backlight and a color filter. Therefore, in recent years, the development of organic EL display devices has been actively promoted.

[0003] By the way, many organic EL display devices have a function of adjusting the brightness (that is, luminance) of the screen (hereinafter referred to as the "brightness adjustment function"). By the brightness adjustment function, for example, when an organic EL display device is adopted for the display of a mobile terminal such as a smartphone, the brightness of the screen is adjusted according to the environment around the operator (user), or when an organic EL display device is adopted for the display of a car navigation device, the brightness of the screen is switched between daytime and nighttime. Regarding this, the adjustment and switching of the brightness may be manually performed by the operator, or may be performed based on, for example, the illuminance detected by an illuminance sensor or a preset time.

[0004] Hereinafter, a value representing the degree of relative brightness that can be adjusted by the brightness adjustment function is referred to as a "brightness level value". The range of values that the brightness level value can take is, for example, in the range of 0 or more and 100 or less, and it is assumed that the greater the brightness level value, the greater the degree of brightness. Also, hereinafter, it is assumed that the higher the signal value (gray scale value) of the input video signal, the higher the luminance becomes.

[0005] Methods for adjusting brightness primarily involve changing the signal value of the data signal (the signal provided to the pixel circuit as a data voltage) according to the brightness level value, and changing the emission duty cycle according to the brightness level value. Here, the emission duty cycle is the percentage of the frame period during which the organic EL element (display element) in the pixel circuit emits light. For example, if the emission duty cycle is 80%, the organic EL element will emit light for 80% of the frame period. In some cases, a method is employed that changes both the signal value of the data signal and the emission duty cycle according to the brightness level value.

[0006] Figure 39 is a diagram illustrating a method for changing the signal value of a data signal according to the brightness level value. The thick dotted line labeled 901, the thick solid line labeled 902, and the solid line labeled 903 indicate how the luminance corresponding to the signal value of the data signal (here, we focus on the data signal corresponding to a certain grayscale value) changes relatively in response to the change in the brightness level value (Figure 40 is similar). As can be seen from Figure 39, with this method, the voltage values ​​of the light emission duty cycle and the anode reset voltage are maintained at constant values ​​regardless of the brightness level value, but the signal value of the data signal is controlled so that the luminance corresponding to the signal value increases as the brightness level value increases. Specifically, changes in the signal value of the data signal are achieved by preparing multiple gamma correction tables that associate the signal value of the input video signal with the signal value of the data signal (the signal provided to the pixel circuit as a data voltage) for each of the multiple grayscale values ​​(for example, 256 grayscale values), and then selecting the gamma correction table to actually use according to the brightness level value.

[0007] Figure 40 illustrates a method for changing the emission duty cycle according to the brightness level value. As can be seen from Figure 40, with this method, the anode reset voltage value and the data signal value are maintained at a constant value regardless of the brightness level value, but the emission duty cycle is controlled to become larger as the brightness level value increases. In this method, the same gamma correction table is used regardless of the brightness level value. Therefore, if the signal value of the input video signal is the same, the signal value of the data signal obtained by gamma correction does not change even if the brightness level value changes.

[0008] Here, we assume that if the light emission control signal applied to the pixel circuit of the organic EL display device is high level, the organic EL element in the pixel circuit remains off, and if the light emission control signal is low level, the organic EL element in the pixel circuit remains emitting light. In this case, if the waveform of the light emission control signal is the waveform indicated by the symbol EMa in Figure 41, the organic EL element remains emitting light during the period indicated by the arrow labeled 92a within one frame period; if the waveform of the light emission control signal is the waveform indicated by the symbol EMb in Figure 41, the organic EL element remains emitting light during the period indicated by the arrow labeled 92b within one frame period; and if the waveform of the light emission control signal is the waveform indicated by the symbol EMc in Figure 41, the organic EL element remains emitting light during the period indicated by the arrow labeled 92c within one frame period. In the method of changing the light emission duty cycle according to the brightness level value, the light emission duty cycle changes by changing the waveform of the light emission control signal in this way according to the brightness level value. As a result, the brightness of the screen changes according to the brightness level value.

[0009] According to the method of changing the signal value of the data signal according to the brightness level value (see Figure 39), when the brightness level value is set so that the luminance is significantly low, many gradations will use the low-current region of the drive transistor that controls the supply of drive current to the organic EL element within the pixel circuit. Since the current variation is relatively large in the low-current region of the drive transistor, brightness unevenness is likely to occur in this case. In addition, since it is necessary to prepare multiple gamma correction tables, the manufacturing cost is higher compared to the method of changing the emission duty cycle according to the brightness level value. For the above reasons, from the viewpoint of suppressing the occurrence of brightness unevenness and the increase in manufacturing costs, it is preferable to adopt the method of changing the emission duty cycle according to the brightness level value.

[0010] Furthermore, a display device that changes the light emission duty cycle to adjust the display brightness is disclosed in Japanese Patent Publication No. 2010-072112. In the display device disclosed in this publication, for example, the brightness of the display panel is adjusted by variably controlling the light emission duty cycle in the range of 1 / 8 to 7 / 8 according to the brightness (brightness of the surrounding environment) detected by a light sensor. In addition, in relation to the following disclosure, Japanese Patent Publication No. 2020-112795 describes adjusting the anode reset voltage to reduce flicker.

[0011] Japanese Patent Publication No. 2010-072112 Japanese Patent Publication No. 2020-112795

[0012] Regarding methods for implementing brightness adjustment, as mentioned above, from the standpoint of suppressing the occurrence of brightness unevenness and increases in manufacturing costs, it is preferable to adopt a method that changes the luminescence duty cycle according to the brightness level value. However, with this method, the display quality may deteriorate in the low-gradation region (the region in which image display is performed based on input video signals with low gradation values). This will be explained below.

[0013] As described above, the pixel circuit of an organic EL display device includes an organic EL element, which is a self-emissive display element. Within the pixel circuit, a parasitic capacitor 932 is formed in parallel with the organic EL element 931, as shown in Figure 42, depending on the wiring layout, etc. The anode of the organic EL element 931 is typically initialized based on an anode reset voltage every frame period. When the anode of the organic EL element 931 is initialized based on the anode reset voltage, the amount of charge held in the parasitic capacitor 932 becomes approximately zero. Therefore, after the anode of the organic EL element 931 is initialized, current first flows through the parasitic capacitor 932 to charge it. As the charge accumulated in the parasitic capacitor 932 increases, the current flowing through the parasitic capacitor 932 decreases, while the current flowing through the organic EL element 931 increases. Consequently, in a pixel circuit where low-gradation display is performed, the current flowing through the organic EL element 931 and the current flowing through the parasitic capacitor 932 change, for example, as shown in Figure 43. In Figure 43, the thick solid line denoted by reference numeral 94 represents the change in current flowing through the organic EL element 931, and the thick dotted line denoted by reference numeral 95 represents the change in current flowing through the parasitic capacitance 932 (the same applies to Figures 44 to 47).

[0014] When gamma correction is performed based on the state when the light emission duty cycle is 100%, the signal value of the data signal to be obtained by gamma correction is set so that the brightness obtained by the light emission of the organic EL element 931 due to the current supplied via the drive transistor, excluding the current flowing through the parasitic capacitance 932, becomes the desired brightness. This setting is performed so that the area of ​​the shaded portion labeled 94a in Figure 44 corresponds to the area of ​​the desired brightness.

[0015] Here, if the light emission duty cycle becomes 30% based on the brightness level value, then 30% of the frame period becomes the light emission period (the period during which the organic EL element 931 should be maintained in a light-emitting state). In this case, if the signal value of the data signal is the same as in the example shown in Figure 44, the luminance obtained from the light emission of the organic EL element 931 will be the luminance corresponding to the area of ​​the shaded portion labeled 94b in Figure 45. Since the current flowing through the organic EL element 931 changes as shown by the thick solid line labeled 94, the area of ​​the shaded portion labeled 94b in Figure 45 will be less than 30% of the area of ​​the shaded portion labeled 94a in Figure 44. In this regard, the luminance that should be obtained is the luminance corresponding to the area of ​​the rectangular portion of the thick dashed line labeled 96 in Figure 46. From the above, the luminance obtained from the light emission of the organic EL element 931 will be lower than the original luminance.

[0016] Incidentally, in a pixel circuit where high-gradation display is performed, the current flowing through the organic EL element 931 and the current flowing through the parasitic capacitance 932 change, for example, as shown in Figure 47. In this example, when the light emission duty cycle is 30%, the brightness that should be obtained corresponds to the brightness of the area of ​​the thick dashed rectangle labeled 97 in Figure 47, whereas the brightness actually obtained corresponds to the brightness of the shaded area labeled 94c in Figure 47. In this case, a large current flows through the organic EL element 931, so the current flowing through the parasitic capacitance 932 becomes relatively small. Therefore, as can be seen from Figure 47, the effect of brightness reduction caused by the current flowing through the parasitic capacitance 932 is almost negligible.

[0017] As described above, when the light emission duty cycle is small, the brightness displayed on the screen becomes significantly lower than the actual brightness, especially in the low-gradation region (hereinafter, this phenomenon may be referred to as "black crushing"). In this regard, the brightness reduction is particularly pronounced in G (green) pixels and R (red) pixels, which have small current values. Furthermore, color shift occurs because the degree of brightness reduction differs for each color: R (red), G (green), and B (blue). Thus, the display quality deteriorates in the low-gradation region.

[0018] Therefore, the following disclosure aims to suppress the decrease in brightness in the low-gradation region and the resulting color shift in a display device (a display device using a display element driven by electric current) that adjusts the brightness of the screen by changing the light emission duty cycle.

[0019] Some embodiments of the present disclosure are display devices using a display element having a first terminal and a second terminal and driven by current, comprising: a display unit including a plurality of data signal lines for transmitting data signals, a plurality of scan signal lines intersecting the plurality of data signal lines, a plurality of light emission control lines intersecting the plurality of data signal lines, a plurality of pixel circuits each corresponding to one of the plurality of data signal lines, at least one of the plurality of scan signal lines, and one of the plurality of light emission control lines, a first power line to which a first power supply voltage is supplied, a second power line to which a second power supply voltage is supplied, and a display element initialization power line to which a display element initialization voltage is supplied for initializing the first terminal of the display element, a data signal line driving circuit for applying the data signals to the plurality of data signal lines, a scan signal line driving circuit for driving the plurality of scan signal lines, a light emission control line driving circuit for driving the plurality of light emission control lines, a power supply circuit for generating the first power supply voltage, the second power supply voltage, and the display element initialization voltage, and a brightness control unit for adjusting the brightness of the display unit based on a brightness level value, wherein each of the plurality of pixel circuits is The brightness control unit includes: a display element provided between the first power line and the second power line, having a first terminal on the first power line side and a second terminal on the second power line side; a control node; a drive transistor provided in series with the display element, having a control terminal connected to the control node, a first conductive terminal on the first power line side, and a second conductive terminal on the second power line side; at least one light emission control transistor as a switching element provided in series with the display element and the drive transistor, having a control terminal connected to a corresponding light emission control line; a display element initialization transistor as a switching element, having a control terminal connected to a corresponding scan signal line, a first conductive terminal connected to the first terminal of the display element, and a second conductive terminal connected to the display element initialization power line; and the brightness control unit includes a light emission duty cycle control unit that controls a light emission duty cycle indicating the proportion of the period during which the at least one light emission control transistor is kept in the ON state within one frame period, based on the brightness level value.The power supply circuit includes an initialization voltage control unit that controls the voltage value of the display element initialization voltage generated by the power supply circuit based on the brightness level value, wherein the light emission duty cycle control unit controls the light emission duty cycle such that the light emission duty cycle increases as the brightness represented by the brightness level value increases, and the initialization voltage control unit controls the voltage value of the display element initialization voltage such that the difference between the voltage value of the display element initialization voltage and the voltage value of the first power supply voltage decreases as the brightness represented by the brightness level value decreases.

[0020] A driving method for a display device according to some embodiments of the present disclosure is a driving method for a display device using a display element having a first terminal and a second terminal and driven by current, wherein the display device includes a display unit including a plurality of data signal lines for transmitting data signals, a plurality of scan signal lines intersecting the plurality of data signal lines, a plurality of light emission control lines intersecting the plurality of data signal lines, a plurality of pixel circuits each corresponding to one of the plurality of data signal lines, at least one of the plurality of scan signal lines and one of the plurality of light emission control lines, a first power line to which a first power supply voltage is supplied, a second power line to which a second power supply voltage is supplied, and a display element initialization power line to which a display element initialization voltage is supplied for initializing the first terminal of the display element, a data signal line driving circuit for applying the data signals to the plurality of data signal lines, a scan signal line driving circuit for driving the plurality of scan signal lines, a light emission control line driving circuit for driving the plurality of light emission control lines, and a power supply circuit for generating the first power supply voltage, the second power supply voltage and the display element initialization voltage, wherein each of the plurality of pixel circuits is The driving method includes: a display element provided between a first power line and a second power line, having a first terminal on the first power line side and a second terminal on the second power line side; a control node; a drive transistor provided in series with the display element, having a control terminal connected to the control node, a first conductive terminal on the first power line side, and a second conductive terminal on the second power line side; at least one light emission control transistor as a switching element provided in series with the display element and the drive transistor, having a control terminal connected to a corresponding light emission control line; and a display element initialization transistor as a switching element, having a control terminal connected to a corresponding scan signal line, a first conductive terminal connected to the first terminal of the display element, and a second conductive terminal connected to the display element initialization power line, wherein the driving method includes a light emission duty cycle control step of controlling a light emission duty cycle indicating the proportion of the period during which the at least one light emission control transistor is kept in the ON state during one frame period, based on a defined brightness level value;The system includes an initialization voltage control step which controls the voltage value of the display element initialization voltage generated by the power supply circuit based on the brightness level value, wherein in the emission duty cycle control step the emission duty cycle is controlled such that the emission duty cycle increases as the brightness represented by the brightness level value increases, and in the initialization voltage control step the voltage value of the display element initialization voltage is controlled such that the difference between the voltage value of the display element initialization voltage and the voltage value of the first power supply voltage decreases as the brightness represented by the brightness level value decreases.

[0021] According to some embodiments of this disclosure, the light emission duty cycle control unit controls the light emission duty cycle so that the brighter the brightness represented by the brightness level value, the larger the light emission duty cycle, and the darker the brightness represented by the brightness level value, the smaller the light emission duty cycle. In such a configuration, the initialization voltage control unit controls the voltage value of the display element initialization voltage so that the difference between the voltage value of the display element initialization voltage and the voltage value of the first power supply voltage becomes smaller the darker the brightness represented by the brightness level value. That is, the smaller the light emission duty cycle, the smaller the difference between the voltage value of the display element initialization voltage and the voltage value of the first power supply voltage. For example, if the first power supply voltage is a high-level power supply voltage and the second power supply voltage is a low-level power supply voltage, the smaller the light emission duty cycle (the darker the brightness represented by the brightness level value), the larger the voltage value of the display element initialization voltage becomes. As a result, when the light emission duty cycle is small, immediately after the first terminal of the display element is initialized, a large amount of charge is held in the parasitic capacitance formed in parallel with the display element. Therefore, compared to the conventional, the current flowing through the parasitic capacitance becomes smaller, and the current flowing through the display element becomes larger. As a result, even in the low-gradation region, brightness close to the brightness that should be obtained can be achieved. Therefore, in a display device that adjusts the brightness of the screen by changing the light emission duty cycle (a display device using a display element driven by electric current), it is possible to suppress the decrease in brightness in the low-gradation region and the resulting color shift.

[0022] This figure shows the relationship between the luminescence duty cycle, the voltage value of the second initialization voltage, and the signal value of the data signal, and the brightness level value in one embodiment. This is a block diagram showing the overall configuration of the organic EL display device according to the above embodiment. This is a block diagram showing the configuration of the scanning side drive circuit in the above embodiment. This is a timing chart for explaining the driving of the luminescence control line in the above embodiment. This is a circuit diagram showing the configuration of the pixel circuit in the above embodiment. This is a timing chart for explaining the operation of the pixel circuit in the above embodiment. This is a block diagram showing the configuration for realizing the brightness adjustment function in the above embodiment. This figure shows an example of a brightness adjustment screen in the above embodiment. This figure shows the basic relationship between the brightness level value, the voltage value of the second initialization voltage, and the voltage value of the low-level power supply voltage in the above embodiment. This figure shows the relationship between the brightness level value, the voltage value of the second initialization voltage, and the voltage value of the low-level power supply voltage in a model in which a phenomenon called black floating occurs in the above embodiment. This figure shows an example of the change in current flowing through the organic EL element and the parasitic capacitance in the above embodiment. This figure compares the brightness that should be obtained and the brightness that is actually obtained when low-gradation display is performed in the above embodiment. This is a graph showing experimental results of measuring the gamma characteristics of W with the luminescence duty cycle set to 53% and the second initialization voltage set to -4.0V. This graph shows the experimental results of measuring the gamma characteristics of RGB with the emission duty cycle set to 53% and the second initialization voltage set to -4.0V. This graph shows the experimental results of measuring chromaticity with the emission duty cycle set to 53% and the second initialization voltage set to -4.0V. This graph shows the experimental results of measuring the gamma characteristics of W with the emission duty cycle set to 53% and the second initialization voltage set to -3.5V. This graph shows the experimental results of measuring the gamma characteristics of RGB with the emission duty cycle set to 53% and the second initialization voltage set to -3.5V. This graph shows the experimental results of measuring chromaticity with the emission duty cycle set to 53% and the second initialization voltage set to -3.5V. This graph shows the experimental results of measuring the gamma characteristics of W with the emission duty cycle set to 53% and the second initialization voltage set to -3.0V.This graph shows the experimental results of measuring the gamma characteristics of RGB with the emission duty cycle set to 53% and the second initialization voltage set to -3.0V. This graph shows the experimental results of measuring chromaticity with the emission duty cycle set to 53% and the second initialization voltage set to -3.0V. This graph shows the experimental results of measuring the gamma characteristics of W with the emission duty cycle set to 8% and the second initialization voltage set to -4.0V. This graph shows the experimental results of measuring the gamma characteristics of RGB with the emission duty cycle set to 8% and the second initialization voltage set to -4.0V. This graph shows the experimental results of measuring chromaticity with the emission duty cycle set to 8% and the second initialization voltage set to -4.0V. This graph shows the experimental results of measuring the gamma characteristics of W with the emission duty cycle set to 8% and the second initialization voltage set to -3.5V. This graph shows the experimental results of measuring the gamma characteristics of RGB with the emission duty cycle set to 8% and the second initialization voltage set to -3.5V. This graph shows the experimental results of measuring chromaticity with the emission duty cycle set to 8% and the second initialization voltage set to -3.5V. This graph shows the experimental results of measuring the gamma characteristics of W with the emission duty cycle set to 8% and the second initialization voltage set to -3.0V. This graph shows the experimental results of measuring the gamma characteristics of RGB with the emission duty cycle set to 8% and the second initialization voltage set to -3.0V. This graph shows the experimental results of measuring chromaticity with the emission duty cycle set to 8% and the second initialization voltage set to -3.0V. This graph shows the experimental results of measuring the gamma characteristics of W with the emission duty cycle set to 8% and the second initialization voltage set to -2.5V. This graph shows the experimental results of measuring the gamma characteristics of RGB with the emission duty cycle set to 8% and the second initialization voltage set to -2.5V. This graph shows the experimental results of measuring chromaticity with the emission duty cycle set to 8% and the second initialization voltage set to -2.5V. This graph shows the experimental results of measuring the gamma characteristics of W with the emission duty cycle set to 8% and the second initialization voltage set to -2.0V. This graph shows the experimental results of measuring the gamma characteristics of RGB with the emission duty cycle set to 8% and the second initialization voltage set to -2.0V. This graph shows the experimental results of measuring chromaticity with the emission duty cycle set to 8% and the second initialization voltage set to -2.0V. This is a circuit diagram showing the configuration of the pixel circuit in the first modified example.This is a block diagram showing the configuration of the brightness control unit in the second modified example. This is a diagram illustrating a method for changing the signal value of a data signal according to the brightness level value. This is a diagram illustrating a method for changing the light emission duty cycle according to the brightness level value. This is a diagram illustrating how the waveform of the light emission control signal is changed according to the brightness level value. This is a diagram illustrating how a parasitic capacitance is formed in parallel with the organic EL element. This is a diagram showing an example of the change in current flowing through the organic EL element and the parasitic capacitance when low-gradation display is performed in a conventional example. This is a diagram illustrating how the signal value of a data signal is set. This is a diagram illustrating how the brightness obtained from the light emission of an organic EL element when the light emission duty cycle is 30% in a conventional example. This is a diagram comparing the brightness that should be obtained and the brightness that is actually obtained when low-gradation display is performed in a conventional example. This is a diagram comparing the brightness that should be obtained and the brightness that is actually obtained when high-gradation display is performed in a conventional example.

[0023] An embodiment will be described below with reference to the attached drawings. In the following, it is assumed that m and n are integers of 2 or greater, i is an integer between 1 and n, and j is an integer between 1 and m.

[0024] <1. Overall Configuration> Figure 2 is a block diagram showing the overall configuration of an organic EL display device 10 according to one embodiment. As shown in Figure 2, the organic EL display device 10 includes a display unit 11, a display control circuit 20, a data-side drive circuit (data signal line drive circuit) 30, a scanning-side drive circuit 40, a power supply circuit 50, and a brightness level value setting unit 60. The scanning-side drive circuit 40 includes a light emission control line drive circuit 41 and a scanning signal line drive circuit 42, as shown in Figure 3. At least a part of the scanning-side drive circuit 40 and the data-side drive circuit 30 may be integrally formed with the display unit 11.

[0025] The display unit 11 is provided with n write control lines SCAN(1) to SCAN(n), (n+1) initialization control lines DIS(0) to DIS(n), n light emission control lines EM(1) to EM(n), and m data signal lines D(1) to D(m). The write control lines SCAN(1) to SCAN(n), initialization control lines DIS(0) to DIS(n), and light emission control lines EM(1) to EM(n) are typically parallel to each other. The write control lines SCAN(1) to SCAN(n) and data signal lines D(1) to D(m) are orthogonal. Each write control line SCAN transmits a write control signal, each initialization control line DIS transmits an initialization control signal, each light emission control line EM transmits a light emission control signal, and each data signal line D transmits a data signal. The display unit 11 is also provided with n × m pixel circuits 15. The n x m pixel circuits 15 constitute an n x m pixel matrix. Hereafter, when distinguishing between the n x m pixel circuits 15, the pixel circuit corresponding to the i-th write control line SCAN(i) and the j-th data signal line D(j) will be referred to as the "i-row j-column pixel circuit". The i-row j-column pixel circuit will be denoted as 15(i,j). Furthermore, hereafter, if necessary, the write control signals given to the write control lines SCAN(1) to SCAN(n), respectively, will also be denoted as SCAN(1) to SCAN(n), the initialization control signals given to the initialization control lines DIS(0) to DIS(n), respectively, the light emission control signals given to the light emission control lines EM(1) to EM(n), respectively, will also be denoted as EM(1) to EM(n), and the data signals given to the data signal lines D(1) to D(m), respectively, will also be denoted as D(1) to D(m).

[0026] Furthermore, the display unit 11 is provided with power supply wiring common to each pixel circuit 15. More specifically, it is provided with a power line that supplies a high-level power supply voltage ELVDD for driving the organic EL element (hereinafter referred to as the "high-level power line"), a power line that supplies a low-level power supply voltage ELVSS for driving the organic EL element (hereinafter referred to as the "low-level power line"), a power line that supplies a first initialization voltage Vini1 for initializing the control node (gate of the drive transistor) in the pixel circuit 15 (hereinafter referred to as the "first initialization power line"), and a power line that supplies a second initialization voltage Vini2 for initializing the anode of the organic EL element (hereinafter referred to as the "second initialization power line"). The second initialization voltage Vini2 is the anode reset voltage mentioned above.

[0027] The operation of each component shown in Figures 2 and 3 will be described below. The brightness level value setting unit 60 sets the brightness level value BL mentioned above for adjusting the brightness of the screen (brightness of the display unit 11), as will be described in more detail later. The brightness level value BL set by the brightness level value setting unit 60 is supplied to the display control circuit 20.

[0028] The display control circuit 20 outputs a data-side control signal Scd that controls the operation of the data-side drive circuit 30, a scanning-side control signal Scs that controls the operation of the scanning-side drive circuit 40, and a power supply voltage control signal SV that controls the operation of the power supply circuit 50, based on an input signal Sin provided from an external source (a signal consisting of an input video signal representing the image to be displayed and various timing signals) and a brightness level value BL provided from the brightness level value setting unit 60.

[0029] The data-side drive circuit 30 drives the data signal lines D(1) to D(m) based on the data-side control signal Scd output from the display control circuit 20. That is, the data-side drive circuit 30 generates m data signals representing the image to be displayed based on the data-side control signal Scd, and applies them to the data signal lines D(1) to D(m).

[0030] As described above, the scanning-side drive circuit 40 includes a light emission control line drive circuit 41 and a scanning signal line drive circuit 42. The display control circuit 20 provides the light emission control line drive circuit 41 with an emission start pulse ESP and an emission clock signal ECK as scanning-side control signals Scs. The display control circuit 20 provides the scanning signal line drive circuit 42 with a gate start pulse GSP and a gate clock signal GCK as scanning-side control signals Scs. Based on the emission start pulse ESP and the emission clock signal ECK, the light emission control line drive circuit 41 applies light emission control signals to the light emission control lines EM(1) to EM(n). Based on the gate start pulse GSP and the gate clock signal GCK, the scanning signal line drive circuit 42 applies write control signals to the write control lines SCAN(1) to SCAN(n) and applies initialization control signals to the initialization control lines DIS(0) to DIS(n). As described above, the light emission control line drive circuit 41 drives the light emission control lines EM(1) to EM(n), and the scan signal line drive circuit 42 drives the write control lines SCAN(1) to SCAN(n) and initialization control lines DIS(0) to DIS(n) as scan signal lines.

[0031] Incidentally, the light emission control line drive circuit 41 is composed of a known shift register, and the emission start pulse ESP given to the first stage of the shift register is transferred to the subsequent stage based on the clock operation of the emission clock signal ECK. As a result, as shown in Figure 4, the light emission control lines EM(1) to EM(n) are supplied with a light emission control signal EM that is maintained at a high level for a period of time corresponding to the pulse width of the emission start pulse ESP. In this regard, the period during which the light emission control signal EM is maintained at a high level is shifted by one line at a time, as shown in Figure 4, based on the operation of the shift register that constitutes the light emission control line drive circuit 41.

[0032] The power supply circuit 50 generates a high-level power supply voltage ELVDD, a low-level power supply voltage ELVSS, a first initialization voltage Vini1, and a second initialization voltage Vini2 based on the power supply voltage control signal SV output from the display control circuit 20. The high-level power supply voltage ELVDD is applied to the high-level power supply line, the low-level power supply voltage ELVSS is applied to the low-level power supply line, the first initialization voltage Vini1 is applied to the first initialization power supply line, and the second initialization voltage Vini2 is applied to the second initialization power supply line.

[0033] As described above, data signals are applied to data signal lines D(1) to D(m), write control signals are applied to write control lines SCAN(1) to SCAN(n), initialization control signals are applied to initialization control lines DIS(0) to DIS(n), and light emission control signals are applied to light emission control lines EM(1) to EM(n). Various power supply voltages are applied to the corresponding power lines, thereby displaying an image based on the input video signal on the display unit 11.

[0034] In this embodiment, the first power supply voltage is realized by the high-level power supply voltage ELVDD, the second power supply voltage is realized by the low-level power supply voltage ELVSS, the control node initialization voltage is realized by the first initialization voltage Vini1, and the display element initialization voltage is realized by the second initialization voltage Vini2. Furthermore, the first power supply line is realized by the high-level power supply line, the second power supply line is realized by the low-level power supply line, the control node initialization power supply line is realized by the first initialization power supply line, and the display element initialization power supply line is realized by the second initialization power supply line.

[0035] <2. Pixel Circuit Configuration and Operation> Figure 5 is a circuit diagram showing the configuration of an i-row, j-column pixel circuit 15(i,j). As shown in Figure 5, the pixel circuit 15(i,j) includes one organic EL element (organic light-emitting diode) L1 as a display element (a display element driven by current), seven transistors (typically thin-film transistors) T1 to T7 (first initialization transistor T1, threshold voltage compensation transistor T2, write control transistor T3, drive transistor T4, first light emission control transistor T5, second light emission control transistor T6, second initialization transistor T7), and one holding capacitor Ca. A parasitic capacitance Cp is formed in parallel with the organic EL element L1. The holding capacitor Ca is a capacitive element consisting of two electrodes (first electrode and second electrode). Transistors T1 to T7 are P-channel type transistors. Transistors T1 to T3 and T5 to T7 function as switching elements. The first initialization transistor T1 and the threshold voltage compensation transistor T2 are double-gate transistors having two control terminals (gate terminals).

[0036] The first conduction terminal of the first initialization transistor T1, the second conduction terminal of the threshold voltage compensation transistor T2, the control terminal of the drive transistor T4, and the second electrode of the holding capacitor Ca are all connected to each other via the control node NC.

[0037] For the first initialization transistor T1, the control terminal is connected to the initialization control line DIS(i-1) in the (i-1)th row, the first conduction terminal is connected to the control node NC, and the second conduction terminal is connected to the first initialization power line. The control node initialization transistor is realized by this first initialization transistor T1. For the threshold voltage compensation transistor T2, the control terminal is connected to the write control line SCAN(i) in the ith row, the first conduction terminal is connected to the second conduction terminal of the drive transistor T4 and the first conduction terminal of the second light emission control transistor T6, and the second conduction terminal is connected to the control node NC. For the write control transistor T3, the control terminal is connected to the write control line SCAN(i) in the ith row, the first conduction terminal is connected to the data signal line D(j) in the jth column, and the second conduction terminal is connected to the first conduction terminal of the drive transistor T4 and the second conduction terminal of the first light emission control transistor T5. For the drive transistor T4, the control terminal is connected to the control node NC, the first conduction terminal is connected to the second conduction terminal of the write control transistor T3 and the second conduction terminal of the first light emission control transistor T5, and the second conduction terminal is connected to the first conduction terminal of the threshold voltage compensation transistor T2 and the first conduction terminal of the second light emission control transistor T6.

[0038] For the first light emission control transistor T5, the control terminal is connected to the i-th row light emission control line EM(i), the first conduction terminal is connected to the high-level power line, and the second conduction terminal is connected to the second conduction terminal of the write control transistor T3 and the first conduction terminal of the drive transistor T4. For the second light emission control transistor T6, the control terminal is connected to the i-th row light emission control line EM(i), the first conduction terminal is connected to the first conduction terminal of the threshold voltage compensation transistor T2 and the second conduction terminal of the drive transistor T4, and the second conduction terminal is connected to the first conduction terminal of the second initialization transistor T7 and the anode of the organic EL element L1. For the second initialization transistor T7, the control terminal is connected to the i-th row initialization control line DIS(i), the first conduction terminal is connected to the second conduction terminal of the second light emission control transistor T6 and the anode of the organic EL element L1, and the second conduction terminal is connected to the second initialization power line. The display element initialization transistor is realized by this second initialization transistor T7. For the holding capacitor Ca, the first electrode is connected to the high-level power line, and the second electrode is connected to the control node NC. For the organic EL element L1, the anode is connected to the second conductive terminal of the second light emission control transistor T6 and the first conductive terminal of the second initialization transistor T7, and the cathode is connected to the low-level power line. The anode corresponds to the first terminal, and the cathode corresponds to the second terminal.

[0039] Regarding the configuration of the pixel circuit 15, in the example shown in Figure 5, all seven transistors T1 to T7 are P-channel type transistors. However, it is also possible to adopt a configuration in which all seven transistors T1 to T7 are N-channel type transistors, or a configuration in which P-channel type transistors and N-channel type transistors are mixed.

[0040] Next, while referring to the timing chart shown in FIG. 6, the operation of the pixel circuit 15(i, j) will be described. Regarding FIG. 6, it is assumed that the period before time t01 and the period after time t06 are the light emission periods (the periods during which the organic EL element L1 in the pixel circuit 15(i, j) should be maintained in the light emission state), and the period from time t01 to time t06 is the non-light emission period (the period during which the organic EL element L1 in the pixel circuit 15(i, j) should be maintained in the non-light emission state).

[0041] Just before time t01, the initialization control signal DIS(i - 1), the initialization control signal DIS(i), and the write control signal SCAN(i) are at a high level, and the light emission control signal EM(i) is at a low level. At this time, the first initialization transistor T1, the threshold voltage compensation transistor T2, the write control transistor T3, and the second initialization transistor T7 are in an off state, and the first light emission control transistor T5 and the second light emission control transistor T6 are in an on state. Therefore, the organic EL element L1 emits light according to the magnitude of the drive current.

[0042] When time t01 arrives, the light emission control signal EM(i) changes from a low level to a high level. As a result, the first light emission control transistor T5 and the second light emission control transistor T6 become off states. Consequently, the supply of current to the organic EL element L1 is cut off, and the organic EL element L1 becomes a non-light emission state.

[0043] When time t02 arrives, the initialization control signal DIS(i - 1) changes from a high level to a low level. As a result, the first initialization transistor T1 becomes an on state. Consequently, the control node NC is initialized (the holding capacitor Ca is initialized) by the first initialization voltage Vini1.

[0044] When time t03 arrives, the initialization control signal DIS(i - 1) changes from a low level to a high level. As a result, the first initialization transistor T1 becomes an off state.

[0045] At time t04, the initialization control signal DIS(i) changes from a high level to a low level. This turns on the second initialization transistor T7, and the second initialization voltage Vini2 is applied to the anode of the organic EL element L1. That is, the anode of the organic EL element L1 is initialized. Also at time t04, the write control signal SCAN(i) changes from a high level to a low level. This turns on the write control transistor T3 and the threshold voltage compensation transistor T2, and the drive transistor T4 enters a diode connection state. As a result, the voltage of the data signal D(j) is applied as the data voltage to the holding capacitor Ca via the diode-connected drive transistor T4. This writes the threshold-compensated data voltage to the holding capacitor Ca, and the gate voltage of the drive transistor T4 is maintained at the voltage of the second electrode of the holding capacitor Ca. If the data voltage is represented by Vdata and the threshold voltage of the drive transistor T4 is represented by Vth (<0), then the gate voltage Vg at this time is expressed by the following equation (1). Vg = Vdata + Vth ... (1) In this way, during the period from time t04 to time t05, data voltages are written to the pixel circuit 15(i,j) while performing internal compensation.

[0046] At time t05, the initialization control signal DIS(i) changes from a low level to a high level. This turns off the second initialization transistor T7. Also at time t05, the write control signal SCAN(i) changes from a low level to a high level. This turns off the write control transistor T3 and the threshold voltage compensation transistor T2.

[0047] At time t06, the light emission control signal EM(i) changes from a high level to a low level. This turns on the first light emission control transistor T5 and the second light emission control transistor T6. As a result, a drive current corresponding to the gate voltage of the drive transistor T4 is supplied to the organic EL element L1, and the organic EL element L1 emits light according to the magnitude of the drive current.

[0048] In addition, in the present embodiment, as will be described later, the light emission duty ratio changes according to the brightness level value BL set by the brightness level value setting unit 60. Therefore, the length of the period during which the light emission control signal EM(i) is maintained at the high level and the length of the period during which the light emission control signal EM(i) is maintained at the low level are variable.

[0049] <3. Brightness adjustment> <3.1 Configuration related to brightness adjustment function>. FIG. 7 is a block diagram showing a configuration for realizing the brightness adjustment function in the present embodiment. As shown in FIG. 7, in the present embodiment, the brightness adjustment function is realized by some components in the display control circuit 20 and the brightness level value setting unit 60.

[0050] The brightness level value setting unit 60 is composed of an illuminance sensor 610, a brightness level value adjustment unit 620, a selection unit 630, a switching unit 640, and a brightness level value determination unit 650. The illuminance sensor 610 detects the illuminance in the vicinity of the display unit 11. The brightness level value adjustment unit 620 displays, on the display unit 11, a brightness adjustment screen 621 such as that shown in FIG. 8 for example, to allow the operator to set the brightness level value BL, and accepts an operation by the operator. The brightness adjustment screen 621 includes a setting bar 622 and a button unit 623 that can be moved on the setting bar 622. The operator can adjust the brightness level value BL by moving the button unit 623 left and right on the setting bar 622.

[0051] The selection unit 630 accepts the operator's selection as to whether to automatically adjust the brightness level value BL or manually (the operator's selection as to whether to turn the illuminance sensor 610 on or off). In other words, in this embodiment, the operator can choose whether to automatically adjust the brightness level value BL or manually. The switching unit 640 switches the connection destination of the brightness level value determination unit 650 between the illuminance sensor 610 and the brightness level value adjustment unit 620. In this regard, if the selection unit 630 selects to automatically adjust the brightness level value BL (turn on the illuminance sensor 610), the switching unit 640 connects the brightness level value determination unit 650 and the illuminance sensor 610. If the selection unit 630 selects to manually adjust the brightness level value BL (turn off the illuminance sensor 610), the switching unit 640 connects the brightness level value determination unit 650 and the brightness level value adjustment unit 620.

[0052] The brightness level value determination unit 650 determines the brightness level value BL according to the connection destination via the switching unit 640. When the brightness level value determination unit 650 is connected to the illuminance sensor 610, the brightness level value BL is determined based on the illuminance detected by the illuminance sensor 610, and this brightness level value BL is provided to the display control circuit 20. When the brightness level value determination unit 650 is connected to the brightness level value adjustment unit 620, the brightness level value BL set by the operator using the brightness level value adjustment unit 620 is provided to the display control circuit 20.

[0053] The display control circuit 20 includes a brightness control unit 200, which consists of a light emission duty cycle control unit 210, an emission start pulse generation unit 212, an initialization voltage control unit 220, and a low-level power supply voltage control unit 230, as components for realizing the brightness adjustment function.

[0054] The light emission duty cycle control unit 210 determines the light emission duty cycle DU according to the brightness level value BL. In this regard, the light emission duty cycle control unit 210 holds, for example, a calculation formula for calculating the light emission duty cycle DU from the brightness level value BL, or a lookup table that associates the brightness level value BL with the light emission duty cycle DU. The light emission duty cycle DU is then determined from the brightness level value BL set by the brightness level value setting unit 60 and the calculation formula or the lookup table.

[0055] The emission start pulse generation unit 212 generates an emission start pulse ESP based on the emission duty cycle DU determined by the emission duty cycle control unit 210. In this embodiment, the pulse width of the emission start pulse ESP corresponds to the length of the non-emission period. Therefore, the smaller the emission duty cycle DU, the longer the pulse width of the emission start pulse ESP, and the larger the emission duty cycle DU, the shorter the pulse width of the emission start pulse ESP.

[0056] The initialization voltage control unit 220 determines the voltage value of the second initialization voltage Vini2 according to the brightness level value BL, and outputs an initialization voltage control signal SINI as a power supply voltage control signal SV so that the second initialization voltage Vini2 with that voltage value is generated in the power supply circuit 50.

[0057] The low-level power supply voltage control unit 230 determines the voltage value of the low-level power supply voltage ELVSS according to the brightness level value BL, and outputs a low-level power supply voltage control signal SVSS as a power supply voltage control signal SV so that the low-level power supply voltage ELVSS with that voltage value is generated in the power supply circuit 50. As will be described later, in this embodiment, there are cases in which the voltage value of the low-level power supply voltage ELVSS is maintained at a constant value regardless of changes in the brightness level value BL, and cases in which the voltage value of the low-level power supply voltage ELVSS may change according to the brightness level value BL. In the case in which the voltage value of the low-level power supply voltage ELVSS is maintained at a constant value regardless of changes in the brightness level value BL, it is not necessary to provide the low-level power supply voltage control unit 230 as described above.

[0058] In this embodiment, the second power supply voltage control unit is implemented by the low-level power supply voltage control unit 230. Furthermore, the light emission duty cycle control step is implemented by the operation of the light emission duty cycle control unit 210, and the initialization voltage control step is implemented by the operation of the initialization voltage control unit 220.

[0059] <3.2 Brightness Adjustment Method> Next, we will explain how the brightness is actually adjusted based on the brightness level value BL set in the brightness level value setting unit 60. Note that the brightness level value BL changes, for example, when the operator makes an adjustment using the brightness adjustment screen 621 or when there is a change in the illuminance detected by the illuminance sensor 610.

[0060] In this embodiment, the relationship between the light emission duty cycle DU, the voltage value of the second initialization voltage Vini2, and the signal value of the data signal, and the brightness level value BL is controlled according to the brightness level value BL, as shown in Figure 1. In Figure 1, the thick dotted line labeled 71, the thick solid line labeled 72, and the solid line labeled 73 indicate how the light emission duty cycle DU, the voltage value of the second initialization voltage Vini2, and the signal value of the data signal are changed relatively in response to the change in the brightness level value BL, respectively.

[0061] The luminescence duty cycle DU is larger when the brightness level value BL is large, and smaller when the brightness level value BL is small. Thus, in this embodiment, a method is employed to change the luminescence duty cycle DU according to the brightness level value BL as a method to realize brightness adjustment. The voltage value of the second initialization voltage Vini2 is smaller when the brightness level value BL is large, and larger when the brightness level value BL is small, in order to suppress a decrease in display quality in the low-gradation region. As can be seen from Figure 1, in this embodiment, the relationship between the brightness level value BL and the voltage value of the second initialization voltage Vini2 is linear. Note that the voltage value of the second initialization voltage Vini2 when the brightness level value BL is at its maximum value corresponds to the voltage value of the conventional second initialization voltage Vini2. The signal value of the data signal (here, we focus on the data signal corresponding to a certain gradation value) is maintained at a constant value (magnitude) regardless of the change in the brightness level value BL.

[0062] Furthermore, the specific relationship between the brightness level value BL, the voltage value of the second initialization voltage Vini2, and the voltage value of the low-level power supply voltage ELVSS is basically as shown in Figure 9. In Figure 9, the thick solid line labeled 78 and the thick dotted line labeled 79 indicate how the voltage values ​​of the second initialization voltage Vini2 and the low-level power supply voltage ELVSS are changed relatively in response to the change in the brightness level value BL (the same applies to Figure 10). As shown in Figure 9, basically, the voltage value of the low-level power supply voltage ELVSS is maintained at a constant value regardless of the change in the brightness level value BL. However, in the range of values ​​that the brightness level value BL can take, in the range where the voltage value of the second initialization voltage Vini2 is greater than the voltage value of the low-level power supply voltage ELVSS, depending on the model, when the difference between the voltage value of the second initialization voltage Vini2 and the voltage value of the low-level power supply voltage ELVSS becomes large, current may flow through the organic EL element L1 due to the forward voltage between the anode and cathode of the organic EL element L1, causing the organic EL element L1 to emit light unnecessarily. In other words, depending on the model, phenomena occur in which the organic EL element L1 emits light even though the grayscale value indicated by the signal value of the data signal is 0, or in which the organic EL element L1 emits light at a brightness higher than the brightness corresponding to the grayscale value indicated by the signal value of the data signal in the low-grayscale region (hereinafter, such phenomena are referred to as "black floating"). Therefore, for such models, the relationship between the brightness level value BL, the voltage value of the second initialization voltage Vini2, and the voltage value of the low-level power supply voltage ELVSS is set to the relationship shown in Figure 10. More specifically, in the range where black level floating does not occur (the range of brightness level value BL), the voltage value of the low-level power supply voltage ELVSS is maintained at a constant value. In the range where black level floating occurs, typically, the voltage value of the low-level power supply voltage ELVSS is set to a higher value as the brightness level value BL decreases, so that the difference between the voltage value of the second initialization voltage Vini2 and the voltage value of the low-level power supply voltage ELVSS remains constant.

[0063] As described above, the light emission duty cycle control unit 210 controls the light emission duty cycle DU such that the light emission duty cycle DU increases as the brightness represented by the brightness level value BL increases. In addition, the initialization voltage control unit 220 controls the voltage value of the second initialization voltage Vini2 such that the voltage value of the second initialization voltage Vini2 increases as the brightness represented by the brightness level value BL decreases. In other words, the initialization voltage control unit 220 controls the voltage value of the second initialization voltage Vini2 such that the difference between the voltage value of the second initialization voltage Vini2 and the voltage value of the high-level power supply voltage ELVDD decreases as the brightness represented by the brightness level value BL decreases. Furthermore, in models where black level floating does not occur, the initialization voltage control unit 220 controls the voltage value of the second initialization voltage Vini2 such that the difference between the voltage value of the second initialization voltage Vini2 and the voltage value of the low-level power supply voltage ELVSS increases as the brightness represented by the brightness level value BL decreases. In models where black level distortion may occur, if the brightness level value BL is greater than a predetermined value, the low-level power supply voltage control unit 230 maintains the voltage value of the low-level power supply voltage ELVSS at a constant value. If the brightness level value BL is less than or equal to the predetermined value, the low-level power supply voltage control unit 230 controls the voltage value of the low-level power supply voltage ELVSS so that the difference between the voltage value of the low-level power supply voltage ELVSS and the voltage value of the high-level power supply voltage ELVDD becomes smaller as the brightness represented by the brightness level value BL becomes dimmer.

[0064] As described above, in this embodiment, the smaller the brightness level value BL, the larger the voltage value of the second initialization voltage Vini2, which serves as the anode reset voltage. Therefore, unlike in the conventional method, immediately after the anode of the organic EL element L1 is initialized, the parasitic capacitance Cp holds charge according to the voltage value of the second initialization voltage Vini2. Specifically, the larger the voltage value of the second initialization voltage Vini2, the more charge is held in the parasitic capacitance Cp. Since the light emission duty cycle DU is set to a smaller value as the brightness level value BL is smaller, the smaller the light emission duty cycle DU, the more charge is held in the parasitic capacitance Cp. When charge is held in the parasitic capacitance Cp immediately after the anode of the organic EL element L1 is initialized, the current flowing through the parasitic capacitance Cp becomes smaller and the current flowing through the organic EL element L1 becomes larger compared to the conventional method.

[0065] From the above, if the anode of the organic EL element L1 is initialized by a sufficiently high voltage value of the second initialization voltage Vini2 and a sufficient charge is held in the parasitic capacitance Cp immediately afterward, then in the case where the signal value of the data signal is the same as in the example shown in Figure 43, the current flowing through the organic EL element L1 and the current flowing through the parasitic capacitance Cp will change as shown in Figure 11, for example. In Figure 11, the thick solid line denoted by reference numeral 75 represents the change in the current flowing through the organic EL element L1, and the thick dotted line denoted by reference numeral 76 represents the change in the current flowing through the parasitic capacitance Cp (the same applies to Figure 12). Comparing Figure 11 and Figure 43, it can be seen that, according to this embodiment, unlike the conventional, most of the current supplied via the drive transistor T4 flows through the organic EL element L1 immediately after the anode of the organic EL element L1 is reset. Here, if the light emission duty cycle DU becomes 30% based on the brightness level value BL, then 30% of the frame period becomes the light emission period. In this case, the brightness that should be obtained corresponds to the area of ​​the rectangular portion indicated by the thick dashed line labeled 78 in Figure 12, whereas the brightness actually obtained corresponds to the area of ​​the shaded portion labeled 77 in Figure 12. From Figure 12, it can be seen that a brightness close to the brightness that should be obtained is actually obtained, and that there is almost no effect of brightness reduction caused by the current flowing through the parasitic capacitance Cp. Thus, even if the luminescence duty cycle DU is small, the aforementioned black crushing does not occur.

[0066] <4. Experimental Results> Here, we will explain the experimental results of measuring the gamma characteristics and chromaticity using three different voltages (-4.0V, -3.5V, -3.0V) in sequence as the second initialization voltage Vini2 with the emission duty cycle set to 53%. The experimental results for the case where the voltage value of the second initialization voltage Vini2 is set to -4.0V are shown in Figures 13 to 15, the experimental results for the case where the voltage value of the second initialization voltage Vini2 is set to -3.5V are shown in Figures 16 to 18, and the experimental results for the case where the voltage value of the second initialization voltage Vini2 is set to -3.0V are shown in Figures 19 to 21. Figures 13, 16, and 19 show the gamma characteristics of W (white), Figures 14, 17, and 20 show the gamma characteristics of RGB (red, green, blue), and Figures 15, 18, and 21 show the chromaticity.

[0067] In Figures 13, 16, and 19, the thick dotted line labeled 80 represents the gamma characteristics corresponding to "gamma value = 2.2", the solid line labeled 81 represents the gamma characteristics of W obtained experimentally, and the shaded area labeled 58 represents the grayscale range in which black crushing is actually visible. In addition, in Figures 14, 17, and 20, the thick dotted line labeled 82 represents the gamma characteristics corresponding to "gamma value = 2.2", the dotted line labeled 83 represents the gamma characteristics of R obtained experimentally, the thick solid line labeled 84 represents the gamma characteristics of G obtained experimentally, and the solid line labeled 85 represents the gamma characteristics of B obtained experimentally. Furthermore, in Figures 15, 18, and 21, the thick dotted line denoted by reference numeral 86 represents chromaticity x, the thick solid line denoted by reference numeral 87 represents chromaticity y, and the shaded area denoted by reference numeral 59 represents the grayscale value range in which color shift (coloring) is actually visible. The same applies to Figures 22 to 36.

[0068] According to the experimental results, black crushing and color shift occurred when the voltage value of the second initialization voltage Vini2 was set to -4.0V and when the voltage value of the second initialization voltage Vini2 was set to -3.5V, but neither black crushing nor color shift occurred when the voltage value of the second initialization voltage Vini2 was set to -3.0V. Therefore, based on these experimental results, it is preferable to set the voltage value of the second initialization voltage Vini2 to -3.0V when the light emission duty cycle is 53%.

[0069] Next, we will explain the experimental results of measuring gamma characteristics and chromaticity using five different voltages (-4.0V, -3.5V, -3.0V, -2.5V, -2.0V) in sequence as the second initialization voltage Vini2 with the emission duty cycle set to 8%. The experimental results for the case where the voltage value of the second initialization voltage Vini2 is set to -4.0V are shown in Figures 22 to 24, the experimental results for the case where the voltage value of the second initialization voltage Vini2 is set to -3.5V are shown in Figures 25 to 27, the experimental results for the case where the voltage value of the second initialization voltage Vini2 is set to -3.0V are shown in Figures 28 to 30, the experimental results for the case where the voltage value of the second initialization voltage Vini2 is set to -2.5V are shown in Figures 31 to 33, and the experimental results for the case where the voltage value of the second initialization voltage Vini2 is set to -2.0V are shown in Figures 34 to 36. Figures 22, 25, 28, 31, and 34 show the gamma characteristics of W (white), Figures 23, 26, 29, 32, and 35 show the gamma characteristics of RGB (red, green, and blue), and Figures 24, 27, 30, 33, and 36 show the chromaticity.

[0070] According to the experimental results, when the voltage value of the second initialization voltage Vini2 was set to -4.0V to -2.5V, black crushing and color shift occurred, but when the voltage value of the second initialization voltage Vini2 was set to -2.0V, neither black crushing nor color shift occurred. Therefore, based on these experimental results, it is preferable to set the voltage value of the second initialization voltage Vini2 to -2.0V when the light emission duty cycle is 8%.

[0071] The above describes experimental results only for cases where the light emission duty cycle is set to 53% and cases where the light emission duty cycle is set to 8%. However, it would be sufficient to determine the correspondence between the light emission duty cycle and the voltage value of the second initialization voltage Vini2 based on the results of experiments conducted with the light emission duty cycle set to various values.

[0072] <5. Effects> According to this embodiment, under control by the light emission duty cycle control unit 210, the brighter the brightness represented by the brightness level value BL set in the brightness level value setting unit 60, the larger the light emission duty cycle DU becomes, and the darker the brightness represented by the brightness level value BL, the smaller the light emission duty cycle DU becomes. In this configuration, under control by the initialization voltage control unit 220, the voltage value of the second initialization voltage (anode reset voltage) Vini2 for initializing the anode of the organic EL element L1 becomes larger the darker the brightness represented by the brightness level value BL. That is, the smaller the light emission duty cycle DU, the larger the voltage value of the second initialization voltage Vini2 becomes. Therefore, when the light emission duty cycle DU is small, immediately after the anode of the organic EL element L1 is initialized, a large amount of charge is held in the parasitic capacitance Cp formed in parallel with the organic EL element L1. Consequently, compared to the conventional method, the current flowing through the parasitic capacitance Cp becomes smaller, and the current flowing through the organic EL element L1 becomes larger. As a result, even in the low-gradation region, brightness close to the brightness that should be obtained can be achieved. Therefore, according to this embodiment, in an organic EL display device 10 that adjusts the brightness of the screen by changing the light emission duty cycle DU, it is possible to suppress the decrease in brightness in the low-gradation region and the resulting color shift.

[0073] <6. Modifications> Modifications of the above embodiment will be described below.

[0074] <6.1 First Modification> In the above embodiment, the pixel circuit 15 with the configuration shown in Figure 5 was used. That is, the control node NC (gate of the drive transistor T4) and the anode of the organic EL element L1 were initialized with different voltages. In contrast, in this modification, the control node NC and the anode of the organic EL element L1 are initialized with the same voltage. That is, one initialization voltage is used to initialize the inside of the pixel circuit 15. Hereafter, this initialization voltage will be denoted by the symbol Vini.

[0075] In this modified example, unlike the configuration shown in Figure 2, the power supply circuit 50 generates a high-level power supply voltage ELVDD, a low-level power supply voltage ELVSS, and one initialization voltage Vini. The initialization voltage Vini is supplied to the pixel circuit 15 by an initialization power line. The initialization voltage Vini realizes the display element initialization voltage, and the initialization power line realizes the display element initialization power line.

[0076] Figure 37 is a circuit diagram showing the configuration of the i-row, j-column pixel circuit 15(i,j) in this modified example. In this modified example, the second conductive terminal of the first initialization transistor T1 and the second conductive terminal of the second initialization transistor T7 are connected to the initialization power line. In this configuration, the control node NC is initialized by the initialization voltage Vini during the period when the initialization control signal DIS(i-1) is maintained at a low level (the period from time t02 to time t03 in Figure 6), and the anode of the organic EL element L1 is initialized by the initialization voltage Vini during the period when the initialization control signal DIS(i) is maintained at a low level (the period from time t04 to time t05 in Figure 6).

[0077] Under the above configuration, the voltage value of the initialization voltage Vini is controlled in the same way as the voltage value of the second initialization voltage Vini2 in the above embodiment. That is, as shown by the thick solid line labeled 72 in Figure 1, the voltage value of the initialization voltage Vini becomes smaller as the brightness level value BL is larger, and larger as the brightness level value BL is smaller. As a result, the deterioration of display quality in the low-gradation region is suppressed, similar to the above embodiment.

[0078] Furthermore, according to this modification, the smaller the brightness level value BL, the higher the initialization voltage Vini of the control node NC will be. Therefore, this modification can only be adopted if the performance of initializing the control node NC is not impaired even if only one initialization voltage Vini is used as the voltage to initialize the inside of the pixel circuit 15.

[0079] <6.2 Second Modification> In the above embodiment, a method was employed to implement brightness adjustment by changing the light emission duty cycle DU according to the brightness level value BL set in the brightness level value setting unit 60. However, the method for implementing brightness adjustment is not limited to this. In this modification, a method is employed to implement brightness adjustment by changing the signal value of the data signal and the light emission duty cycle DU according to the brightness level value BL.

[0080] In this modified example, the luminescence duty cycle DU is controlled to be larger as the brightness level value BL increases, and smaller as the brightness level value BL decreases, similar to the above embodiment. The signal value of the data signal is controlled to be larger as the brightness level value BL increases, and smaller as the brightness level value BL decreases. To achieve this, in this modified example, as shown in Figure 38, a data-side control signal generation unit 240 that generates the data-side control signal Scod is included in the brightness control unit 200 within the display control circuit 20. The data-side control signal generation unit 240 controls the signal value of the data signal so that the brightness represented by the brightness level value BL increases, the brightness corresponding to the signal value of the data signal increases. In this modified example, the data signal value control unit is realized by this data-side control signal generation unit 240.

[0081] Under the configuration described above, in this modified example, as in the above embodiment, the voltage value of the second initialization voltage Vini2 is controlled such that it becomes smaller as the brightness level value BL is larger, and larger as the brightness level value BL is smaller. This provides the same effects as in the above embodiment.

[0082] <7. Others> In the above embodiments and modifications, an organic EL display device was used as an example for explanation, but the above disclosure can be applied to display devices other than organic EL display devices, as long as they use a display element driven by electric current. Examples of display elements driven by electric current include the organic EL element, i.e., organic light-emitting diode, mentioned above, as well as inorganic light-emitting diodes and quantum dot light-emitting diodes (QLEDs).

[0083] 10...Organic EL display device 11...Display unit 15...Pixel circuit 20...Display control circuit 30...Data side drive circuit (data signal line drive circuit) 41...Light emission control line drive circuit 60...Brightness level value setting unit 200...Brightness control unit 210...Light emission duty cycle control unit 212...Emission start pulse generation unit 220...Initialization voltage control unit 230...Low level power supply voltage control unit 610...Illuminance sensor 620...Brightness level value adjustment unit 630...Selection unit 640...Switching unit 650...Brightness level value determination unit BL...Brightness level value Cp...Parasitic capacitance DU...Light emission duty cycle L1...Organic EL element T1...First initialization transistor T2...Threshold voltage compensation transistor T3...Write control transistor T4...Drive transistor T5...First light emission control transistor T6...Second light emission control transistor T7...Second initialization transistor

Claims

1. A display device using a display element having a first terminal and a second terminal and driven by current, comprising: a display unit including a plurality of data signal lines for transmitting data signals, a plurality of scan signal lines intersecting the plurality of data signal lines, a plurality of light emission control lines intersecting the plurality of data signal lines, a plurality of pixel circuits each corresponding to one of the plurality of data signal lines, at least one of the plurality of scan signal lines, and one of the plurality of light emission control lines, a first power line to which a first power supply voltage is supplied, a second power line to which a second power supply voltage is supplied, and a display element initialization power line to which a display element initialization voltage is supplied for initializing the first terminal of the display element, a data signal line driving circuit for applying the data signals to the plurality of data signal lines, a scan signal line driving circuit for driving the plurality of scan signal lines, a light emission control line driving circuit for driving the plurality of light emission control lines, a power supply circuit for generating the first power supply voltage, the second power supply voltage, and the display element initialization voltage, and a brightness control unit for adjusting the brightness of the display unit based on a brightness level value, wherein each of the plurality of pixel circuits is The brightness control unit includes: a display element provided between the first power line and the second power line, having a first terminal on the first power line side and a second terminal on the second power line side; a control node; a drive transistor provided in series with the display element, having a control terminal connected to the control node, a first conductive terminal on the first power line side, and a second conductive terminal on the second power line side; at least one light emission control transistor as a switching element provided in series with the display element and the drive transistor, having a control terminal connected to a corresponding light emission control line; a display element initialization transistor as a switching element, having a control terminal connected to a corresponding scan signal line, a first conductive terminal connected to the first terminal of the display element, and a second conductive terminal connected to the display element initialization power line; and the brightness control unit includes a light emission duty cycle control unit that controls a light emission duty cycle indicating the proportion of the period during which the at least one light emission control transistor is kept in the ON state within one frame period, based on the brightness level value.A display device comprising: an initialization voltage control unit that controls the voltage value of the display element initialization voltage generated by the power supply circuit based on the brightness level value; the light emission duty cycle control unit controls the light emission duty cycle such that the light emission duty cycle increases as the brightness represented by the brightness level value increases; and the initialization voltage control unit controls the voltage value of the display element initialization voltage such that the difference between the voltage value of the display element initialization voltage and the voltage value of the first power supply voltage decreases as the brightness represented by the brightness level value decreases.

2. The display device according to claim 1, characterized in that the initialization voltage control unit controls the voltage value of the display element initialization voltage such that the difference between the voltage value of the display element initialization voltage and the voltage value of the second power supply voltage increases as the brightness represented by the brightness level value becomes dimmer.

3. The display device according to claim 1, wherein the brightness control unit further includes a second power supply voltage control unit that controls the voltage value of the second power supply voltage generated by the power supply circuit based on the brightness level value, and if the brightness level value is greater than a predetermined value, the second power supply voltage control unit maintains the voltage value of the second power supply voltage at a constant value, and if the brightness level value is less than or equal to the predetermined value, the second power supply voltage control unit controls the voltage value of the second power supply voltage such that the difference between the voltage value of the second power supply voltage and the voltage value of the first power supply voltage becomes smaller the dimmer the brightness represented by the brightness level value.

4. The display device according to claim 3, characterized in that, if the brightness level value is less than or equal to the predetermined value, the second power supply voltage control unit controls the voltage value of the second power supply voltage so that the difference between the voltage value of the second power supply voltage and the voltage value of the display element initialization voltage becomes constant.

5. The display device according to claim 1, wherein the power supply circuit further generates a control node initialization voltage for initializing the control node, the display unit further includes a control node initialization power line to which the control node initialization voltage is supplied, and each of the plurality of pixel circuits further includes a control node initialization transistor as a switching element having a control terminal connected to a corresponding scan signal line, a first conductive terminal connected to the control node, and a second conductive terminal connected to the control node initialization power line.

6. The display device according to claim 1, wherein each of the plurality of pixel circuits further includes a control node initialization transistor as a switching element having a control terminal connected to a corresponding scanning signal line, a first conductive terminal connected to the control node, and a second conductive terminal connected to the display element initialization power line.

7. The display device according to claim 1, characterized in that the initialization voltage control unit controls the voltage value of the display element initialization voltage so that the relationship between the brightness level value and the voltage value of the display element initialization voltage is linear.

8. The display device according to claim 1, further comprising an illuminance sensor for detecting the illuminance near the display unit, wherein the brightness level value is determined based on the illuminance detected by the illuminance sensor.

9. The display device according to claim 1, further comprising a brightness level value adjustment unit for the operator to adjust the brightness level value.

10. The display device according to claim 1, further comprising: an illuminance sensor for detecting the illuminance near the display unit; a brightness level value adjustment unit for the operator to adjust the brightness level value; and a selection unit for selecting whether to automatically adjust the brightness level value or manually, wherein when the selection unit selects to automatically adjust the brightness level value, the brightness control unit adjusts the brightness of the display unit based on a brightness level value determined based on the illuminance detected by the illuminance sensor; and when the selection unit selects to manually adjust the brightness level value, the brightness control unit adjusts the brightness of the display unit based on a brightness level value determined by the operator using the brightness level value adjustment unit.

11. The display device according to claim 1, further comprising a data signal value control unit that controls the signal value of the data signal such that the brightness corresponding to the signal value of the data signal increases as the brightness represented by the brightness level value increases.

12. The display device according to claim 1, wherein the at least one light emission control transistor includes a first light emission control transistor having a control terminal connected to the corresponding light emission control line, a first conductive terminal connected to the first power supply line, and a second conductive terminal connected to the first conductive terminal of the drive transistor, and a second light emission control transistor having a control terminal connected to the corresponding light emission control line, a first conductive terminal connected to the second conductive terminal of the drive transistor, and a second conductive terminal connected to the second power supply line via the display element.

13. The display device according to claim 1, characterized in that the first power supply voltage is a high-level power supply voltage, the second power supply voltage is a low-level power supply voltage, and the initialization voltage control unit controls the voltage value of the display element initialization voltage such that the voltage value of the display element initialization voltage increases as the brightness represented by the brightness level value becomes dimmer.

14. The display device according to any one of claims 1 to 13, characterized in that the display element is an organic light-emitting diode, an inorganic light-emitting diode, and a quantum dot light-emitting diode.

15. A method for driving a display device using a display element having a first terminal and a second terminal and driven by current, wherein the display device includes a display unit including a plurality of data signal lines for transmitting data signals, a plurality of scan signal lines intersecting the plurality of data signal lines, a plurality of light emission control lines intersecting the plurality of data signal lines, a plurality of pixel circuits each corresponding to one of the plurality of data signal lines, at least one of the plurality of scan signal lines and one of the plurality of light emission control lines, a first power line to which a first power supply voltage is supplied, a second power line to which a second power supply voltage is supplied, and a display element initialization power line to which a display element initialization voltage is supplied for initializing the first terminal of the display element, a data signal line driving circuit for applying the data signals to the plurality of data signal lines, a scan signal line driving circuit for driving the plurality of scan signal lines, a light emission control line driving circuit for driving the plurality of light emission control lines, and a power supply circuit for generating the first power supply voltage, the second power supply voltage and the display element initialization voltage, wherein each of the plurality of pixel circuits is The driving method includes: a display element provided between a first power line and a second power line, having a first terminal on the first power line side and a second terminal on the second power line side; a control node; a drive transistor provided in series with the display element, having a control terminal connected to the control node, a first conductive terminal on the first power line side, and a second conductive terminal on the second power line side; at least one light emission control transistor as a switching element provided in series with the display element and the drive transistor, having a control terminal connected to a corresponding light emission control line; and a display element initialization transistor as a switching element, having a control terminal connected to a corresponding scan signal line, a first conductive terminal connected to the first terminal of the display element, and a second conductive terminal connected to the display element initialization power line; the driving method includes: a light emission duty cycle control step of controlling a light emission duty cycle indicating the proportion of the period during which the at least one light emission control transistor is kept in the ON state within one frame period, based on a defined brightness level value; and an initialization voltage control step of controlling the voltage value of the display element initialization voltage generated by the power supply circuit, based on the brightness level value.A driving method characterized in that, in the light emission duty cycle control step, the light emission duty cycle is controlled such that the light emission duty cycle increases as the brightness represented by the brightness level value increases, and in the initialization voltage control step, the voltage value of the display element initialization voltage is controlled such that the difference between the voltage value of the display element initialization voltage and the voltage value of the first power supply voltage decreases as the brightness represented by the brightness level value decreases.

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