Display device

Abstract

It has: a light-emitting element having a pixel portion arranged in a matrix, where the luminance of each pixel portion varies according to the magnitude of the current and the input period of the current; and a pixel circuit portion that causes the light-emitting element to emit light with a luminance corresponding to the gray scale obtained from the image signal. The pixel circuit portion has: a first driving portion that drives the light-emitting element when the gray scale of the image is in the range of gray scales below the first boundary value and does not drive the light-emitting element when the gray scale of the image is in the range of gray scales higher than the first boundary value; and a second driving portion that does not drive the light-emitting element when the gray scale of the image is in the range of gray scales below the second boundary value and drives the light-emitting element when the gray scale of the image is in the range of gray scales higher than the second boundary value.

Description

Technical Field

[0001] The present disclosure relates to a display device in which a pixel portion having a self-luminous light-emitting element such as a light emitting diode (LED) and organic electroluminescence (EL) is arranged in a matrix, and image data is written into each pixel portion by a selection signal and a light emission signal, and the display is performed with high definition and high contrast. Background Art

[0002] Display devices of the prior art are described, for example, in Patent Documents 1 and 2.

[0003] Prior Art Documents

[0004] Patent Documents

[0005] Patent Document 1: Japanese Patent Application Laid-Open No. 2003-058106

[0006] Patent Document 2: Japanese Patent Application Laid-Open No. 2004-133240 Summary of the Invention

[0007] The display device of the present disclosure has the following structure: a light-emitting element having a plurality of pixel portions arranged in a matrix, the brightness of each of the plurality of pixel portions varying according to the magnitude of a current and the input period of the current; and a pixel circuit portion that causes the light-emitting element to emit light with a brightness corresponding to a gradation obtained from an image signal.

[0008] The pixel circuit portion includes:

[0009] a first driving portion that drives the light-emitting element when the gradation obtained from the image signal is in a range of gradations below a first boundary value, and does not drive the light-emitting element when the gradation obtained from the image signal is in a range of gradations higher than the first boundary value; and

[0010] a second driving portion that does not drive the light-emitting element when the gradation obtained from the image signal is in a range of gradations below a second boundary value lower than the first boundary value, and drives the light-emitting element when the gradation obtained from the image signal is in a range of gradations higher than the second boundary value. Brief Description of the Drawings

[0011] According to the following detailed description and the drawings, the objects, features, and advantages of the present disclosure will become clearer.

[0012] Figure 1 It is an electrical circuit diagram showing a schematic structure of a display device according to an embodiment of the present disclosure.

[0013] Figure 2 It represents Figure 1 an electrical circuit diagram of the structure of the pixel portion of the display device shown

[0014] Figure 3 a graph showing the relationship before synthesis between the current input to the light-emitting element and the gray scale

[0015] Figure 4 a graph showing the relationship between the current output to the light-emitting element and the emission wavelength of the light-emitting element

[0016] Figure 5 It is used to explain Figure 1 a timing chart of the operation of the display device shown

[0017] Figure 6 a graph showing the relationship between the gray scale and the emission wavelength of the light-emitting element when the light-emitting element is driven by a drive signal obtained by synthesizing the drive signal of the first drive unit and the drive signal of the second drive unit in the boundary region between low gray scale and high gray scale

[0018] Figure 7 It represents Figure 1 an electrical circuit diagram of other structures of the pixel portion of the display device shown Detailed implementation manners

[0019] First, a display device having a structure that is the basis of the display device of the present disclosure will be described

[0020] The display device having a structure that is the basis of the display device of the present disclosure includes n×m pixels (n is the number of rows, m is the number of columns, both are positive integers) arranged in a matrix, n gate signal lines, m source signal lines, a gate signal line drive circuit, and a source signal line drive circuit. The source signal (image signal) supplied from the source signal line drive circuit is written into the pixel portion that becomes a selected state according to the gate signal (pixel selection signal) supplied via the n gate signal lines from the gate signal line drive circuit via the m source signal lines

[0021] Each pixel portion includes a pixel circuit as a light emission control portion. The light emission control portion has a thin film transistor (TFT). The light emission control signal transmitted in n light emission control signal lines arranged in parallel with the gate signal lines is supplied to the light-emitting element via the pixel circuit. Through this light emission control signal, the adjustment of the brightness of the entire display area and the on / off control of the display are performed. One of these gate signal lines, source signal lines, and light emission control signal lines is allocated to each pixel portion

[0022] Each of the aforementioned pixel portions includes a light-emitting element and a light-emitting control unit. The light-emitting control unit has: a light-emitting control element that controls the on / off of the light-emitting element through a light-emitting control signal; a switching element composed of a TFT for inputting a source signal to a driving element; a driving element connected to the switching element and composed of a TFT that controls the light-emitting intensity of the light-emitting element; and a holding capacitor for holding the voltage of the source signal input to the gate electrode of the driving element until the next rewrite (during one frame). The gate electrode of the driving element is connected to the drain electrode of the switching element, the source electrode of the driving element is connected to the positive power supply voltage signal line (VDD signal line), the drain electrode of the driving element is connected to the source electrode of the light-emitting control element, and the drain electrode of the light-emitting control element is connected to the positive electrode of the light-emitting element. The negative electrode of the light-emitting element is connected to the negative power supply voltage signal line (VSS signal line).

[0023] The light-emitting element is composed of a self-luminous element such as a light-emitting diode (LED) element and an organic electroluminescence (EL) element, and emits light with a brightness corresponding to the magnitude of the current flowing from the anode to the cathode. Regarding the driving element, during the activation period of the gate signal line connected to the gate electrode of the switching element, the source signal (image signal) from the source signal line is input to the gate electrode of the driving element, and the drain current of the driving element flowing through the light-emitting element is controlled according to the gate voltage of the driving element. The light-emitting intensity of the light-emitting element is controlled according to this drain current to represent gray scale.

[0024] In the aforementioned prior arts described in Patent Documents 1 and 2, gray scale control is performed only by the current flowing through the light-emitting element. Therefore, the low gray scale region is driven with a low current. In a light-emitting element such as an LED, the luminous efficiency is extremely reduced in the low current region, and variations in characteristics such as changes in luminous chromaticity (luminous wavelength) become large. Therefore, the display quality may be reduced. Especially when aiming for high contrast, the influence becomes significant.

[0025] In the prior art described in the aforementioned Patent Documents 1 and 2, gray-scale control is performed only by the current flowing through the light-emitting element. Therefore, the low-gray-scale region is driven with a low current, and the high-gray-scale region is driven with a high current, and the set range of the current flowing through the light-emitting element becomes wide. In particular, in the low-gray-scale region, the driving element needs to control a minute current, and the reproducibility of the gray scale deteriorates. Further, in the case where the chromaticity (emission wavelength) of a light-emitting element such as an LED has a current dependence, since the emission chromaticity shifts at the maximum gray scale and the minimum gray scale, the reproducibility of the chromaticity deteriorates. On the other hand, in order to narrow the set range of the current flowing through the light-emitting element, there is a driving method for controlling the gray scale by a combination of a plurality of pulses with different periods and LED current values. However, in the gray scale corresponding to the boundary between the low gray scale and the high gray scale, which is equivalent to the driving of a plurality of pulses based on the driving element, the current flowing through the light-emitting element changes discontinuously from a high current to a low current. Therefore, if the emission wavelength of the LED has a current dependence, the emission wavelength of the LED changes discontinuously, and the shift of the emission chromaticity becomes significant. In particular, in the case of displaying a high-definition image that requires high contrast, the display quality may be degraded.

[0026] In particular, the emission luminance of an LED is proportional to the integrated value related to the time of the LED current input to the LED during a certain period such as a frame period. For example, in order to set the luminance ratio (i.e., contrast) between the maximum gray scale and the minimum gray scale to 10,000:1, when the maximum emission luminance is normalized to 1, it is necessary to control the integrated value related to the time of the LED current in the range of 1 to 0.0001. However, in the case of controlling the emission luminance (i.e., gray scale) of the LED only by the LED current, the set range of the LED current value becomes wide from 1 to 0.0001, and it is necessary to use a low-current region where the luminous efficiency is extremely reduced, which is not preferable. This tendency becomes more significant when aiming for high contrast. On the other hand, in the driving method for controlling the gray scale by a combination of a plurality of pulses with different periods and LED current values, for example, in the case of controlling with two types of pulses, in order to achieve the contrast of 10,000:1, the set range of the LED current value can be narrowed to 1 to 0.01. However, since the emission chromaticity of the LED element has a characteristic of changing depending on the LED current value, in the case of controlling the gray scale by a combination of a plurality of pulses with different periods and LED current values, in the gray scale corresponding to the boundary of the driving based on the plurality of pulses, the current flowing through the LED changes discontinuously and significantly. Therefore, if the emission wavelength of the LED has a current dependence, the emission wavelength of the LED element may change discontinuously.

[0027] Hereinafter, preferred embodiments of the present disclosure will be described in detail with reference to the accompanying drawings.

[0028] Figure 1This is an electrical circuit diagram showing the schematic structure of a display device according to an embodiment of the present disclosure. Figure 2 It shows Figure 1 an electrical circuit diagram of the structure of the pixel portion of the display device shown. In addition, Figure 1 for ease of illustration, the first power supply line VDD and the second power supply line VSS are omitted and shown. The display device 10 of the present embodiment has a self-luminous type light-emitting element 11 whose luminance varies according to the magnitude of the current and the input period of the current. In addition, the display screen 13 is composed of a plurality (= n × m) of pixel portions 12 arranged in a matrix of n rows × m columns (where n and m are positive integers).

[0029] The display device 10 has a pixel circuit portion A ( Figure 2 shown), and when an image signal is input from an external device, as will be described later Figure 3 shown, based on the gray scale obtained from the image signal, the driving from the low gray scale driving portion (first driving portion) and the high gray scale driving portion (second driving portion) is switched and selected to cause the light-emitting element to emit light.

[0030] As Figure 2 and Figure 3 shown, the pixel circuit portion A has: a low gray scale driving portion A1 that drives the light-emitting element 11 when the gray scale obtained from the image signal is in the range of gray scales below the first boundary value and does not drive the light-emitting element 11 when the gray scale obtained from the image signal is in the range of gray scales higher than the first boundary value; and a high gray scale driving portion A2 that does not drive the light-emitting element 11 when the gray scale obtained from the image signal is in the range of gray scales below the second boundary value lower than the first boundary value and drives the light-emitting element 11 when the gray scale obtained from the image signal is in the range of gray scales higher than the second boundary value. These low gray scale driving portion A1 and high gray scale driving portion A2 are independently selected for driving.

[0031] The display device 10 includes: m data signal lines Sig(1) to (m) (collectively referred to as "data signal lines Sig"), arranged for each column in the matrix arrangement of a plurality of pixel portions 12; n low-gray-scale scanning lines GL(L)(1) to (n) (collectively referred to as "low-gray-scale scanning lines GL(L)"), arranged for each row in the matrix arrangement; high-gray-scale scanning lines GL(H)(1) to (n) (collectively referred to as "high-gray-scale scanning lines GL(H)"), arranged for each row in the matrix arrangement; low-gray-scale light emission control lines EMI(L)(1) to (n) (collectively referred to as "low-gray-scale light emission control lines EMI(L)"), arranged for each row in the matrix arrangement; high-gray-scale light emission control lines EMI(H)(1) to (n) (collectively referred to as "high-gray-scale light emission control lines EMI(H)"), arranged for each row in the matrix arrangement; a signal line driving circuit 14 that outputs an image signal to the data signal lines Sig; a scanning line driving circuit 15 that outputs a selection signal to the low-gray-scale scanning lines GL(L) and the high-gray-scale scanning lines GL(H); a first power supply line VDD to which power supply power is supplied; and a second power supply line VSS having a potential different from the potential of the first power supply line VDD (for example, a given positive potential), such as a given negative potential or a ground potential. When the potential of the second power supply line VSS is a ground potential, the pixel circuit portion A and its peripheral circuits operate substantially at one potential (positive potential), so the circuit structure is likely to be simplified.

[0032] In the present embodiment, the duty ratio of the light emission period per frame of the light emission control signal is set in the signal line driving circuit 14 so that all gray levels are 0 to 255, a predetermined first boundary value is gray level 64, and a predetermined second boundary value is gray level 32. These boundary values are an example and are set to appropriate values according to the gray level characteristics required by the image signal based on specifications such as the contrast of the display device.

[0033] In addition, since all gray levels are 0 to 255, the number of gray levels is 256. Therefore, the highest gray level can be 255 or 256 based on 0.

[0034] In this embodiment, the structure may also be as follows: when the gray level obtained from the image signal is in the range of gray levels below the second boundary value, the low gray level driving unit A1 drives the light emitting element 11 with the first current pulse; when the gray level obtained from the image signal is in the range of gray levels higher than the second boundary value and below the first boundary value, the low gray level driving unit A1 drives the light emitting element 11 with the second current pulse, and the high gray level driving unit A2 drives the light emitting element 11 with the third current pulse (the third current pulse that is time-division multiplexed with the second current pulse and follows it with a time interval) that has been time-division multiplexed with the second current pulse; when the gray level obtained from the image signal is in the range of gray levels higher than the first boundary value, the high gray level driving unit A2 drives the light emitting element 11 with the fourth current pulse. In this case, when the gray level obtained from the image signal is in the range higher than the second boundary value and below the first boundary value (the intermediate gray level range), the current value input to the light emitting element 11 and the integral value (denoted as IGm) related to the current input period can be adjusted to an intermediate value (IGl > IGm > Igh) between the current value during low gray level driving below the second boundary value and the integral value (denoted as Igl) related to the current input period, and the current value during high gray level driving higher than the first boundary value and the integral value (denoted as Igh) related to the current input period. At the same time, by synthesizing the driving signals from the two driving units, the low gray level driving unit A1 and the high gray level driving unit A2, it is easy to suppress the discontinuous change in the emission wavelength caused by the discontinuous change in the current flowing through the light emitting element near the boundary between the low gray level region and the high gray level region.

[0035] Each pixel section 12 has a pixel circuit section A, and the pixel circuit section includes: a scanning transistor Tg(L) for low gray levels, having a gate terminal connected to a scanning line GL(L) for low gray levels and a source terminal connected to a data signal line Sig; a scanning transistor Tg(H) for high gray levels, having a gate terminal connected to a scanning line GL(H) for high gray levels and a source terminal connected to the data signal line Sig; a capacitor C(L) for low gray levels, having one end connected to a drain terminal of the scanning transistor Tg(L) for low gray levels and the other end connected to a first power supply line VDD; a capacitor C(H) for high gray levels, having one end connected to a drain terminal of the scanning transistor Tg(H) for high gray levels and the other end connected to the first power supply line VDD; a driving transistor Td(L) for low gray levels, having a gate terminal connected to one end of the capacitor C(L) for low gray levels and a source terminal connected to the first power supply line VDD; a driving transistor Td(H) for high gray levels, having a gate terminal connected to one end of the capacitor C(H) for high gray levels and a source terminal connected to the first power supply line VDD; a light emission control transistor Ts(L) for low gray levels, having a gate terminal connected to a light emission control line EMI(L) for low gray levels, a drain terminal connected to an anode electrode of the light emitting element 11, and a source terminal connected to a drain terminal of the driving transistor Td(L) for low gray levels; and a light emission control transistor Ts(H) for high gray levels, having a gate terminal connected to a light emission control line EMI(H) for high gray levels, a drain terminal connected to the anode electrode of the light emitting element 11, and a source terminal connected to a drain terminal of the driving transistor Td(H) for high gray levels.

[0036] As described above, the low gray level driving section A1 has: a scanning transistor Tg(L) for low gray levels, having a gate terminal connected to a scanning line GL(L) for low gray levels and a source terminal connected to a data signal line Sig; a capacitor C(L) for low gray levels, having one end connected to a drain terminal of the scanning transistor Tg(L) for low gray levels and the other end connected to a first power supply line VDD; a driving transistor Td(L) for low gray levels, having a gate terminal connected to one end of the capacitor C(L) for low gray levels and a source terminal connected to the first power supply line VDD; and a light emission control transistor Ts(L) for low gray levels, having a gate terminal connected to a light emission control line EMI(L) for low gray levels, a drain terminal connected to an anode electrode of the light emitting element 11, and a source terminal connected to a drain terminal of the driving transistor Td(L) for low gray levels.

[0037] The high gray-scale driving unit A2 includes: a high gray-scale scanning transistor Tg(H) whose gate terminal is connected to the high gray-scale scanning line GL(H) and whose source terminal is connected to the data signal line Sig; a high gray-scale capacitor C(H) one end of which is connected to the drain terminal of the high gray-scale scanning transistor Tg(H) and the other end of which is connected to the first power supply line VDD; a high gray-scale driving transistor Td(H) whose gate terminal is connected to one end of the high gray-scale capacitor C(H) and whose source terminal is connected to the first power supply line VDD; and a high gray-scale light-emitting control transistor Ts(H) whose gate terminal is connected to the high gray-scale light-emitting control line EMI(H), whose drain terminal is connected to the anode electrode of the light-emitting element 11, and whose source terminal is connected to the drain terminal of the high gray-scale driving transistor Td(H).

[0038] The light-emitting element 11 is connected in series between the drain terminal of the low gray-scale light-emitting control transistor Ts(L), the drain terminal of the high gray-scale light-emitting control transistor Ts(H), and the second power supply line VSS. That is, the anode electrode of the light-emitting element 11 is connected in parallel to the drain terminal of the low gray-scale light-emitting control transistor Ts(L) and the drain terminal of the high gray-scale light-emitting control transistor Ts(H). In addition, the cathode electrode of the light-emitting element 11 is connected to the second power supply line VSS. Each pixel unit 12 is configured to include the light-emitting element 11 and the aforementioned pixel circuit unit A.

[0039] A power supply voltage of, for example, about +3V of a positive electrode is applied to the first power supply line VDD, and a power supply voltage of, for example, a ground potential (about 0V) or a negative potential such as about -3V to -5V, which is lower than the voltage of the first power supply line VDD, is applied to the second power supply line VSS. For example, when the potential difference between the voltage of the first power supply line VDD and the voltage of the second power supply line VSS is 8V, if the voltage of the first power supply line VDD is 3V, the voltage of the second power supply line VSS is -5V, and if the voltage of the first power supply line VDD is 8V, the voltage of the second power supply line VSS is 0V.

[0040] The aforementioned low gray-scale scanning transistor Tg(L), high gray-scale scanning transistor Tg(H), low gray-scale driving transistor Td(L), high gray-scale driving transistor Td(H), low gray-scale light-emitting control transistor Ts(L), and high gray-scale light-emitting control transistor Ts(H) can be implemented by, for example, p-channel TFTs. By inputting a low-level signal (L signal) to the gate electrode, conduction occurs between the source and drain, and it becomes a conducting state, allowing current to flow.

[0041] A TFT is a three-terminal device having a semiconductor film made of amorphous silicon (a-Si), low-temperature polycrystalline silicon (LTPS), etc., and having a gate electrode as a gate terminal, a source electrode as a source terminal, and a drain electrode as a drain terminal. By applying a voltage of a given potential to the gate electrode, it functions as a switching element (gate transfer element) through which current flows in the semiconductor film (channel) between the source electrode and the drain electrode.

[0042] The low-gray-scale light-emission control transistor Ts(L) and the high-gray-scale light-emission control transistor Ts(H) input light-emission control signals for controlling the light-emission period from the light-emission control lines EMI(L) and EMI(H) to the gate electrode, and supply drive currents from the low-gray-scale drive transistor Td(L) and the high-gray-scale drive transistor Td(H) to the light-emitting element 11 during the active period of the input light-emission control signals.

[0043] A low-gray-scale capacitor C(L) as a capacitive element is connected in parallel between the gate terminal and the source terminal of the low-gray-scale drive transistor Td(L). In addition, a high-gray-scale capacitor C(H) as a capacitive element is connected in parallel between the gate terminal and the source terminal of the high-gray-scale drive transistor Td(H). The low-gray-scale capacitor C(L) functions as a holding capacitor for holding the voltage of the image signal input to the gate terminal of the low-gray-scale drive transistor Td(L) until the next rewrite (for a period of one frame), and the high-gray-scale capacitor C(H) functions as a holding capacitor for holding the voltage of the image signal input to the gate terminal of the high-gray-scale drive transistor Td(H) until the next rewrite (for a period of one frame).

[0044] The image data signal is supplied from the signal line driving circuit 14 to each pixel section 12 via the data signal line Sig. The scan signal is supplied from the scan line driving circuit 15 to each pixel section 12 via each scan line GL(L) and GL(H). The light-emission control signal is supplied from the scan line driving circuit 15 to each pixel section 12 via each light-emission control line EMI(L) and EMI(H).

[0045] As the light-emitting element 11, a self-luminous element such as a microchip-type light-emitting diode, a single-chip-type light-emitting diode, an organic EL, an inorganic EL, or a semiconductor laser element can be adopted.

[0046] Each pixel portion 12 may also be composed of a sub-pixel for red light emission, a sub-pixel for green light emission, and a sub-pixel for blue light emission. The sub-pixel for red light emission has a red light-emitting element composed of a red LED or the like, the sub-pixel for green light emission has a green light-emitting element composed of a green LED or the like, and the sub-pixel for blue light emission has a blue light-emitting element composed of a blue LED or the like. For example, these sub-pixels may be arranged in a column direction or in a row direction.

[0047] Figure 3 It is a graph showing the relationship between the drive current from the low gray-scale drive unit A1 and the drive current from the high gray-scale drive unit A2 with respect to the gray-scale of the light-emitting element 11. In this figure, the vertical axis is the current flowing through the light-emitting element 11 (relative value; relative value when the maximum value is normalized to 1), and the horizontal axis is the gray-scale from 0 to 255. Considering the relationship between the drive current of the light-emitting element 11 and the emission luminance, and the relationship between the gray-scale set from the input-output characteristics of the display device and the current flowing through the light-emitting element 11, the drive current input from the low gray-scale drive unit A1 to the light-emitting element 11 and the drive current input from the high gray-scale drive unit A2 to the light-emitting element 11 are set. In the low gray-scale region below the second boundary value, only the drive current is supplied from the low gray-scale drive unit A1 to the light-emitting element 11, and the input period (about several μ to several 100 μ seconds) of the first current pulse input from the low gray-scale drive unit A1 to the light-emitting element 11 is short, and the peak value (current level) increases as the gray-scale increases. In addition, the integral value related to the time of the first current pulse during one frame period also increases as the gray-scale increases.

[0048] When the gray-scale obtained from the image signal is in the low gray-scale region below the second boundary value, only the drive current is supplied from the low gray-scale drive unit A1, and the input period (about several μ to several 100 μ seconds) of the first current pulse input from the low gray-scale drive unit A1 to the light-emitting element 11 is short, and the peak value (current level) increases as the gray-scale increases. In addition, the integral value related to the time of the first current pulse during one frame period also increases as the gray-scale increases.

[0049] When the gray-scale obtained from the image signal is in the high gray-scale region higher than the first boundary value, only the drive current is supplied from the high gray-scale drive unit A2 to the light-emitting element 11, and the input period (about several milliseconds (m) to several 10 milliseconds (m)) of the fourth current pulse input from the high gray-scale drive unit A2 to the light-emitting element 11 is long, and the peak value increases as the gray-scale increases. In addition, the integral value related to the time of the fourth current pulse during one frame period also increases as the gray-scale increases.

[0050] When the gray level obtained from the image signal is in the gray level region of an intermediate gray level that is higher than the second boundary value and equal to or lower than the first boundary value, drive currents are supplied from the low gray level drive unit A1 and the high gray level drive unit A2 to the light emitting element 11 respectively. That is, the input period (for example, the duty ratio in one frame period) of the second current pulse input from the low gray level drive unit A1 to the light emitting element 11 is the same as that of the first current pulse, and the peak value decreases as the gray level increases. The input period of the third current pulse input from the high gray level drive unit A2 to the light emitting element 11 is the same as that of the fourth current pulse, and the peak value increases as the gray level increases. In addition, the sum of the integral value related to the time of the second current pulse and the integral value related to the time of the third current pulse in one frame period also increases as the gray level increases, and at the second boundary value and the first boundary value, the integral values related to the time of the current pulses in the gray levels before and after these boundary values are smoothly connected.

[0051] In addition, in the gray level region of the intermediate gray level that is higher than the second boundary value and equal to or lower than the first boundary value, the drive current generated by the low gray level drive unit A1 decreases as the gray level increases. On the other hand, the drive current generated by the high gray level drive unit A2 increases as the gray level increases. Thus, as Figure 3 shown by the dotted line, it is possible to suppress the discontinuous change of the drive current in the gray level region of the intermediate gray level corresponding to the switching part between the drive by the low gray level drive unit A1 and the drive by the high gray level drive unit A2. As a result, it is possible to suppress the discontinuous change of the emission wavelength near the boundary between the low gray level region and the high gray level region.

[0052] Therefore, in the light emitting element 11, in the gray level region of the intermediate gray level that is higher than the second boundary value and equal to or lower than the first boundary value, drive currents are input to the light emitting element 11 from both the low gray level drive unit A1 and the high gray level drive unit A2. As a result, as shown in Figure 6 described later, the light emitting element 11 can suppress the discontinuous change of the emission wavelength at the boundary between the low gray level region and the high gray level region, and more specifically, in the intermediate gray level region that is higher than the second boundary value and equal to or lower than the first boundary value, and can reduce the deviation of the emission chromaticity.

[0053] Next, the operation of the display device 10 will be described.

[0054] Figure 5 is for explaining Figure 1Timing chart of the operation of the display device shown. Since the multiple pixel units 12 arranged in an n-row and m-column matrix have the same structure, as an example, the operation of one pixel on the scan lines GL(L)(1) and GL(H)(1) will be described. In the signal line driving circuit 14 and the scan line driving circuit 15, a total of two image signals, a low gray level and a high gray level, are written for each pixel. The scan line GL(L) for the low gray level is set to the active state, and writing to the driving transistor Td(L) for the low gray level is performed. After that, the scan line GL(H) for the high gray level is set to the active state, and writing to the driving transistor Td(H) for the high gray level is performed. The write data is alternately supplied to the driving transistor Td(L) for the low gray level and the driving transistor Td(H) for the high gray level from the data signal line Sig in a time-division manner.

[0055] When the light-emitting element 11 emits light in the gray level region C (a high gray level region higher than the first boundary value), a high gray level driving voltage Vg(H) is written to the driving transistor Td(H) for the high gray level, and a cut-off voltage Vg(L) is written to the driving transistor Td(L) for the low gray level. In addition, when the light-emitting element 11 emits light in the gray level region A (a low gray level region below the second boundary value), a low gray level driving voltage Vg(L) is written to the driving transistor Td(L) for the low gray level, and a cut-off voltage Vg(H) is written to the driving transistor Td(H) for the high gray level. In addition, when the light-emitting element 11 emits light in the gray level region B (a gray level region of an intermediate gray level higher than the second boundary value and below the first boundary value), a low gray level driving voltage Vg(L) is written to the driving transistor Td(L) for the low gray level, and a high gray level driving voltage Vg(H) is written to the driving transistor Td(H) for the high gray level. Regarding the light emission control, by inputting a pulse signal for the light emission period with a low duty ratio (a pulse signal for the first current pulse, a pulse signal for the second current pulse) to the light emission control line EMI(L) for the low gray level, and inputting a pulse signal for the light emission period with a high duty ratio (a pulse signal for the third current pulse, a pulse signal for the fourth current pulse) to the light emission control line EMI(H) for the high gray level, a current pulse is output to the light-emitting element 11.

[0056] At this time, the driving signal is as described above Figure 3As shown, when the gray level obtained from the image signal is in the range below the first boundary value, the low gray level driving unit A1 drives the light emitting element 11 with a low gray level voltage, and when the gray level obtained from the image signal is in the range higher than the first boundary value, the light emitting element 11 is not driven. In addition, when the gray level obtained from the image signal is in the range below the second boundary value, the high gray level driving unit A2 does not drive the light emitting element 11, and when the gray level obtained from the image signal is in the range higher than the second boundary value, the light emitting element 11 is driven with a high gray level voltage. In the region where the gray level obtained from the image signal is higher than the second boundary value and below the first boundary value, the light emitting element 11 is continuously driven in a time-division manner from both the low gray level driving unit A1 and the high gray level driving unit A2 during one frame period.

[0057] Therefore, for the light emitting element 11, when the gray level obtained from the image signal is below the second boundary value, it is only driven by the driving signal based on the low gray level driving unit A1, and when the gray level obtained from the image signal is in the range higher than the first boundary value, it is only driven by the driving signal based on the high gray level driving unit A2. Moreover, when the gray level obtained from the image signal is higher than the second boundary value and below the first boundary value, it is driven by the driving signal synthesized from the driving signal of the low gray level driving unit A1 and the driving signal of the high gray level driving unit A2.

[0058] Figure 6It is a graph showing the relationship between the gray scale and the emission wavelength of the light emitting element 11 when the light emitting element 11 is driven by a drive signal obtained by synthesizing the drive signal of the low gray scale drive unit A1 and the drive signal of the high gray scale drive unit A2 in the boundary region between the low gray scale and the high gray scale (the gray scale region higher than the second boundary value and lower than the first boundary value). As in the display device 10 of the present embodiment, in order to achieve high contrast, it is necessary to drive the light emitting element 11 in a wide luminance range. In the present embodiment, the light emission pulse period of the low gray scale drive unit A1 is driven at a high peak in a range of, for example, about 0.1% to 10% of the duty ratio lower than the light emission pulse period of the high gray scale drive unit A2, so that the current flowing through the light emitting element 11 in the low gray scale region can be increased. As a result, in the low current region where the light emission efficiency of the light emitting element 11 is poor and the characteristic deviation is large, a stable light emission luminance can be easily obtained even when not used for a long time within one frame period. In addition, since the current range with respect to all gray scales of the light emitting element 11 can be narrowed down to about 0.01 to 1.0 (relative value), the variation range of the emission wavelength can be suppressed in a narrow wavelength region in the change of the emission chromaticity depending on the current flowing through the light emitting element 11, that is, in the range of all gray scales from 0 to 255. Further, by driving the boundary region between the low gray scale and the high gray scale (the gray scale region of the intermediate gray scale) with a drive signal obtained by synthesizing the drive signal of the low gray scale drive unit A1 and the drive signal of the high gray scale drive unit A2, the discontinuous change of the current flowing through the light emitting element 11 in the boundary region can be suppressed, and thus the discontinuous change of the emission wavelength depending on the current flowing through the light emitting element 11 can be suppressed.

[0059] The display device of the present disclosure is configured by arranging a plurality of substrates 1 horizontally and vertically on the same plane. The substrate 1 is mounted with Figure 1 a plurality of light emitting elements 11 as shown, and by bonding their sides to each other (tiling) with an adhesive material or the like, a composite and large display device, so-called a multi-display, can be realized. In such a large-screen display device, the discontinuous change of the emission wavelength due to the current dependence of the light emitting element can also be suppressed, and a high-definition image can be displayed with high contrast.

[0060] Figure 7 It is an electrical circuit diagram showing another structure of the pixel portion of the display device of the present disclosure. In addition, the same reference numerals are given to the parts corresponding to the foregoing embodiments.

[0061] The display device 10 of the present embodiment may also have the following structure. The display device 10 includes: m low-gray-scale data signal lines Sig(L)(1) to (m) (collectively referred to as "data signal lines Sig(L)"), arranged for each column in the matrix arrangement of a plurality of pixel portions 12; a first power supply line VDD to which power supply power is supplied; a second power supply line VSS to which a voltage of a potential lower than the potential of the first power supply line VDD (e.g., a given positive potential), such as a given negative potential or a ground potential, is supplied; m high-gray-scale data signal lines Sig(H)(1) to (m) (collectively referred to as "data signal lines Sig(H)"), arranged for each column in the matrix arrangement of a plurality of pixel portions 12; n scan lines GL(1) to (n) (collectively referred to as "scan lines GL"), arranged for each row in the matrix arrangement; n low-gray-scale light emission control lines EMI(L), arranged for each row in the matrix arrangement; n high-gray-scale light emission control lines EMI(H), arranged for each row in the matrix arrangement; data signal lines Sig(L); a signal line driving circuit 14 that outputs an image signal to the data signal lines Sig(H); and a scan line driving circuit 15 that outputs a selection signal to the scan lines GL. When the potential of the second power supply line VSS is a ground potential, the pixel circuit portion A1 and its peripheral circuits operate at substantially one potential (positive potential), so the circuit structure can be easily simplified.

[0062] As Figure 7 shown, each pixel portion 12 may also have the following structure. Each pixel portion 12 has a pixel circuit portion B, and the pixel circuit portion B includes a low-gray-scale driving portion B1 and a high-gray-scale driving portion B2. The low-gray-scale driving portion B1 has: a low-gray-scale scan transistor Tg(L) whose gate terminal is connected to the scan line GL and whose source terminal is connected to the low-gray-scale data signal line Sig(L); a low-gray-scale capacitor C(L) one end of which is connected to the drain terminal of the low-gray-scale scan transistor Tg(L) and the other end of which is connected to the first power supply line VDD; a low-gray-scale driving transistor Td(L) whose gate terminal is connected to one end of the low-gray-scale capacitor C(L) and whose source terminal is connected to the first power supply line VDD; and a low-gray-scale light emission control transistor Ts(L) whose gate terminal is connected to the low-gray-scale light emission control line EMI(L), whose drain terminal is connected to the anode electrode of the light emitting element 11, and whose source terminal is connected to the drain terminal of the low-gray-scale driving transistor Td(L).

[0063] The high gray-scale driving unit B2 includes: a scanning transistor Tg(H) for high gray-scale, with its gate terminal connected to the scanning line GL and its source terminal connected to the data signal line Sig(H) for high gray-scale; a capacitor C(H) for high gray-scale, with one end connected to the drain terminal of the scanning transistor Tg(H) for high gray-scale and the other end connected to the first power supply line VDD; a driving transistor Td(H) for high gray-scale, with its gate terminal connected to one end of the capacitor C(H) for high gray-scale and its source terminal connected to the first power supply line VDD; and a light-emitting control transistor Ts(H) for high gray-scale, with its gate terminal connected to the light-emitting control line EMI(H) for high gray-scale, its drain terminal connected to the anode electrode of the light-emitting element 11, and its source terminal connected to the drain terminal of the driving transistor Td(H) for high gray-scale.

[0064] The light-emitting element 11 is connected in series between the drain terminal of the scanning transistor Ts(L) for low gray-scale and the drain terminals of the scanning transistor Ts(H) for high gray-scale and the second power supply line VSS. That is, the anode electrode of the light-emitting element 11 is connected in parallel with the drain terminal of the scanning transistor Ts(L) for low gray-scale and the drain terminals of the scanning transistor Ts(H) for high gray-scale. In addition, the cathode electrode of the light-emitting element 11 is connected to the second power supply line VSS.

[0065] In this embodiment, similar to the aforementioned Figure 1 , Figure 2 embodiment, they are common in the following aspects: having two sets of driving transistors Td(L), Td(H), light-emitting control transistors Ts(L), Ts(H) for low gray-scale and high gray-scale, and synthesizing their outputs to drive the light-emitting element 11. Among them, in this embodiment, by having two data signal lines Sig(L), Sig(H) for low gray-scale and high gray-scale and making the scanning line GL one, data signals are written to the driving transistor Td(L) for low gray-scale and the driving transistor Td(H) for high gray-scale at the same timing. That is, the low gray-scale driving unit B1 and the high gray-scale driving unit B2 that make up the pixel circuit unit B are simultaneously selected and driven. Thereby, the light-emitting element 11 can be driven at a higher frame frequency, and data signals are written to the two driving transistors Td(L), Td(H) for low gray-scale / high gray-scale simultaneously through one scanning signal (gate signal). Therefore, compared with the case of writing low gray-scale and high gray-scale sequentially, the writing time per frame can be shortened.

[0066] For example, when the gray-scale obtained from the image signal is in the range of being higher than the second boundary value and less than or equal to the first boundary value, the low gray-scale driving unit B1 drives the light-emitting element 11 with a second current pulse, and the high gray-scale driving unit B2 drives the light-emitting element 11 with a third current pulse that is time-division multiplexed with the second current pulse. At this time, it is not necessary to switch between the low gray-scale driving unit B1 and the high gray-scale driving unit B2, so the writing time per frame can be shortened.

[0067] The display device of the present disclosure arranges a plurality of substrates 1 horizontally and vertically on the same plane. The substrate 1 is mounted with Figure 1 the plurality of light-emitting elements 11 shown, and by bonding their sides to each other using an adhesive material or the like, a composite and large display device, so-called multi-display, can be realized.

[0068] According to the display device of the present disclosure, even in a low gray-scale region, the light-emitting element can be driven with a high current. Therefore, it is necessary to operate the light-emitting element in a region with low luminous efficiency and large characteristic deviation, and a high-quality image can be displayed with a high contrast. In addition, when the gray scale obtained from the image signal is in the range higher than the second boundary value and below the first boundary value, the light-emitting element is driven by a drive signal obtained by synthesizing the drive signal of the low gray-scale drive unit and the drive signal of the high gray-scale drive unit. As a result, even for a light-emitting element having a current dependence on the emission wavelength, a discontinuous change in the emission wavelength near the boundary between the low gray-scale region and the high gray-scale region can be suppressed, and a high-quality image can be displayed with a high contrast.

[0069] The display device of the present disclosure can implement the following modes (1) to (9) in the above-described embodiment.

[0070] (1) It may also be configured such that the maximum value of the current for driving the light-emitting element 11 by the low gray-scale drive units A1 and B1 is equal to or higher than the maximum value of the current for driving the light-emitting element 11 by the high gray-scale drive units A2 and B2. In this case, even in the low gray-scale region, the light-emitting element can be driven in a higher current region. Therefore, it is not necessary to operate the light-emitting element in a low current region with low luminous efficiency and large characteristic deviation, and a higher-quality image can be displayed with a higher contrast.

[0071] (2) It may also be configured such that the chromaticity of the light emission of the light-emitting element 11 changes according to the magnitude of the current. In this case, since the low gray-scale drive units A1 and B1 and the high gray-scale drive units A2 and B2 can be driven in a time-division manner, and the drive current of the low gray-scale drive units A1 and B1 is increased, it is not necessary to operate the light-emitting element 11 in a low current region where the change in chromaticity (emission wavelength) is large. Therefore, the display device of the present disclosure is suitable for the light-emitting element 11 whose chromaticity of light emission changes according to the magnitude of the current.

[0072] (3) It may also be configured such that the light-emitting element 11 is a light-emitting diode. In this case, the light-emitting diode has the advantages of being able to be driven with a low voltage of about 2 to 3.5V and having a long lifespan. In addition, since the chromaticity of light emission is likely to change according to the magnitude of the current, especially in the low current region, the structure of the display device of the present disclosure is preferable.

[0073] (4) The gray value obtained from the image signal that exceeds the first boundary value and is below the second boundary value may also be in the range of 1 / 8 to 1 / 4 of the maximum gray value. In this case, the gray region that exceeds the first boundary value and is below the second boundary value becomes the intermediate gray region, and it is easy to drive the intermediate gray region in the high-current region by time-division driving. In addition, in the intermediate gray region, the light-emitting element is driven by a driving signal that synthesizes the driving signal of the low-gray driving unit and the driving signal of the high-gray driving unit. As a result, even for a light-emitting element having a current dependence on the emission wavelength, it is possible to suppress the discontinuous change of the emission wavelength near the boundary between the low-gray region and the high-gray region, and to display a high-quality image with a high contrast.

[0074] (5) It may also be a structure in which the maximum gray value is 256, and the gray value obtained from the image signal that exceeds the first boundary value and is below the second boundary value is in the range of 32 to 64. In this case, since the maximum gray value is 256, it is possible to accurately represent the subtle differences in color and brightness. The gray value obtained from the image signal that exceeds the first boundary value and is below the second boundary value is in the range of 32 to 64, so it is easy to drive the low-gray region and the intermediate gray region as high-current regions by time-division driving.

[0075] (6) It may also be configured to include a duty ratio control unit that controls the duty ratio for low gray levels during the low-gray emission period W11 in one frame period and the duty ratio for high gray levels during the high-gray emission period W12 in one frame period. The duty ratio control unit makes the ratio of the duty ratio for low gray levels to the duty ratio for high gray levels constant according to the brightness adjustment signal for the entire plurality of pixel units 12, and changes the duty ratio for low gray levels and the duty ratio for high gray levels. According to this structure, the duty ratio control unit is configured to make the ratio of the duty ratio for low gray levels to the duty ratio for high gray levels constant on the premise that the duty ratio for low gray levels and the duty ratio for high gray levels change, so the performance such as the operability and operation speed of the duty ratio control unit is improved, and the circuit structure can be simplified. In this case, the duty ratio control unit may also control the duty ratio for low gray levels and the duty ratio for high gray levels during the intermediate-gray emission period in one frame period.

[0076] The duty ratio control unit may also be a control circuit unit included in the scan line driving circuit 15. In addition, when the scan line driving circuit 15 is a driving element such as an IC (Integrated Circuit) or an LSI (Large Scale Integrated Circuit), the duty ratio control unit may also be a program software included in the RAM (Random Access Memory) unit or the ROM (Read Only Memory) unit included in the driving element. In addition, the duty ratio control unit may also be a driving element separate from the signal line driving circuit 14 and the scan line driving circuit 15, or a program software included in the RAM unit or the ROM unit included in the driving element. In addition, the duty ratio control unit may also be a control circuit formed on an external circuit board.

[0077] (7) It may also be configured to include a duty ratio control unit that controls the duty ratio for low grayscale during one frame period of the low grayscale emission period W11 and the duty ratio for high grayscale during one frame period of the high grayscale emission period W12. The duty ratio control unit changes the ratio of the duty ratio for low grayscale to the duty ratio for high grayscale according to the luminance adjustment signal for the entire plurality of pixel units 12, and also changes the duty ratio for low grayscale and the duty ratio for high grayscale. The duty ratio control unit is configured to change the ratio of the duty ratio for low grayscale to the duty ratio for high grayscale on the premise of changing the duty ratio for low grayscale and the duty ratio for high grayscale, so that the duty ratio for low grayscale and the duty ratio for high grayscale can be controlled with high precision. In this case, the duty ratio control unit may also control the duty ratio for low grayscale and the duty ratio for high grayscale during one frame period of the intermediate grayscale emission period.

[0078] (8) It may also be configured to include: a substrate having a pixel unit arrangement surface on which a plurality of pixel units 12 are arranged, a surface opposite to the pixel unit arrangement surface, and side surfaces; and a driving unit arranged on the opposite surface side for driving the low grayscale driving units A1, B1 and the high grayscale driving units A2, B2. In this case, it is not necessary to arrange the driving unit at the frame portion of the pixel unit arrangement surface of the substrate, so that a high-quality image can be displayed with a large area. In addition, when the side surfaces of a plurality of display devices are combined with each other to form a composite display device, the combined portion is not easily noticeable, and the continuity of the image in the combined portion can be maintained.

[0079] (9) The substrate 2 may also be configured on the side and have a structure of side wirings that connect the low gray-scale driving parts A1, B1 and the high gray-scale driving parts A2, B2 to the driving part. In this case, through conductors such as vias for connecting the low gray-scale driving parts A1, B1, the high gray-scale driving parts A2, B2 and the driving part do not need to be provided on the substrate. As a result, an area for setting through conductors does not need to be secured in the pixel part area, so that the area of the display device can be reduced, and the display device can be miniaturized.

[0080] The display device of the present disclosure can be preferably implemented not only for LEDs as light-emitting elements, but also for display devices that mount other light-emitting elements having current dependence such as organic LEDs, organic ELs, etc. as light-emitting elements.

[0081] In the display device according to the present disclosure, when the gray scale obtained from the image signal is higher than the second boundary value and within the range of the gray scale up to the first boundary value, the light-emitting element is driven by a driving signal obtained by synthesizing the driving signal of the low gray-scale driving part and the driving signal of the high gray-scale driving part. As a result, even for a light-emitting element having current dependence on the emission wavelength, a discontinuous change in the emission wavelength near the boundary between the low gray-scale region and the high gray-scale region can be suppressed, and a high-quality image can be displayed with high contrast.

[0082] -Industrial Applicability-

[0083] The display device of the present disclosure can be applied to various electronic devices. Examples of such electronic devices include a composite and large display device (multi-display), an automotive route guidance system (automotive navigation system), a marine route guidance system, an aircraft route guidance system, a smart phone terminal, a mobile phone, a tablet terminal, a personal digital assistant (PDA), a camera, a digital still camera, an electronic manual, an e-book, an electronic dictionary, a personal computer, a copying machine, a terminal device of a game device, a television, a product display label, a price display label, an industrial programmable display device, an automotive audio, a digital audio player, a facsimile machine, a printer, a cash automated teller machine (ATM), a vending machine, a digital display watch, a smart watch, a guidance display device provided at a station, an airport, etc.

[0084] The present disclosure can be implemented in various other ways without departing from its spirit or main features. Therefore, the foregoing embodiments are merely illustrative in all respects, the scope of the present disclosure is shown in the claims, and there is no limitation in the main text of the specification. Further, all modifications and changes belonging to the claims are within the scope of the present disclosure.

[0085] -Symbol Explanation-

[0086] 10 Display device

[0087] 11 Light-emitting element

[0088] 12 Pixel section

[0089] 13 Display screen

[0090] 14 Signal line driving circuit

[0091] 15 Scan line driving circuit

[0092] A, B pixel circuit sections

[0093] A1, B1 low gray-scale driving sections

[0094] A2, B2 high gray-scale driving sections

[0095] Sig Data signal line

[0096] GL(L) Scan line for low gray-scale

[0097] GL(H) Scan line for high gray-scale

[0098] EMI(L) Light emission control line for low gray-scale

[0099] EMI(H) Light emission control line for high gray-scale

[0100] VDD First power supply line

[0101] VSS Second power supply line

[0102] Tg(L) Scan transistor for low gray-scale

[0103] Tg(H) Scan transistor for high gray-scale

[0104] C(L) Capacitor for low gray-scale

[0105] C(H) Capacitor for high gray-scale

[0106] Td(L) Driving transistor for low gray-scale

[0107] Td(H) Driving transistor for high gray-scale

[0108] Ts(L) Light emission control transistor for low gray-scale

[0109] Ts(H) Light emission control transistor for high gray-scale.

Claims

1. A display device includes: a light-emitting element having a plurality of pixel portions arranged in a matrix, wherein the luminance of each of the plurality of pixel portions varies according to the magnitude of a current and the input period of the current; and a pixel circuit section that causes the light-emitting element to emit light with a luminance corresponding to the gray level obtained from the image signal, The pixel circuit section includes: a first driving section that drives the light-emitting element when the gray level obtained from the image signal is in a range of gray levels below a first boundary value, and does not drive the light-emitting element when the gray level obtained from the image signal is in a range of gray levels higher than the first boundary value; and a second driving section that does not drive the light-emitting element when the gray level obtained from the image signal is in a range of gray levels below a second boundary value lower than the first boundary value, and drives the light-emitting element when the gray level obtained from the image signal is in a range of gray levels higher than the second boundary value.

2. The display device according to claim 1, wherein when the gray level obtained from the image signal is in a range of gray levels below the second boundary value, the first driving section drives the light-emitting element with a first current pulse, when the gray level obtained from the image signal is in a range of gray levels higher than the second boundary value and below the first boundary value, the first driving section drives the light-emitting element with a second current pulse, and the second driving section drives the light-emitting element with a third current pulse that is time-division multiplexed with the second current pulse, and the third current pulse follows immediately after the second current pulse with a time interval therebetween, when the gray level obtained from the image signal is in a range of gray levels higher than the first boundary value, the second driving section drives the light-emitting element with a fourth current pulse.

3. The display device according to claim 1 or 2, wherein the first driving section and the second driving section are independently selected for driving.

4. The display device according to claim 3, Wherein it includes: data signal lines disposed for each column in the matrix arrangement of the plurality of pixel sections; a first power supply line supplied with a power supply voltage; a second power supply line supplied with a power supply voltage lower than the power supply voltage; low gray level scan lines disposed for each row in the matrix arrangement; low gray level light emission control lines disposed for each row in the matrix arrangement; high gray level scan lines disposed for each row in the matrix arrangement; high gray level light emission control lines disposed for each row in the matrix arrangement; a signal line driving circuit that outputs an image signal to the data signal lines; and a scan line driving circuit that outputs selection signals to the low gray level scan lines and the high gray level scan lines, The first driving section has: a low gray level scan transistor having a gate terminal connected to the low gray level scan line and a source terminal connected to the data signal line; a low gray level capacitor having one end connected to the drain terminal of the low gray level scan transistor and the other end connected to the first power supply line; a low gray level driving transistor having a gate terminal connected to the one end of the low gray level capacitor and a source terminal connected to the first power supply line; and a low gray level light emission control transistor having a gate terminal connected to the low gray level light emission control line, a drain terminal connected to the anode electrode of the light-emitting element, and a source terminal connected to the drain terminal of the low gray level driving transistor, The second driving section has: A scanning transistor for high gray levels, having a gate terminal connected to the scanning line for high gray levels and a source terminal connected to the data signal line; A capacitor for high gray levels, having one end connected to the drain terminal of the scanning transistor for high gray levels and the other end connected to the first power supply line; A driving transistor for high gray levels, having a gate terminal connected to the one end of the capacitor for high gray levels and a source terminal connected to the first power supply line; and A light emission control transistor for high gray levels, having a gate terminal connected to the light emission control line for high gray levels, a drain terminal connected to the anode electrode of the light emitting element, and a source terminal connected to the drain terminal of the driving transistor for high gray levels, wherein the light emitting element is connected between the drain terminal of the light emission control transistor for low gray levels and the drain terminals of the light emission control transistors for high gray levels and the second power supply line.

5. The display device according to claim 4, wherein the potential of the second power supply line is a ground potential.

6. The display device according to claim 1, wherein the first driving unit and the second driving unit are selectively driven simultaneously.

7. The display device according to claim 6, wherein it includes: Signal lines for low gray levels, disposed for each column in the matrix arrangement of the plurality of pixel units; A first power supply line to which a power supply voltage is supplied; A second power supply line to which a power supply voltage lower than the power supply voltage is supplied; Signal lines for high gray levels, disposed for each column in the matrix arrangement of the plurality of pixel units; Scanning lines, disposed for each row in the matrix arrangement; Light emission control lines for low gray levels, disposed for each row in the matrix arrangement; Light emission control lines for high gray levels, disposed for each row in the matrix arrangement; A signal line driving circuit for outputting an image signal to the signal lines for low gray levels and the signal lines for high gray levels; and A scanning line driving circuit for outputting a selection signal to the scanning lines, wherein the first driving unit has: A scanning transistor for low gray levels, having a gate terminal connected to the scanning line and a source terminal connected to the signal line for low gray levels; A capacitor for low gray levels, having one end connected to the drain terminal of the scanning transistor for low gray levels and the other end connected to the first power supply line; A driving transistor for low gray levels, having a gate terminal connected to the one end of the capacitor for low gray levels and a source terminal connected to the first power supply line; and A light emission control transistor for low gray levels, having a gate terminal connected to the light emission control line for low gray levels, a drain terminal connected to the anode electrode of the light emitting element, and a source terminal connected to the drain terminal of the driving transistor for low gray levels, wherein the second driving unit has: A scanning transistor for high gray levels, having a gate terminal connected to the scanning line and a source terminal connected to the signal line for high gray levels; A capacitor for high gray levels, having one end connected to the drain terminal of the scanning transistor for high gray levels and the other end connected to the first power supply line; A driving transistor for high gray levels, having a gate terminal connected to the one end of the capacitor for high gray levels and a source terminal connected to the first power supply line; and A light emission control transistor for high gray levels, having a gate terminal connected to the light emission control line for high gray levels, a drain terminal connected to the anode electrode of the light emitting element, and a source terminal connected to the drain terminal of the driving transistor for high gray levels, The light-emitting element is connected between the drain electrode of the light-emitting control transistor for low gray levels, the drain electrode of the light-emitting control transistor for high gray levels, and the second power supply line.

8. The display device according to claim 7, wherein, The potential of the second power supply line is a ground potential.

9. The display device according to claim 1, wherein, The duty ratio during one frame period of the low gray level light-emitting period in which the first driving unit performs the low gray level light-emitting operation is smaller than the duty ratio during one frame period of the high gray level light-emitting period in which the second driving unit performs the high gray level light-emitting operation.

10. The display device according to claim 9, wherein, The duty ratio during one frame period of the low gray level light-emitting period is 0.1 to 10%.

11. The display device according to claim 1, wherein, The maximum value of the current with which the first driving unit drives the light-emitting element is equal to or greater than the maximum value of the current with which the second driving unit drives the light-emitting element.

12. The display device according to claim 1, wherein, The chromaticity of the light emitted by the light-emitting element varies according to the magnitude of the current.

13. The display device according to claim 12, wherein, The light-emitting element is a light-emitting diode.

14. The display device according to claim 1, wherein, The gray level value that exceeds the first boundary value and is equal to or less than the second boundary value is within the range of 1 / 8 to 1 / 4 of the maximum gray level value.

15. The display device according to claim 14, wherein, The maximum gray level value is 256, and the gray level value that exceeds the first boundary value and is equal to or less than the second boundary value is within the range of 32 to 64.

16. The display device according to claim 9, wherein, A duty ratio control unit is provided, and the duty ratio control unit controls the duty ratio for low gray levels during one frame period of the low gray level light-emitting period and the duty ratio for high gray levels during one frame period of the high gray level light-emitting period. The duty ratio control unit makes the ratio of the duty ratio for low gray levels to the duty ratio for high gray levels constant according to a brightness adjustment signal with respect to the entirety of the plurality of pixel units, and varies the duty ratio for low gray levels and the duty ratio for high gray levels.

17. The display device according to claim 9, wherein, A duty ratio control unit is provided, and the duty ratio control unit controls the duty ratio for low gray levels during one frame period of the low gray level light-emitting period and the duty ratio for high gray levels during one frame period of the high gray level light-emitting period. The duty ratio control unit varies the ratio of the duty ratio for low gray levels to the duty ratio for high gray levels according to a brightness adjustment signal with respect to the entirety of the plurality of pixel units, and varies the duty ratio for low gray levels and the duty ratio for high gray levels.

18. The display device according to claim 1, wherein, It includes: A substrate having a pixel unit disposition surface on which a plurality of the pixel units are disposed, a surface opposite to the pixel unit disposition surface, and side surfaces; and A driving unit disposed on one side of the opposite surface and driving the first driving unit and the second driving unit.

19. The display device according to claim 18, wherein the substrate includes side wirings which are disposed on the sides and connect the first driving unit and the second driving unit to the driving unit.

20. A composite display device comprises a plurality of the display devices according to any one of claims 1 to 19, and the composite display device is formed by combining the sides of the plurality of the display devices with each other.

Images