Patent Grant
11908361
The following disclosure relates to a display device and a driving method for the display device, and more specifically to a display device including pixel circuits each including a display element driven by current, such as an organic EL element, and a driving method for the display device.
In recent years, an organic EL display device including pixel circuits each including an organic EL element has been put to practical use. The organic EL element is also called an organic light-emitting diode (OLED), and is a self-emissive display element that emits light at luminance depending on current flowing therethrough. As such, since the organic EL element is a self-emissive display element, the organic EL display device can easily achieve slimming down, a reduction in power consumption, an increase in luminance, etc., compared to a liquid crystal display device that requires a backlight, a color filter, and the like.
As drive systems for the organic EL display device, there are known a passive matrix system (also called a simple matrix system.) and an active matrix system. An organic EL display device that adopts the passive matrix system is simple in structure, but has difficulty in increasing size and increasing resolution. On the other hand, an organic EL display device that adopts the active matrix system (hereinafter, referred to as “active matrix-type organic EL display device”.) can easily achieve an increase in size and an increase in resolution compared to the organic EL display device that adopts the passive matrix system.
In the active matrix-type organic EL display device, a plurality of pixel circuits are formed in matrix form. The pixel circuits of the active matrix-type organic EL display device each typically include an input transistor that selects a pixel; and a drive transistor that controls supply of current to an organic EL element. Note that in the following, current flowing through the organic EL element from the drive transistor may be referred to as “drive current”.
For the organic EL display device, as the drive transistor, typically, a thin-film transistor (TFT) is adopted. However, a threshold voltage of the thin-film transistor changes by degradation. Multiple drive transistors are provided in a display unit, and the level of degradation differs between drive transistors, and thus, variations occur in the threshold voltage. As a result, variations in luminance occur, degrading display quality. In addition, the current efficiency (light-emission efficiency) of the organic EL element decreases over time. Thus, even if constant current is supplied to the organic EL element, luminance gradually decreases over time. As a result of those facts, burn-in occurs. Hence, there are proposed various types of processes (compensation processes) for compensating for degradation of the drive transistor or the organic EL element. Note that there are both of a case in which both the drive transistor and the organic EL element are targets for a compensation process and a case in which only one of the drive transistor and the organic EL element is a target for a compensation process. Hence, in the following, for the sake of convenience, a circuit element that is a target for a compensation process may be referred to as “compensation-target circuit element”.
As one scheme for a compensation process, an external compensation scheme is known. According to the external compensation scheme, in order to detect a characteristic of a compensation-target circuit element, the magnitude of current flowing through the compensation-target circuit element under a predetermined condition is measured by a circuit provided external to a pixel circuit. Then, based on a result of the measurement, a video signal is corrected. By this, degradation of the compensation-target circuit element is compensated for.
Now, terms used in this specification will be described. A series of processes for measuring current flowing through a compensation-target circuit element under a predetermined condition to detect a characteristic of the compensation-target circuit element are referred to as “characteristic detection monitoring”. In relation to this, a period during which the characteristic detection monitoring is performed is referred to as “characteristic detection period”, a row that is a target for the characteristic detection monitoring is referred to as “monitoring row”, and a voltage provided to the pixel circuit upon characteristic detection monitoring out of data voltages provided to a pixel circuit through a data line is referred to as “monitoring voltage”. In addition, for the sake of convenience, a characteristic of the drive transistor is referred to as “TFT characteristic”, and a characteristic of the organic EL element is referred to as “OLED characteristic”. In addition, a period during which a compensation process can be performed to perform display at uniform luminance in all pixels for any gradation level is referred to as “compensable period”. In addition, degradation of the compensation-target circuit element may be represented as “degradation of a pixel”. In addition, in order to determine at what level of luminance each organic EL element is to emit light by a compensation process, there is a need to determine luminance that is used as a reference upon the determination of the level of luminance. Hence, luminance used as a reference for determining display luminance of each organic EL element (each display element) after degradation compensation is referred to as “reference luminance”.
Meanwhile, luminance after a compensation process or the length of the compensable period depends on a mode of a compensation process, which will be described below. Note that in
As one mode of a compensation process, there is a mode in which compensation is performed with reference to the luminance level of the most degraded pixel. In this mode, as shown in
Further, there is a mode in which compensation is performed with reference to an initial luminance level to prevent a reduction in luminance associated with proceeding of degradation of a pixel (degradation of a compensation-target circuit element) (see
In consideration of the above, in order to achieve a balance between luminance and the length of a compensable period, there is proposed a mode in which compensation is performed with reference to a luminance level corresponding to an average degree of degradation of all pixels (see
[Patent Document 1] Japanese Laid-Open Patent Publication No. 2009-141302
[Patent Document 2] Japanese Laid-Open Patent Publication No. 2003-177713
As described above, when compensation is performed with reference to a luminance level corresponding to an average degree of degradation of all pixels, speeding-up of degradation of a pixel is suppressed and uniformity of luminance on the entire screen is maintained. However, when variations in the degree of degradation of pixels in the entire display unit increase, since the compensation current remarkably increases for a pixel with great degradation like a pixel Pb of
An object of the following disclosure is therefore to implement a compensation process that obtains a sufficient length of a compensable period in a display device including pixel circuits each including a display element (typically, an organic EL element) driven by current.
A display device according to some embodiments of the present disclosure is a display device including a plurality of pixel circuits each including a display element driven by current and a drive transistor for controlling current to be supplied to the display element, the display device including:
a degree-of-degradation obtaining circuit configured to determine a degree of degradation representing a level of degradation of a compensation-target circuit element included in each of K pixel circuits, the compensation-target circuit element being at least one of the display element and the drive transistor, and the K pixel circuits being some or all of the plurality of pixel circuits;
an index value calculating circuit configured to calculate, as an index value, a value depending on a deviation determined based on degrees of degradation of the K pixel circuits;
a reference luminance setting circuit configured to set reference luminance based on the index value, the reference luminance being luminance used as a reference for determining display luminance of the display element after degradation compensation; and
a compensation computing circuit configured to compensate for degradation of the compensation-target circuit elements by correcting input video signals based on the reference luminance and the degree of degradation of each of the K pixel circuits, upon generating video signals to be supplied to the plurality of pixel circuits.
A display device according to some other embodiments of the present disclosure is a display device including a plurality of pixel circuits each including a display element driven by current and a drive transistor for controlling current to be supplied to the display element, the display device including:
a degree-of-degradation obtaining circuit configured to determine a degree of degradation representing a level of degradation of a compensation-target circuit element included in each of K pixel circuits, the compensation-target circuit element being at least one of the display element and the drive transistor, and the K pixel circuits being some or all of the plurality of pixel circuits;
an index value calculating circuit configured to calculate, as an index value, a value depending on a deviation determined based on degrees of degradation of the K pixel circuits;
a reference current setting circuit configured to set a reference current corresponding to reference luminance based on the index value, the reference luminance being luminance used as a reference for determining display luminance of the display element after degradation compensation; and
a compensation computing circuit configured to compensate for degradation of the compensation-target circuit elements by correcting input video signals based on the reference current and the degree of degradation of each of the K pixel circuits, upon generating video signals to be supplied to the plurality of pixel circuits.
A driving method (for a display device) according to some embodiments of the present disclosure is a driving method for a display device including a plurality of pixel circuits each including a display element driven by current and a drive transistor for controlling current to be supplied to the display element, the driving method including:
a degree-of-degradation calculating step of determining a degree of degradation representing a level of degradation of a compensation-target circuit element included in each of K pixel circuits, the compensation-target circuit element being at least one of the display element and the drive transistor, and the K pixel circuits being some or all of the plurality of pixel circuits;
an index value calculating step of calculating, as an index value, a value depending on a deviation determined based on degrees of degradation of the K pixel circuits;
a reference value setting step of setting, as a reference value, reference luminance or reference current corresponding to the reference luminance, based on the index value, the reference luminance being luminance used as a reference for determining display luminance of the display element after degradation compensation; and
a compensation computing step of compensating for degradation of the compensation-target circuit elements by correcting input video signals based on the reference value and the degree of degradation of each of the K pixel circuits, upon generating video signals to be supplied to the plurality of pixel circuits.
According to some embodiments of the present disclosure, reference luminance (luminance used as a reference for determining display luminance of each display element after degradation compensation) is set based on variations in the degree of degradation of compensation-target circuit elements. Thus, for example, when the variations in the degree of degradation are small, the reference luminance is set to luminance corresponding to an average degree of degradation, by which speeding-up of degradation of the compensation-target circuit elements can be suppressed while a remarkable darkening of display is suppressed, and when the variations in the degree of degradation is large, the reference luminance is set to luminance lower than the luminance corresponding to the average degree of degradation, by which further speeding-up of degradation of a remarkably degraded compensation-target circuit element can be suppressed. As such, not only when variations in the degree of degradation are small, but also when variations in the degree of degradation are large, speeding-up of degradation of compensation-target circuit elements can be suppressed. From the above, a display device including pixel circuits each including a display element driven by current implements a compensation process that obtains a sufficient length of a compensable period.
Embodiments will be described below with reference to the accompanying drawings. Note that in the following, it is assumed that M and N are integers greater than or equal to 2, i is an integer between 1 and N, inclusive, and j is an integer between 1 and M, inclusive.
<1.1 Overall Configuration and Summary>
In the display unit 30 there are disposed M data lines S(1) to S(M) and N scanning lines G1(1) to G1(N) intersecting the M data lines S(1) to S(M). In addition, in the display unit 30 there are disposed N monitoring control lines G2(1) to G2(N) so as to have a one-to-one correspondence with the N scanning lines G1(1) to G1(N). The scanning lines G1(1) to G1(N) and the monitoring control lines G2(1) to G2(N) are parallel to each other. Furthermore, in the display unit 30 there are provided N×M pixel circuits 310 at respective intersections of the N scanning lines G1(1) to G1(N) and the M data lines S(1) to S(M). By thus providing the N×M pixel circuits 310, a pixel matrix of N rows×M columns is formed in the display unit 30. In addition, in the display unit 30 there are disposed a high-level power line (not shown) that supplies a high-level power supply voltage ELVDD and a low-level power line (not shown) that supplies a low-level power supply voltage ELVSS.
Note that in the following, when the M data lines S(1) to S(M) do not need to be distinguished from each other, the data line is simply given reference character S. Likewise, when the N scanning lines G1(1) to G1(N) do not need to be distinguished from each other, the scanning line is simply given reference character G1, and when the N monitoring control lines G2(1) to G2(N) do not need to be distinguished from each other, the monitoring control line is simply given reference character G2.
The control circuit 10 receives image data VDb sent from an external source and monitoring data MO outputted from the source driver 20, and performs a compensation computation process (described later) on the image data VDb based on the monitoring data MO, thereby generating digital video signals (image data having been subjected to the compensation computation process) VDa to be provided to the source driver 20. Note that the monitoring data MO is data representing the value of current measured to detect a TFT characteristic or an OLED characteristic. The control circuit 10 also controls the operation of the source driver 20 by providing the digital video signals VDa and source control signals SCTL to the source driver 20, and controls the operation of the gate driver 32 by providing gate control signals GCTL to the gate driver 32. The source control signals SCTL include a source start pulse signal, a source clock signal, a latch strobe signal, etc. The gate control signals GCTL include a gate start pulse signal, a gate clock signal, an output enable signal, etc.
The gate driver 32 is connected to the N scanning lines G1(1) to G1(N) and the N monitoring control lines G2 (1) to G2(N). The gate driver 32 is composed of a shift register, a logic circuit, and the like. The gate driver 32 drives the N scanning lines G1(1) to G1(N) and the N monitoring control lines G2(1) to G2(N), based on the gate control signals GCTL outputted from the control circuit 10.
The source driver 20 is connected to the M data lines S(1) to S(M). The source driver 20 selectively performs the operation of driving the data lines S(1) to S(M) and the operation of measuring currents flowing through the data lines S(1) to S(M). That is, as shown in
By driving the N scanning lines G1(1) to G1(N), the N monitoring control lines G2(1) to G2(N), and the M data lines S(1) to S(M) in the above-described manner, an image based on the image data VDb sent from the external source is displayed on the display unit 30. Upon the display, a compensation computation process is performed on the image data VDb based on the monitoring data MO, by which degradation of drive transistors or organic EL elements is compensated for.
<1.2 Pixel Circuits and the Source Driver>
Next, the pixel circuits 310 and the source driver 20 will be described in detail. The source driver 20 performs the following operation when functioning as the data line driving unit 210. The source driver 20 receives source control signals SCTL outputted from the control circuit 10 and applies, as data voltages, video signal voltages generated based on target luminance to the respective M data lines S(1) to S(M). Regarding this, the source driver 20 sequentially holds digital video signals VDa indicating voltages to be applied to the respective data lines S, at timing at which a pulse of a source clock signal is generated, triggered by a pulse of a source start pulse signal. Then, the held digital video signals VDa are converted into analog voltages at timing at which a pulse of a latch strobe signal is generated. The converted analog voltages are simultaneously applied as data voltages to all data lines S(1) to S(M). When the source driver 20 functions as the current monitoring unit 220, the source driver 20 applies a monitoring voltage to the data lines S(1) to S(M), obtains, as analog data, currents flowing through the data lines S(1) to S(M), and converts the analog data into digital data. The converted digital data is outputted as monitoring data MO from the source driver 20.
The input transistor T1 is connected at its control terminal to a scanning line G1(i), connected at its first conductive terminal to the data line S(j), and connected at its second conductive terminal to a control terminal of the drive transistor T2 and a first electrode of the capacitor Cst. The drive transistor T2 is connected at its control terminal to the second conductive terminal of the input transistor T1 and the first electrode of the capacitor Cst, connected at its first conductive terminal to a high-level power line and a second electrode of the capacitor Cst, and connected at its second conductive terminal to a first conductive terminal of the monitoring control transistor T3 and an anode terminal of the organic EL element 311. The monitoring control transistor T3 is connected at its control terminal to a monitoring control line G2(i), connected at its first conductive terminal to the second conductive terminal of the drive transistor T2 and the anode terminal of the organic EL element 311, and connected at its second conductive terminal to the data line S(j). The capacitor Cst is connected at its first electrode to the control terminal of the drive transistor T2 and the second conductive terminal of the input transistor T1, and connected at its second electrode to the first conductive terminal of the drive transistor T2 and the high-level power line. The organic EL element 311 is connected at its anode terminal to the second conductive terminal of the drive transistor T2 and the first conductive terminal of the monitoring control transistor T3, and connected at its cathode terminal to a low-level power line.
As shown in
A digital video signal VDa is provided to an input terminal of the DA converter 21. The DA converter 21 converts the digital video signal VDa into an analog voltage. The analog voltage is a video signal voltage or a monitoring voltage. An output terminal of the DA converter 21 is connected to a non-inverting input terminal of the operational amplifier 22. Accordingly, a video signal voltage or a monitoring voltage is provided to the non-inverting input terminal of the operational amplifier 22. An inverting input terminal of the operational amplifier 22 is connected to the data line S(j). The switch 24 is provided between the inverting input terminal and output terminal of the operational amplifier 22. The capacitor 23 is provided in parallel to the switch 24 and between the inverting input terminal and output terminal of the operational amplifier 22. An input and output control signal DWT included in source control signals SCTL is provided to a control terminal of the switch 24. The output terminal of the operational amplifier 22 is connected to an input terminal of the AD converter 25.
In a configuration such as that described above, when the input and output control signal DWT is at a high level, the switch 24 is in an on state, and a short-circuit state occurs between the inverting input terminal and output terminal of the operational amplifier 22. At this time, the operational amplifier 22 functions as a buffer amplifier. By this, a voltage (a video signal voltage or a monitoring voltage) provided to the non-inverting input terminal of the operational amplifier 22 is applied to the data line S(j). When the input and output control signal DWT is at a low level, the switch 24 is in an off state, and the inverting input terminal and output terminal of the operational amplifier 22 are connected to each other through the capacitor 23. At this time, the operational amplifier 22 and the capacitor 23 function as an integrator circuit. By this, an output voltage from the operational amplifier 22 is a voltage depending on current flowing through the data line S(j). The AD converter 25 converts the output voltage from the operational amplifier 22 into a digital value. Data obtained after the conversion is sent as monitoring data MO to the control circuit 10.
Note that although, in the present embodiment, it is configured such that a signal line for supplying data voltages (a video signal voltage and a monitoring voltage) and a signal line for measuring current are a shared signal line, the configuration is not limited thereto. A configuration can also be adopted in which a signal line for supplying data voltages and a signal line for measuring current are provided independently of each other. In addition, for the configuration of the pixel circuit 310, too, other configurations than the configuration shown in
<1.3 Compensation Process>
A compensation process of the present embodiment will be described below.
<1.3.1 Outline>
The current monitoring unit 220 measures, for each of the N×M pixel circuits 310 in the display unit 30, current flowing through a compensation-target circuit element under a predetermined condition. Then, the current monitoring unit 220 outputs monitoring data MO representing the measurement values of the currents.
The degree-of-degradation calculating circuit 110 calculates a degree of degradation X representing the level of degradation of each compensation-target circuit element, based on the monitoring data MO. In other words, the degree-of-degradation calculating circuit 110 calculates a degree of degradation X based on current measured by the current monitoring unit 220. That is, the degree-of-degradation calculating circuit 110 performs conversion from a current value to a degree of degradation X. Regarding this, for a transistor, a curve representing an I-V characteristic (current-voltage characteristic) changes due to degradation, for example, from a curve given reference character 61 in
Meanwhile, when only the drive transistor T2 is treated as a compensation-target circuit element, a degree of degradation X is calculated based on a current value measured to detect a TFT characteristic, and when only the organic EL element 311 is treated as a compensation-target circuit element, a degree of degradation X is calculated based on a current value measured to detect an OLED characteristic, and when both the drive transistor T2 and the organic EL element 311 are treated as compensation-target circuit elements, a degree of degradation X is calculated based on a current value measured to detect a TFT characteristic and a current value measured to detect an OLED characteristic.
The degrees of degradation (data on the degrees of degradation) X for one screen calculated by the degree-of-degradation calculating circuit 110 are saved in the frame memory 120.
The variation coefficient calculating circuit 130 calculates a variation coefficient CV of the degrees of degradation X based on the degrees of degradation X for one screen held in the frame memory 120. Here, assuming that the number of the pixel circuits 310 in the display unit 30 is K (i.e., N×M=K), a specific way of determining the variation coefficient CV will be described below. Note that with p being an integer between 1 and K, inclusive, the degree of degradation of each of the K pixel circuits 310 is represented by Xp.
First, as shown in the following equation (1), an average degree of degradation Xave is calculated by dividing a total sum of the degrees of degradation Xp of the K pixel circuits 310 by K.
Then, a total sum of K “squares of a difference between the degree of degradation Xp and the average degree of degradation Xave” is determined, and the square root of a value obtained by dividing the total sum by K is determined. That is, a standard deviation σ of the degrees of degradation X is calculated by the following equation (2):
Finally, as shown in the following equation (3), by dividing the standard deviation σ by the average degree of degradation Xave, a variation coefficient CV of the degrees of degradation X is calculated.
The variation coefficient CV calculated in the above-described manner is provided to the reference luminance setting circuit 140.
The reference luminance setting circuit 140 sets the above-described reference luminance SB based on the degrees of degradation X for one screen held in the frame memory 120 and the variation coefficient CV calculated by the variation coefficient calculating circuit 130. Note that a detailed description of the reference luminance setting circuit 140 will be made later.
The compensation computing circuit 150 performs a compensation computation process on input video signals (image data sent from an external source) VDb, based on the degree of degradation X of each pixel circuit 310 and the reference luminance SB set by the reference luminance setting circuit 140. By this, the input video signals VDb are corrected so as to compensate for degradation of pixels, and digital video signals VDa to be supplied to the N×M pixel circuits 310 in the display unit 30 are generated. As above, the compensation computing circuit 150 corrects input video signals VDb based on reference luminance SB and the degree of degradation X of each of the N×M pixel circuits 310 upon generating digital video signals VDa to be supplied to the N×M pixel circuits 310, thereby compensating for degradation of pixels (degradation of compensation-target circuit elements). Note that a further detailed description of processes performed by the compensation computing circuit 150 will be made later.
The data line driving unit 210 generates data voltages based on the digital video signals (image data having been subjected to the compensation computation process) VDa generated by the compensation computing circuit 150, and applies the data voltages to the data lines S.
Note that although, in the present embodiment, it is assumed that current values are obtained for all pixels (pixel circuits 310) by characteristic detection monitoring, the configuration is not limited thereto. By obtaining current values by characteristic detection monitoring, with a plurality of pixels serving as one unit, it becomes possible to reduce memory capacity for holding the degrees of degradation X. In this case, compensation accuracy decreases, but when a high-resolution panel whose pixel size is very small is adopted, it is difficult for a viewer to visually recognize a difference between an image having been subjected to a compensation process for a case in which current values are obtained for all pixels and an image having been subjected to a compensation process for a case in which current values are obtained with a plurality of pixels serving as one unit. Therefore, when a high-resolution panel is adopted, by obtaining current values by characteristic detection monitoring with a plurality of pixels serving as one unit, an effect of cost reduction is obtained.
As above, the degree-of-degradation calculating circuit 110 may calculate a degree of degradation X for all of the N×M pixel circuits 310, or may calculate a degree of degradation X for some of the N×M pixel circuits 310, with a plurality of pixel circuits 310 serving as one unit. Here, when it is assumed that K pixel circuits 310 are targets for calculation of the degree of degradation X by the degree-of-degradation calculating circuit 110, the variation coefficient calculating circuit 130, the reference luminance setting circuit 140, and the compensation computing circuit 150 perform the above-described processes based on the degrees of degradation X of the K pixel circuits 310.
<1.3.2 Characteristic Detection Monitoring>
Next, characteristic detection monitoring will be described.
During the detection preparation period Ta, a scanning line G1(i) is brought into an active state, and a monitoring control line G2(i) is maintained in a non-active state. By this, the input transistor T1 goes into an on state, and the monitoring control transistor T3 is maintained in an off state. In addition, during the detection preparation period Ta, a monitoring voltage Vmg(i, j) is applied to a data line S(j). Note that the monitoring voltage Vmg(i, j) does not indicate a given fixed voltage, and the magnitude of the monitoring voltage Vmg(i, j) differs between when a TFT characteristic is detected and when an OLED characteristic is detected. That is, the monitoring voltage used here is a concept including both a monitoring voltage for detecting a TFT characteristic (hereinafter, referred to as “TFT characteristic measurement voltage”.) and a monitoring voltage for detecting an OLED characteristic (hereinafter, referred to as “OLED characteristic measurement voltage”.). When the monitoring voltage Vmg(i, j) is the TFT characteristic measurement voltage, the drive transistor T2 goes into an on state. When the monitoring voltage Vmg(i, j) is the OLED characteristic measurement voltage, the drive transistor T2 is maintained in an off state.
During the current measurement period Tb, the scanning line G1 (i) is brought into a non-active state and the monitoring control line G2(i) is brought into an active state. By this, the input transistor T1 goes into an off state and the monitoring control transistor T3 goes into an on state. Here, when the monitoring voltage Vmg(i, j) is the TFT characteristic measurement voltage, the drive transistor T2 is maintained in an on state and current does not flow through the organic EL element 311. Thus, as indicated by an arrow given reference character 7 in
During the video signal voltage writing period Tc, the scanning line G1(i) is brought into an active state and the monitoring control line G2(i) is brought into a non-active state. By this, the input transistor T1 goes into an on state and the monitoring control transistor T3 goes into an off state. In addition, during the video signal voltage writing period Tc, a data voltage based on target luminance is applied to the data line S(j). By this, the drive transistor T2 goes into an on state. As a result, as indicated by an arrow given reference character 9 in
<1.3.3 Setting of Reference Luminance>
Next, the setting of reference luminance (luminance used as a reference for determining display luminance of each organic EL element 311 after degradation compensation) will be described. The organic EL display device according to the present embodiment is characterized in that the way of setting reference luminance upon a compensation process varies depending on the magnitude of variations in the degree of degradation X, which will be described with reference to
In the present embodiment, when variations in the degree of degradation X are relatively small, compensation is performed with reference to a luminance level corresponding to an average degree of degradation of all pixels as shown in
Note that as described above, the degree-of-degradation calculating circuit 110 may calculate a degree of degradation X for some of the N×M pixel circuits 310.
In this case, the above-described pre-compensation average luminance is average luminance for a case in which organic EL elements 311 included in some of the N×M pixel circuits 310 emit light based on a predetermined gradation value in a state in which degradation compensation is not performed. From the above, when it is assumed that K pixel circuits 310 are targets for calculation of a degree of degradation X, if the variation coefficient CV is less than or equal to the threshold value, then the reference luminance is set to average luminance (pre-compensation average luminance) of K organic EL elements 311 included in the K pixel circuits 310 for a case in which the K organic EL elements 311 emit light based on a predetermined gradation value in a state in which degradation compensation is not performed, and if the variation coefficient CV is greater than the threshold value, then the reference luminance is set to luminance lower than the pre-compensation average luminance.
The setting of the reference luminance is performed, for example, such that a correspondence between the variation coefficient and the reference luminance satisfies a correspondence represented by a bold solid line of
Meanwhile, the pre-compensation average luminance changes over time. Therefore, in order to calculate the reference luminance, there is a need to hold information indicating a correspondence such as that shown in
The average luminance calculating unit 142 calculates the above-described pre-compensation average luminance Bave, based on the degrees of degradation X for one screen held in the frame memory 120. For the calculation, first, a degree of degradation X of each pixel circuit 310 (e.g., the amount of change in threshold voltage from an initial state) is read from the frame memory 120. Then, an I-V characteristic (current-voltage characteristic) of the drive transistor T2 in each pixel circuit 310 is determined from the read degree of degradation X. For example, in a case in which the amount of change in threshold voltage from an initial state is treated as the degree of degradation X, the I-V characteristic is obtained by shifting an I-V characteristic in an initial state based on the amount of change in threshold voltage. When the light-emission efficiency of the organic EL element 311 has not been reduced, display luminance is proportional to the amount of current, and thus, a luminance-voltage characteristic (a relationship between a voltage applied to the control terminal of the drive transistor T2 and display luminance) of each pixel circuit 310 is obtained based on an I-V characteristic of the drive transistor T2 in each pixel circuit 310 and a relationship between the amount of current and display luminance of the organic EL element 311. Furthermore, display luminance of each pixel when a voltage corresponding to a predetermined gradation value is provided to the control terminal of the drive transistor T2 in each pixel circuit 310 (i.e., display luminance of each pixel for a case in which the organic EL element 311 included in each pixel circuit 310 emits light based on a predetermined gradation value in a state in which degradation compensation is not performed) is calculated from the luminance-voltage characteristic of each pixel circuit 310. Then, by dividing a total sum of the display luminance of all pixels by the number of pixels (the number of the pixel circuits 310), pre-compensation average luminance Bave is calculated. Note that display luminance for a case in which the light-emission efficiency of the organic EL element 311 has been reduced is obtained by multiplying display luminance for a case in which the light-emission efficiency of the organic EL element 311 has not been reduced by reduced light-emission efficiency (a value less than 1) which is estimated from the degree of degradation X. Thus, when the light-emission efficiency of the organic EL element 311 has been reduced, a luminance-voltage characteristic of each pixel circuit 310 is obtained based on an I-V characteristic of the drive transistor T2 in each pixel circuit 310 and a “relationship between the amount of current and display luminance of the organic EL element 311” that takes into account a reduction in light-emission efficiency.
The parameter holding unit 144 is, for example, a register and holds parameters PV for determining an adjustment factor AF based on the variation coefficient CV of the degrees of degradation X. More specifically, in order that a graph whose horizontal axis is a value that the variation coefficient CV can take and whose vertical axis is a value that the adjustment factor AF for calculating reference luminance can take (a graph representing a correspondence between the variation coefficient CV and the adjustment factor AF) (see
The adjustment factor calculating unit 146 calculates an adjustment factor AF based on the variation coefficient CV of the degrees of degradation X by referring to the parameters PV held in the parameter holding unit 144.
In a case shown in
The reference luminance calculating unit 148 calculates reference luminance SB by multiplying the pre-compensation average luminance Bave calculated by the average luminance calculating unit 142 by the adjustment factor AF calculated by the adjustment factor calculating unit 146.
Based on the reference luminance SB set in the above-described manner, a compensation computation process by the compensation computing circuit 150 is performed. By this, degradation of pixels (degradation of compensation-target circuit elements) is compensated for.
<1.3.4 Processes Performed by the Compensation Computing Circuit>
Processes performed by the compensation computing circuit 150 (see
First, target luminance corresponding to a gradation value indicated by an input video signal VDb is determined. The target luminance is luminance indicating at what level of brightness the organic EL element 311 is to emit light so that degradation compensation is performed, and is luminance determined for each organic EL element 311. For example, the target luminance differs between an organic EL element 311 that is to emit light (display) based on the gradation value “30” and an organic EL element 311 that is to emit light (display) based on the gradation value “100”. Target luminance Lx is determined by the following equation (4):
Lx=SB×(Gx/Gm)γ (4)
For the above equation (4), SB represents reference luminance set by the reference luminance setting circuit 140, Gx represents a gradation value indicated by an input video signal VDb, Gm represents a predetermined gradation value used upon calculation of pre-compensation average luminance Bave by the average luminance calculating unit 142, and γ represents a gamma value that defines a relationship between the gradation value and luminance in the organic EL display device.
Next, the magnitude of current (the amount of current) to be supplied to the organic EL element 311 is determined. For the determination, first, a relationship between the amount of current and display luminance of the organic EL element 311 is determined taking into account a reduction in light-emission efficiency which is estimated from the degree of degradation X read from the frame memory 120. Then, based on the relationship, the magnitude of current (the amount of current) to be supplied to the organic EL element 311 is determined from the target luminance Lx determined by the above equation (4).
Thereafter, a gate voltage corresponding to the magnitude of current (the amount of current) to be supplied to the organic EL element 311 is determined based on an I-V characteristic of the drive transistor T2 obtained after degradation (which is obtained by shifting an I-V characteristic in an initial state depending on the degree of degradation X).
<1.4 Effects>
According to the present embodiment, a variation coefficient CV of the degrees of degradation X of compensation-target circuit elements is calculated, and reference luminance (luminance used as a reference for determining display luminance of each organic EL element 311 after degradation compensation) SB is set based on the variation coefficient CV. When the variation coefficient CV is less than or equal to a threshold value which is prepared in advance, i.e., when variations in the degree of degradation X are relatively small, the reference luminance SB is set to pre-compensation average luminance Bave. In this case, the magnitude of compensation current is relatively small, and thus, speeding-up of degradation of pixels (degradation of compensation-target circuit elements) is suppressed. In addition, a remarkable darkening of display is also suppressed. When the variation coefficient CV is greater than the threshold value, i.e., when variations in the degree of degradation X are relatively large, the reference luminance SB is set to luminance lower than the pre-compensation average luminance Bave. In this case, even if there is a remarkably degraded pixel compared to other pixels, since supply of large compensation current to the remarkably degraded pixel is suppressed, speeding-up of degradation of the pixel is suppressed. As such, not only when variations in the degree of degradation X are small, but also when variations in the degree of degradation X are large, speeding-up of degradation of pixels is suppressed. From the above, according to the present embodiment, an organic EL display device implements a compensation process that obtains a sufficient length of a compensable period. That is, in the organic EL display device, while uniformity of luminance on the entire screen is secured, rapid degradation of pixels is suppressed over known display devices.
In addition, in the present embodiment, the reference luminance SB is set based on the variation coefficient CV of the degrees of degradation X as described above. Since the variation coefficient CV is a dimensionless numerical value, by using the variation coefficient CV, relative evaluation of variations in the degree of degradation X can be performed regardless of the magnitude of a numerical value indicating the degree of degradation X. Therefore, for example, it becomes unnecessary to perform the operation of adjusting a threshold value for each device or for each model, etc.
<1.5 Variants>
<1.5.1 First Variant>
In the first embodiment, the setting of reference luminance SB by the reference luminance setting circuit 140 is performed based on a variation coefficient CV of the degrees of degradation X. However, the configuration is not limited thereto. For example, the setting of reference luminance SB may be performed based on the standard deviation or variance of the degrees of degradation X or may be performed based on the maximum deviation of the degrees of degradation X (a difference between a maximum degree of degradation and a minimum degree of degradation). That is, the configuration may be such that an index value calculating circuit (in the first embodiment, the variation coefficient calculating circuit 130) is provided that calculates, as an index value, a value depending on a deviation determined based on the degrees of degradation X of some or all of the N×M pixel circuits 310, and the setting of reference luminance SB by the reference luminance setting circuit 140 is performed based on the index value calculated by the index value calculating circuit.
<1.5.2 Second Variant>
In the first embodiment, when the variation coefficient CV is less than or equal to the threshold value, the reference luminance SB is set to the pre-compensation average luminance Bave, and when the variation coefficient CV is greater than the threshold value, the reference luminance SB is set to luminance lower than the pre-compensation average luminance Bave (see
<1.5.3 Third Variant>
In the first embodiment, there is provided only one threshold value to be compared with the variation coefficient CV upon setting the reference luminance SB. However, the configuration is not limited thereto, and two or more threshold values may be provided. By this, for example, as shown in
A second embodiment will be described. The following mainly describes only differences from the first embodiment.
<2.1 Overall Configuration, etc.>
An overall configuration in the present embodiment is the same as the overall configuration in the first embodiment (see
<2.2 Compensation Process>
A compensation process of the present embodiment will be described below.
<2.2.1 Outline>
The reference current setting circuit 160 sets reference current SC corresponding to reference luminance (luminance used as a reference for determining display luminance of each organic EL element 311 after degradation compensation), based on the degrees of degradation X (Xr, Xg, and Xb) for one screen held in the frame memory 120 and a variation coefficient CV calculated by the variation coefficient calculating circuit 130. Note that the reference current for the pixel circuit 310R for red is represented by SCr, the reference current for the pixel circuit 310G for green is represented by SCg, and the reference current for the pixel circuit 310B for blue is represented by SCb.
The compensation computing circuit 150 performs a compensation computation process on an input video signal (image data sent from an external source) VDb, based on the degrees of degradation X of each pixel circuit 310 and the reference currents SC set by the reference current setting circuit 160. By this, the input video signals VDb are corrected so as to compensate for degradation of pixels, and digital video signals VDa to be supplied to the N×M pixel circuits 310 in the display unit 30 are generated. As above, the compensation computing circuit 150 corrects an input video signal VDb based on reference currents SC and the degrees of degradation X of each of the N×M pixel circuits 310 upon generating digital video signals VDa to be supplied to the N×M pixel circuits 310, thereby compensating for degradation of pixels (degradation of compensation-target circuit elements). Note that a further detailed description of processes performed by the compensation computing circuit 150 will be made later.
As described above, in the present embodiment, unlike the first embodiment, reference currents SC are set based on the variation coefficient CV of the degrees of degradation X, and a compensation computation process is performed based on the reference currents SC and the degrees of degradation X of each pixel circuit 310.
Note that in the present embodiment, too, the degrees of degradation X calculated by the degree-of-degradation calculating circuit 110 may be the degrees of degradation X of some of the N×M pixel circuits 310. Accordingly, processes by the variation coefficient calculating circuit 130, the reference current setting circuit 160, and the compensation computing circuit 150 may be performed based on the degrees of degradation X of some of the N×M pixel circuits 310.
Meanwhile, since one pixel is composed of a pixel circuit 310R for red, a pixel circuit 310G for green, and a pixel circuit 310B for blue as described above, for the degree of degradation X, too, the degree of degradation Xr for red, the degree of degradation Xg for green, and the degree of degradation Xb for blue are determined. However, it is conceivable that if reference luminance is set for each color based on those degrees of degradation Xr, Xg, and Xb and a compensation process is performed based on the reference luminance, then white balance gets thrown off. For example, when reference luminance for blue is set to a value higher than that of reference luminance for red and green, an image with an overall bluish cast is displayed. Hence, the variation coefficient calculating circuit 130 calculates a variation coefficient CV common to all colors, based on the degrees of degradation X for all colors (the degree of degradation Xr of the pixel circuit 310R for red, the degree of degradation Xg of the pixel circuit 310G for green, and the degree of degradation Xb of the pixel circuit 310B for blue). The reference current setting circuit 160 first calculates base reference current used as a base for calculating reference current for each color, based on the variation coefficient CV common to all colors which is calculated by the variation coefficient calculating circuit 130. Then, based on the base reference current, the reference current setting circuit 160 sets, for each color, the reference current according to light-emission efficiency. That is, the reference current setting circuit 160 sets the reference current SCr for red, the reference current SCg for green, and the reference current SCb for blue.
In the present embodiment, for example, as shown in
<2.2.2 Reference Current Setting Circuit>
The average current calculating unit 162 calculates the pre-compensation average current Cave based on the degrees of degradation X for one screen held in the frame memory 120. For the calculation, first, pre-compensation average luminance Bave is calculated in the same manner as in the first embodiment. Then, the pre-compensation average current Cave is determined from the pre-compensation average luminance Bave, based on a relationship between the amount of current and display luminance of the organic EL element 311.
As in the first embodiment, the parameter holding unit 164 holds, as parameters PV, a value, on the horizontal axis, of a critical point on a graph for determining an adjustment factor AF (see
The reference current calculating unit 168 calculates the above-described base reference current by multiplying the pre-compensation average current Cave calculated by the average current calculating unit 162 by the adjustment factor AF calculated by the adjustment factor calculating unit 166. Based on the base reference current, the reference current calculating unit 168 calculates the reference current SC for each color, taking into account light-emission efficiency for each of red, green, and blue. That is, the reference current SCr for red, the reference current SCg for green, and the reference current SCb for blue are calculated by the reference current calculating unit 168.
Based on the reference currents SCr, SCg, and SCb for the respective colors set in the above-described manner, a compensation computation process by the compensation computing circuit 150 is performed. By this, degradation of pixels (degradation of compensation-target circuit elements) are compensated for.
<2.2.3 Processes Performed by the Compensation Computing Circuit>
Processes performed by the compensation computing circuit 150 (see
First, target current corresponding to a gradation value indicated by an input video signal VDb is determined. The target current is current to be supplied to an organic EL element 311 whose light-emission efficiency has not been reduced, so that the organic EL element 311 emits light at the above-described target luminance (luminance that indicates at what level of brightness the organic EL element 311 is to emit light so that degradation compensation is performed, and that is determined for each organic EL element 311). The target current Cx is determined by the following equation (5):
Cx=SCr×(Gx/Gm)γ (5)
For the above equation (5), SCr represents the reference current (the reference current for red) set by the reference current setting circuit 160, Gx represents a gradation value indicated by an input video signal VDb, Gm represents a predetermined gradation value used upon calculation of a pre-compensation average current Cave by the average current calculating unit 162, and y represents a gamma value that defines a relationship between the gradation value and luminance of the organic EL display device.
Then, taking into account a reduction in light-emission efficiency which is estimated from a degree of degradation X read from the frame memory 120, the magnitude of current (the amount of current) to be actually supplied to the organic EL element 311 is determined from the target current Cx determined by the above equation (5).
Thereafter, a gate voltage corresponding to the magnitude of current (the amount of current) to be actually supplied to the organic EL element 311 is determined based on an I-V characteristic of the drive transistor T2 obtained after degradation (which is obtained by shifting an I-V characteristic in an initial state based on the degree of degradation X).
The same processes are also performed for data on the pixel circuit 310G for green and the pixel circuit 310B for blue.
<2.3 Effects>
According to the present embodiment, a variation coefficient CV of the degrees of degradation X of compensation-target circuit elements is calculated, and the base reference current is calculated based on the variation coefficient CV. Then, based on the base reference current, the reference current SC is set for each color, taking into account light-emission efficiency for each color. When the variation coefficient CV is less than or equal to a threshold value which is prepared in advance, i.e., when variations in the degree of degradation X are relatively small, the base reference current is set to the pre-compensation average current Cave. In this case, the magnitude of the compensation current is relatively small, and thus, speeding-up of degradation of pixels (degradation of compensation-target circuit elements) is suppressed. In addition, a remarkable darkening of display is also suppressed. When the variation coefficient CV is greater than the threshold value, i.e., when variations in the degree of degradation X are relatively large, the base reference current is set to current smaller than the pre-compensation average current Cave. In this case, even if there is a remarkably degraded pixel compared to other pixels, since supply of large compensation current to the remarkably degraded pixel is suppressed, speeding-up of degradation of the pixel is suppressed. As such, not only when variations in the degree of degradation X are small, but also when variations in the degree of degradation X are large, speeding-up of degradation of pixels is suppressed. From the above, according to the present embodiment, an organic EL display device that performs color display implements a compensation process that obtains a sufficient length of a compensable period. In addition, since the reference current SC is set for each color taking into account light-emission efficiency, white balance does not get thrown off by a compensation process.
<2.4 Variant>
In the second embodiment, a variation coefficient CV common to all colors is calculated based on the degrees of degradation X for all colors (the degree of degradation Xr of the pixel circuit 310R for red, the degree of degradation Xg of the pixel circuit 310G for green, and the degree of degradation Xb of the pixel circuit 310B for blue). However, the configuration is not limited thereto, and a variation coefficient CV common to all colors may be calculated based on the degree of degradation for green (the degree of degradation of the pixel circuit 310G for green) Xg, which will be described below as a variant of the second embodiment.
For the organic EL element 311 in the pixel circuit 310, generally, phosphorescent materials are adopted as light-emitting materials for red and green, and fluorescent materials are adopted as light-emitting materials for blue. The light-emission efficiency of phosphorescent materials is three times or more higher than the light-emission efficiency of fluorescent materials, but the phosphorescent materials are generally very low in thermal stability. In addition, green has a shorter wavelength than red and thus is energetically unstable. Therefore, an organic EL element that emits green light consequently degrades earlier than an organic EL element that emits blue light or an organic EL element that emits red light. Hence, in the present variant, a variation coefficient CV common to all colors is calculated based on the degree of degradation Xg for green.
The average current calculating unit 162 calculates the pre-compensation average current Cave based on the degree of degradation Xg for green among the degrees of degradation X for one screen held in the frame memory 120. The reference current calculating unit 168 calculates the above-described base reference current by multiplying the pre-compensation average current Cave calculated by the average current calculating unit 162 by an adjustment factor AF calculated by the adjustment factor calculating unit 166. In the present variant, the variation coefficient CV and the pre-compensation average current Cave are calculated based on the degree of degradation Xg for green, and thus, the base reference current calculated by the reference current calculating unit 168 is the reference current SCg for green. The reference current SCr for red is calculated based on the base reference current (the reference current SCg for green), taking into account a difference in light-emission efficiency between green and red. Likewise, the reference current SCb for blue is calculated based on the base reference current (the reference current SCg for green), taking into account a difference in light-emission efficiency between green and blue.
According to the present variant described above, the amount of computation required to calculate a variation coefficient CV can be reduced compared to that in the second embodiment. Regarding this, since the variation coefficient CV is calculated based on the degree of degradation Xg for green, taking into account early degradation of an organic EL element that emits green light, a reduction in compensation accuracy associated with the reduction in the amount of computation is suppressed.
In each of the above-described embodiments (including the variants), an organic EL display device is described as an example, but the display device is not limited thereto. The content of the present disclosure can be applied to any display device including display elements driven by current (display elements whose luminance or transmittance is controlled by current). For example, the content of the present disclosure can also be applied to an inorganic EL display device including inorganic light-emitting diodes, a QLED display device including quantum dot light emitting diodes (QLEDs), etc.
In the first embodiment, in an organic EL display device that performs monochrome display, reference luminance is set based on an index value (a variation coefficient CV of the degrees of degradation X), and a compensation computation process is performed based on the reference luminance. However, in the organic EL display device that performs monochrome display, as in the second embodiment, the configuration may be such that a reference current corresponding to reference luminance is set based on an index value (a variation coefficient CV of the degrees of degradation X) and a compensation computation process is performed based on the reference current. In this case, the reference current setting circuit 160 shown in
In the second embodiment, in an organic EL display device that performs color display, reference currents corresponding to reference luminance are set based on an index value (a variation coefficient CV of the degrees of degradation X), and a compensation computation process is performed based on the reference currents. However, in the organic EL display device that performs color display, as in the first embodiment, the configuration may be such that reference luminance is set based on an index value (a variation coefficient CV of the degrees of degradation X) and a compensation computation process is performed based on the reference luminance. In this case, the setting of reference luminance SB by the reference luminance setting circuit 140 shown in
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2020/038181 | 10/8/2020 | WO |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2022/074797 | 4/14/2022 | WO | A |
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| Number | Date | Country |
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| Number | Date | Country | |
|---|---|---|---|
| 20230368708 A1 | Nov 2023 | US |
