JP2008151946A - Electroluminescence display device - Google Patents

Abstract

The power consumption of an EL display device is reduced.
A data signal to be applied to an element driving transistor Tr2 for controlling a current flowing through the EL element is adjusted in accordance with a use environment such as external light luminance, temperature, and the degree of deterioration of the EL element, and a driving power source The power supply voltage difference (CV margin) between the cathode power supply and the cathode power supply is adjusted. In particular, the power supply voltage difference between the drive power supply and the cathode power supply is reduced by raising the cathode power supply voltage in response to lowering the data signal and lowering the operating voltage of the EL element. Thereby, not only the current drop due to the data signal but also the power consumption corresponding to the CV shift voltage is reduced.
[Selection] Figure 4

Description

  The present invention relates to a display device including a light emitting element such as an organic electroluminescence (EL) element.

  An EL display device provided with an EL element, which is a light emitting element having a diode structure, in each pixel is advantageous in that it is a self-luminous type and is thin and has low power consumption, such as a liquid crystal display device (LCD) and a CRT. Attention has been focused on as a display device that can replace the display device, and research has been underway.

  In addition, an active matrix EL display device that includes a switching element such as a thin film transistor (TFT) that individually controls the EL element in each pixel and controls the EL element for each pixel is expected as a high-definition display device.

In such an EL display device, the luminance required for the display device varies greatly depending on the usage environment of the device. For example, as shown in FIG. 10, when the outside light luminance is high, such as a sunny outdoor day, the luminance necessary for the display device is very large in order to make the display image visible. In the example of FIG. 10, the luminance required in a sunny outdoor environment is 250 cd / m ² . On the other hand, the luminance required in an environment where the outside light luminance is low indoors or at night is about 25 cd / m ² , and the required luminance for viewing is 10 times different.

  Therefore, in Patent Document 1 and the like, in order to realize high display quality (high contrast display) even in a sunny outdoor environment, it is proposed to increase the amount of current flowing through the EL element to emit light with high brightness. Has been.

  In addition, when used in an indoor environment, the external light luminance is low, and thus the visibility is good even when the display device does not emit light with high luminance. Furthermore, if the luminance of the display device is high when the luminance of the external light is low, there is a possibility that the display screen may be dazzled and the visual feeling may be impaired. Is preferred.

JP 2006-30318 A

  As the demand for lower power consumption of electronic devices becomes stronger, EL display devices used as displays of such devices are strongly desired to further reduce power consumption without reducing display quality. When the luminance required for visual recognition is low, the power consumption can be reduced by reducing the light emission luminance by reducing the amount of current flowing to the EL element, but it is desirable to further reduce the power consumption while maintaining the display quality. It is.

  The present invention has been made in view of the above problems, and an object thereof is to enable reduction of power consumption of an EL display device.

  In another aspect of the present invention, the present invention provides an electroluminescence display device, which includes a display unit including a plurality of pixels, a control unit that controls display operation in the display unit, and a use condition of the display device. Each of the plurality of pixels includes an electroluminescence element and an element driving transistor connected to the electroluminescence element, and the source and drain of the element driving transistor and the electroluminescence element The element driving transistor is connected in series between the driving power source and the cathode power source, the element driving transistor operates in a saturation region, and controls a current flowing through the electroluminescence element according to a data signal, and the control unit includes the determination unit A data signal adjusting unit that adjusts the data signal according to the determination result in And a voltage adjustment unit that adjusts a power supply voltage difference between the drive power supply and the cathode power supply according to the determination result, and the operation voltage of the electroluminescence element is made lower than an operation reference value by adjusting the data signal In this case, the voltage adjustment unit reduces the power supply voltage difference between the drive power supply and the cathode power supply from a reference value in accordance with a decrease in the operating voltage.

  In another aspect of the present invention, in the electroluminescence display device, the voltage adjustment unit includes the amount of decrease in the operating voltage and the amount of decrease in the absolute value of the pinch-off voltage of the element driving transistor that is decreased by adjusting the data signal. Accordingly, the power supply voltage difference between the drive power supply and the cathode power supply is reduced.

  In another aspect of the present invention, an electroluminescence display device includes a display unit including a plurality of pixels, and a drive unit that controls a display operation in the display unit, and each of the plurality of pixels includes: An electroluminescence element and an element driving transistor connected to the electroluminescence element, the source and drain of the element driving transistor and the electroluminescence element being connected in series between a driving power source and a cathode power source; The driving transistor operates in a saturation region and controls a current flowing through the electroluminescence element in accordance with a data signal. The driving unit determines a use condition of the display device, and a determination result in the determination unit The data signal adjustment unit for adjusting the data signal and the determination result in the determination unit. And a voltage adjustment unit that adjusts a power supply voltage difference between the drive power supply and the cathode power supply, and the voltage adjustment unit includes a current-voltage characteristic line of the electroluminescence element and a current-voltage characteristic of the element drive transistor. The power supply voltage difference between the drive power supply and the cathode power supply is set so that the voltage difference between the operating voltage of the electroluminescent element corresponding to the intersection with the line and the pinch-off voltage of the element drive transistor is within a reference range. adjust.

  In another aspect of the present invention, the electroluminescence display device further includes a photosensor that detects an external light luminance of the display device, and the determination unit compares the detected external light luminance with a reference luminance. When the luminance is lower than the reference luminance, the data signal adjustment unit adjusts the data signal so as to reduce the amount of current flowing through the electroluminescence element, and the voltage adjustment unit increases the power supply voltage of the cathode power supply. Let

  In another aspect of the present invention, a temperature sensor is further provided, and the determination unit determines a slope of a current-voltage characteristic of the electroluminescence element according to the detected temperature, and the slope is smaller than a reference value. The voltage adjustment unit raises the power supply voltage of the cathode power supply from a reference value.

  In another aspect of the present invention, in the electroluminescence display device, when the inclination is larger than a reference value, the voltage adjusting unit lowers the power supply voltage of the cathode power supply from the reference value.

  In another aspect of the present invention, in the electroluminescence display device, the determination unit includes a current characteristic detection unit for detecting a current characteristic in each pixel, and the determination unit does not emit light from the electroluminescence element. When the inspection off display signal for the level and the inspection on display signal for the emission level are supplied, the cathode current of the electroluminescence element according to the inspection off display signal and the inspection on display signal A corresponding on / off current difference from the cathode current of the electroluminescence element is detected, and a change in use condition is determined from a change in current characteristics based on the on / off current difference.

In another aspect of the present invention, in the electroluminescence display device, a cathode power supply line that supplies the cathode power supply to the electroluminescence element is a vertical scanning direction that intersects a horizontal scanning direction of the plurality of pixels, or a horizontal scanning direction. Are provided for each column or row.

  According to the present invention, the data signal to be applied to the element driving transistor for controlling the current flowing to the EL element of each pixel is adjusted according to the use environment such as the external light brightness, temperature, and the degree of deterioration of the EL element. The power supply voltage difference between the drive power supply and the cathode power supply is adjusted. In particular, when the data signal is lowered to lower the operating voltage of the EL element, the voltage difference (cathode voltage margin) between the pinch-off voltage of the element driving transistor and the operating voltage of the EL element increases due to the lowering of the operating voltage. In the present invention, expansion of the margin is suppressed by reducing the power supply voltage difference between the drive power supply and the cathode power supply. In the EL display device, the power consumption in each pixel is represented by the product of the current (Ioled) flowing through the EL element and the power supply voltage difference (PVDD−CV) between the drive power supply and the cathode power supply. Therefore, according to the present invention, it is possible to further reduce power consumption by reducing not only the cathode current but also the power supply voltage difference.

  The best mode for carrying out the present invention (hereinafter referred to as an embodiment) will be described below with reference to the drawings.

  In the present embodiment, the display device is specifically an active matrix organic EL display device, and a display unit including a plurality of pixels is formed on the EL panel 100. FIG. 1 is a diagram showing an example of an equivalent circuit of the active matrix EL display device according to this embodiment. In the display portion of the EL panel 100, a plurality of pixels are arranged in a matrix, and in the horizontal (H) scanning direction (row direction) of the matrix, a selection line (gate line GL) 10 from which selection signals are sequentially output is provided. In the vertical (V) scanning direction (column direction), a data line 12 (DL) from which a data signal (Vsig) is output and an organic EL element (hereinafter simply referred to as an “EL element”) that is a driven element are formed. 18), a power supply line 16 (VL) for supplying the drive power supply PVDD is formed.

  Each pixel is provided in a region corresponding to the intersection of these lines. Each pixel includes an EL element 18 as a driven element, and a selection transistor Tr1 (hereinafter, “ Selection Tr1 ”), a storage capacitor Cs, and an element drive transistor Tr2 (hereinafter referred to as“ element drive Tr2 ”) constituted by a p-channel TFT are provided.

  In the selection Tr1, the drain is connected to the data line 12 for supplying the data voltage (Vsig) to the pixels arranged in the vertical scanning direction, and the gate is connected to the gate line 10 for selecting the pixels arranged on one horizontal scanning line. The source is connected to the gate of the element drive Tr2.

  The source of the element drive Tr2 is connected to the power supply line 16, and the drain is connected to the anode of the EL element 18. The cathode of the EL element is formed in common for each pixel and is connected to a cathode power source CV.

  The EL element 18 has a diode structure and includes a light emitting element layer between a lower electrode and an upper electrode. The light-emitting element layer includes, for example, a light-emitting layer containing at least an organic light-emitting material, and can adopt a single-layer structure or a multilayer structure of two layers, three layers, or four layers or more depending on the material characteristics used for the light-emitting element layer. . In the present embodiment, the lower electrode is patterned into individual shapes for each pixel, functions as the anode, and is connected to the element drive Tr2. Further, the upper electrode functions in common with a plurality of pixels as a cathode.

  In addition, a driving circuit for driving each pixel can be provided around the display portion of the EL panel 100, and this driving circuit sequentially outputs a selection signal to each gate line 10 in the example of FIG. The driver 220V and an H driver 220H for supplying the data signal Vsig corresponding to the display content of the pixel selected by the gate line 10 to the data line 12 are provided.

  The selection Tr1 of the corresponding row is controlled to be turned on by the selection signal sequentially output, and the data signal Vsig supplied to the data line 12 is supplied to the gate of the element driving Tr2 and the storage capacitor Cs via the selection Tr1 that is turned on. . The holding capacitor Cs holds a charge corresponding to the supplied data signal Vsig for a predetermined period, whereby the gate potential of the element drive Tr2 is held at a potential corresponding to the data signal Vsig for a predetermined period, and according to the gate potential. The current Ids supplied from the power source line 16 to the EL element 18 by the element driving Tr2 is controlled. A current Ioled corresponding to the supplied current Ids flows through the EL element 18 and emits light with a luminance corresponding to the amount of current.

  In such an EL panel 100, the Vd-Id characteristic of the element drive Tr2 of each pixel and the VI characteristic of the EL element 18 are superimposed as shown in FIG. In FIG. 2, the right end of the horizontal axis is the drive power supply voltage PVDD, and the left side is the cathode voltage CV. The vertical axis represents the drain current Ids for the VI characteristics of the TFT and the anode-cathode current Ioled for the VI characteristics of the EL element.

  The TFT characteristics A and B are the relationship between the drain-source voltage Vds and the drain current Ids of the element drive Tr2 which operates by receiving the data signal Vsig set corresponding to the use conditions A and B of the display device at the gate. It is. For example, the use condition A is a case where the brightness of outside light is high and the required brightness for viewing the EL panel is high, such as outdoors on a sunny day, and the EL element 18 emits light with higher brightness than usual in order to make the display of the same content as normal visible. The absolute value Vsigmax of the maximum value of the data signal Vsig is set to be large (VsigmaxA). On the other hand, the use condition B is, for example, a case where the brightness of the outside light is low indoors and the required brightness for viewing the EL panel is low. 18 may be emitted, and the absolute value of the data signal Vsigmax is set small (VsigmaxB).

  Here, in the present embodiment, when performing display, an object is to drive the EL element at a constant current by preventing a drastic change in the drain current Ids due to the fluctuation of the drain-source voltage Vds. The current is supplied to the EL element 18.

  This saturation region is an operation region where the operation threshold value Vth, the gate-source voltage Vgs, and the drain-source voltage Vds of the element driving Tr2 satisfy Vgs−Vth <Vds, and the drain current is changed even when Vds changes. A change in Ids is small, and a drain current Ids corresponding to the voltage of the data signal Vsig supplied to the gate of the element drive Tr2 can be obtained. On the other hand, when the voltage condition of Vgs−Vth> Vds is satisfied, the element drive Tr2 operates in a linear region, and the drain current Ids changes linearly according to Vds. The drain-source voltage Vds that becomes the boundary between the linear region and the saturation region is a pinch-off voltage, and this pinch-off voltage has an absolute value that decreases as the absolute value of the data signal Vsig supplied to the gate of the element drive Tr2 decreases. (Refer to TFT pinch-off characteristics in FIG. 2).

  Since the EL element 18 is connected in series between the element drive Tr2, the drive power supply PVDD, and the cathode power supply CV, the intersections Pa and Pb between the VI characteristic line of the EL element 18 and the TFT characteristic lines A and B are When the data signals VsigmaxA and VsigmaxB corresponding to the respective use conditions A and B are supplied to the element drive Tr2, they become the operating points Pa and Pb of the EL element 18, and the EL element 18 has element driving at the operating points Pa and Pb. A current Ioled equal to the drain current Ids of Tr2 flows, and the EL element 18 emits light with a luminance corresponding to this Ioled.

  Usually, the drive power supply voltage PVDD is determined so as to correspond to the highest Vsigmax A, and the cathode power supply voltage CV is determined by considering the voltage drop caused by the current flowing through the element drive Tr2 and the EL element 18, and the element drive Tr2 at the highest Vsigmax A. Even when driven, the voltage is set to a low voltage that allows a sufficient current to flow through the EL element 18. As described above, in the present embodiment, display is performed by operating the element drive Tr2 in the saturation region, so that the operating voltage (Pa) and the pinch-off voltage of the EL element 18 corresponding to the TFT characteristics A shown in FIG. Is equivalent to a margin of potential difference between the anode electrode and the cathode electrode of the EL element 18, that is, a settable range of the cathode power supply voltage CV (hereinafter referred to as CV margin).

  However, when the maximum value Vsigmax of the data signal is lowered as in TFT characteristics B shown in FIG. 2 according to the external light luminance or the like, the voltage (PVDD−CV) required for the operation of the EL element 18 is also the operating current. Ioled also decreases, and it can be seen that the CV margin (potential difference between the pinch-off voltage and the EL operating voltage) set in accordance with the TFT characteristic A is increased as compared with the TFT characteristic A (use condition A). That is, if the cathode power supply voltage CV is kept constant as in the prior art, power saving can be achieved by reducing the voltage value of the data signal Vsigmax, but the CV margin is expanded and the cathode current flows unnecessarily. I can't prevent that.

  Therefore, in this embodiment, as shown in FIG. 3, the data signal Vsigmax is changed according to the use conditions, and the difference between the drive power supply voltage PVDD and the cathode power supply voltage CV is set so as not to increase the CV margin accordingly. Adjust. That is, when the data signal Vsigmax is lowered according to the use conditions, the cathode power supply voltage CV is increased in consideration of the decrease in the operating voltage of the EL element 18 and the decrease in the TFT pinch-off voltage, PVDD−CV is decreased, and CV The margin is adjusted so as to be within a predetermined range even if the data signal Vsigmax changes. Here, by lowering the data signal Vsigmax, the operating point of the EL element 18 is lower than the reference operating value, and this reference operating value is an absolute value with respect to the pinch-off voltage of the corresponding TFT characteristic, for example. The voltage value is greater than 1V and less than | PVDD−CV |. Furthermore, this reference | standard operation value can be set to the voltage value of the EL element 18 in the maximum light emission luminance state implement | achieved by the panel as an example. In the present embodiment, when the operating voltage of such an EL element 18 is made lower than the reference operating value, the difference between PVDD and CV is reduced below the reference operating condition according to the decrease.

The CV margin is in line with the pinch-off characteristics of the TFT.
Vsigmax = (PVDD−CV) −Vth + CV margin (1)
It is preferable to determine so that Further, the optimum value of the CV margin varies depending on the change in the VI characteristic of the EL element with time and the temperature characteristic, but it is desirable to determine it within 0.5 <CV margin <5V. The CV margin is preferably determined mainly in consideration of manufacturing variations and temperature dependence of the VI characteristics of the EL element 18. The manufacturing variation (ΔVelm) of the VI characteristics of the EL element requires a margin of about 1.5V, and the temperature dependency (ΔVelt) requires a margin of about 1.5V. Further, manufacturing variations (ΔVtftm) and temperature characteristics (temperature dependence) (ΔVtftt) of the pinch-off characteristics of the TFT also affect the CV margin, and each requires a margin of about 0.5V. More specifically, 0.2V <ΔVelm <2V, 0.2V <ΔVelt <2V, 0.1V <ΔVtftm + ΔVtftt <1V may be considered. As a result, since it can be considered that CV margin≈ΔVelm + ΔVelt + ΔVtftt + ΔVtftt, 0.5 <CV margin <5V is obtained as described above.

  When the cathode power supply voltage CV is increased, the VI characteristic of the EL element 18 is shifted to the right in FIG. 3 (in the direction approaching the TFT pinch-off voltage) by that amount. Therefore, the intersection (EL element operating point Pb) between the corresponding VI characteristic B of the EL element 18 and the corresponding characteristic (here, characteristic B) of the TFT shifts to the right, so that the CV margin is reduced accordingly. In the example of FIG. 3, a margin comparable to the CV margin in the case of TFT characteristics A is obtained.

  FIG. 4 conceptually shows the effect of reducing the power consumption by adjusting the data signal Vsigmax according to the usage situation and the effect of reducing the power consumption by adjusting the CV margin. Here, the power consumption is represented by the product of the voltage V and the current I as represented by the area surrounded by the dotted line in FIG. Therefore, for example, the data signal VsigmaxA used in an environment with strong external light is reduced by setting the absolute value of the data signal Vsigmax to VsigmaxB (| VsigmaxA |> | VsigmaxB |) in an environment with low external light. Power consumption is reduced by the amount of drain current Ids (Ioled). Further, by increasing the cathode power supply voltage CV so that the CV margin becomes substantially constant, the power consumption is reduced by the shift of the cathode power supply voltage CV. Therefore, in a usage environment where the outside light is weak, the data signal Vsigmax is lowered below the reference condition and the cathode power supply voltage CV is raised, thereby reducing the power consumption in each pixel in both the current direction and the voltage direction. It becomes possible to reduce. In FIG. 4, for the convenience of illustration, the area surrounded by the dotted line is the power consumption. However, the power consumption of each pixel in the case of the TFT characteristic A is the intersection of the TFT characteristic line A and the VI characteristic line A of the EL element. It is represented by a square area with the origin of the graph as a diagonal point. When the cathode power supply voltage CV is not shifted, the power consumption of each pixel in the case of the TFT characteristic B is a quadrangle whose diagonal is the intersection of the TFT characteristic line B and the VI characteristic line A of the EL element and the origin of the graph. When the cathode power supply voltage CV is shifted as in the present embodiment, a rectangular area whose diagonal is the intersection of the TFT characteristic line B and the VI characteristic line B of the EL element and the origin is consumed. It becomes electric power.

  Such adjustment of the CV margin can be executed by adopting a cathode power supply voltage adjustment unit 300 as shown in FIG. 5 as an example. The cathode power supply adjustment unit 300 in FIG. 5 includes a power supply circuit 320 that generates a plurality of cathode power supply voltages CV, and a switch 322 that switches a power supply voltage output to the cathode electrode of the EL element 18. The number of cathode power supplies to be generated is not particularly limited, but at least two types of adjustment of the data signal Vsigmax are provided. In this case, one is a sufficiently low voltage CVa (−3.5 V or the like) that can correspond to the case where the required luminance for visual recognition is the maximum, and the other is, for example, a sufficient voltage CVb ( 0V). In this case, the switch 322 may be provided corresponding to each of the power supplies CVa and CVb. Of course, the voltage is not limited to two, and another voltage CVref may be provided for reference, or a variable resistor or the like may be provided in the power supply circuit to generate an arbitrary cathode power supply voltage CV. In this case, it is also possible to supply the cathode electrode of the display unit directly from a power supply circuit that outputs an arbitrary cathode power supply voltage without using the switch 322. Note that the switching of the cathode power supply voltage CV output to the cathode electrode is executed according to the determination result in the use condition determination unit of the display device as described later.

  Next, an overall configuration of an EL display device including a use condition determination unit will be described. The display device includes an EL panel 100, and the panel 100 includes a display unit including a pixel circuit illustrated in FIG. 1 on a substrate such as glass, and a driving unit 200 for driving the display unit. In the example of FIG. 6, as a part of the drive unit 200, an H driver 220H and a V driver 220V for driving each pixel of the display unit are arranged around the display unit (a substrate together with a circuit of the pixel unit). It can also be built on top).

  The panel driving unit 200 including the H and V drivers 220H and 220V and generating and supplying necessary signals and power to the driver and the display unit further includes a signal processing circuit, a power circuit, and the like. In addition, in this embodiment, in addition, a sensor 230 for use condition detection, a use condition determination unit 240, a data signal adjustment unit 250, and a cathode power supply voltage adjustment unit 300 are provided. The driving unit 200 having a plurality of circuits can be configured by one or a plurality of integrated circuits (driver ICs). The driver IC is formed on the same substrate on which the display unit is formed, for example, COG (chip on glass). ) Method.

  The signal processing unit of the driving unit 200 includes a necessary timing signal generating unit from a video signal supplied from the outside, a vertical and horizontal synchronizing signal, a dot clock, and a data signal generating unit that generates a data signal of a desired level. The generated data signal is further supplied to a data signal adjusting unit 250 that adjusts the maximum value Vsigmax according to the apparatus use conditions. The panel drivers 220H and 220V can be built in the periphery of the display unit as shown in FIG. 6, but may be built in a driver IC in which the drive unit 200 is integrated.

  The power supply circuit of the drive unit 200 can be built in the driver IC or can be configured by using a dedicated power supply circuit IC. The power supply circuit includes the above-described cathode power supply adjustment unit circuit 300, drive power supply, and the like. A circuit for boosting the power source for operating the PVDD, H, and V drivers 220H and 220V by using a power source supplied from the outside is supplied, and the generated power source voltage is supplied to each unit (supplied from the outside to each unit as it is) In some cases).

  In the present embodiment, the display panel 100 includes a sensor 230 for detecting the use environment of the display device and determining the use condition. As described above, the sensor 230 can be provided in the driver IC in which the driving unit 200 is integrated, or an independent sensor can be mounted in the driver IC. The sensor 230 is, for example, a photosensor that detects external light luminance as shown in the figure, and the external light luminance detected by the sensor 230 is supplied to the determination unit 240.

  The determination unit 240 determines the necessary viewing brightness of the panel according to the ambient light brightness detected by the sensor 230, and determines whether the data adjustment unit 250 adjusts the maximum value Vsigmax of the data signal Vsig according to the necessary viewing brightness. And instructs the cathode power supply voltage adjusting unit 300 whether or not to adjust the cathode power supply voltage. Here, an example of a procedure for adjusting Vsigmax and the cathode voltage according to the use conditions of the apparatus will be specifically described with reference to FIG.

Here, the external light luminance is adopted as the use condition as described above, and the sensor 230 measures the external light (S1). The determination unit 240 determines whether or not the measured external light luminance is greater than the threshold value (S2). The threshold is a predetermined luminance set as a reference, for example, an intermediate value of visibility required luminance as shown in FIG. 10 (150cm / m ² about: outdoor or bright about indoor fogging) be employed such as it can. The determination unit 240 determines that the external light luminance is high when the measured luminance is larger than the reference luminance and the number of times that the reference luminance is exceeded exceeds the specified number of times, and the data signal adjustment unit 250 determines the maximum value Vsigmax of the data signal. Is increased (S3). Specifically, it may be higher than the reference Vsigmax or higher than Vsigmax at the time of low luminance. Further, the cathode power supply voltage adjustment unit 300 makes the cathode power supply voltage CV lower than the reference CV or lower than the CV at the time of low luminance according to the increase of the data signal Vsigmax (S4).

  If the measured brightness is smaller than the reference brightness or the number of times the reference brightness is exceeded is less than or equal to the specified number, it is determined that the external light brightness is low, and the data signal adjustment unit 250 uses the maximum value Vsigmax of the data signal as a reference or high value. Lower than Vsigmax at the time of luminance (S5). Further, the cathode power supply voltage adjustment unit 300 increases the cathode power supply voltage CV from the reference or high luminance CV according to the decrease of the data signal Vsigmax (S6).

  In this way, by adjusting the data signal Vsigmax in accordance with the external light luminance and adjusting the cathode power supply voltage accordingly, it is possible to achieve both suppression of power consumption and display quality. In particular, when the data signal Vsigmax is lowered, the cathode power supply voltage is raised in consideration of the operating voltage (voltage at the operating point) of the EL element that is lowered thereby, thereby ensuring power consumption in the case of low luminance. Can be suppressed.

  The data signal Vsigmax and the cathode power supply voltage may be set in three stages, and the second stage in the middle may be assigned in the case of the reference luminance range. In this case, the external light luminance is compared with the high luminance determination threshold value and the low luminance determination threshold value, and if within the reference range, the data signal Vsigmax is set as the reference value, and the cathode power supply voltage is also set as the reference value. When the external light luminance exceeds the high luminance determination threshold, the data signal Vsigmax is increased and the cathode power supply voltage CV is decreased. When the low luminance determination threshold is exceeded, the data signal Vsigmax is lowered and the cathode power supply voltage CV is raised. By doing so, it is possible to always reduce the power consumption with the optimum light emission luminance. When it is determined that the external light has high brightness, the cathode power supply voltage CV may be kept at the reference value. This is because reducing the cathode power supply voltage CV leads to an increase in power consumption, and it is not necessary to change the reference cathode power supply voltage if the CV margin is secured as described above.

  In addition to the adjustment of the data signal Vsigmax and the cathode power supply voltage CV according to the external light luminance, the adjustment may be performed in consideration of a power saving request made to the device and other use conditions. FIG. 8 shows an example of a procedure for adjusting the data signal Vsigmax and the cathode power supply voltage CV in consideration of usage conditions other than luminance.

  In this example, when there is a power saving request from the CPU or the like that controls the entire apparatus in the display device (S10), the external light is measured (S11), and the determination unit 240 changes the driving condition according to the external light luminance. Determine whether or not. If the external light luminance is low as a result of the determination, the data signal Vsigmax is decreased (S13), and the cathode power supply voltage CV is increased to reduce the PVDD-CV voltage difference (S15). This makes it possible to reliably reduce power consumption when there is a power saving request. If there is a power saving request, the data signal Vsigmax is not changed (does not increase) or decreases according to the request even if the external light luminance is high (S13). If Vsigmax is lowered, the cathode power supply voltage CV is lowered correspondingly (S15).

  In addition to external light, a temperature sensor may be provided to measure the device temperature (S14), and the CV margin may be adjusted according to the measured temperature (S15). The VI characteristic of the EL element has temperature dependence, and the gradient of the characteristic becomes smaller as the temperature becomes lower. Therefore, when the temperature is lower than the reference, the slope of the VI characteristic of the EL element becomes small, and the potential difference between the intersection of the TFT characteristic and the EL VI characteristic and the pinch-off voltage of the TFT, that is, the CV margin increases. . Therefore, by increasing the cathode power supply voltage CV in accordance with the decrease in the slope of the VI characteristic of the EK element, it is possible to suppress the expansion of the CV margin and to reduce the power consumption by the increase in CV.

  Further, since the slope of the VI characteristic of the EL element 18 also changes with time, the length (life) of the driving time is measured, and the slope corresponding to the measured driving time is determined (S14). Accordingly, the CV margin may be adjusted (S15). Here, the VI characteristics of the EL element tend to have a tendency that the inclination becomes smaller due to deterioration with time as the driving time becomes longer. In this case, the power consumption can be reduced by increasing the cathode power supply voltage CV according to the driving time.

  It should be noted that the change in the VI characteristics due to the aging of the EL elements can be detected from the result of measuring the difference ΔIcv between the cathode currents Icvon and Icvoff obtained by supplying the on display signal for inspection and the off display signal to each EL element. it can. Although it is possible to estimate the VI characteristic change of the EL element from the integrated value of the driving time, an actual on-off characteristic change can be obtained by supplying an on / off signal for inspection as a data signal and measuring the cathode power supply at that time. It can be detected with high accuracy.

  Here, the display signal for inspection is a data signal for operating the element drive Tr2 in the saturation region in the same manner as in the normal display for both the on display and the off display. In an environment where the VI characteristics of the EL element change with time (deterioration) and the inclination becomes small, the value of ΔIcv becomes correspondingly small, so that the change in inclination of the VI characteristic due to the change over time can be surely known. .

  Further, by obtaining a difference ΔIcv not only from the cathode current Icvon corresponding to the ON display signal but also from the cathode power source Icvoff corresponding to the OFF display signal, the detection error of the cathode current detection amplifier takes the difference between Icvon and Icvoff. It is canceled by this, and an accurate measurement can be performed.

  It should be noted that the cathode current inspection is performed in the normal display mode by adopting a method such as executing one line at a rate during the vertical blanking period of the video signal. It is possible to detect ΔIcv for the screen. Further, when such an inspection is executed, the cathode electrode as described in FIG. 1 is not shared by all pixels, but an independent cathode electrode line CVL is provided for each column as shown in FIG. It is advantageous in terms of detection speed and the like to detect Icv obtained in the cathode electrode line for each column. In the horizontal scanning direction, a cathode electrode line CVL may be provided for each row, and Icv may be detected for each row.

  Further, the CV margin may be adjusted according to the display content. This is effective in displaying a still image. In still image display, the data signal Vsigmax in one frame period is detected. If the value is smaller than the reference Vsigmax (S14), the cathode power supply voltage CV is increased. (S15) Method. Such a CV margin adjustment can also reliably reduce power consumption.

  Further, in the above description, the configuration of the display unit 100 has been described by taking the circuit configuration as shown in FIG. 1 as an example. However, similarly to the configuration in which the cathode electrode line CVL is provided for each column as shown in FIG. Can be applied. In this case, as described above, the cathode current can be detected for each cathode electrode line, and the CV margin can be adjusted for each cathode power supply line.

  In the pixel circuit described above, a p-channel TFT is used as the element driving transistor, but an n-channel TFT may be used. Further, in the above pixel circuit, an example in which a configuration including two transistors, that is, a selection transistor and a drive transistor, is employed as a transistor for one pixel has been described. However, the transistors are not limited to the two types and the circuit configuration described above. .

1 is an equivalent circuit diagram illustrating a schematic circuit configuration of an EL display device according to an embodiment of the present invention. It is a figure explaining the CV margin which concerns on embodiment of this invention. It is a figure explaining the adjustment method of the CV margin which concerns on embodiment of this invention. It is a figure explaining the significance of adjustment of a CV margin concerning an embodiment of the present invention. It is a figure explaining the cathode power supply voltage adjustment part which concerns on embodiment of this invention. It is a figure which shows schematic structure of EL display apparatus containing the determination part of the use condition which concerns on embodiment of this invention. It is a figure which shows an example of the procedure which detects external light brightness | luminance and adjusts the data signal Vsigmax and the cathode power supply voltage CV. It is a figure which shows an example of the procedure which adjusts the data signal Vsigmax and the cathode power supply voltage CV in consideration of use conditions other than a brightness | luminance and a brightness | luminance. FIG. 2 is an equivalent circuit diagram illustrating a schematic circuit configuration different from that of FIG. 1 of the EL display device according to the embodiment of the present invention. It is a figure explaining the visual recognition required brightness | luminance in an EL panel according to external light brightness | luminance.

Explanation of symbols

  10 gate line (GL), 12 data line (DL), 14 capacitance line, 16 power supply line (VL), 18 EL element, 100 EL panel, 200 drive unit, 220H H driver, 220V V driver, 230 sensor, 240 judgment Unit, 250 data signal adjustment unit (data adjustment unit), 300 cathode power supply voltage adjustment unit (CV adjustment unit), 320 cathode power supply circuit, 322 switch.

Claims (10)

An electroluminescence display device,
A display unit including a plurality of pixels, a control unit that controls a display operation in the display unit, and a determination unit that determines a use condition of the display device,
Each of the plurality of pixels is
An electroluminescence element and an element driving transistor connected to the electroluminescence element,
The source and drain of the element driving transistor and the electroluminescence element are connected in series between a driving power source and a cathode power source, and the element driving transistor operates in a saturation region, and the electroluminescence element operates according to a data signal. Control the current flowing through the element,
The controller is
A data signal adjustment unit that adjusts the data signal according to a determination result in the determination unit;
A voltage adjustment unit that adjusts a power supply voltage difference between the drive power source and the cathode power source according to a determination result in the determination unit;
When the operation voltage of the electroluminescence element is made lower than the operation reference value by adjusting the data signal, the voltage adjustment unit is configured to supply power voltages of the drive power supply and the cathode power supply according to a decrease in the operation voltage. An electroluminescence display device, wherein the difference is reduced from a reference value. The electroluminescent display device according to claim 1,
The voltage adjustment unit is configured to reduce a difference between the power supply voltage of the drive power supply and the cathode power supply according to a decrease in the operating voltage and a decrease in an absolute value of the pinch-off voltage of the element drive transistor that is decreased by adjusting the data signal. An electroluminescence display device characterized by reducing the size. An electroluminescence display device,
A display unit including a plurality of pixels, and a drive unit that controls a display operation in the display unit,
Each of the plurality of pixels is
An electroluminescence element and an element driving transistor connected to the electroluminescence element,
The source and drain of the element driving transistor and the electroluminescence element are connected in series between a driving power source and a cathode power source, and the element driving transistor operates in a saturation region, and the electroluminescence element operates according to a data signal. Control the current flowing through the element,
The drive unit includes: a determination unit that determines a use condition of the display device; a data signal adjustment unit that adjusts the data signal according to a determination result of the determination unit; and a determination result of the determination unit A voltage adjustment unit for adjusting a power supply voltage difference between the drive power supply and the cathode power supply,
The voltage adjusting unit is a voltage between an operating voltage of the electroluminescent element corresponding to an intersection of a current-voltage characteristic line of the electroluminescent element and a current-voltage characteristic line of the element driving transistor, and a pinch-off voltage of the element driving transistor. An electroluminescence display device, wherein a power supply voltage difference between the drive power supply and the cathode power supply is adjusted so that the difference falls within a reference range. In the electroluminescence display device according to any one of claims 1 to 3,
The electroluminescence display device according to claim 1, wherein the voltage adjustment unit reduces the potential difference between the driving power source and the cathode power source by increasing a voltage of the cathode power source. In the electroluminescence display device according to any one of claims 1 to 4,
Furthermore, a photo sensor for detecting the external light luminance of the display device is provided,
The determination unit compares the detected external light luminance with a reference luminance,
If it is smaller than the reference brightness,
The data signal adjustment unit adjusts the data signal so as to reduce the amount of current flowing through the electroluminescence element,
The electroluminescence display device, wherein the voltage adjusting unit increases the cathode power supply voltage. The electroluminescence display device according to claim 5,
The determination unit compares the detected external light luminance with a reference luminance,
If it is larger than the reference brightness,
The electroluminescence display device, wherein the data signal adjustment unit adjusts the data signal so as to increase an amount of current flowing through the electroluminescence element. In the electroluminescent display device according to any one of claims 1 to 6,
In addition, it has a temperature sensor,
The determination unit determines the slope of the current-voltage characteristics of the electroluminescence element according to the detected temperature,
When the inclination is smaller than a reference value, the voltage adjustment unit raises the power supply voltage of the cathode power supply from a reference value. The electroluminescent display device according to claim 7.
When the inclination is larger than a reference value, the voltage adjusting unit lowers the power supply voltage of the cathode power supply from a reference value. In the electroluminescence display device according to any one of claims 1 to 8,
The determination unit includes a current characteristic detection unit for detecting a current characteristic in each pixel,
The determination unit supplies the inspection off-display signal that sets the electroluminescence element to a non-emission level and the inspection on-display signal that sets the emission level to the electroluminescence element according to the inspection off-display signal The on-off current difference between the cathode current of the electroluminescent element and the cathode current of the electroluminescence element according to the on-display signal for inspection is detected, and a change in use condition according to a change in current characteristics is determined based on the on-off current difference An electroluminescent display device characterized by: In the electroluminescence display device according to any one of claims 1 to 9,
A cathode power supply line for supplying the cathode power supply to the electroluminescence element is provided for each column or row in the vertical scanning direction or the horizontal scanning direction of the plurality of pixels. .

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