JP2008014968A - Electroluminescence display device, and driving method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To effectively attain the fluctuation compensation of a threshold voltage of an element driven transistor in an EL display device. <P>SOLUTION: The EL display device is provided with: the element drive transistor T4 for supplying a current from a drive power source PVdd to an EL element 10; a current switch transistor T5 which is inserted between the element drive transistor T4 and the EL element 10; and a short-circuiting transistor T3 for controlling whether or not the transistor T4 is diode connected between the gate and drain of the element drive transistor T4. A selection transistor T1 for controlling whether or not a data signal is to be supplied to the control end of the drive transistor T4 is provided and a holding capacitor Cs is provided between the transistor T1 and the element drive transistor T4. A discharge element for ejecting the gate charge of the element drive transistor T4 to a discharge control line DS is provided between the current switch transistor T5 side of the element drive transistor T4 and the discharge control line DS. The operation of the discharge element is controlled by the potential of the discharge control line DS. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a pixel circuit of an electroluminescence display device that controls a drive current supplied to an electroluminescence element provided in each pixel according to a data signal.

  An EL display device using an electroluminescence (hereinafter referred to as EL) element that is a self-luminous element as a display element for each pixel can provide a self-luminous display device that does not require a backlight, and the device can be thinned. It has advantages such as low power consumption, and has attracted attention as a display device that replaces a display device such as a liquid crystal display (LCD) or CRT. In particular, a current-driven organic EL element using an organic compound as a light-emitting material can achieve high luminance by improving the material, and has a very high degree of freedom in selecting a light-emitting color.

  In addition, since the EL elements are individually controlled for each pixel, a so-called active matrix EL display device in which a switch element such as a thin film transistor (TFT) is used for each pixel as a pixel circuit enables high-definition display.

  In an active matrix EL display device, a plurality of gate lines extend in the row direction (horizontal scanning direction) on the substrate, and a plurality of data lines and power supply lines extend in the column direction (vertical scanning direction). Each of the pixels is provided with an EL element, a selection TFT, an element driving TFT, and a storage capacitor. By selecting the gate line, the selection TFT is turned on, the data voltage (voltage video signal) on the data line is charged to the holding capacitor, the element driving TFT is operated with this voltage, and the power from the power supply line is supplied to the EL element. Supply.

  However, in such a pixel circuit, if the threshold voltage of the element driving TFTs of the pixel circuits arranged in a matrix varies, the current supplied to the EL element varies, the light emission luminance varies, and the display quality deteriorates. There is a problem. However, it is difficult to make the characteristics of the TFTs constituting the pixel circuit of the entire display panel the same, and it is difficult to prevent variations in the on / off threshold.

  Therefore, it is desirable to prevent the influence on the display of the variation in the threshold value in the element driving TFT.

  Here, a circuit for compensating for fluctuations in the threshold value of the TFT is proposed in the following Patent Document 1.

  However, in Patent Document 1, the threshold value of the element driving TFT that supplies the current from the power source to the EL element by supplying the current signal to the data line or the reference signal in advance is used as the signal from the data line. Compensates accordingly. In an active matrix type flat display device such as an active matrix type LCD that has been widely used conventionally, a data signal is usually supplied to each pixel from a data line. In order to supply a special reference signal or current signal from the data line, a large change is required in the peripheral drive circuit for driving the data line.

  In view of this, the present applicant has proposed in Japanese Patent Application Laid-Open No. H11-228707 to compensate for variations in threshold values of element driving TFTs without outputting a special signal to the peripheral driving circuit for driving the data line.

Special Table 2002-514320 JP 2005-326828 A

  In Patent Document 2, the element drive transistor is diode-connected at a predetermined timing, so that the data signal is held after the gate voltage of the element drive transistor is set in advance to a voltage lower than the power supply voltage by the threshold voltage of the element drive transistor. The voltage is supplied to the gate of the element driving transistor through the capacitor. Therefore, the influence of the threshold voltage of the element driving transistor can be removed from the current (current supplied to the EL element) that flows through the transistor according to the voltage applied to the gate of the element driving transistor. Here, in Patent Document 2, unnecessary charges accumulated in the gates of the element driving transistors are once discharged prior to threshold compensation, and for this reason, the element driving transistors are connected in a diode-connected state. Unnecessary charges are discharged as dark current from the transistor to the cathode power source CV of the EL element through the EL element.

  However, in the configuration of Patent Document 2, it is necessary to pass a current through the EL element in order to discharge the gate charge of the element driving transistor, and the EL element emits light during this discharge period, and the contrast is lowered. As an example, in Patent Document 2, a diode-connected element driving transistor and an EL element operate during a discharge period (about 1 μs), and a large through current of about 8 μA flows. In one vertical scanning (V) period, this becomes 1 nA or less during the 1V period, but the EL element emits light, leading to a decrease in contrast. In addition, since the potential of the current discharge side electrode of the EL element varies depending on the light emitting state of the EL element, there is a possibility that the discharge becomes insufficient depending on the operating state of the EL element. Since the driving time affects the lifetime of the EL element, in order to extend the lifetime as much as possible, there is a demand to omit light emission that does not contribute to the original display.

  The present invention effectively compensates for variations in the threshold voltage of an element driving transistor of a pixel circuit in an electroluminescence display device.

  The present invention relates to an electroluminescence display device, in which a pixel circuit for controlling the electroluminescence element of each pixel supplies a drive current corresponding to a potential of a control terminal from a drive power source to the electroluminescence element A drive transistor, a current switch transistor provided between the element drive transistor and the electroluminescence element, for turning on and off the drive current, a short-circuit transistor for controlling whether the element drive transistor is diode-connected, and a data line A selection transistor that controls whether or not to supply a data signal from the control terminal of the element driving transistor, a holding capacitor provided between the selection transistor and the control terminal of the element driving transistor, and the holding Capacitor side potential of the capacitor A discharge element connected between a capacitance potential control transistor to be controlled, the current switch transistor side of the element driving transistor, and a discharge control line, and discharging the charge at the control end of the element driving transistor to the discharge control line And having.

  In another aspect of the present invention, in the display device, the discharge element is a diode-connected discharge transistor or a diode element, and the discharge element operates according to a potential of the discharge control line, and A forward current flows from the element driving transistor toward the discharge control line from the current switch transistor side.

  In another aspect of the present invention, in the display device, the capacitance potential control transistor is provided between the selection transistor side of the storage capacitor and the discharge control line, and is supplied to a control terminal thereof. By turning on in response to the above, the selection transistor side of the storage capacitor is electrically connected to the discharge control line.

  In another aspect of the present invention, in the display device, the control terminal of the current switch transistor is connected to the same light emission control line as the control terminal of the capacitance potential control transistor, and the control signal supplied to the light emission control line When turned on in response, the current from the element driving transistor is supplied to the electroluminescence element.

  In another aspect of the present invention, there is provided a method for driving the display device, wherein the capacitance potential control transistor is turned off, the selection transistor and the short-circuit transistor are turned on, and then the discharge element is turned on. The charge accumulated at the control terminal of the element driving transistor is discharged to the discharge control line through the discharge element. After the discharge, the discharge element is controlled to be off, and the control terminal of the element driving transistor is controlled by the on-controlled short-circuit transistor in a state where the potential on the selection transistor side of the storage capacitor is the potential of the data signal. The voltage is different from the power supply voltage of the drive power supply by the threshold voltage of the element drive transistor. Next, the selection transistor and the short-circuit transistor are turned off, and the current switch transistor is turned on, and the current that the element driving transistor flows to the electroluminescence element is supplied via the current switch transistor, The luminescence element is operated.

  According to the present invention, by connecting a discharge element to the current switch transistor side of the element drive transistor of each pixel and selectively operating the discharge element, unnecessary charges accumulated in the gate of the element drive transistor are It is possible to discharge without going through the electroluminescence element. Since the potential of the electrode on the current discharge side of the EL element varies depending on the display content, when discharging is performed via the EL element, the amount of discharge depends on the electrode potential on the current discharge side. For example, in the immediately preceding frame, when the EL element emits light with high brightness, or when another EL element emits light with high brightness, the electrode potential on the current discharge side becomes high and the amount of charge to be discharged is large. . Therefore, when discharging is performed through the EL element, there is a possibility that the discharge is not sufficient or the discharge amount varies from pixel to pixel. When such a variation in the discharge amount occurs, it affects the light emission luminance of the next frame and causes a variation in luminance. However, the discharge element connected to a discharge control line different from the EL element as in the present invention. By discharging, it is possible to reliably discharge regardless of display contents. For this reason, it is not necessary to cause a dark current to flow to cause the electroluminescence element to emit light for discharging, and it is possible to prevent a decrease in contrast and contribute to an improvement in the lifetime of the EL element.

  In the present invention, a short-circuit transistor for controlling whether or not the element drive transistor is diode-connected is provided, and the control terminal voltage of the drive transistor is set to the data voltage and the drive by turning on the short-circuit transistor with the selection transistor turned on. It can be set according to the threshold voltage of the transistor. Therefore, a drive current corresponding to the voltage of the data signal can be supplied to the EL element regardless of the threshold voltage of the drive transistor. Accordingly, it is possible to prevent luminance variation due to variation in threshold values of the element driving transistor for each pixel. Further, each pixel may be supplied with a data signal at a desired timing from the data line via the selection transistor, and it is not necessary to provide a special configuration in the peripheral drive circuit for driving the data line.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  FIG. 1 shows a circuit configuration per pixel of the EL display device according to the embodiment. FIG. 2 shows a schematic circuit configuration of an active matrix EL display device having such a pixel circuit. In the following description, a display device using a current-driven organic EL element as an EL element will be described as an example. Other elements can be similarly applied as long as they are current-driven elements.

  A plurality of pixels are arranged in a matrix on a display unit on a panel substrate such as glass, and each pixel is provided with an EL element 10 as a display element, and a pixel circuit for controlling the EL element 10 for each pixel is provided. It is provided for each pixel. Note that a part of a driving circuit for controlling each pixel circuit may be built in the periphery of the display unit on the same panel substrate as the display unit as shown in FIG. When crystalline silicon such as polycrystalline silicon is employed as the active layer of each transistor of each pixel circuit to be described later, a peripheral driver circuit (horizontal driver, vertical driver) using a transistor using the same polycrystalline silicon as the active layer. ) Can be built in the same substrate as the display portion.

  A data line DL and a power supply line VL extend in the vertical scanning direction of the panel, and a data signal (data voltage Vsig) about the display luminance of the pixel is output to the data line DL. This data signal of each pixel circuit is supplied via T1. The power supply line VL is connected to the drive power supply PVdd, and supplies a current from the drive power supply to the element drive transistor T4 of each circuit.

  In the horizontal scanning direction of the panel, a gate line GL for sequentially scanning each pixel for each row is formed for each row, and a light emission control line (light emission set line) ES and a discharge control line DS are also formed for each row. It is connected to each pixel circuit.

  Each pixel circuit is supplied with a data signal from the EL element 10, an element driving transistor T 4 that supplies a driving current corresponding to the potential of the gate (control terminal) from the driving power source PVdd to the EL element 10, and a data signal from the data line DL. A transistor T1 is provided. In addition, a storage capacitor Cs is provided between the selection transistor T1 and the gate of the element driving transistor T4. A short-circuit transistor T3 is connected between the gate and drain of the element driving transistor to control whether the element driving transistor T4 is diode-connected. A current switch transistor T5 is provided between the element driving transistor T4 and the EL element 10 to control on / off of the supply of the driving current from the power source PVdd to the EL element 10. Further, a capacitance potential control transistor T2 is provided between the selection transistor side of the storage capacitor Cs and the discharge control line DS, and further, between the current switch transistor T5 side of the element driving transistor T4 and the discharge control line DS. Is provided with a diode-connected discharge transistor T6. Note that the gates of the capacitance potential control transistor T2 and the current switch transistor T5 are both connected to the light emission control line ES.

  As described above, in the example of FIG. 1, the pixel circuit includes six transistors (thin film transistors: TFT), the EL element 10, and the storage capacitor Cs. Of the six transistors, only the element driving transistor T4 is a p-channel transistor. The remaining five transistors are n-channel transistors.

  The EL element 10 includes a light-emitting element layer having a light-emitting layer containing at least an organic light-emitting material between an anode and a cathode. The light-emitting element layer has a single-layer structure, a hole transport layer / light emission, depending on the material used. A three-layer structure of layer / electron transport layer and a multilayer structure of four layers such as a hole injection layer / hole transport layer / light emitting layer / electron transport layer or more can be employed. Light emission is obtained when holes injected from the anode and electrons injected from the cathode recombine in the light emitting element layer, and the excited luminescent molecules return to the ground state. In this embodiment, the cathode of the EL element is composed of an electrode common to each pixel, and the common cathode is connected to a cathode power source CV having a low potential (−9.5 V). One anode is formed of a pixel electrode formed in an individual shape for each pixel and connected to the element driving transistor T4 via the current switch transistor T5. Therefore, a current corresponding to the voltage applied to the gate of the element driving transistor T4 is supplied from the driving power source PVdd to the anode of each EL element 10, and the EL element 10 emits light with a luminance corresponding to the supplied current.

  Hereinafter, the connection structure of each pixel circuit will be described more specifically.

  First, the drain of the selection transistor T1 is connected to the data line DL, and the source of the selection transistor T1 is connected to one electrode of the storage capacitor Cs.

  The gate of the selection transistor T1 is connected to the gate line GL. The gate line GL is also connected to the gate of the short-circuit transistor T3. The drain of the short-circuit transistor T3 is connected to the gate of the element drive transistor T4, and the source of the short-circuit transistor T3 is connected to the drain of the element drive transistor T4. Has been. The element driving transistor T4 has a source connected to the power supply line VL and a drain connected to the anode of the EL element 10 via the current switch transistor T5.

  A diode-connected discharge transistor T6 is connected between the drain of the element drive transistor T4 (current switch transistor T5 side) and the discharge control line DS. Specifically, the gate and drain of the discharge transistor T6 are connected to the drain of the element driving transistor T4, and the source of the discharge transistor T6 is connected to the discharge control line DS. The source / drain of the capacitance potential control transistor T2 is connected to the selection transistor T1 side of the storage capacitor Cs and the discharge control line DS, and its gate is connected to the current control line ES. For this reason, the capacitance potential control transistor T2 is turned on in response to the light emission control signal (H level) output to the light emission control line ES, and the potential on the selection transistor T1 side of the storage capacitor Cs is set to the potential of the discharge control line DS ( The reference voltage is fixed. When the light emission control signal is at the H level, the current switch transistor T5 is also turned on at the same time, and the current from the element driving transistor T4 is supplied to the EL element 10, so that the EL element 10 has a brightness corresponding to the supplied current. Flashes on.

  Next, the operation of this circuit will be further described with reference to FIG. As shown in FIG. 3, the pixel circuit includes (0) a previous frame light emission period, (i) a preset period, (ii) a discharge period, (iii) a gate potential reset period (data writing period) of the element driving transistor T4, ( iv) The state of the light emission period of the next frame is passed.

(0) Previous frame emission period (GL = L level, ES = H level, DS = H level)
When a control signal of H level (for example, 8V) is output to the light emission control line ES, the current switch transistor T5 is turned on, and the drive current from the power supply line VL is supplied to the EL element 10 via the element drive transistor T4. The EL element 10 emits light. At this time, the H level reference voltage signal having the same potential as that of the drive power source PVdd is output to the discharge control line DS (for example, 2 V). For this reason, the potential Vn on the selection transistor T1 side of the storage capacitor Cs is fixed to the potential (2V) of the reference voltage signal (Vn = PVdd) via the capacitance potential control transistor T2 that is turned on together with the current switch transistor T5. For this reason, during the light emission period, an off leak current is generated in the select transistor T1 that is controlled to be off, and the potential of the storage capacitor Cs is prevented from fluctuating. When the reference voltage signal is output to the discharge control line DS, the reference voltage signal is also applied to the source of the discharge transistor T6, and the potential of the gate drain of the discharge transistor T6 that is diode-connected is the power supply voltage. Since it is lower than PVdd by at least the threshold voltage of the element drive transistor T4, the discharge transistor T6 is off.

(I) Preset period (GL = H level, ES = L level, DS = H level)
After the light emission control line ES is set to L level (for example, -7V) and the current switch transistor T5 and the capacitance potential control transistor T2 are turned off, an H level (for example, 8V) selection signal is output to the gate line GL. As a result, the selection transistor T1 and the short-circuit transistor T3 are turned on. In addition, by setting the light emission control line ES to the L level and then shifting the timing to set the GL to the H level, it is possible to turn on the short-circuit transistor T3 after the current switch transistor T5 is completely turned off. Through current is prevented from flowing through the cathode power source CV.

  When the short-circuit transistor T3 is turned on, the gate and drain of the element drive transistor T4 are short-circuited, and the transistor T4 is diode-connected in the forward direction with respect to the drive power supply Pvdd. For this reason, the potential Vg of the gate of the element driving transistor T4 (on the element driving transistor T4 side of the storage capacitor Cs) is lower than the power supply voltage PVdd by the threshold voltage Vtp of the element driving transistor T4 (Vg = PVdd− | Vtp |). It changes to become.

  At the same time, since the selection transistor T1 is also turned on, the data signal (Vsig) supplied to the data line DL is supplied to the selection transistor T1 side of the storage capacitor Cs, and the selection transistor T1 side potential Vn of the storage capacitor Cs is (0 ) Vn = PVdd in the period changes to Vn = Vsig. Therefore, the storage capacitor Cs is charged with a voltage corresponding to | Vsig− (PVdd− | Vtp |) |. This preset period is set to a period necessary for writing the data signal Vsig to all the data lines of the display portion, and shifts to a discharge period after completion.

(Ii) Discharge period (GL = H level, ES = L level, DS = L level)
At an arbitrary timing when charges corresponding to the threshold voltage Vtp and the data signal Vsig are written to the storage capacitor Cs to some extent, an L level (for example, −7 V) discharge control signal is output to the discharge control line DS. At this time, the gate line GL is maintained at the H level and the light emission control line ES is maintained at the L level, the selection transistor T1 and the short-circuit transistor T3 are turned on, and the potential control transistor T2 and the current switch transistor T5 are turned off. Note that the element driving transistor T4 operates because it is diode-connected by the short-circuit transistor T3. In this state, when a discharge control signal is output, the discharge transistor T6 is turned on, and the current from the drive power source PVdd is changed to the drive transistor T4 and the discharge transistor in the state where the voltage Vn = Vsig on the selection transistor T1 side of the storage capacitor Cs. It flows to the discharge control line DS via T6. For this reason, the electric charge held at the gate (and drain: the connection side with the current switch transistor T5) of the drive transistor T4 is extracted. As a result, the data signal component of the previous frame is completely removed, and the gate voltage Vg of the drive transistor T4 becomes a predetermined low voltage corresponding to the discharge control signal level.

  In this discharge period, a sufficient discharge process can be performed in a short period of about 1 μs per 1 H period (about 60 μs).

(Iii) Reset data writing period (GL = H level, ES = L level, DS = H level)
After discharging by controlling the discharge transistor T6 to be on, the potential of the discharge control line DS is changed from the electric control signal level to the reference voltage signal level of H level (for example, 2V). As a result, the discharge transistor T6 is turned off. The selection transistor T1 and the short-circuit transistor T3 remain on-controlled, and the data signal Vsig is supplied to the data line DL. Therefore, the gate potential Vg of the element driving transistor T4 is reset to a potential (Vg = PVdd− | Vtp |) lower than the power supply voltage PVdd by the threshold voltage Vtp of the element driving transistor T4. At the same time, the data signal (Vsig) supplied from the data line DL is supplied to the selection transistor T1 side of the storage capacitor Cs, and | Vsig− (PVdd− | Vtp |) | is charged in the storage capacitor Cs. Further, the potential Vm on the current switch transistor T5 side of the element driving transistor T4 is set to PVdd− | Vtp | equal to Vg because the short-circuit transistor T3 is on.

(Iv) Light emission period (GL = L level, ES = H level)
When the writing of the data signal to the storage capacitor Cs is completed, the gate line GL changes from the H level to the L level, and the selection transistor T1 and the short-circuit transistor T3 are turned off.

  Here, the gate potential Vg of the element driving transistor T4 is equal to the potential | Vsig− (PVdd− | Vtp |) | corresponding to the electric charge held by the storage capacitor Cs, while the drain (on the current switch transistor T5 side) of the element driving transistor T4. ) Is set to PVdd− | Vtp |. As described above, since the short-circuit transistor T3 is controlled to be off at this time, the influence of PVdd− | Vtp | is canceled in the potential difference between the gate and the drain of the element driving transistor T4.

  Here, the light emission control line ES becomes H level, the current switch transistor T5 is turned on, the capacitance potential control transistor T2 is turned on, and the potential Vn on the selection transistor T1 side of the storage capacitor Cs is set to a reference voltage equal to the drive power source PVdd. Fixed. For this reason, the potential Vn on the selection transistor T1 side of the storage capacitor and the gate potential Vg of the element driving transistor T4 are shifted by PVdd. Therefore, the drain current of the transistor T4 determined by the gate voltage cancels the influence of the threshold voltage Vtp of the element driving transistor T4, and becomes a current according to the data signal Vsig. Further, as will be described later, since the same reference potential as that of the drive power supply is set to Vn by the discharge control line DS provided separately from the power supply line VL for supplying current to each pixel, the element drive transistor does not vary depending on the position of the pixel. The gate voltage Vg of T4 can be controlled. Therefore, the drive current supplied to the EL element 10 when the current switch transistor T5 is turned on is not affected by variations in the threshold voltage Vtp of the element drive transistor T4, and the light emission luminance of the EL element 10 varies in the threshold voltage Vtp. Therefore, it is possible to prevent the emission luminance from being affected by the voltage distribution of the power supply line VL.

  Even when switching to the light emission period, a through current flows from the drive power supply PVdd to the EL element by changing the light emission control line ES to the H level after shifting the gate line GL to the L level and shifting the timing. Is preventing.

  By sequentially repeating the above driving for each horizontal scanning (1H) period and for each row, discharge and data writing are executed for each pixel during the 1H period selected by the pixel. From the horizontal scanning period, until the same row is selected again in the next frame, the potentials of the gate line GL, the light emission control line ES, and the discharge control line DS are maintained, and light emission at the EL element 10 is maintained.

  Here, in this embodiment, as described above, each pixel circuit is driven line-sequentially. That is, the write timing of the data signal from the data line DL is simultaneously executed for the pixels on the same row. In this line sequential drive, a circuit for holding each data signal to be output to the corresponding data line DL is provided in the H driver shown in FIG. 2, and a timing signal or the like generated from this holding circuit based on a horizontal start signal is used. This can be achieved by outputting the data signal Vsig simultaneously to each data line DL.

  By adopting line sequential driving in this way, it is possible to secure a sufficient data writing period for each pixel following the discharge process executed during the 1H period. As described above, a short period of about 1 μs is sufficient for the discharge period, for example, and a data write period following this discharge period can be secured about 10 μs per 1H period by adopting line sequential driving. It is possible to execute sufficient writing.

  In the above example, the diode-connected discharge transistor T6 is used as the discharge element, but instead of the discharge transistor as shown in FIG. 4, the current switch transistor T5 side of the element drive transistor T4 and the discharge control line DS are used. A diode D1 may be provided between the two. When the diode D1 is employed, the anode of the diode D1 is connected to the current switch transistor T5 side of the element driving transistor T4, and the cathode is connected to the discharge control line DS. The pixel circuit of FIG. 4 can be driven by the same method as in the configuration of FIG. 1 to obtain the same effect.

  Next, the operation of the discharge element of the present embodiment will be described with reference to FIG. FIG. 5A shows a configuration adopted in Patent Document 2, that is, a configuration in which the gate charge of the element driving transistor T4 is discharged through the EL element (EL) before writing the data signal of the next frame. FIG. 5B shows a configuration in which a discharge element (T6) is provided separately from the EL element 10 as in the present embodiment.

  The cathode potential on the current discharge side of the EL element varies depending on the display content. Therefore, as shown in FIG. 5A, when discharging is performed via the EL element, the amount of discharge depends on the cathode potential CV. Even if the connected power supply voltage is a sufficiently low potential, such as −9.5 V, for example, the cathode voltage CV is, for example, when the EL element emits light with high brightness in the immediately preceding frame, or other EL elements. Is emitting high brightness, the cathode potential CV is locally higher than the original power supply voltage (for example, −9.5 V) in the panel. Therefore, when discharging is performed through the EL element, there is a possibility that the discharge is not sufficient or the discharge amount varies from pixel to pixel. Further, the gate / drain voltage Vg of the element driving transistor T4 may vary due to the influence of the cathode potential CV.

  On the other hand, if the gate charge of the element driving transistor T4 is discharged to the discharge control line DS via the discharge element T6 as in the present embodiment shown in FIG. 5B, the cathode voltage CV of the EL element 10 is reduced. Not affected. The discharge control line DS can be controlled to a constant potential regardless of the light emitting state of the EL element by being a wiring independent of the cathode wiring of the EL element. Further, since only unnecessary charges in the pixel circuit are discharged, the amount of current flowing is very small compared to the cathode wiring of the EL element. Therefore, by setting the discharge control line DS to a voltage low enough to turn on the diode-connected discharge transistor (or diode D1) T6 (for example, −7 V equal to the L level of the gate line or the light emission control line), the discharge is controlled. The element can be reliably operated to perform discharge.

  Further, by turning off the discharge element and setting the discharge control line DS to the reference voltage (for example, 2 V) having the same potential as that of the drive power supply PVdd as described above during the light emission period of the EL element, the current of the element drive transistor T4 is set. It is also possible to prevent the potential Vm on the switch transistor T5 side from fluctuating due to a voltage drop in the power supply line VL or the like. That is, the power supply line VL is arranged for each pixel in the column direction as shown in FIG. 2. If a large amount of current flows through the EL elements in the corresponding column, the potential of the power supply line VL in that column is locally May be reduced. When the pixel is located away from the PVdd terminal, the actual voltage of the power supply line VL is lower than that of the pixel located closer to the voltage drop. Therefore, there is a possibility that the drive current supplied to the EL element 10 of the corresponding pixel varies. However, current does not flow through the discharge control line DS at least during the off period of the discharge element, and it is easy to maintain a stable potential regardless of the position of the pixel on the panel. Therefore, if a reference voltage as high as the power supply voltage PVdd is applied to the discharge control line DS, the potential Vm on the current switch transistor T5 side of the element driving transistor T4 can be maintained at the same high potential in each pixel. It becomes possible.

  Further, during the OFF period of the discharge element, the discharge control line DS is set to the same voltage as the drive power supply PVdd, and the capacitor potential control transistor T2 is turned on, so that the potential Vn on the selection transistor T1 side of the storage capacitor Cs is fixed to PVdd. can do. For this reason, the capacitance potential control and the discharge can be controlled by the same discharge control line DS, and the decrease in the aperture ratio can be minimized. Further, by making the H level reference voltage signal of the discharge control line DS equal to the drive power supply PVdd, it can be generated by being supplied from the PVdd terminal having the same signal level, and the L level in the above example. For example, -7 is set, which is equal to the L level of the gate line GL and the light emission control line ES. Therefore, it can be created using a power source that determines the L level of these GL and ES. Therefore, it is not necessary to use a separate power source for generating a discharge control signal and a reference voltage signal output to the discharge control line DS for each pixel.

It is a figure which shows the structure of the pixel circuit of the EL display apparatus which concerns on embodiment of this invention. It is a figure explaining the schematic structure of the whole EL display device concerning this embodiment of the present invention. FIG. 10 is a timing diagram illustrating a method for driving an EL display device according to an embodiment of the present invention. It is a figure which shows the other example of the pixel circuit of the EL display apparatus which concerns on embodiment of this invention. It is a figure explaining the effect | action of the discharge element which concerns on embodiment of this invention.

Explanation of symbols

  10 EL element (organic EL element), Cs retention capacity, CV cathode power supply, DL data line, DS discharge control line, D1 discharge diode, ES light emission set line, GL gate line, PVdd drive power supply, T1 selection transistor, T2 capacitance potential Control transistor, T3 short-circuit transistor, T4 element drive transistor, T5 current switch transistor, T6 discharge transistor, Vg Gate voltage of element drive transistor, Vsig data voltage.

Claims (7)

An electroluminescence display device,
A pixel circuit for controlling the electroluminescence element of each pixel,
An element drive transistor for supplying a drive current corresponding to the potential of the control terminal from the drive power supply to the electroluminescence element;
A current switch transistor provided between the element driving transistor and the electroluminescent element, for turning on and off the driving current;
A short-circuit transistor for controlling whether the element driving transistor is diode-connected, or
A selection transistor for controlling whether to supply a data signal from a data line to the control terminal of the element driving transistor;
A storage capacitor provided between the selection transistor and a control terminal of the element driving transistor;
A capacitor potential control transistor for controlling the selection transistor side potential of the storage capacitor;
A discharge element connected between the current switch transistor side of the element driving transistor and a discharge control line, and discharging the charge at the control end of the element driving transistor to the discharge control line;
An electroluminescence display device comprising: The display device according to claim 1,
The discharge element is a diode-connected discharge transistor or a diode element,
The electroluminescent display device, wherein the discharge element operates in accordance with a potential of the discharge control line and allows a forward current to flow from the current switch transistor side of the element driving transistor toward the discharge control line. The electroluminescence display device according to claim 1 or 2,
The capacitance potential control transistor is provided between the selection transistor side of the storage capacitor and the discharge control line, and is turned on in response to a control signal supplied to a control terminal thereof, whereby the storage capacitor An electroluminescence display device, wherein a selection transistor side is electrically connected to the discharge control line. The electroluminescent display device according to claim 3.
The control terminal of the current switch transistor is connected to the same light emission control line as the control terminal of the capacitance potential control transistor, and when turned on according to a control signal supplied to the light emission control line, the current from the element driving transistor Is supplied to the electroluminescence element. A pixel circuit for controlling the electroluminescent element of each pixel,
An element drive transistor for supplying a drive current corresponding to the potential of the control terminal from the drive power supply to the electroluminescence element;
A current switch transistor provided between the element driving transistor and the electroluminescent element, for turning on and off the driving current;
A short-circuit transistor for controlling whether the element driving transistor is diode-connected, or
A selection transistor for controlling whether to supply a data signal from a data line to the control terminal of the element driving transistor;
A storage capacitor provided between the selection transistor and a control terminal of the element driving transistor;
A capacitor potential control transistor for controlling the selection transistor side potential of the storage capacitor;
A discharge element connected between the current switch transistor side of the element drive transistor and a discharge control line;
A driving method of an electroluminescence display device having
The capacitance potential control transistor is turned off, the selection transistor and the short-circuit transistor are turned on, and then the discharge element is turned on to store the charge accumulated in the control terminal of the element driving transistor. Through the discharge control line,
After the discharge, the discharge element is controlled to be off, and the control terminal of the element driving transistor is controlled by the on-controlled short-circuit transistor in a state where the potential on the selection transistor side of the storage capacitor is the potential of the data signal. The voltage is different from the power supply voltage of the drive power supply by a threshold voltage of the element drive transistor,
The selection transistor and the short-circuit transistor are turned off, and the current switch transistor is turned on, and the current that the element driving transistor supplies to the electroluminescence element is supplied through the current switch transistor, and the electroluminescence element is A driving method of an electroluminescence display device, characterized by being operated. In the driving method of the electroluminescence display device according to claim 5,
A driving method of an electroluminescence display device, wherein a reference voltage signal having the same potential as that of the driving power source is output to the discharge control line during an ON period of the current switch transistor. In the driving method of the electroluminescence display device according to claim 6,
The capacitance potential control transistor is provided between the selection transistor side of the storage capacitor and the discharge control line, and when the ON operation is performed according to a control signal applied to a control terminal thereof, the capacitance potential control transistor is connected to the discharge control line. A driving method of an electroluminescence display device, comprising: outputting a reference voltage signal, and fixing a potential on the selection transistor side of the storage capacitor to a potential corresponding to the reference voltage signal.

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