JP2007148129A - Display apparatus and driving method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a display apparatus capable of correcting the deterioration of a light emitting element with the lapse of time or the characteristic dispersion of a drive transistor and reducing the number of constitutional elements of a pixel circuit as much as possible. <P>SOLUTION: The display apparatus comprises a pixel array part 1, scanner parts 4, 5 and a signal part 3. Each pixel 2 comprises a sampling transistor T1, a pixel capacitor C1, a drive transistor T5, and a switching transistor T4 for connecting a light emitting element EL and the drive transistor T5 to a power supply line VL. The scanner parts 4, 5 respectively output control signals to a first scanning line WS and a second scanning line DS and ON/OFF control the sampling transistor T1 and the switching transistor T4 to execute correcting operation for correcting the pixel capacitor C1 for canceling dependency on the threshold voltage of the drive transistor T5 and sampling operation for writing a video signal in the corrected pixel capacitor C1. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a flat panel display device that displays an image by current-driving light emitting elements arranged for each pixel, and a driving method thereof. More specifically, the present invention relates to a so-called active matrix display device that controls the amount of electricity supplied to a light emitting element such as an organic EL element by an insulated gate field effect transistor provided in each pixel circuit, and a driving method thereof.

  In an image display device such as a liquid crystal display, an image is displayed by arranging a large number of liquid crystal pixels in a matrix and controlling the transmission intensity or reflection intensity of incident light for each pixel in accordance with image information to be displayed. This also applies to an organic EL display using an organic EL element as a pixel, but unlike a liquid crystal pixel, the organic EL element is a self-luminous element. Therefore, the organic EL display has advantages such as higher image visibility than the liquid crystal display, no backlight, and high response speed. Further, the luminance level (gradation) of each light emitting element can be controlled by the value of the current flowing therethrough, and is greatly different from a voltage control type such as a liquid crystal display in that it is a so-called current control type.

In the organic EL display, similarly to the liquid crystal display, there are a simple matrix method and an active matrix method as driving methods. Although the former has a simple structure, there is a problem that it is difficult to realize a large-sized and high-definition display. Therefore, the active matrix method is actively developed at present. In this method, a current flowing through a light emitting element in each pixel circuit is controlled by an active element (generally a thin film transistor or TFT) provided in the pixel circuit, and is described in the following patent documents.
JP 2003-255856 A JP 2003-271095 A JP 2004-133240 A JP 2004-029791 A JP 2004-093682 A

  A conventional pixel circuit is arranged at a portion where a row scanning line for supplying a control signal and a column signal line for supplying a video signal intersect, and includes at least a sampling transistor, a capacitor, a drive transistor, and a light emitting element. . The sampling transistor conducts in response to the control signal supplied from the scanning line and samples the video signal supplied from the signal line. The capacitor unit holds an input voltage corresponding to the sampled video signal. The drive transistor supplies an output current during a predetermined light emission period in accordance with the input voltage held in the capacitor unit. In general, the output current depends on the carrier mobility and threshold voltage of the channel region of the drive transistor. The light emitting element emits light with luminance according to the video signal by the output current supplied from the drive transistor.

  The drive transistor receives the input voltage held in the capacitor portion at the gate, causes an output current to flow between the source and the drain, and energizes the light emitting element. In general, the light emission luminance of a light emitting element is proportional to the amount of current applied. Further, the output current supply amount of the drive transistor is controlled by the gate voltage, that is, the input voltage written in the capacitor. The conventional pixel circuit controls the amount of current supplied to the light emitting element by changing the input voltage applied to the gate of the drive transistor in accordance with the input video signal.

Here, the operating characteristic of the drive transistor is expressed by the following Equation 1.
Ids = (1/2) μ (W / L) Cox (Vgs−Vth) ² Formula 1
In the transistor characteristic formula 1, Ids represents a drain current flowing between the source and the drain, and is an output current supplied to the light emitting element in the pixel circuit. Vgs represents a gate voltage applied to the gate with reference to the source, and is the above-described input voltage in the pixel circuit. Vth is the threshold voltage of the transistor. Μ represents the mobility of the semiconductor thin film constituting the channel of the transistor. In addition, W represents the channel width, L represents the channel length, and Cox represents the gate capacitance. As is apparent from the transistor characteristic equation 1, when the thin film transistor operates in the saturation region, if the gate voltage Vgs increases beyond the threshold voltage Vth, the thin film transistor is turned on and the drain current Ids flows. In principle, as shown in the above transistor characteristic equation 1, if the gate voltage Vgs is constant, the same amount of drain current Ids is always supplied to the light emitting element. Therefore, if video signals of the same level are supplied to all the pixels constituting the screen, all the pixels should emit light with the same luminance, and the uniformity of the screen should be obtained.

  However, in practice, the current-voltage characteristics of the light emitting element deteriorate with time, and the luminance of the light emitting element changes due to this influence. The conventional pixel circuit cannot absorb or correct the fluctuation of the current-voltage characteristic (IV characteristic) of the light emitting element, which is a problem to be solved.

  A more important problem is that thin film transistors (TFTs) formed of a semiconductor thin film such as polysilicon have variations in individual device characteristics. In particular, the threshold voltage Vth is not constant and varies from pixel to pixel. As apparent from the transistor characteristic equation 1 described above, if the threshold voltage Vth of each drive transistor varies, even if the gate voltage Vgs is constant, the drain current Ids varies and the luminance varies from pixel to pixel. , Damage the screen uniformity. Conventionally, a pixel circuit incorporating a function for canceling variations in threshold voltages of drive transistors has been developed, and is disclosed in, for example, Patent Document 3 described above.

  However, a conventional pixel circuit incorporating a function for canceling variations in threshold voltage (threshold voltage correction function) has a complicated configuration, which has been an obstacle to pixel miniaturization or high definition. In addition, a conventional pixel circuit having a threshold voltage correcting function has a relatively large number of constituent elements, resulting in a decrease in yield.

  In view of the above-described problems of the prior art, the present invention is capable of correcting deterioration with time of light emitting elements and variation in characteristics of drive transistors, and an active matrix display device in which the number of constituent elements of a pixel circuit is reduced as much as possible. An object is to provide a driving method. In order to achieve this purpose, the following measures were taken. That is, the display device according to the present invention includes a pixel array unit, a scanner unit, and a signal unit. The pixel array unit includes scanning lines arranged in rows and signal lines arranged in columns, and matrix-like pixels arranged at a portion where both intersect, and the signal unit displays video on the signal lines. The scanner unit supplies a control signal to the first scanning line and the second scanning line to sequentially scan the pixels for each row. Each pixel includes a sampling transistor, a pixel capacitor connected thereto, a drive transistor connected thereto, a light emitting element connected thereto, and a switching transistor connecting the drive transistor to a power supply line. The sampling transistor conducts in response to a control signal supplied from the first scanning line and samples the signal potential of the video signal supplied from the signal line into the pixel capacitor, and the pixel capacitor is connected to the sampled video signal. An input voltage is applied between the gate and source of the drive transistor according to the signal potential of the drive transistor, the drive transistor supplies an output current according to the input voltage to the light emitting element, and the output current is The light emitting element emits light with a luminance corresponding to a signal potential of the video signal by an output current supplied from the drive transistor during a light emission period, and the switching transistor has a second voltage dependency. Conducts in response to a control signal supplied from the scanning line and connects the drive transistor to the power supply line during the light emission period, and does not emit light Between it becomes non-conductive, decoupling the drive transistor from the power supply line. The scanner unit outputs a control signal to each of the first scan line and the second scan line, and controls on / off of the sampling transistor and the switching transistor to cancel the dependency of the output current on the threshold voltage. A correction operation for correcting the pixel capacitance and a sampling operation for writing the signal potential of the video signal to the corrected pixel capacitance are executed.

  Specifically, the signal unit switches the video signal between a fixed potential and a signal potential in accordance with the correction operation and the sampling operation, so that the potential necessary for the correction operation and the sampling operation is changed to each value. The pixel is supplied via a signal line. In this case, the signal unit supplies the fixed potential to the signal line in accordance with the correction operation, and then switches to the signal potential in accordance with the sampling operation.

  The power supply line is arranged in the pixel array unit in parallel with the first scanning line and the second scanning line, and the scanner unit includes a power supply line scanner that scans the power supply line in the same manner as the scanning line. Thus, a potential necessary for the correction operation is supplied to each pixel through the power supply line. In this case, the power supply line scanner switches the power supply potential supplied from the power supply line during the period during which the correction operation is performed from the normal power supply potential supplied during the light emission period, so that the potential necessary for the correction operation is changed. Each pixel is supplied via the power line. Preferably, the scanner unit outputs a control signal to each of the first scanning line and the second scanning line in a horizontal scanning period assigned to the row of the pixel, and thus performs the correction operation within the horizontal scanning period. And the sampling operation.

  The present invention also includes a pixel array section, a scanner section, and a signal section, and the pixel array section is disposed at a portion where the scanning lines arranged in rows and the signal lines arranged in columns intersect with each other. The signal unit supplies a video signal to the signal line, and the scanner unit supplies a control signal to the first scanning line and the second scanning line to sequentially set the pixel for each row. Each pixel that is scanned includes a sampling transistor, a pixel capacitor connected thereto, a drive transistor connected thereto, a light emitting element connected thereto, and a switching transistor connecting the drive transistor to a power supply line In the driving method of the apparatus, the sampling transistor conducts in response to a control signal supplied from a first scanning line and receives a signal potential of a video signal supplied from the signal line in the pixel capacitor. The pixel capacitor applies an input voltage between the gate and source of the drive transistor according to the signal potential of the sampled video signal, and the drive transistor outputs an output current according to the input voltage. The output current is dependent on the threshold voltage of the drive transistor, and the light emitting element is set to the signal potential of the video signal by the output current supplied from the drive transistor during the light emission period. The switching transistor emits light in accordance with the brightness, and the switching transistor is turned on in response to the control signal supplied from the second scanning line to connect the drive transistor to the power supply line during the light emission period, and is turned off during the non-light emission period. The drive transistor is disconnected from the power supply line, and the scanner unit applies control signals to the first scanning line and the second scanning line, respectively. A correction operation for correcting the pixel capacitance in order to cancel the dependence of the output current on the threshold voltage by controlling on / off of the sampling transistor and the switching transistor, and outputting the video to the corrected pixel capacitance. A sampling operation for writing the signal potential of the signal is executed.

  According to the present invention, the scanner unit that performs line-sequential scanning of the pixel array unit of the display device performs on / off control of the sampling transistor and the switching transistor included in each pixel, and the threshold voltage correction operation of the drive transistor and the video signal sampling operation Is running. As described above, since this display device can suppress variations in threshold voltages of drive transistors included in each pixel, uniform image quality without unevenness and roughness can be obtained. In addition, the pixel capacitance included in each pixel applies an input voltage between the gate and the source of the drive transistor in accordance with the signal potential of the sampled video signal. Since the gate-source voltage of the drive transistor is kept constant by the pixel capacitance, the drive transistor operates as a constant current source, and the current flowing through the light emitting element does not change. Therefore, even if the IV characteristics of the light emitting element deteriorate, a constant current always flows and the luminance of the light emitting element does not change. As described above, a pixel circuit capable of coping with variations in characteristics of drive transistors and deterioration with time of IV characteristics of light emitting elements includes a sampling transistor, a drive transistor, a switching transistor, and a pixel capacitor. The pixel circuit included in the display device of the present invention includes three transistors and one capacitor, and is configured with a total of four elements. Since the pixel circuit of the present invention is formed with a small number of elements of three transistors and one pixel capacitance, high definition and high yield can be expected. As a result, the display device of the present invention can be configured with three gate lines and three power supply lines per RGB three pixels, so that the ratio of the power supply lines and gate lines to each pixel can be reduced, and high definition can be achieved. In addition, a high yield can be expected.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. First, in order to clarify the background of the present invention, a general configuration of an active matrix display device will be described with reference to FIG. As shown in the figure, this display device includes a pixel array 1, a horizontal selector 3, and a write scanner 4. The pixel array 1 is integrally formed on one panel. The horizontal selector 3 and the light scanner 4 may be built in the panel or may be externally attached. The pixel array 1 is composed of scanning lines WS arranged in rows, signal lines SL arranged in columns, and pixel circuits 2 arranged at the intersection of the two. The scanning lines WS are connected to the write scanner 4 and sequentially output control signals to sequentially select the pixel circuits 2 in units of rows. The horizontal selector 3 is connected to each signal line SL, and writes a video signal to the selected pixel circuit 2.

  FIG. 2 is a circuit diagram showing an example of the pixel circuit 2 shown in FIG. The pixel circuit 2 has the simplest configuration, and includes two transistors T1 and T5, one pixel capacitor C1, and one light emitting element EL. The sampling transistor T1 is an N-channel thin film transistor. The drive transistor T5 is a P-channel type thin film transistor. The pixel capacitor C1 is a thin film capacitor. The light emitting element EL is, for example, a two-terminal element (diode) having an organic EL thin film as a light emitting layer. These elements T1, T5, C1, and EL are integrally formed on an insulating substrate constituting the panel.

  The sampling transistor T1 is connected between the signal line SL and the gate of the drive transistor T5. The gate of the sampling transistor T1 is connected to the write scanner 4 through the scanning line WS. A pixel capacitor C1 is connected to the gate of the drive transistor T5. The source of the drive transistor T5 is connected to the power supply Vcc. The drain of the drive transistor T5 is connected to the anode of the light emitting element EL. The cathode of the light emitting element EL is grounded.

  In the horizontal scanning period, the sampling transistor T1 is applied with a control signal from the write scanner 4 and becomes conductive. As a result, the sampling transistor T1 samples the video signal supplied from the horizontal selector 3 to the signal line SL and writes it to the pixel capacitor C1. The drive transistor T5 supplies the drain current Ids to the light emitting element EL according to the video signal written in the pixel capacitor C1. As a result, the light emitting element EL emits light with a luminance corresponding to the video signal.

  In the method shown in FIG. 2, the output current Ids flowing through the light emitting element EL is controlled by changing the gate application voltage Vgs of the drive transistor in accordance with the video signal. In this example, the source of the P-channel type sampling transistor T5 is connected to the power source Vcc and is designed to always operate in the saturation region, so that it becomes a constant current source that operates according to the above-described equation 1. That is, the P-channel drive transistor T5 can always supply a constant output current Ids to the light emitting element EL according to the voltage Vgs between the gate and the source without depending on the potential of the drain connected to the light emitting element EL side.

  FIG. 3 is a graph showing the IV characteristics of the light emitting element EL. A light-emitting element typified by an organic EL element or the like has a tendency that an IV characteristic changes with time, and a solid line indicates an initial state while a dotted line indicates an IV characteristic after change with time. In the graph, the voltage V is an anode voltage. Corresponding to FIG. 2, the anode voltage V is the drain voltage of the drive transistor T5. On the other hand, the current I is the output current Ids supplied from the drive transistor T5. As described above, the pixel circuit 2 in FIG. 2 can always supply a constant output current Ids to the light emitting element EL without the drive transistor T5 depending on the drain voltage. Therefore, even if the IV characteristic of the light emitting element EL changes with time, it is possible to supply a constant current without being affected by this. Therefore, no change in luminance occurs in the light emitting element EL.

  FIG. 4 is a circuit diagram showing another example of the conventional pixel circuit 2. In order to facilitate understanding, parts corresponding to those of the prior art shown in FIG. The difference is that the drive transistor T5 is not an P-channel type but an N-channel type. In this case, the source side of the drive transistor T5 is connected to the anode side of the light emitting element EL. Therefore, the source potential is affected by the change over time of the IV characteristic of the light emitting element EL and fluctuates. The gate / source voltage Vgs changes with time of the light emitting element. As a result, the amount of output current Ids flowing through the light emitting element EL changes, and the light emission luminance changes. In addition, the threshold voltage Vth varies for each pixel circuit in the drive transistor T5. Therefore, as shown in Equation 1 above, the drain current Ids varies due to variations in Vgs and Vth, and the light emission luminance changes from pixel to pixel.

  A display device that corrects the deterioration of the light emitting element with time and the variation in characteristics of the drive transistor has already been developed by the applicant, and a prior development example is shown in FIG. As shown in the figure, this display device includes a pixel array 1, a horizontal selector 3, a write scanner 4, a drive scanner 5, a correction scanner 7, and a second correction scanner 8. The pixel array 1 includes pixel circuits 2 arranged in a matrix. In order to simplify the illustration, one pixel circuit 2 is shown. The pixel circuit 2 includes five transistors T1 to T5, one pixel capacitor C1, and one light emitting element EL, and has a relatively large number of elements. The pixel circuit 2 is driven by four scanning lines WS, DS, AZ, and AZ2, one signal line SL, and four power lines Vcc, Vss, Vofs, and Vcat. . There are nine control lines in total, which puts pressure on the area occupied by pixels. The scanning line WS is scanned by the write scanner 4, the DS is scanned by the drive scanner 5, AZ is controlled by the correction scanner 7, and AZ2 is controlled by the second correction scanner 8. An input signal (Vsig) is supplied from the horizontal selector 3 to the signal line SL. In this example, all transistors T1 to T5 are N-channel type. The source S of the drive transistor T5 as the center is connected to the anode of the light emitting element EL. The cathode of the light emitting element EL is connected to Vcat. The drain of the drive transistor T5 is connected to Vcc via the switching transistor T4. The gate of the switching transistor T4 is connected to the scanning line DS. The gate G of the drive transistor T5 is connected to the signal line SL via the sampling transistor T1. The gate of the sampling transistor T1 is connected to the scanning line WS. The gate G of the drive transistor T5 is connected to Vofs via the switching transistor T3. The gate of the switching transistor T3 is connected to the scanning line AZ2. A pixel capacitor C1 is connected between the gate G and the source S of the drive transistor T5. The source S of the drive transistor T5 is connected to Vss via the switching transistor T2. The gate of the switching transistor T2 is connected to the scanning line AZ.

  FIG. 6 is a timing chart for explaining the operation of the pixel circuit 2 shown in FIG. The change of ON / OFF of the transistors T1 to T4 along the time axis J is shown. The on / off control of T1 to T4 is performed by the corresponding scanner via the corresponding scanning line. This timing chart also shows changes in the potentials of the gate G and source S of the drive transistor T5. Since the transistor T4 is turned on before the timing J1, the output current is supplied to the light emitting element EL via the drive transistor T5 and the light emitting state is obtained.

  At timing J1, the transistor T3 is turned on, and the gate G of the drive transistor T5 is lowered to Vofs. Further, since the switching transistor T2 is turned on, the source S of the drive transistor T5 drops to Vss. Since Vss is lower than the threshold voltage Vthel of the light emitting element EL, no current flows through the light emitting element EL and a non-light emitting period starts. The potential difference between Vofs and Vss is larger than the threshold voltage Vth of the drive transistor T5. In this way, the threshold voltage correction operation is prepared by setting the potential across the pixel capacitor C1.

  At timing J2, the switching transistor T2 is turned off. As a result, the source S of the drive transistor T5 is disconnected from Vss and starts to rise. When current flows from the drive transistor T5 to the pixel capacitor C1, and the voltage Vgs at both ends is just equal to the threshold voltage Vth of the drive transistor T5, it is cut off. As a result, a voltage corresponding to the threshold voltage Vth of the drive transistor T5 is written to both ends of the pixel capacitor C1. The threshold cancellation operation is performed as described above.

  The switching transistor T4 is turned off at timing J3, and the switching transistor T3 is also turned off at timing J4. At this point, all the transistors T1 to T4 are turned off.

  At timing J5, the sampling transistor T1 is turned on, and the video signal Vsig supplied from the signal line SL is written to the gate G of the drive transistor T5. The sampling transistor T1 is turned off at timing J6 when the horizontal scanning period (1H) assigned to the pixel circuit 2 elapses. Signal writing was performed during this period J5-J6.

  Thereafter, the process proceeds to timing J7 and the switching transistor T4 is turned on. As a result, the drive transistor T5 is connected to the power supply Vcc and supplies an output current. The value of this output current is controlled to be constant by the input voltage Vgs held in the pixel capacitor C1. Light emission starts when the potential of the source S of the drive transistor T5 starts to rise and exceeds the threshold voltage Vthel of the light emitting element EL. Due to the bootstrap effect, the gate potential of the drive transistor T5 also rises in conjunction with the rise of the source potential. The gate / source voltage Vgs of the drive transistor T5 is always held constant by the pixel capacitor C1.

  Hereinafter, the operation of the pixel circuit according to the prior development shown in FIGS. 5 and 6 will be described in detail with reference to FIGS. First, the light emitting state of the light emitting element EL is a state in which only the switching transistor T4 is turned on as shown in FIG. At this time, since the drive transistor T5 is set to operate in the saturation region, the current Ids flowing through the light emitting element EL takes a value represented by the characteristic equation 1 according to the gate-source voltage Vgs of the drive transistor T5.

  Next, the switching transistor T3 and the switching transistor T2 are turned on in the non-light emitting period. At this time, the gate voltage of the drive transistor T5 is charged to Vofs and the source voltage is charged to Vss. The gate-source voltage of the drive transistor T5 takes a value of Vofs−Vss, and a corresponding current Ids ′ flows from Vcc to Vss. (FIG. 8) Here, in order to make the light emitting element EL emit no light, the voltage Vofs and Vss are set so that the voltage Vel applied to the light emitting element EL becomes smaller than the sum of the threshold voltage Vthel and the cathode voltage Vcat of the light emitting element EL. Need to be set. Either the switching transistor T3 or the switching transistor T2 may be turned on first.

  Further, the switching transistor T2 is turned off (FIG. 9). Since the equivalent circuit of the light emitting element EL is represented by a diode Tel and a capacitor Cel as shown in FIG. 10, as long as Vel ≦ Vcat + Vthel (the leakage current of the light emitting element EL is considerably smaller than the current flowing through the drive transistor T5). The current of the drive transistor T5 is used to charge the pixel capacitor C1 and the light emitting element capacitor Cel. At this time, the anode voltage Vel of the light emitting element (that is, the source voltage of the drive transistor) rises with time as shown in FIG. After a predetermined time has elapsed, the gate-source voltage of the drive transistor T5 takes a value of Vth. At this time, Vel = Vofs−Vth ≦ Vcat + Vthel.

After the threshold cancel operation is finished, the switching transistors T4 and T3 are turned off. By turning off the switching transistor T4 prior to the switching transistor T3, it is possible to suppress fluctuations in the gate voltage of the drive transistor T5. Next, the sampling transistor T1 is turned on and the gate voltage of the drive transistor T5 is set to the signal voltage Vsig (FIG. 12). At this time, the gate-source voltage of the drive transistor T5 is determined by the pixel capacitance C1, the parasitic capacitance Cel of the light emitting element EL, and the parasitic capacitance C2 of the drive transistor T5 as shown in the following Expression 2. However, since the light emitting element capacitance Cel is larger than the pixel capacitance C1 and the parasitic capacitance C2, the gate / source voltage Vgs of the drive transistor T5 is approximately Vsig + Vth. However, for convenience, Vofs = 0.

  After the writing is completed, the switching transistor T4 is turned on to raise the drain voltage of the drive transistor T5 to the power supply voltage Vcc. Since the gate-source voltage of the drive transistor T5 is constant, the drive transistor T5 causes a constant current Ids ″ to flow through the light emitting element EL, and Vel rises to a voltage Vx at which a current of Ids ″ flows through the light emitting element EL. The element EL emits light (FIG. 13).

  In this circuit as well, the IV characteristic of the light emitting element EL changes as the light emission time becomes longer. Therefore, the potential at point B in the figure also changes. However, since the gate / source voltage of the drive transistor T5 is maintained at a constant value, the current flowing through the light emitting element EL does not change. Therefore, even if the IV characteristic of the light emitting element EL deteriorates, the constant current Ids always flows, and the luminance of the light emitting element EL does not change.

  Here, a power supply line and a gate line in the pixel circuit according to the preceding development will be considered. In this pixel circuit, 12 power lines per RGB trio are composed of Vcc, Vofs, Vss, Vsig and 4 gate lines are WS, AZ, AZ2, and DS. Is a large percentage. Therefore, it is difficult in terms of high definition and high yield of the panel.

  In the present invention, the circuit configuration shown in FIG. This circuit configuration is composed of three transistors and one capacitor per pixel, and the power supply lines are three gate lines and three power supply lines per RGB trio.

  As shown in the figure, the display device according to the present embodiment includes a pixel array unit 1, a scanner unit, and a signal unit. The scanner unit includes a write scanner 4, a drive scanner 5, and a power line scanner 9. The signal unit is composed of a horizontal selector 3. The pixel array section 1 includes scanning lines WS and DS arranged in rows, signal lines SL arranged in columns, and matrix pixel circuits 2 arranged at portions where the two intersect. The horizontal selector 3 constituting the signal unit supplies the video signal Sig to the signal line SL. The write scanner 4 constituting the scanner unit supplies the control signal WS to the first scanning line WS, and the drive scanner 5 included in the scanner unit also supplies the control signal DS to the second scanning line DS. The pixel circuit 2 is scanned every time. Each pixel circuit 2 includes a sampling transistor T1, a pixel capacitor C1 connected thereto, a drive transistor T5 connected thereto, a light emitting element EL connected thereto, and a switching transistor connecting the drive transistor T5 to the power supply line VL. Including T4. The sampling transistor T1 conducts in response to the control signal WS supplied from the first scanning line WS and samples the signal potential Vsig of the video signal Sig supplied from the signal line SL into the pixel capacitor C1. The pixel capacitor C1 applies an input voltage Vgs between the gate G and the source S of the drive transistor T5 according to the signal potential Vsig of the sampled video signal Sig. The drive transistor T5 supplies an output current Ids corresponding to the input voltage Vgs to the light emitting element EL. This output current Ids is dependent on the threshold voltage Vth of the drive transistor T5. The light emitting element EL is connected between the source S of the drive transistor T5 and the cathode potential Vcat, and emits light with luminance according to the signal potential Vsig of the video signal Sig by the output current Ids supplied from the drive transistor T5 during the light emission period. . The switching transistor T4 is turned on in response to the control signal DS supplied from the second scanning line DS, connects the drive transistor T5 to the power supply line VL during the light emission period, and becomes non-conductive during the non-light emission period and turns off the drive transistor T5. Disconnect from the power line VL.

  As a feature of the present invention, the write scanner 4 and the drive scanner 5 constituting the scanner unit output control signals WS and DS to the first scanning line WS and the second scanning line DS, respectively, and the sampling transistor T1 and the switching transistor T4 are provided. A correction operation that corrects the pixel capacitor C1 to cancel the dependency of the output current Ids on the threshold voltage Vth by performing on / off control, and a sampling operation that writes the signal potential Vsig of the video signal Sig to the corrected pixel capacitor C1. Execute. In this case, the horizontal selector 3 constituting the signal unit switches the video signal Sig between the fixed potential Vofs and the signal potential Vsig in accordance with the correction operation and the sampling operation, and is thus necessary for the correction operation and the sampling operation. Is supplied to each pixel circuit 2 through the signal line SL. Specifically, the horizontal selector 3 supplies the fixed potential Vofs to the signal line SL in accordance with the correction operation, and then switches to the signal potential Vsig in accordance with the sampling operation.

  The power supply line VL is arranged in the pixel array unit 1 in parallel with the first scanning line WS and the second scanning line DS. As described above, the scanner unit includes the power supply line scanner 9 that scans the power supply line VL in the same manner as the scanning lines WS and DS, so that the potentials Vcc and Vss necessary for the correction operation are supplied to each pixel circuit 2. Supply via line VL. Specifically, the power supply line scanner 9 switches the power supply potential Vss supplied from the power supply line VL during the correction operation period from the normal power supply potential Vcc supplied during the light emission period, so that it is necessary for the correction operation. Such potential is supplied to each pixel circuit 2 via the power supply line VL. In the present embodiment, the scanner unit outputs a control signal to each of the first scanning line WS and the second scanning line DS in the horizontal scanning period assigned to the row of the pixel, and thus the correction described above within the horizontal scanning period. The operation and sampling operation are executed.

  FIG. 15 is a timing chart for explaining the operation of the display device shown in FIG. On-off changes of the sampling transistor T1 and the switching transistor T4 are represented along the time axis J. In addition, a change in power supply voltage appearing on the power supply line VL and a change in signal voltage appearing on the signal line SL are also shown. In addition, the potential change of the gate G and the source S of the drive transistor T5 is also shown.

  As shown in the figure, the light emission period is from the timing J1 to the timing J8 and the timing J1 to J8 is the non-light emission period. The threshold voltage correction operation is performed during the threshold cancellation period from the timing J4 to J5. Further, the above-described sampling operation is performed in timing J6 to J7 in the signal writing period. In addition, the timing J1 to J4 is a preparation period for correction.

  First, at timing J1, the switching transistor T4 is turned off. As a result, the drive transistor T5 is disconnected from the power supply potential Vcc, so that the potentials of the gate G and the source S are lowered. The potential of the source S is just a level obtained by adding the threshold voltage Vthel to the cathode potential Vcat of the light emitting element EL. After the power supply potential is switched from Vcc to Vss at timing J2, the sampling transistor T1 and the switching transistor T4 are turned on at timing J3. At this time, the power supply potential is continuously Vss and the signal line SL is at a predetermined fixed potential Vofs. When the sampling transistor T1 is turned on, the fixed potential Vofs is written to the gate G of the drive transistor T5. Further, when the switching transistor T4 is turned on, the source S of the drive transistor T5 is lowered to Vss.

  Thereafter, at timing J4, the power supply voltage is switched from Vss to Vcc. As a result, a current flows from the drive transistor T5 into the pixel capacitor C1, and the potential of the source S starts to rise. At this time, the light emitting element EL is in a reverse bias state, and thus does not emit light. The drive transistor T5 is cut off when the voltage between the gate G and the source S of the drive transistor T5 is just Vth. Therefore, a voltage corresponding to the threshold voltage Vth is written into the pixel capacitor C1.

  Thereafter, after the switching transistor T4 is turned off at timing J5, the signal line SL is switched from the fixed potential Vofs to the signal potential Vsig at timing J6. At this time, since the sampling transistor T1 is continuously in the on state, the signal potential Vsig is written into the pixel capacitor C1 in such a manner that the signal potential Vsig is added to the threshold voltage Vth. At timing J7, the sampling transistor T1 is turned off, and the signal writing operation is completed. Thereafter, at timing J8, the switching transistor T4 is turned on and the light emission period starts.

  Hereinafter, the operation of the pixel circuit according to the present invention shown in FIGS. 14 and 15 will be described in detail with reference to FIGS. First, the light emitting state of the light emitting element EL is a state in which only the switching transistor T4 is turned on as shown in FIG. At this time, since the drive transistor T5 is designed to operate in the saturation region, the value of the current flowing through the light emitting element EL takes the value represented by the characteristic equation 1 according to the gate-source voltage Vgs of the drive transistor T5. .

  Next, the switching transistor T4 is turned off in the non-light emitting period (FIG. 17). Since the current is not supplied from the power supply voltage to the cathode by turning off the switching transistor T4, the light emitting element EL is extinguished, and the source voltage of the drive transistor T5 is the sum of the cathode voltage Vcat and the threshold voltage Vthel of the light emitting element EL, that is, Vcat + Vthel. Value.

  Thereafter, the power supply voltage is set to Vss, the signal voltage is set to Vofs, and the sampling transistor T1 and the switching transistor T4 are turned on (FIG. 18). By turning on the sampling transistor T1, the gate voltage of the drive transistor T5 is charged to a value of Vofs. Further, since Vss is smaller than Vcat + Vthel, point A in the figure is the source potential of the drive transistor T5, and point B is the drain. Further, since Vofs−Vss is larger than the threshold voltage Vth of the drive transistor T5, the current flows as shown in the figure, and the potential at the point B is charged to the value Vss. Here, since Vss is smaller than the sum Vcat + Vthel of the cathode voltage Vcat and the threshold voltage Vthel of the light emitting element EL, that is, Vss ≦ Vthel + Vcat, the light emitting element EL does not emit light.

  In this state, the power supply voltage is further set to Vcc (FIG. 19). With this operation, point B in the figure again becomes the source voltage of the drive transistor T5, and point A becomes the drain voltage. Since the equivalent circuit of the light emitting element EL is represented by a diode Tel and a capacitor Cel as shown in the figure, as long as Vel ≦ Vcat + Vthel (the leakage current of the light emitting element EL is considerably smaller than the current flowing through the drive transistor T5), The current of the drive transistor T5 is used to charge the pixel capacitor C1 and the light emitting element capacitor Cel. At this time, Vel rises with time. After a predetermined time has elapsed, the gate-source voltage of the drive transistor T5 takes a value of Vth. At this time, Vel = Vofs−Vth ≦ Vcat + Vthel.

  After a predetermined time has elapsed, the switching transistor T4 is turned off, and the signal line is set to Vsig, and a desired signal voltage is written to the gate of the drive transistor T5 (FIG. 20). At this time, the gate-source voltage of the drive transistor T5 is determined by the pixel capacitance C1, the parasitic capacitance Cel of the light emitting element EL, and the parasitic capacitance C2 of the transistor T5 as shown in Equation 2 above. However, since the light emitting element capacitance Cel is larger than the pixel capacitance C1 and the parasitic capacitance C2, the gate / source voltage Vgs of the drive transistor T5 is approximately Vsig + Vth.

  After the writing is completed, the sampling transistor T1 is turned off and the switching transistor T4 is turned on to raise the drain voltage of the drive transistor to the power supply voltage Vcc. Since the gate-source voltage of the drive transistor T5 is constant, the drive transistor T5 causes a constant current Ids ″ to flow through the light emitting element EL, and Vel rises to a voltage Vx at which a current of Ids ″ flows through the light emitting element EL. The element EL emits light (FIG. 21).

  In this circuit as well, the IV characteristic of the light emitting element EL changes as the light emission time becomes longer. Therefore, the potential at point B in the figure also changes. However, since the gate / source voltage of the drive transistor T5 is maintained at a constant value, the current flowing through the light emitting element EL does not change. Therefore, even if the IV characteristic of the light emitting element EL deteriorates, the constant current Ids always flows, and the luminance of the light emitting element EL does not change. The value of the power source in the present invention is binary. As a result, the existing gate driver can be used, and the cost can be reduced.

  A modification of the present invention is shown in FIG. In this modification, the operation timing of the switching transistor T4 is different from that of the above embodiment. In this modification, the margin of the threshold correction period can be extended by the rise time of the switching transistor T4.

  According to the present invention, since the threshold variation of the drive transistor can be suppressed, a uniform image quality without unevenness and roughness can be obtained. According to the present invention, since the power source is a pulse having a binary value, an existing gate driver can be used, and the cost can be reduced. Since the pixel circuit of the present invention is formed with a small number of elements such as three transistors and one pixel capacity, high definition and high yield can be expected. Since the pixel circuit of the present invention is composed of three gate lines and three power supply lines per RGB trio, the ratio of the power supply and the gate line to the pixel can be reduced. Yield can be expected. According to the present invention, since the gate / source voltage of the drive transistor is maintained at a constant value, the current flowing through the light emitting element EL does not change. Therefore, even if the IV characteristic of the light emitting element EL deteriorates, the constant current Ids always flows, and the luminance of the light emitting element EL does not change.

It is a block diagram which shows the general structure of a display apparatus. It is a circuit diagram which shows the reference example of a display apparatus. It is a graph with which it uses for operation | movement description of the display apparatus shown in FIG. It is a circuit diagram which shows another reference example of a display apparatus. It is a circuit diagram which shows the display apparatus concerning prior development. 6 is a timing chart for explaining the operation of the display device shown in FIG. 5. FIG. 6 is a circuit diagram for explaining an operation of the display device shown in FIG. 5. It is a circuit diagram similarly used for operation | movement description. It is a circuit diagram similarly used for operation | movement description. It is a circuit diagram similarly used for operation | movement description. It is a graph similarly provided for operation | movement description. It is a circuit diagram similarly used for operation | movement description. It is a circuit diagram similarly used for operation | movement description. It is a block diagram which shows the display apparatus concerning this invention. FIG. 15 is a timing chart for explaining the operation of the display device shown in FIG. 14. FIG. 15 is a pixel circuit diagram for explaining the operation of the display device according to the present invention shown in FIG. 14. It is a pixel circuit diagram for the same explanation of operation. It is a pixel circuit diagram for the same explanation of operation. It is a pixel circuit diagram for the same explanation of operation. It is a pixel circuit diagram for the same explanation of operation. It is a pixel circuit diagram for the same explanation of operation. It is a timing chart which shows the modification of the display apparatus concerning this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Pixel array part, 2 ... Pixel circuit, 3 ... Horizontal selector, 4 ... Write scanner, 5 ... Drive scanner, 9 ... Power supply line scanner, T1 ... Sampling transistor , T4... Switching transistor, T5... Drive transistor, EL... Light emitting element, C1.

Claims (7)

Including a pixel array unit, a scanner unit, and a signal unit,
The pixel array section includes scanning lines arranged in rows and signal lines arranged in columns, and matrix-like pixels arranged in a portion where both intersect,
The signal unit supplies a video signal to the signal line,
The scanner unit supplies control signals to the first scanning line and the second scanning line to sequentially scan pixels for each row,
Each pixel includes a sampling transistor, a pixel capacitor connected thereto, a drive transistor connected thereto, a light emitting element connected thereto, and a switching transistor connecting the drive transistor to a power line,
The sampling transistor conducts in response to a control signal supplied from the first scanning line and samples the signal potential of the video signal supplied from the signal line into the pixel capacitor,
The pixel capacitor applies an input voltage between the gate and the source of the drive transistor according to the signal potential of the sampled video signal,
The drive transistor supplies an output current corresponding to the input voltage to the light emitting element, and the output current has a dependency on a threshold voltage of the drive transistor,
The light emitting element emits light with luminance according to the signal potential of the video signal by an output current supplied from the drive transistor during a light emission period,
The switching transistor is turned on in response to a control signal supplied from the second scanning line, connects the drive transistor to the power supply line during the light emission period, and is turned off during the non-light emission period. Disconnect from the line,
The scanner unit outputs a control signal to each of the first scanning line and the second scanning line, and controls on / off of the sampling transistor and the switching transistor to cancel the dependency of the output current on the threshold voltage. A display device comprising: a correction operation that corrects the pixel capacitance; and a sampling operation that writes a signal potential of the video signal to the corrected pixel capacitance.   The signal unit switches the video signal between a fixed potential and a signal potential in accordance with the correction operation and the sampling operation, and thereby signals the potential necessary for the correction operation and the sampling operation to each pixel. The display device according to claim 1, wherein the display device is supplied through a line.   The display device according to claim 2, wherein the signal unit supplies the fixed potential to a signal line in accordance with the correction operation, and then switches to the signal potential in accordance with the sampling operation. The power supply line is arranged in the pixel array unit in parallel with the first scanning line and the second scanning line,
The scanner unit includes a power supply line scanner that scans the power supply line in the same manner as a scanning line, and supplies a potential necessary for the correction operation to each pixel through the power supply line. The display device according to claim 1.   The power supply line scanner switches a power supply potential supplied from the power supply line during a period during which the correction operation is performed from a normal power supply potential supplied during the light emission period, so that a potential necessary for the correction operation is changed to each pixel. The display device according to claim 4, wherein the power supply line is supplied to the display device.     The scanner unit outputs a control signal to each of the first scanning line and the second scanning line in a horizontal scanning period assigned to the row of the pixel, so that the correction operation and the sampling are performed in the horizontal scanning period. The display device according to claim 1, wherein an operation is executed. A pixel array unit, a scanner unit, and a signal unit, wherein the pixel array unit includes a matrix of pixels arranged at a portion where the scanning lines arranged in rows and the signal lines arranged in columns intersect with each other. The signal unit supplies a video signal to the signal line, and the scanner unit supplies a control signal to the first scanning line and the second scanning line to sequentially scan the pixels for each row. Is a driving method of a display device including a sampling transistor, a pixel capacitor connected thereto, a drive transistor connected thereto, a light emitting element connected thereto, and a switching transistor connecting the drive transistor to a power supply line. There,
The sampling transistor conducts in response to a control signal supplied from the first scanning line and samples the signal potential of the video signal supplied from the signal line into the pixel capacitor,
The pixel capacitor applies an input voltage between the gate and source of the drive transistor according to the signal potential of the sampled video signal,
The drive transistor supplies an output current corresponding to the input voltage to the light emitting element, and the output current has a dependency on a threshold voltage of the drive transistor;
The light emitting element emits light with a luminance according to the signal potential of the video signal by an output current supplied from the drive transistor during a light emission period,
The switching transistor is turned on in response to a control signal supplied from the second scanning line, connects the drive transistor to the power supply line during the light emission period, and becomes non-conductive during the non-light emission period. Disconnect from the line,
The scanner unit outputs a control signal to each of the first scanning line and the second scanning line, and controls on / off of the sampling transistor and the switching transistor to cancel the dependency of the output current on the threshold voltage. A display device driving method, comprising: performing a correction operation for correcting the pixel capacitance; and a sampling operation for writing a signal potential of the video signal in the corrected pixel capacitance.

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