JP2011180552A - Image display device and method of driving the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem of an image display device including a pixel circuit in which a drive transistor is constituted by an n-channel TFT, wherein the number of control signal lines for controlling the pixel circuit increases and the pixel aperture ratio decreases. <P>SOLUTION: A switch 72 is connected in series to a capacitor 64 with one terminal connected to a data line 50 and the other terminal connected to the gate of the drive transistor 62. The switch 74 is connected between the source of the drive transistor 62 and the ground potential GND. A control signal line 52b is wired to each row of the pixel circuit 40. The control signal line 52b provides a control signal to each of the switches 72, 74 in the pixel circuit 40 at the row where the control signal line is wired. By sharing the control signal line for these two switches 72, 74, the area of the control signal lines in a screen where the pixel circuit is arranged decreases and the pixel aperture ratio can be increased. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an image display device in which pixel circuits each having a light emitting element and a plurality of switches for controlling the operation thereof are arranged in a matrix, and a driving method thereof.

  Conventionally, an organic EL display which is a display device using an organic EL (Electro-Luminescence) element has been proposed. Since the organic EL element is a self-luminous element, a backlight necessary for a liquid crystal display device is unnecessary, and it is suitable for thinning and has a feature that a viewing angle is close to 180 °. Therefore, the organic EL display is highly expected to be put to practical use as a next-generation display device.

  Currently proposed organic EL displays mainly use current-controlled organic EL elements that utilize the phenomenon of generating light when holes and electrons injected into a light emitting layer recombine. The organic EL display can adopt an active matrix system similarly to a liquid crystal display (LCD), and a plurality of pixel circuits are arranged corresponding to the pixels. The pixel circuit is configured using, for example, a thin film transistor (TFT) formed of amorphous silicon, polycrystalline silicon, or the like. Each pixel circuit is provided with an organic EL element such as an organic light-emitting diode (OLED), and the luminance of each pixel can be controlled in accordance with a current flowing through the organic EL element.

In an active matrix image display device, a drive transistor is provided in each pixel circuit, and a light emitting element of the pixel circuit is driven. The drain-source current I _DS of the driving transistor changes according to the gate-source voltage V _GS , and a gradation current corresponding to the gradation of the pixel is obtained by applying a potential corresponding to the pixel value to the gate. . This gradation current is supplied to the organic EL element as a drive current.

FIG. 9 is a circuit diagram of a pixel circuit described in Patent Document 1 below. The drive transistor Q01 of this pixel circuit is composed of a p-channel TFT. Q01 is disposed between the anode of the organic EL element OLED, which is a diode-type light emitting element, and the drive voltage source V _OLED that supplies a predetermined positive voltage. The cathode of the OLED is connected to the ground potential GND. The source of Q01 is connected to V _OLED and the drain of Q01 is connected to the anode of OLED via switch 2. The gate of Q01 is connected to the data line 8 via the capacitor 4 and the switch 6, V _GS corresponding to the voltage and V _OLED of the data signal applied to the data line 8 are set, corresponding to the V _GS I _DS is supplied to the OLED.

  If the driving transistor can be composed of an n-channel TFT, a conventional amorphous silicon process can be used in TFT fabrication (Patent Document 2). In addition, high-speed driving is possible.

FIG. 10 is a circuit diagram of a pixel circuit described in Patent Document 2 below in which the drive transistor Q02 is configured by an n-channel TFT. When Q02 is an n-channel, its drain is connected to the V _OLED via the switch 2 and its source is connected to the anode of the OLED. The source potential fluctuates due to a change with time in the current-voltage (IV) characteristics of the OLED, and accordingly, the V _GS set when a data signal is applied to the data line 8 is changed, and the light emission luminance of the OLED. This also has the disadvantage of changing. This inconvenience can be avoided by providing a switch 10 between the source of Q02 and GND, turning on the switch 10 when setting V _GS and setting the source potential to GND.

US Pat. No. 6,229,506 JP 2004-361640 A

  When the driving transistor is composed of an n-channel TFT, the switch 10 which is unnecessary when the driving transistor is composed of a p-channel TFT is necessary, and the number of switches in the pixel circuit is increased. As the number of switches increases and the number of signal lines that supply control signals to the switches increases, the area that the signal lines occupy on the matrix arrangement of the pixel circuit increases. There was a problem that the area decreased.

  The present invention has been made to solve the above-described problems, and improves the pixel aperture ratio in an image display apparatus having a pixel circuit having a switch for connecting a connection point between a driving transistor and a light emitting element to a reference voltage source. It is another object of the present invention to provide a method for driving the image display device.

  An image display device according to the present invention supplies a plurality of pixel circuits arranged in a matrix and a data signal arranged in each column of the matrix arrangement of the pixel circuits and having a voltage corresponding to a luminance value to the pixel circuits. A control signal line that is arranged in each row of the matrix arrangement of the pixel circuit and supplies a control signal to a plurality of switches provided in the pixel circuit, and first, second, and third reference voltage sources The pixel circuit has one terminal connected to the first reference voltage source and emits light with an intensity corresponding to the drive current supplied to the other terminal, and a voltage corresponding to the data signal And a second current terminal connected to the second reference voltage source, the control signal being applied to the data signal. The drive current according to the A drive transistor generated between the current terminals, a first capacitor connected between the data line and the control terminal, and a second capacitor connected between the first current terminal and the control terminal. A capacitor, a first switch capable of intermittent connection between the second current terminal and the second reference voltage source, and a second switch capable of intermittent connection between the control terminal and the second current terminal. A third switch connected in series to the first capacitor and capable of switching capacitive coupling and separation between the data line and the control terminal; the first current terminal; A fourth switch that can be intermittently connected to the three reference voltage sources, and the control signal line provided in each row corresponding to the third and fourth switches is common to both the switches This is a single wiring.

  In a preferred aspect of the present invention, the first capacitor has one terminal connected to the data line, the other terminal connected to the control terminal via the third switch, and the second capacitor The image display apparatus has one terminal connected to the first current terminal and the other terminal connected to the control terminal.

  In another preferred aspect of the present invention, the first capacitor has one terminal connected to the data line via the third switch, the other terminal connected to the control terminal, and the second capacitor. The capacitor has one terminal connected to the first current terminal, the other terminal connected to the one terminal of the first capacitor, and the first terminal between the first current terminal and the control terminal. An image display device connected in series with the capacitor.

  In the driving method of the image display device according to the present invention, the pixel circuits are sequentially selected in units of rows, and the writing operation for writing the luminance information corresponding to the data signal to the pixel circuits in the selected row is performed. In the light emitting operation for causing the light emitting element to emit light at an intensity according to luminance information, the writing operation for the selected row sets the first to fourth switches in a conductive state and determines the reference level of the luminance value. A first step of applying a predetermined voltage to the data line; a second step of switching the first switch to a non-conducting state following the first step; and a second switch following the second step Switching to a non-conductive state, applying the data signal to the data line and setting the control terminal to a potential corresponding to the data signal; and the third step And a fourth step of switching the third and fourth switches to a non-conducting state subsequent to the first step, and the light emission operation causes the first switch to be in a conducting state after the fourth step. A method comprising the steps of:

  According to the present invention, a pixel circuit structure and a driving method for controlling the third switch and the fourth switch with a common control signal line are realized, and the wiring space of the control signal line is reduced in the image display device. The pixel aperture ratio is improved, and the image display device can be driven with stable characteristics of light emission luminance with respect to the data signal, thereby ensuring image quality.

It is a schematic diagram which shows the structure of the outline of the organic electroluminescent display which concerns on embodiment of this invention. 1 is a circuit diagram illustrating a pixel circuit according to a first embodiment. FIG. 3 is a circuit diagram schematically illustrating a state of the pixel circuit in the first embodiment in a precharge period. FIG. 3 is a circuit diagram schematically illustrating a state of the pixel circuit in the first embodiment in a Vth writing period. FIG. 3 is a circuit diagram schematically illustrating a state of the pixel circuit in the first embodiment in a Vth writing period. FIG. 3 is a circuit diagram schematically showing a state of the pixel circuit in the light emitting period in the light emitting period in the first embodiment. It is a signal waveform diagram which shows the change of the various signals in the drive method of the organic EL display which concerns on embodiment of this invention. It is a circuit diagram which shows the pixel circuit of 2nd Embodiment. It is a circuit diagram of the conventional pixel circuit. It is a circuit diagram of the conventional pixel circuit which comprised the drive transistor by n channel TFT.

  Hereinafter, an organic EL display 20 according to an embodiment of the present invention (hereinafter referred to as an embodiment) will be described with reference to the drawings.

[First Embodiment]
The organic EL display 20 is an active matrix display device and is mounted as a display panel on a television, a personal computer, a mobile terminal, a mobile phone, or the like. FIG. 1 is a schematic diagram showing a schematic configuration of the organic EL display 20. The organic EL display 20 includes a display unit 22, a data electrode drive circuit 24, a scan electrode drive circuit 26, a control circuit 28, and a drive voltage source 30.

  In the display unit 22, a plurality of pixel circuits 40 are arranged in a matrix corresponding to the pixels. Further, in the display unit 22, signal lines from the data electrode drive circuit 24 and the scan electrode drive circuit 26 to each pixel circuit 40 and a power supply line from the reference voltage source to each pixel circuit 40 are formed. Specifically, the data line 50 is wired from the data electrode driving circuit 24 to each column of the matrix arrangement of the pixel circuit 40. A plurality of control signal lines 52 are wired from the scan electrode driving circuit 26 to each row of the matrix arrangement of the pixel circuit 40. Power supply lines 54 and 56 are wired from the ground potential GND and the drive voltage source 30 to each pixel circuit 40, respectively. The pixel circuit 40 is applied with various signals and power through these signal lines and power supply lines and is driven in an active matrix.

  The data electrode drive circuit 24 outputs a data signal having a voltage corresponding to the luminance value based on the image signal to the data line 50 during the writing period of the image signal to the pixel circuit 40, and the data signal passes through the data line 50. To the pixel circuit 40.

  The scan electrode drive circuit 26 generates a control signal for the gate electrode of a thin film transistor (TFT) constituting various switches in the pixel circuit 40. The control signal is supplied to the pixel circuit 40 via the control signal line 52.

  The control circuit 28 operates based on a clock signal input to the organic EL display 20 and a synchronization signal such as a vertical synchronization signal and a horizontal synchronization signal, and controls the operation of each part of the organic EL display 20. For example, the control circuit 28 buffers an image signal input to the organic EL display 20 in a memory (not shown), reads the image signal line by line from the memory during a writing period to the pixel circuit 40, and the data electrode driving circuit 24. To supply. The control circuit 28 cooperates with the scan electrode driving circuit 26 to select and control the pixel circuit 40 in units of rows in the writing period, and to control the operation of the pixel circuit 40 in the light emitting period.

The driving voltage source 30 supplies a positive voltage V _OLED .

  FIG. 2 is a circuit diagram showing the pixel circuit 40. The pixel circuit 40 includes an OLED 60, a driving transistor 62, first and second capacitors 64 and 66, and first to fourth switches 68, 70, 72, and 74.

  The OLED 60 is a light emitting element that emits light with an intensity corresponding to the drive current, and the cathode electrode of the OLED 60 is connected to a ground potential GND that is a first reference voltage source via a power line 56. The anode electrode is connected to the drive voltage source 30 as the second reference voltage source via the drive transistor 62 and the like, and is supplied with a drive current.

The drive transistor 62 is a TFT, and a voltage corresponding to the data signal is applied to the gate electrode which is a control terminal, and a current (grayscale current) corresponding to the gate-source voltage V _GS is generated between the drain and the source. The drain - source current _{I DS} is supplied to OLED60 as the drive current. Here, the drive transistor 62 is composed of an n-channel TFT. In this case, the source electrode is a first current terminal connected to the anode electrode of the OLED 60, and the drain electrode is a second current terminal connected to the drive voltage source 30. It becomes.

  The capacitor 64 is connected between the data line 50 and the gate electrode of the driving transistor 62. More specifically, the capacitor 64 has one terminal connected to the data line 50 and the other terminal connected to the gate electrode of the drive transistor 62 via the third switch 72.

  The capacitor 66 has one terminal connected to the source electrode of the driving transistor 62 and the other terminal connected to the gate electrode of the driving transistor 62.

  The first switch 68 is provided between the drain electrode of the drive transistor 62 and the drive voltage source 30 and can be intermittently connected between them.

  The second switch 70 is connected between the gate electrode and the drain electrode of the driving transistor 62, and enables the connection between them.

  The third switch 72 is connected in series to the capacitor 64 and can switch capacitive coupling and separation between the data line 50 and the gate electrode of the driving transistor 62. In this embodiment, the switch 72 has one terminal connected to the capacitor 64 and the other terminal connected to the gate electrode of the drive transistor 62.

The fourth switch 74 is connected between the source electrode of the driving transistor 62 and the third reference voltage source, and enables the connection between them. In the present embodiment, the ground potential GND is used as the third reference voltage source. The voltage V _REF of the third reference voltage source can be basically set within a range in which the OLED 60 is not turned on when it is applied to the anode electrode of the OLED 60. Therefore, the ground potential GND is not necessarily required. However, when the first reference voltage source is shared, there is an advantage that the power supply line is shared.

  Each switch 68, 70, 72, 74 is configured by using a TFT, and on / off thereof is controlled by a control signal applied to the gate electrode of the TFT from the scan electrode driving circuit 26 via the control signal line 52. The Three control signal lines 52a, 52b, and 52c are provided for the four switches provided in each pixel circuit 40. Each control signal line 52a, 52b, 52c extends through the display unit 22 in the row direction. The control signal line 52 in each row is connected to the corresponding switch in the pixel circuit 40 in the corresponding row. In the pixel circuit 40, the control signal line 52a is connected to the switch 68, and the control signal line 52b is connected to the switch 70. In the pixel circuit 40, the control signal line 52 c is connected to the switch 72 and the switch 74, and a common control signal is applied to the switches 72 and 74.

3 to 6 are circuit diagrams schematically showing the state of the pixel circuit 40 in the operation of the organic EL display 20. 3 to 6 show the on / off states of the switches in the main process of operation. FIG. 7 is a signal waveform diagram showing changes in various signals in the operation. In FIG. 7, the horizontal axis represents time, and the vertical axis represents voltage. In the figure, the control signals S _A , S _B , S _C applied to the control signal lines 52 and the data signal S _D applied to the data line 50 are shown. , And the gate-source voltage V _GS of the drive transistor 62 are shown side by side in the vertical direction. Here, the control signals _{S A, S B, S C} is the signal applied to the control signal lines 52a, 52 b, 52c, when the control signal for each switch corresponding is High (H) level, turned on, Turns off at Low (L) level. The voltage of the data signal _SD is configured to increase as the luminance value increases. The operation of the pixel circuit 40 will be described with reference to FIGS.

  In the operation of the organic EL display 20, the writing period Twrt is sequentially set for each row of the pixel circuit 40, and an image signal is written into the pixel circuit 40 in the period Twrt. Then, a light emission period Tilm is set subsequent to the writing period Twrt, and the OLED 60 emits light with an intensity corresponding to the image signal written in the preceding writing period Twrt in the period Tilm.

  In the write period Twrt, a precharge operation, a Vth write operation, and a data write operation are performed in order.

In the precharge operation period Tpre, the pixel circuit 40 is set to the state shown in FIG. Precharge period Tpre the control signals _{S A, S B, S C} is set to H level, the switch 68, 70, 72, 74 is set to the ON state. In this state, the driving transistor 62 has a diode connection in which the gate electrode and the drain electrode are connected, and the gate potential of the driving transistor 62 held by the capacitor 66 in the previous frame becomes the voltage V _OLED of the driving voltage source 30. Reset. The source electrode of the drive transistor 62 is set to the ground potential GND through the switch 74. Note that the drawing current during the precharge operation flows into the ground potential GND via the switch 74, so that the OLED 60 does not emit light.

In the period Tvth of the Vth writing operation, the pixel circuit 40 is set to the state shown in FIG. Vth write period Tvth the control signal _S B, while the _{S C} was maintained at H level, the control signal _{S A} is switched to L level, the switch 70, 72, 74 is turned on, the switch 68 is set to the OFF state. Since the gate electrode and the drain electrode of the driving transistor 62 are short-circuited by the switch 70, V _GS is automatically set to the threshold voltage (Vth) of the driving transistor 62. Since the pixel circuit 40 includes the switch 74, the source potential of the driving transistor 62 at this time can be set to a constant potential. That is, when the switch 74 is in the on state, the source potential of the drive transistor 62 is set to the ground potential GND. As a result, the gate potential of the drive transistor 62 is set to Vth. In the periods Tpre and Tvth, a voltage S _D (black) corresponding to the black of the image is applied to the data line 50.

In the data write operation period Tdata, the pixel circuit 40 is set to the state shown in FIG. In the data writing period Tdata, first to the data line 50 while applying a S _{D (black),} to turn off the switch 70 a control signal S _B to the L level. That is, the switches 72 and 74 are set to the on state and the switch 70 is set to the off state. Thereafter, the gradation voltage S _D (data) corresponding to the luminance value of the pixel is applied to the data line 50. Then, charge movement occurs in the capacitors 64 and 66 in accordance with the voltage change of the data line 50, and the potential of the gate electrode of the drive transistor 62 changes toward a value corresponding to the gradation voltage S _D (data). The period Tdata is set to such a length that the potential change of the data line 50 is sufficiently transmitted to the gate electrode of the drive transistor 62. The source potential of the drive transistor 62 is set to the ground potential GND via the switch 74.

The last period Tdata, by switching the control signal _{S C} to the L level, the switch 70, 72, 74 are turned off. At this time, the gate electrode of the drive transistor 62 is disconnected from the data line 50, and the potential of the gate electrode is not affected by the subsequent potential change of the data line 50, and V _{GS at} this time is held by the capacitor 66.

When the writing of the image signal is completed in the period Tdata, the light emission period Tilm is started. In the light emission period Tilm, the pixel circuit 40 is set to the state shown in FIG. Once in the light emission period Tilm, the control signal _{S A} is switched to H level, the switch 68 is turned on, the switch 70, 72, 74 is turned off. The drive transistor 62 supplies a drain-source current I _DS corresponding to the gate-source voltage V _GS set in the period Tdata to the OLED 60, and the OLED 60 emits light with brightness corresponding to the current amount.

[Second Embodiment]
The organic EL display according to the second embodiment has a difference from the first embodiment in the pixel circuit. Other configurations and driving methods are basically the same as those described in the first embodiment. Therefore, in this embodiment, the same reference numerals as those in the first embodiment are used for the constituent elements having the same functions as those in the first embodiment, and the description in the first embodiment is used here. Differences will be described.

  FIG. 8 is a circuit diagram showing the pixel circuit 80. The pixel circuit 80 includes an OLED 60, a drive transistor 62, first and second capacitors 64 and 66, and first to fourth switches 68, 70, 72, and 74.

  The pixel circuit 80 is different from the pixel circuit 40 in that the capacitors 64 and 66 and the switch 72 are connected to each other. In the pixel circuit 80, the capacitor 64 has one terminal connected to the data line 50 via the switch 72 and the other terminal connected to the gate electrode of the drive transistor 62. The switch 72 has one terminal connected to the data line 50 and the other terminal connected to the capacitor 64. The capacitor 66 has one terminal directly connected to the source electrode of the drive transistor 62 and the other terminal connected to a connection point between the capacitor 64 and the switch 72. That is, the capacitor 66 is connected in series with the capacitor 64 between the gate electrode and the source electrode of the driving transistor 62.

  The control signal line 52 for supplying a control signal to each switch 68, 70, 72, 74 is the same as in the first embodiment, and in particular, a common control signal line 52b is provided for the switches 72, 74. It is the same.

  20 organic EL display, 22 display unit, 24 data electrode drive circuit, 26 scan electrode drive circuit, 28 control circuit, 30 drive voltage source, 40, 80 pixel circuit, 50 data line, 52, 52a, 52b, 52c control signal line 54, 56 Power line, 60 OLED, 62 Drive transistor, 64, 66 Capacitor, 68, 70, 72, 74 Switch.

Claims (4)

A plurality of pixel circuits arranged in a matrix;
A data line disposed in each column of the matrix arrangement of the pixel circuit and supplying a data signal having a voltage corresponding to a luminance value to the pixel circuit;
A control signal line that is arranged in each row of the matrix arrangement of the pixel circuit and supplies a control signal to a plurality of switches provided in the pixel circuit;
First, second and third reference voltage sources;
Have
The pixel circuit includes:
A light emitting element having one terminal connected to the first reference voltage source and emitting light with an intensity corresponding to a driving current supplied to the other terminal;
A control terminal to which a voltage according to the data signal is applied; a first current terminal connected to the other terminal of the light emitting element; and a second current terminal connected to the second reference voltage source. A drive transistor that generates the drive current according to the data signal between the current terminals;
A first capacitor connected between the data line and the control terminal;
A second capacitor connected between the first current terminal and the control terminal;
A first switch capable of intermittently connecting between the second current terminal and the second reference voltage source;
A second switch capable of switching between the control terminal and the second current terminal;
A third switch connected in series to the first capacitor and capable of switching capacitive coupling and separation between the data line and the control terminal;
A fourth switch capable of intermittently connecting between the first current terminal and the third reference voltage source;
Have
The control signal line provided in each row corresponding to the third and fourth switches is a single wiring common to both the switches,
An image display device characterized by the above. The image display device according to claim 1,
The first capacitor has one terminal connected to the data line and the other terminal connected to the control terminal via the third switch,
The second capacitor has one terminal connected to the first current terminal and the other terminal connected to the control terminal;
An image display device characterized by the above. The image display device according to claim 1,
The first capacitor has one terminal connected to the data line via the third switch, and the other terminal connected to the control terminal,
The second capacitor has one terminal connected to the first current terminal, the other terminal connected to the one terminal of the first capacitor, and between the first current terminal and the control terminal. Connected in series with the first capacitor;
An image display device characterized by the above. 4. The driving method for driving the image display device according to claim 1, wherein the pixel circuits are sequentially selected in units of rows, and the data is supplied to the pixel circuits in the selected row. In a writing operation for writing luminance information according to a signal and a light emitting operation for causing the light emitting element to emit light with an intensity according to the written luminance information,
The write operation for the selected row is:
A first step of turning on the first to fourth switches and applying a predetermined voltage for determining a reference level of the luminance value to the data line;
A second step of switching the first switch to a non-conductive state following the first step;
A third step of switching the second switch to a non-conductive state following the second step, applying the data signal to the data line, and setting the control terminal to a potential corresponding to the data signal;
A fourth step of switching the third and fourth switches to a non-conductive state subsequent to the third step;
Have
The light emitting operation includes a step of bringing the first switch into a conductive state in a state after the fourth step;
A driving method of an image display device characterized by the above.

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