JP2008310075A - Image display device - Google Patents

Image display device Download PDF

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Publication number
JP2008310075A
JP2008310075A JP2007158253A JP2007158253A JP2008310075A JP 2008310075 A JP2008310075 A JP 2008310075A JP 2007158253 A JP2007158253 A JP 2007158253A JP 2007158253 A JP2007158253 A JP 2007158253A JP 2008310075 A JP2008310075 A JP 2008310075A
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Japan
Prior art keywords
driver transistor
voltage
current
transistor
light emitting
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Pending
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JP2007158253A
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Japanese (ja)
Inventor
Shinya Ono
晋也 小野
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Panasonic Corp
パナソニック株式会社
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Priority to JP2007158253A priority Critical patent/JP2008310075A/en
Publication of JP2008310075A publication Critical patent/JP2008310075A/en
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Abstract

<P>PROBLEM TO BE SOLVED: To constitute a pixel circuit by using only an N channel type transistor, the pixel circuit having a current light emitting element connected to the source of a driver transistor. <P>SOLUTION: An image display device has an array of a plurality of pixel circuits 10 each of which has: a current light emitting element D1; the driver transistor Q1 supplying a current to the current light emitting element D1; a holding capacitor C1 holding a voltage determining the amount of the current that the driver transistor Q1 supplies; and a write switch Q2 writing a voltage corresponding to an image signal to the holding capacitor C1, wherein transistors constituting each of the pixel circuits 10 are N channel type transistors and each of the pixel circuits 10 has an enable switch Q4 connected between the source of the driver transistor Q1 and the anode of the current light emitting element D1 and also has an auxiliary capacitor C2 connected between the source of the driver transistor Q1 and a predetermined power line 26. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an active matrix display device using a current light emitting element.

  An organic EL display device in which a large number of organic electroluminescence (EL) elements that emit light by themselves is arranged is expected as a next-generation image display device because a backlight is not required and the viewing angle is not limited.

  The organic EL element is a current light-emitting element that controls luminance by the amount of current that flows, and there are a simple matrix method and an active matrix method as a method for driving the organic EL element. Although the former has a simple pixel circuit, it is difficult to realize a large and high-definition display. Therefore, in recent years, active matrix type organic EL display devices in which pixel circuits each having a driver transistor for driving a current light emitting element are arranged for each organic EL element have been actively developed.

The driver transistor and its peripheral circuit are generally formed using a thin film transistor, and there are a thin film transistor using polysilicon and a thin film transistor using amorphous silicon. Amorphous silicon thin-film transistors have weaknesses such as low mobility and large change in threshold voltage over time, but they are suitable for large organic EL display devices because they have good uniformity of mobility voltage and are easy and inexpensive to enlarge. ing. In addition, a method for overcoming the change with time of the threshold voltage, which is a weak point of the amorphous silicon thin film transistor, by devising the pixel circuit has been studied. For example, Patent Document 1 includes a pixel circuit that can display a stable image without affecting the amount of current flowing through a light-emitting element even when the threshold voltage of a thin film transistor changes. An organic EL display device is disclosed.
Special table 2002-514320 gazette

  However, the pixel circuit described in Patent Document 1 is configured using a P-channel transistor. On the other hand, as an amorphous silicon thin film transistor for a large-sized image display device, only an N-channel transistor has been put into practical use. Therefore, it is necessary to configure an image circuit using only the N-channel transistor. Further, in order to easily manufacture the organic EL element, a circuit configuration in which the anode of the organic EL element is connected to the source of the driver transistor and the cathode of the organic EL element of each image circuit is connected to the common electrode is desirable. Furthermore, a pixel compensation circuit for a source grounding operation is required in order to suppress unevenness in light emission luminance caused by a voltage drop due to a current flowing during light emission of an organic EL and an electric resistance of a power supply line.

  SUMMARY An advantage of some aspects of the invention is that it provides an image display device in which a pixel circuit in which a current light emitting element is connected to a source of a driver transistor is configured using only an N-channel transistor. .

  In order to achieve the above object, the present invention provides a current light emitting element, a driver transistor that causes a current to flow through the current light emitting element, a holding capacitor that holds a voltage that determines the amount of current that the driver transistor passes, and a voltage that corresponds to an image signal. Is an image display device in which a plurality of pixel circuits each having a write switch for writing to a holding capacitor are arranged, the transistors constituting each of the pixel circuits are N-channel transistors, and each of the pixel circuits is connected to a current light emitting element. An enable switch inserted in a current path through which a current flows; and an auxiliary capacitor that suppresses voltage fluctuation of a terminal of the holding capacitor opposite to the terminal to which the write switch is connected. Connect an enable switch between the anode of the light emitting element and the driver switch. Characterized in that connected to the auxiliary capacitor between the source and the predetermined power supply line of Njisuta. With this configuration, it is possible to provide an image display device in which a pixel circuit in which a current light emitting element is connected to the source of a driver transistor is configured using only N-channel transistors.

  Each of the pixel circuits of the image display device according to the present invention includes a separation switch connected between the gate of the driver transistor and one terminal of the holding capacitor, and a connection between the gate of the driver transistor and the drain of the driver transistor. A configuration having a gate-drain connection switch provided is desirable.

  According to the present invention, it is possible to provide an image display device in which a pixel circuit in which a current light emitting element is connected to the source of a driver transistor is configured using only an N-channel transistor.

  Hereinafter, an active matrix image display device according to an embodiment of the present invention will be described with reference to the drawings. Note that an active matrix type organic EL display device that emits light from an organic EL element using a thin film transistor will be described here as an image display device. However, the present invention relates to an active matrix that uses a light emitting element that controls luminance according to the amount of current that flows. Applicable to all types of image display devices.

(Embodiment)
FIG. 1 is a schematic diagram showing a configuration of an organic EL display device according to an embodiment of the present invention. The organic EL display device according to the present embodiment includes a large number of pixel circuits 10 arranged in a matrix, and scanning lines that supply the pixel circuit 10 with a scanning signal Scn, a reset signal Rst, an enable signal Enbl, and a merge signal Mrg. A drive circuit 11, a data line drive circuit 12 that supplies a data signal Data corresponding to an image signal to the pixel circuit 10, and a power supply line drive circuit 14 that supplies power to the pixel circuit 10 are provided. In the present embodiment, description will be made assuming that the pixel circuits 10 are arranged in a matrix of n rows and m columns.

  The scanning line driving circuit 11 supplies the scanning signal Scn independently to the scanning lines 41 commonly connected to the pixel circuits 10 arranged in the row direction in FIG. 1, and also to the pixel circuits 10 arranged in the row direction. A reset signal Rst is independently supplied to the commonly connected reset line 42, and an enable signal Enbl is supplied to each of the enable lines 43 commonly connected to the pixel circuits 10 arranged in the row direction. A merge signal Mrg is supplied independently to each merge line 44 commonly connected to the pixel circuits 10 arranged in the row direction. Further, the data line driving circuit 12 supplies the data signals Data independently to the data lines 20 commonly connected to the pixel circuits 10 arranged in the column direction in FIG. In the present embodiment, the number of scanning lines 41, reset lines 42, enable lines 43, and merge lines 44 is n, and the number of data lines 20 is m.

  The power supply line drive circuit 14 supplies power to the high voltage side power supply line 24 and the low voltage side power supply line 25 that are commonly connected to all the pixel circuits 10. Further, a reference voltage is supplied to a reference voltage line 26 commonly connected to all the pixel circuits 10 as a predetermined power supply line.

  FIG. 2 is a circuit diagram of the pixel circuit 10 in the embodiment of the present invention. The pixel circuit 10 according to the present embodiment includes an organic EL element D1 that is a current light emitting element, a driver transistor Q1 that passes current through the organic EL element D1, and a holding capacitor C1 that holds a voltage that determines the amount of current flowing through the driver transistor Q1. A transistor Q2 that is a write switch for writing a voltage corresponding to the image signal to the holding capacitor C1, a transistor Q3 that is a gate-drain connection switch connected between the gate and drain of the driver transistor Q1, A transistor Q4 which is an enable switch inserted in a current path for passing a current to the organic EL element D1, and a holding capacitor C1 when detecting the threshold voltage Vth of the driver transistor Q1 or writing a voltage to the holding capacitor C1 Turn off the gate of driver transistor Q1 And a transistor Q5 which is separation switch for separating. In addition, the pixel circuit 10 according to the present embodiment further includes an auxiliary capacitor C2 that suppresses voltage fluctuation at a terminal opposite to the terminal to which the transistor Q2 is connected among the terminals of the holding capacitor C1. The auxiliary capacitor C2 is used to superimpose the data voltage Vdata on the threshold voltage Vth of the driver transistor Q1. Here, the driver transistor Q1 and the transistors Q2 to Q5 constituting the pixel circuit 10 are all N-channel thin film transistors.

  A transistor Q4, which is an enable switch, is connected between the source of the driver transistor Q1 and the anode of the organic EL element D1, and an auxiliary capacitor C2 is connected between the source of the driver transistor Q1 and a reference voltage line 26, which is a predetermined power supply line. Has been. That is, the drain of the driver transistor Q1 is connected to the high voltage side power supply line 24, the source of the driver transistor Q1 is connected to the drain of the transistor Q4, the source of the transistor Q4 is connected to the anode of the organic EL element D1, and the organic EL element The cathode of D1 is connected to the low-voltage power supply line 25, and an auxiliary capacitor C2 is connected between the source of the driver transistor Q1 and the reference voltage line 26. Here, the voltage supplied to the high voltage side power supply line 24 is, for example, 0 (V), and the voltage supplied to the low voltage side power supply line 25 is, for example, −20 (V). It is important that the reference voltage is constant without fluctuation, and the voltage value itself can be set arbitrarily. Therefore, for example, the high voltage side power line 24 or the low voltage side power line 25 may be used as the reference voltage line 26.

  One terminal of the holding capacitor C1 is connected to the source of the driver transistor Q1, and the other terminal of the holding capacitor C1 is connected to the gate of the driver transistor Q1 through the transistor Q5 and is connected to the data line through the transistor Q2. 20 is connected. That is, the transistor Q5, which is a separation switch, is connected between the gate of the driver transistor Q1 and one terminal of the holding capacitor C1. A transistor Q3 is connected between the gate and drain of the driver transistor Q1. The gate of the transistor Q2 is connected to the scanning line 41, the gate of the transistor Q3 is connected to the reset line 42, the gate of the transistor Q4 is connected to the enable line 43, and the gate of the transistor Q5 is connected to the merge line 44. .

  Next, the operation of the pixel circuit 10 in the present embodiment will be described. FIG. 3 is a timing chart showing the operation of the pixel circuit 10 in the embodiment of the present invention. In the present embodiment, each of the pixel circuits 10 detects the threshold voltage Vth of the driver transistor Q1 within one field period, and writes the data signal Data corresponding to the image signal to the holding capacitor C1. An operation of causing the organic EL element D1 to emit light is performed based on the voltage written in the holding capacitor C1. For convenience, a period for detecting the threshold voltage Vth is a threshold detection period T1, a period for writing the data signal Data is a writing period T2, and a period for causing the organic EL element D1 to emit light is a light emission period T3. Will be explained. Note that the threshold detection period T1, the writing period T2, and the light emission period T3 are defined for each of the pixel circuits 10, and it is necessary to match the phases of the above three periods for all the pixel circuits 10. There is no. In the present embodiment, the phases of the three periods are made to coincide with each other for the pixel circuits 10 arranged in the row direction, and each writing period T2 is overlapped for the pixel circuits 10 arranged in the column direction. In order to avoid this, the phases of the three periods are shifted and driven. By driving by shifting the phase in this way, the time of the light emission period T3 can be set longer, which is desirable for improving the image display luminance.

(Threshold detection period T1)
FIG. 4 is a diagram for explaining the operation in the threshold detection period T1 of the image display device according to the embodiment of the present invention. In FIG. 4, the transistors Q2 to Q5 of FIG. 2 are replaced with switches SW2 to SW5, respectively, for the sake of explanation.

  At the first time t11 of the threshold detection period T1, the reset signal Rst is set to high level to turn on the switch SW3, and the gate of the driver transistor Q1 is connected to the high voltage side power supply line 24. Then, the driver transistor Q1 is turned on, a current flows, and the organic EL element D1 emits light.

  Immediately after that, at time t12, the enable signal Enbl is set to low level to turn off the switch SW4. However, since the driver transistor Q1 remains on, the electric charge of the holding capacitor C1 is discharged and the auxiliary capacitor C2 is charged, so that the source voltage Vs of the driver transistor Q1 starts to rise. When the gate-source voltage Vgs of driver transistor Q1 becomes equal to threshold voltage Vth, driver transistor Q1 is turned off. Therefore, the source voltage Vs of the driver transistor Q1 is

Thus, the voltage VC1 of the holding capacitor C1 becomes equal to the threshold voltage Vth. In this way, the voltage Vth is held in the holding capacitor C1 and the auxiliary capacitor C2. Since the organic EL element D1 emits light regardless of the image signal between time t11 and time t12, it is desirable to set this time interval as short as possible, and in this embodiment, it is set to 1 μs or less. .

(Writing period T2)
FIG. 5 is a diagram for explaining the operation in the writing period T2 of the image display device according to the embodiment of the present invention.

  At time t21 in the writing period T2, the merge signal Mrg is set to the low level to turn off the switch SW5. At time t22, the scanning signal Scn is set to the high level to turn on the switch SW2. At this time, the voltage Vdata corresponding to the image signal supplied to the data line 20 is applied to one terminal of the holding capacitor C1. Therefore, the voltage VC1 of the holding capacitor C1 is increased by a voltage obtained by dividing the voltage Vdata by the holding capacitor C1 and the auxiliary capacitor C2,

It becomes.

  At time t23 when the writing operation of the pixel circuit 10 is completed, the scanning signal Scn is returned to the low level to turn off the switch SW2, and at time t24, the reset signal Rst is set to the low level to turn off the switch SW3. Further, at time t25, the merge signal Mrg is set to the high level to turn on the switch SW5. As a result, the gate-source voltage Vgs of the driver transistor Q1 becomes equal to the voltage VC1 of the holding capacitor C1.

(Light emission period T3)
FIG. 6 is a diagram for explaining the operation in the writing period T3 of the image display apparatus according to the embodiment of the present invention.

  At time t31, the enable signal Enbl is set to the high level and the switch SW4 is turned on. Then, a current flows through the organic EL element D1, and the organic EL element D1 emits light with a luminance corresponding to the image signal. At this time, the current Ipxl flowing through the organic EL element D1 is

It becomes. Β is a coefficient determined depending on the mobility μ of the driver transistor Q1, the gate insulating film capacitance Cox, the channel length L, and the channel width W.

It is represented by

  Thus, the term of the threshold voltage Vth is not included in the current Ipxl flowing through the organic EL element D1. Therefore, even when the threshold voltage of the driver transistor Q1 varies with time, the current Ipxl flowing through the organic EL element D1 is not affected by this, and the organic EL element D1 emits light with luminance corresponding to the image signal. Can be made.

  Further, since the luminance of the organic EL element D1 is determined by the voltage of the holding capacitor C1, it is necessary to drive so that the voltage of the holding capacitor C1 does not fluctuate unexpectedly. Therefore, by controlling each transistor based on the sequence shown in FIG. 3, the voltage of the holding capacitor C1 can be reliably controlled.

  As described above, according to the present embodiment, the pixel circuit 10 in which the organic EL element D1 is connected to the source of the driver transistor Q1 and the cathode of the organic EL element D1 is commonly connected to the low-voltage side power supply line. It can be configured using only N-channel transistors. As described above, the pixel circuit in this embodiment is optimal when a large-sized display device is formed using an amorphous silicon thin film transistor, but it is desirable even when a polysilicon thin film transistor is used.

  In the present embodiment, for the pixel circuits 10 arranged in the row direction, the phases of the three periods of the threshold detection period T1, the writing period T2, and the light emission period T3 are made to coincide with each other and arranged in the column direction. The configuration in which the pixel circuit 10 is driven by shifting the phases of the three periods so that the writing periods T2 do not overlap has been described. However, the present invention is not limited to this. For example, one field period is divided into three periods including a threshold detection period T1, an address period T2, and a light emission period T3, and all the pixel circuits 10 are synchronized. It may be driven.

  Note that the numerical values such as voltage values shown in the present embodiment are merely examples, and it is desirable that these numerical values are set appropriately and optimally depending on the characteristics of the organic EL element, the specifications of the image display device, and the like. .

  According to the image display device of the present invention, a pixel circuit in which a current light emitting element is connected to the source of a driver transistor can be configured using only an N-channel transistor, and an active circuit using the current light emitting element can be formed. It is useful as a matrix type image display device.

The schematic diagram which shows the structure of the organic electroluminescent display apparatus in embodiment of this invention. Circuit diagram of a pixel circuit in an embodiment of the present invention Timing chart showing operation of pixel circuit in the embodiment of the present invention The figure for demonstrating the operation | movement in the threshold value detection period of the image display apparatus in embodiment of this invention. The figure for demonstrating the operation | movement in the writing period of the image display apparatus in embodiment of this invention. The figure for demonstrating the operation | movement in the light emission period of the image display apparatus in embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Pixel circuit 11 Scan line drive circuit 12 Data line drive circuit 14 Power line drive circuit 20 Data line 24 High voltage side power line 25 Low voltage side power line 26 Reference voltage line 41 Scan line 42 Reset line 43 Enable line 44 Merge line D1 Organic EL element C1 Holding capacitor C2 Auxiliary capacitor Q1 Driver transistor Q2, Q3, Q4, Q5 Transistor SW2, SW3, SW4, SW5 Switch

Claims (2)

  1. A current light emitting element; a driver transistor for passing a current through the current light emitting element; a holding capacitor for holding a voltage for determining an amount of current flowing through the driver transistor; and a write switch for writing a voltage corresponding to an image signal to the holding capacitor; An image display device in which a plurality of pixel circuits having
    The transistors constituting each of the pixel circuits are N-channel transistors,
    Each of the pixel circuits suppresses voltage fluctuations between an enable switch inserted in a current path for passing a current to the current light emitting element and a terminal on the opposite side of the terminal of the holding capacitor to which the write switch is connected. And an auxiliary capacitor
    An image display, wherein the enable switch is connected between a source of the driver transistor and an anode of the current light emitting element, and the auxiliary capacitor is connected between the source of the driver transistor and a predetermined power supply line. apparatus.
  2. Each of the pixel circuits includes an isolation switch connected between the gate of the driver transistor and one terminal of the holding capacitor, and a gate connected between the gate of the driver transistor and the drain of the driver transistor. The image display device according to claim 1, further comprising a drain connection switch.
JP2007158253A 2007-06-15 2007-06-15 Image display device Pending JP2008310075A (en)

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Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2011004646A1 (en) 2009-07-10 2011-01-13 シャープ株式会社 Display device
WO2017121105A1 (en) * 2016-01-15 2017-07-20 京东方科技集团股份有限公司 Pixel circuit, drive method, display panel and display device

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2011004646A1 (en) 2009-07-10 2011-01-13 シャープ株式会社 Display device
US8605077B2 (en) 2009-07-10 2013-12-10 Sharp Kabushiki Kaisha Display device
WO2017121105A1 (en) * 2016-01-15 2017-07-20 京东方科技集团股份有限公司 Pixel circuit, drive method, display panel and display device
US10311783B2 (en) 2016-01-15 2019-06-04 Boe Technology Group Co., Ltd. Pixel circuit, method for driving the same, display panel and display device

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