JP2008046261A - Active matrix display device and its drive method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an active matrix display device which has a function to switch the current drive or voltage drive for taking advantage of the current drive system and the voltage drive system. <P>SOLUTION: This active matrix display device has a display 100, a signal line drive circuit 300, a scanning line drive circuit 200, and a system controller 400. It switches the signal writing in the pixel circuit 101 to the current writing state or to the voltage writing state depending on the sizes of the signals for writing in the pixel circuit 101, and switches to the voltage writing state when writing signals to obtain gradations lower than the predetermined gradation. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an active matrix display device and a driving method thereof. For example, the present invention is suitable for a display device using an organic electroluminescence element (hereinafter referred to as an organic EL element) as a display element, and in particular, improves the definition of gradation. It has been improved to be able to.

  Active matrix display devices using organic EL elements have been developed. In this apparatus, the characteristics of the drive transistor for driving the organic EL element are required to be substantially the same between the pixels. However, since the transistor is usually formed on an insulator such as a glass substrate, variations in transistor characteristics are likely to occur.

To solve this problem, a threshold cancellation type circuit and a current copy type circuit have been proposed (see Patent Documents 1 and 2). According to these circuits, the influence of the threshold value of the drive transistor on the drive current for driving the display element can be eliminated. Therefore, even if the threshold value of the driving transistor varies between pixels, it is possible to reduce the influence of such variations on the driving current supplied to the organic EL element.
US Pat. No. 6,229,506 B1 US Pat. No. 6,373,454B1

  The above threshold cancellation type circuit and current copy type circuit can eliminate the influence of the threshold value of the driving transistor on the driving current of the display element. These circuits can be said to be a current driving method and a voltage driving method when classified by a method of writing a signal to a driving capacitor.

Here, the inventors focused on the following points,
(1) In the case of the current driving method, when the driving current is large, there is good driving capability. However, when a signal for display with a low drive current, that is, low luminance display (low gradation display) is written to the drive capacitor, the drive capability is lowered, and the capacity of the signal line or pixel cannot be set to a predetermined value. That is, the signal writing to the drive capacity is insufficient, and the quality of the low gradation image quality is lowered. This is a quality degradation with respect to contrast.

(2) On the other hand, in the case of the voltage drive method, when a signal is written to the drive capacitor, voltage-current conversion is performed for writing. However, there is a problem that the voltage-current conversion circuit is affected by variations in characteristics of the thin film transistor, leading to display unevenness. However, in the case of the voltage driving method, when a signal for low luminance display (low gradation display) is written to the driving capacitor, relatively stable writing can be obtained.

  SUMMARY OF THE INVENTION An object of the present invention is to provide an active matrix display device and method capable of switching between a current driving method and a voltage driving method as a signal writing method for a driving capacity, and taking advantage of the advantages of both methods. And

  In one aspect of the present invention, a plurality of pixel circuits arranged in a matrix on a substrate and including a display element and a drive capacitor for driving the elements are selected for each row of the plurality of pixel circuits. Including a writing period setting circuit for setting a signal writing period, a driving circuit for supplying a writing signal to a driving capacitor in each pixel circuit in the row in which the signal writing period is set, and a row of the plurality of pixel circuits A light emission period setting circuit that applies a pixel on control signal and sets a light emission period during which the display element is driven, and the drive circuit provides a voltage drive method when the write signal is applied to the drive capacitor. It is configured to be switchable to either one of the current driving method, and the switching point for the luminance of the voltage driving method and the current driving method can be changed.

  According to the above means, the quality of the low gradation image quality can be improved and the reliability of the apparatus can be obtained.

  Embodiments of the present invention will be described below with reference to the drawings. In FIG. 1, the overall schematic configuration of the apparatus of the present invention will be described. A display unit 100 is constructed at the center of the glass substrate 11. In the display unit 100, a plurality of pixel circuits 101 are arranged in a matrix. A specific configuration example of the pixel circuit 101 will be described later.

  The display unit 100 is provided with a plurality of signal lines wired for each column of pixel circuits, and a signal line driving circuit 300 for outputting a video signal (write signal) is connected to the plurality of signal lines. ing. The display unit 100 is provided with a plurality of scanning lines wired for each row of the plurality of pixel circuits 101. The plurality of scanning lines are connected to a scanning line driving circuit 200 that outputs a scanning signal. Yes. The scanning line driving circuit 200 includes a writing period setting circuit 211 and a light emission period setting circuit 212 described later.

  The entire display device is centrally controlled by the system controller 400. The system controller 400 may be disposed on the glass substrate 11 or may be provided outside. Various processing programs are included in the system controller 400, and by these processing programs, a startup process at power-on, a signal process at normal operation, and the like are executed. The system controller 400 receives a synchronization signal and a clock of a video signal from the outside, and operates in synchronization with the video signal.

  The system controller 400 controls the scanning line driving circuit 200 and the signal line driving circuit 300. The signal line driving circuit 300 includes an image data register unit 301, a driving method control unit 501, and a voltage / current output switching unit 502.

  The image data register unit 301 receives an external video signal under the control of the system controller 400 and outputs a write signal (corresponding to pixel data) to the pixel circuits 100 in each column of the display unit 100. At this time, the writing signal passes through the voltage / current output switching unit 502 that operates under the control of the driving method control unit 501, and is written into the driving capacity of the pixel circuit 101 in either the voltage driving method or the current driving method. It is.

  The writing period setting circuit 211 of the scanning line driving circuit 200 sets a writing period for each row so that the writing signal is written to the appropriate pixel circuit 101. The light emission period setting circuit 212 sets a period during which the light emitting element of the pixel circuit 101 including the drive capacitor in which the write signal is written emits light.

  FIG. 2 shows an example of the driving method control unit 501 and the voltage / current output switching unit 502.

  In the image data register unit 301, for example, video signals for one row of the pixel circuit are written every horizontal period. From the image data register unit 301, write signals are output simultaneously to each signal line (each column) in synchronism with the horizontal synchronization signal in units of one row.

  On the other hand, the driving method control unit 501 determines an average gradation for one frame or a fraction of a frame. For this purpose, for example, the average value of the current flowing through the entire signal line is determined. Alternatively, the average gradation may be determined based on the total value or histogram of data supplied to the image data register unit 301.

  Each signal line (column) of the image data register unit 301 is connected to the voltage output unit 50A and the current output unit 50B. In FIG. 2, the voltage output unit 50A and the current output unit 50B connected to the signal lines in the first column are shown as representatives. Similar circuits are also connected to the signal lines in the other columns. The voltage output unit 50A is a circuit that outputs a voltage corresponding to the value of the write signal of the output signal line of the image data register unit 301. The current output unit 50B is a circuit that outputs a current corresponding to the value of the write signal of the output signal line of the image data register unit 301. Outputs of the voltage output unit 50A and the current output unit 50B are input to the selection unit 50C. The selection unit 50C outputs the output of the voltage output unit 50A to the voltage signal line SIG_V or the output of the current output unit 50B to the current signal line SIG_I according to the determination signal from the previous drive method control unit 501. Decide whether to output.

  In addition, the determination signal of the driving method control unit 501 is also given to the scanning line driving circuit 200. In the scanning line driving circuit 200, the driving capacity in the pixel circuit can be switched to a current driving state or a voltage driving state by switching the levels of the scanning lines SIG1 and SIG2 in accordance with a determination signal as described later.

FIG. 3 shows the relationship between gradation and luminance / current in order to explain the concept of the present invention. Now, it is assumed that the display device has a display capability of 32 gradations. Here, it is assumed that the maximum luminance of the display device is “1” (for example, 300 cd / m ² ) of the setting position (this maximum luminance is set when the display device is designed). In this case, the actual operation is set to obtain the next operation. The driving method control unit 501 maintains the current driving mode until the luminance decreases and the luminance a (representation capability of about 8 gradations) is reached. However, when the luminance is lower than a (for example, the average current value flowing through the signal line is 100 nA), the mode is switched to the voltage driving mode. On the other hand, when it is determined that the luminance is equal to or higher than a, the drive method control unit 501 switches to the current drive mode.

Further, it is assumed that the display device has a display capability of 32 gradations, and the maximum luminance of the display device is “2” (for example, 150 cd / m ² ) of the setting position (this maximum luminance is set when the display device is designed. ) In this case, the actual operation is set to obtain the next operation. That is, the driving method control unit 501 maintains the current driving mode until the luminance decreases and the luminance a (expressing ability of about 16 gradations) is reached. However, when the luminance is lower than a (for example, the average current value flowing through the signal line is 100 nA), the mode is switched to the voltage driving mode. This is because when the write current is 100 nA, insufficient writing to the pixel circuit occurs. If current driving is further continued in the region where the writing shortage occurs, the gradation should originally be lowered, but the gradation in the display portion is not lowered and becomes excessive. That is, the quality of the brightness contrast is lowered.

  However, according to the present invention, when the setting of the maximum luminance is lowered, the setting method is to shift the gradation that switches from the current driving to the voltage driving mode from the 8th gradation to the 16th gradation. This setting can be made by adjusting the output characteristics or sensitivity of the drive system control unit 501. As a result, when the current drive is switched to the voltage drive, a linear characteristic can be obtained in the luminance-gradation characteristics. In other words, the quality of the contrast is improved up to the low gradation. When voltage driving is performed on the pixel circuit, voltage driving for 16 gradations may be performed. For this reason, a circuit for generating a gradation voltage related to voltage driving may be simple.

  Here, in this apparatus, it is necessary to monitor the state in which the luminance changes and to switch the driving method described above. Various methods are possible as a method of monitoring the change in luminance.

  For example, (1) there is a method of obtaining an average luminance or spectrum for each frame and determining an appropriate driving method for each frame based on the value. (2) Alternatively, there is a method in which one frame is divided into a plurality of regions, an average luminance or spectrum for each region is obtained, and an appropriate driving method is determined for each region based on the value. (3) Alternatively, there is a method of selecting the method (2) above when the percentage of the moving image area is large in the image, and the method (1) above when the percentage of the still image area is large.

  Further, for example, in an apparatus employing the method (1), the user may be able to turn this operation on or off. Similarly, in an apparatus adopting the method (2), the user may be able to turn this operation on or off. Similarly, in an apparatus employing the method (3), the user may be able to turn this operation on or off. Further, the display device may have all the methods (1), (2), and (3), and the user may select them.

<Next, an example of a pixel circuit>
FIG. 4 shows a typical configuration example of the pixel circuit 101 in the first column and the first row. The source of the driving transistor TR is connected to the first power supply line to which the anode voltage PVDD is applied. A capacitor C1 as a drive capacitor is connected between the gate and the anode of the drive transistor TR. The drain of the driving transistor TR is connected to the signal line SIG_1 via the switch SW1. A switch SW2 is connected between the gate and drain of the driving transistor TR. The driving transistor TR and the switches SW1 and SW2 are composed of thin film transistors. The gates of the switches SW1 and SW2 are connected to the scanning line SG2 that performs pixel selection.

  The drain of the drive transistor TR is connected to the source of the switch SW3 formed of a thin film transistor. The drain of the switch SW3 is connected to a display element OELD1 that is an organic EL element. The gate of the switch SW3 is supplied with a pixel on / off control signal and connected to the scanning line SIG1. The pixel on / off control signal is output from the light emission period setting circuit 212 of the scanning line driving circuit 200 as described above.

  When the switches SW1 and SW2 are turned on by the pixel selection signal, the signal voltage is charged in the capacitor C1 according to the signal current flowing through the signal line SIG_I. When the switches SW1 and SW2 are turned off and the switch SW3 is turned on, a current corresponding to the signal voltage stored in the capacitor C1 flows to the display element OELD1 through the drive transistor TR and the switch SW3. The amount of light emission is substantially proportional to the current flowing through the display element OELD1. The switch SW3 is controlled by a lighting on / off control signal. The above is the operation in the current drive mode under a high gradation state.

  Here, the gate of the driving transistor TR is further connected to the drain of the switch SW4 via the capacitor C2. The source of the switch SW4 is connected to the voltage signal line SIG_V connected to the signal line driver circuit 300. The gate of the switch SW4 is connected to the scanning line SIG2 that performs pixel selection.

  In the voltage drive mode, the scanning line SIG2 is at a high level and the scanning line SIG1 is at a low level. For this reason, the switch SW4 is turned on and SW2 and SW3 are turned off. Here, when a voltage signal is supplied to the voltage signal line SIG_V, the voltage signal is speeded up by the capacitor C2 and charged to the capacitor C1. Thereafter, in the pixel display period, the switch SW4 is turned off and the switch SW3 is turned on. As a result, a current corresponding to the signal voltage stored in the capacitor C1 flows to the display element OELD1 via the drive transistor TR and the switch SW3. The above is the operation in the voltage driving mode under the state where the gradation is low.

  The present invention is not limited to the above embodiment. FIG. 5 shows another configuration example of the pixel circuit. In the pixel circuit of FIG. 4, the signal line driver circuit 300 requires the voltage output unit 50A and the current output unit 50B. However, when the pixel circuit of FIG. 5 is used, only the current output unit 50A is required. Therefore, only the current signal line SIG is provided. In FIG. 5, the same parts as those in FIG. In the pixel circuit of FIG. 5, a switch SW5 is further connected between the drain of the switch SW4 and the power supply line of the anode voltage PVDD. The scanning line SIG3 for pixel selection is connected to the gate of the switch SW5 from the writing period setting circuit 211 of the scanning line driving circuit 200. Other parts have the same configuration as the pixel circuit of FIG.

  The operation of the circuit of FIG. 5 is the same as the operation of the circuit of FIG. 4 in the current drive mode with a high gradation. In the current drive mode, the switches SW4 and SW5 are off. When a signal is written to the drive capacitor C1 in the voltage drive mode with a low gradation, the switches SW4 and SW5 are turned on and the switches SW1 and SW2 are turned off. When a signal current is supplied from the signal line SIG, a signal flows through the switch SW4, and current / voltage conversion is performed by the switch SW5. The voltage at this time is charged to the drive capacitor C1 via the capacitor C2. The subsequent operation is an operation in the light emission period and is the same as the example described in FIG.

  The present invention is not limited to the above embodiment. FIG. 6 shows another configuration example of the pixel circuit. In the pixel circuit of FIG. 4, the signal line driver circuit 300 requires a voltage output unit and a current output unit. However, when the pixel circuit of FIG. 6 is used, only the voltage output unit is required.

Therefore, only the voltage signal line SIG is provided. In FIG. 6, the same parts as those in FIG. In the pixel circuit of FIG. 5, the gate of the switch SW5 is connected to the scanning line SG3. However, in the pixel circuit of FIG. 6, the gate of the switch SW5 is connected to the scanning line SG2. Other portions have the same configuration as the pixel circuit of FIG.

  When a signal is written to the drive capacitor C1 in the voltage drive mode with a low gradation, the switch SW4 is turned on and the switches SW1, SW2, and SW5 are turned off. When a voltage signal is supplied from the voltage signal line SIG_V, a signal flows through the switch SW4, and the voltage at this time is charged to the drive capacitor C1 through the capacitor C2. The subsequent operation is an operation in the light emission period and is the same as the example described in FIG.

  On the other hand, in the current drive mode when the gradation is high, the transistor SW4 is turned off, and the transistors SW1, SW2, and SW5 are turned on. Therefore, a current corresponding to the voltage signal voltage flows from the voltage signal line SIG_V through the switch SW2, and this current charges the capacitors C2 and C1. As a result, a voltage proportional to the value of the voltage signal is charged in the capacitor C1. The subsequent light emission period is the same as the operation of the previous embodiment.

  The present invention is not limited to the above embodiment. FIG. 7 shows a portion that functions when the maximum luminance setting position described in FIG. 3 is changed. In the above embodiments, the maximum luminance setting position has been described as being set and designed at the time of manufacturing the apparatus at either the setting position “1” or the setting position “2” shown in FIG. However, the functional block shown in FIG. 7 is configured as a countermeasure when the maximum brightness setting position can be changed by the user even after the product is manufactured. Taking such measures is useful for reducing power consumption.

  For example, the maximum luminance setting position is determined by adjusting the luminance in the white display state of the display device, for example. The set position information at this time is given to the system controller 400 as a maximum brightness adjustment signal via the adjustment unit. In accordance with the set position information, the system controller 400 adjusts the amplification gain of the current flowing through the display element of the apparatus, the power supply voltage, or both through the gain adjustment section or the power supply voltage adjustment section 410. In response to this, the maximum brightness adjustment position shown in FIG. 3 changes. For example, it is set to any one between the setting position “1” and the setting position “2”.

  Thus, if a linear luminance change from gradation 32 to gradation 0 is to be obtained, the gradation that becomes the switching point between voltage driving and current driving must also be switched. For this reason, in this embodiment, the system controller 400 adjusts the sensitivity with respect to the input of the drive system control unit 501 to move the switching point. The relational data between the degree of change of the switching point and the maximum luminance setting position is measured in advance by experiments and stored in a table in a memory, for example. Therefore, when the maximum brightness adjustment position is adjusted by the adjustment data, sensitivity adjustment data for automatically moving the switching point is determined. This sensitivity adjustment data adjusts, for example, the output gain or input gain of the drive system control unit.

  The present invention is not limited to the above embodiment. In the above embodiment, switching between the current drive mode and the voltage drive mode is performed according to the gradation. However, the current writing mode and the voltage driving mode may be switched by dividing the signal writing period in time. For example, as shown in FIG. 8A, it is assumed that a current of level i5 needs to flow during the writing period T0 in the prior art as a writing signal. In such a case, in this system, as shown in FIG. 8B, a current of level i4 is supplied during the writing period T1, and driving is performed at the voltage value V1 in the period T2. However, the current level i4 and the voltage value V1 are varied according to the current level i5.

  Alternatively, as shown in FIG. 8C, the current level im during current driving and the voltage value V1 during voltage driving may be fixed, and the current driving period may be varied according to the current level i5. That is, the pulse width of the current level im is varied.

  The control data for obtaining the above-described current level, voltage value, and pulse width is stored in, for example, a lookup table. This control data is, for example, data obtained in advance by experiments in accordance with the level (current value, voltage value) of the write signal. The lookup table is used as a memory for converting a write signal output from the image data register unit 301.

  Note that the present invention is not limited to the above-described embodiment as it is, and can be embodied by modifying the constituent elements without departing from the scope of the invention in the implementation stage. Further, various inventions can be formed by appropriately combining a plurality of constituent elements disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, you may combine suitably the component covering different embodiment.

1 is a diagram showing a schematic configuration of an active matrix display device according to the present invention. The figure which shows the specific structural example of the principal part of this invention. FIG. 3 is an explanatory diagram of gradation versus current characteristics shown to explain the basic concept of the present invention. FIG. 6 illustrates a configuration example of a pixel circuit according to the present invention. FIG. 10 is a diagram showing another configuration example of a pixel circuit according to the present invention. FIG. 10 is a diagram showing still another configuration example of a pixel circuit according to the present invention. The figure which shows the principal part in other embodiment of this invention. The figure shown in order to demonstrate operation | movement of further another embodiment of this invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 11 ... Glass substrate, 100 ... Display part, 101 ... Pixel circuit, 200 ... Scanning line drive circuit, 300 ... Signal line drive circuit, 400 ... System controller, 501 ... Drive system control part.

Claims (11)

A plurality of pixel circuits arranged in a matrix on a substrate and including a display element and a drive capacitor for driving the element;
A write period setting circuit configured to perform selection for each row of the plurality of pixel circuits and set a signal writing period in the selected row; and a write signal to a driving capacitor in each pixel circuit in the row in which the signal writing period is set A drive circuit that provides
A light emission period setting circuit that selects a row of the plurality of pixel circuits, gives a pixel on control signal, and sets a light emission period in which the display element is driven;
Have
The drive circuit is configured to be switchable between a voltage drive method and a current drive method when the write signal is applied to the drive capacitor, and a switching point for luminance of the voltage drive method and the current drive method. An active matrix display device characterized by being variable. The drive circuit includes a signal line drive circuit that drives a signal line provided for each column of the plurality of pixel circuits,
The signal line driving circuit includes a voltage output unit and a current output unit to which signals of each column are supplied,
A selection unit that selects one of the voltage output unit and the current output unit and supplies the selected signal to the corresponding pixel circuit;
2. The active matrix display device according to claim 1, wherein the pixel circuit has a current writing path and a voltage writing path with respect to the drive capacitor. The drive circuit includes a signal line drive circuit that drives a signal line provided for each column of the plurality of pixel circuits,
A current signal output from the signal line driver circuit to each column is supplied to the pixel circuit,
2. The active matrix display device according to claim 1, wherein the pixel circuit is provided with a current writing path and a voltage writing path for the drive capacitor, and one of the paths is switched and selected. The drive circuit includes a signal line drive circuit that drives a signal line provided for each column of the plurality of pixel circuits,
A voltage signal output to each column from the signal line driver circuit is supplied to the pixel circuit,
2. The active matrix display device according to claim 1, wherein the pixel circuit is provided with a current writing path and a voltage writing path with respect to the driving capacitor, and one of the paths is switched and selected. The condition for the drive circuit to shift from the current writing method to the voltage writing method is as follows:
2. The active matrix display device according to claim 1, wherein an average current value of a signal line provided for each column of the plurality of pixel circuits is approximately 100 nA. The condition for the drive circuit to shift from the current writing method to the voltage writing method is as follows:
2. The active matrix display device according to claim 1, wherein an average value of a write signal of a signal line provided for each column of the plurality of pixel circuits is a value indicating a predetermined gradation or less.   7. The active matrix display device according to claim 6, wherein the predetermined gradation is adjusted and set by the drive system control unit in accordance with a preset maximum luminance.   The driving circuit includes a driving method control unit that switches between a voltage driving method and a current driving method, and the writing period setting circuit, and the switching point set by the driving method control unit and the writing period setting circuit is determined by a system controller. 2. The active matrix display device according to claim 1, wherein the active matrix display device is adjusted by a control signal. The driving circuit includes a driving method control unit that switches between a voltage driving method and a current driving method and the writing period setting circuit, and the driving method control unit and the writing period setting circuit provide a period during which the writing signal is supplied to the driving capacitor. The active matrix type display device according to claim 1, wherein the current drive system is switched to the voltage drive system.   The driving circuit includes a driving method control unit that switches between a voltage driving method and a current driving method and the writing period setting circuit, and the driving method control unit and the writing period setting circuit provide a period during which the writing signal is supplied to the driving capacitor. 2. The active matrix according to claim 1, wherein the current drive system is switched to the voltage drive system, and the write signal is pulse-width modulated in accordance with the level and applied to the drive capacitor in the current drive system. Type display device. A plurality of pixel circuits arranged in a matrix on the substrate and including display elements and drive capacitors for driving the elements, and a selection for each row of the plurality of pixel circuits are performed, and a signal writing period is set in the selected row A driving circuit that includes a writing period setting circuit and applies a writing signal to a driving capacitor in each pixel circuit of the row in which the signal writing period is set; and a pixel on control signal is selected by selecting a row of the plurality of pixel circuits A light emission period setting circuit for setting a light emission period in which the display element is driven, and a driving method of a display device for driving the display element.
The drive circuit is
Set the switching point for the brightness of the voltage drive method and current drive method,
A driving method of an active matrix display device, wherein when the write signal is applied to the driving capacitor, switching is performed between a voltage driving method and a current driving method at the switching point.

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