JP2003316326A - Display device and its driving method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an active matrix type display device which is highly precise, has a low driving voltage, low electric power consumption and a long service life, and its driving method. <P>SOLUTION: A plurality of comparison lines which are extended along an X direction and supply comparison signals, are connected to a scanning line driving circuit of the display device. A pixel is constituted of a writing transistor that takes in image signals through a data line when a scanning line, to which the pixel is connected, is selected, a holding capacitor which holds the image signals being taken into the writing transistor, a comparison section which compares the voltage levels of the comparison signals being supplied from the comparison line and the image signals being held in the holding capacitor and a driving transistor which is controlled by the output signals generated by the comparison section in accordance with the comparison result of the voltage levels and controls luminance of the display pixels. The comparison signals are set as step voltages which are gradually increasing or decreasing or triangular wave voltages. The output signals generated by the comparison section are set as the signals made by converting the image signals to time width signals. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a display device and a driving method thereof, and more particularly to a high definition, low driving voltage, low power consumption and long life active matrix type display device and a driving method thereof. Regarding

[0002]

2. Description of the Related Art Recently, as a flat panel display which replaces a liquid crystal display (LCD), an organic E using an organic electroluminescence (EL) element in a pixel portion.
The L display is receiving attention. Since this organic EL display is a self-luminous element, it has high visibility,
In addition, since it has a feature that it can be driven at a low voltage, research for practical use is being actively conducted.

An organic EL device is provided with a layer containing an organic compound capable of emitting EL light (hereinafter referred to as "organic layer"), an anode, and a cathode, and the organic compound contained in the organic layer is referred to as "E".
Methods for obtaining L emission include singlet excitation (fluorescence) and triplet excitation (phosphorescence).

An organic EL display using such an organic EL element in a pixel portion is an LCD according to its driving system.
Similar to the passive matrix (simple matrix)
Type and active matrix type. Of these, a passive matrix type organic EL display is composed of a plurality of anodes provided on a transparent substrate, a plurality of cathodes intersecting with the anodes, and a light emitting unit including an organic layer sandwiched between these electrodes, 1 for the light emitting part at the intersection of the anode and cathode
One pixel is formed as a unit, and a plurality of pixels are arranged to form a display unit. Then, the display device is configured by connecting the external drive circuit and the display unit via a connecting unit formed by extending the anode and the cathode from the display unit to the periphery of the substrate.

While the organic EL display adopting the passive matrix type drive system has an advantage that the structure is simple, in order to increase the number of pixels of the matrix, it is necessary to increase the instantaneous brightness of each pixel, and the life of the display and There is a problem in terms of power consumption.

On the other hand, an active matrix type organic E
An L display has a plurality of gate electrodes on a transparent substrate and a plurality of source electrodes intersecting the gate electrodes, and an active element (generally, a thin film transistor (TFT)) is provided in each intersection region of these electrodes. And a storage capacitor for image signals are connected to form a switching element, and an organic light emitting layer and a counter electrode are sequentially stacked on the switching element to form a pixel, and a plurality of such pixels are arranged. By doing so, the display portion is formed. Then, the external drive circuit and the display unit are connected to each other through the connection unit formed by extending these electrodes (gate electrode, source electrode, and counter electrode) from the display unit to the periphery of the substrate, and the display device is It is configured.

FIG. 5 is a diagram for explaining a configuration example of a conventional active matrix type organic EL display, FIG. 5 (a) is an example of a circuit diagram of this display, and FIG. 5 (b) is this. It is an example of a circuit diagram inside a pixel provided in a display.

This active matrix type organic EL
In the display, the pixel 55 is connected to the scanning line 53 and the data line 54 at each intersection of the XY matrix of the scanning line 53 in the X direction and the data line 54 in the Y direction connected to the data line driving circuit 51 and the scanning line driving circuit 52. Is provided.

A circuit inside the pixel 55 includes a writing transistor Tr1 which controls a switching function when an image signal is written in the pixel 55, and a storage capacitor C which holds the image signal.
And a drive transistor Tr2 driven by the voltage of the storage capacitor C. The transistor in the pixel 55 having such a structure is often a TFT formed of low-temperature polycrystalline silicon, and due to restrictions on the operation of the TFT and the manufacturing of the organic EL element, for example, JP-A-8-2
As disclosed in Japanese Patent No. 41048, the drive transistor Tr2 is generally a p-channel type.

The driving system of the active matrix type organic EL display is roughly classified into an analog gradation system and a digital gradation system. The digital gradation system includes modulation methods such as time division modulation and area modulation.

In the analog gradation method, in FIG. 5, the image signal written from the data line driving circuit 51 to the pixel 55 is an analog voltage or analog current, and the gradation is controlled by this voltage value or current value, and this image is A signal is applied to the gate of the drive transistor Tr2 and the drive transistor Tr2 is turned on (ON), so that a current flows through the pixel.

On the other hand, in the digital gradation method, the pixel signal written from the data line driving circuit 51 to the pixel 55 has a binary value of ON (ON) and OFF (OFF), and gradation control is performed according to the time width of the image signal. The method is called time modulation, and pixel 5
The method of dividing 5 into a plurality of sub-pixels and controlling the gradation by the number of lighting is called area gradation. Examples of time modulation include, for example, Japanese Patent No. 2784615 and JP 2002-00.
The subfield method disclosed in Japanese Patent No. 6808 is known.

FIG. 6 is a timing chart when gradation is performed by the subfield method. One frame is divided into a plurality of subfields, and each subfield is set so that its lighting time is a power of two. The gradation control is performed by a combination of lighting and non-lighting of these subfields.

In the subfield method, since the deviation of the lighting time within one frame, which is a combination of subfields constituted by powers of 2, is not constant, a pseudo contour phenomenon occurs during reproduction of a moving image. That is, when the time interval of the lighting subfield within one frame becomes long in order to express a certain gray level, the brightness of the lighting subfield of the next one frame is added to the brightness of the previous lighting subfield to be recognized. As a result, a pseudo contour is generated in the image. In order to avoid this phenomenon, various measures have been taken such as changing the arrangement of the subfields, or allocating the subfields to a time distribution different from the power of 2.

[0015]

Generally, in an active matrix type organic EL display, a T formed on a glass substrate as a transistor which is an active element.
Although FT is used, amorphous silicon and low temperature polycrystalline silicon used for forming a TFT have a lower crystallinity and a larger variation in TFT characteristics than a single crystal. When forming a low temperature polycrystalline silicon TFT on a large glass substrate, a laser annealing method is usually used to avoid problems such as thermal deformation of the glass substrate, but it is uniform on a large area glass substrate. It is not easy to irradiate the substrate with laser energy, and it is inevitable that the crystallization state of the low-temperature polycrystalline silicon varies depending on the location in the substrate.

Due to such variations in crystallinity,
Even with TFTs formed on the same substrate, the threshold voltage (Vth) between pixels on the substrate varies by several hundred mV. Therefore, even if a signal is written to different pixels with the same voltage, the current flowing in each pixel is different for each pixel, and in analog gradation in which gradation is expressed by applying an analog voltage to the drive transistor, a sufficiently high quality image is displayed as a display. There is a problem that cannot be displayed.

On the other hand, the digital gradation is driven by only two values, that is, a voltage sufficiently larger than Vth of the driving transistor or a sufficiently small voltage, and gradation control is performed by the conduction time of the driving transistor. Although the difference in voltage is small and the luminance distribution on the entire screen is sufficiently small and the image quality is good, the number of gradations cannot be increased due to the driving method in the subfield method or area gradation method in digital gradation. is there.

Further, in order to increase the number of gradations in the subfield method, one frame needs to be divided into more subfields, and the address speed (the speed of writing an image signal to a pixel) must be very high. There is a problem of not becoming.

Further, in order to increase the number of gradations in the area gradation method, it is necessary to divide the pixel into a larger number of sub-pixels, which causes a problem of lowering the aperture ratio.

The present invention has been made in view of the above problems, and an object thereof is an active matrix type display device having high definition, low driving voltage, low power consumption and long life, and the same. It is to provide a driving method.

[0021]

In order to achieve such an object, the present invention provides a scanning signal connected to a plurality of scanning lines extending in the X direction. , A data line driving circuit connected to a plurality of data lines extending in the Y direction and sequentially supplying an image signal of a voltage level corresponding to the luminance information to the data line, and these scanning lines. A display device comprising a pixel provided at each intersection with a data line, wherein a plurality of comparison lines extending in the X direction and supplying a comparison signal to the pixel are connected to the scanning line drive circuit. The pixel is connected to the scan line and the data line, and when the scan line is selected, a first active element that captures the image signal via the data line and a first active element are provided. Hold the captured image signal And lifting means, and comparing means for comparing the voltage level of the image signal held in the holding means and the comparison signal supplied from the connected the comparison line in the comparison line,
A second active element controlled by an output signal generated according to the comparison result of the voltage levels by the comparison means, and a display pixel whose brightness is controlled by the second active element. .

The invention described in claim 2 is the same as claim 1.
In the display device described in (3), the comparison signal is a step voltage or a triangular wave voltage that gradually increases or decreases, and the output signal generated by the comparison means is a signal obtained by converting the image signal into a time width signal. Is characterized by.

The invention described in claim 3 is the same as claim 1
Alternatively, in the display device according to 2, the comparison signal is
It is characterized in that the image signals are sequentially supplied from the scanning lines to the holding means after being stored.

Further, in the invention described in claim 4, in the display device according to any one of claims 1 to 3, an analog voltage is applied to the data line driving circuit, and an address in one frame of the pixel is generated. The number of times is one.

According to a fifth aspect of the present invention, a scan line drive circuit connected to a plurality of scan lines extending in the X direction and sequentially supplying a scan signal to the scan lines, and a plurality of data lines extending in the Y direction are provided. A data line drive circuit that sequentially supplies an image signal of a voltage level corresponding to the luminance information to the data line,
A display provided with a pixel provided at each intersection of the scanning line and the data line, and a plurality of comparison lines connected to the scanning line drive circuit and extending in the X direction to supply a comparison signal to the pixel. A method of driving a device, wherein the pixel captures the image signal via the data line when a scanning line connected to the pixel is selected, holds the image signal, and outputs the image signal from the comparison line. Comparing the voltage levels of the supplied comparison signal and the held image signal, generating an output signal according to the result of the comparison, and controlling the brightness of the display pixel based on the output signal. To do.

The invention described in claim 6 is the same as claim 5
In the method for driving a display device according to the item 1, the comparison signal is a step voltage or a triangular wave voltage that gradually increases or decreases, and the output signal is a signal obtained by converting the image signal into a time width signal. To do.

[0027]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below.

FIG. 1 is a diagram for explaining a configuration example of an active matrix type display device of the present invention.
1A is an example of a circuit diagram of a display device, and FIG. 1B is an example of a circuit diagram inside a pixel included in the display device.

In this display device, a pixel 16 is provided at each intersection of the XY matrix of the scanning line 13 in the X direction connected to the data line driving circuit 11 and the scanning line driving circuit 12, the comparison line 14 and the data line 15 in the Y direction. , The scanning line 13, the comparison line 14, and the data line 15 are connected to each other. In this display device, an analog voltage is applied to the data line driving circuit 11, a step voltage or a triangular wave voltage is input to the comparison line 14, and the number of addresses in one frame is one. The comparison signal input to 14 is independently supplied to each comparison line 14.

The circuit inside the pixel 16 compares the writing transistor T1 provided at the intersection of the scanning line 13 and the data line 15 for taking in the image signal into the pixel, the image signal holding capacitor C1 and the image signal. It is composed of a comparison unit D1 connected to a comparison line 14 for comparing a signal, a drive transistor T2 for driving the pixel 16 and an organic EL element (not shown), and the drive transistor T2 is controlled by pulse width modulation. .

In general, T1 is NMOS and T2 is
Is composed of a PMOS, but the type of transistor is not limited to these. Further, in FIG. 1B, the storage capacitor C1 is grounded, but it may be set to an arbitrary constant potential.

The pixel circuit used in the active matrix type display device of the present invention shown in FIG. 1B has the following features. First, since the control method of the drive transistor T2 is pulse width modulation, the drive transistor T2 operates only in two states of conduction and cutoff, which is an advantage of the digital gradation, and the Vth variation of the TFT is small. Since the influence is small, the difference in the voltage applied to the organic EL element for each pixel 16 is small, the luminance distribution of the entire pixel 16 is sufficiently small, and as a result, the image quality is good.

Secondly, the resolution of the pulse width of each pixel 16 in one frame is determined by the analog voltage of the data line driving circuit 11 and the step voltage or triangular wave voltage input to the comparison line 14, so that the number of high gradations is high. This is an easily obtained point.

Thirdly, the number of addresses in one frame is one, and the comparison signal input to the comparison line 14 is independently supplied to each comparison line 14, whereby the maximum lighting time in one frame is increased. Is a point that can be optimally determined.

FIG. 2 is a diagram for explaining a sequence for driving an arbitrary pixel included in the display device of the present invention shown in FIG. When driving the pixel, first, the scanning line is selected and the writing transistor T1 is turned on.
Then, it is brought into a conductive state (S1), and an image signal is input from the data line (S2). At this time, the image signal is stored in the storage capacitor C1 via the writing transistor T1 in the conductive state. In this state, the scanning line is not selected, and the write transistor T1 is turned off to be in the cutoff state (S).
3), the pixel holds the image signal written therein. It should be noted that the image signal setting of the data line may be ended as it is (S4), and the image information writing to another pixel may be subsequently performed.

Next, a step voltage (or a triangular wave voltage) is input to the comparison line as a comparison signal, the image signal stored in the storage capacitor C1 and the comparison signal are compared by the comparison unit D1, and the magnitude relationship is dealt with. Let drive transistor T
Controls continuity / interruption of 2. That is, when the comparison signal is larger than the image signal, T2 is turned on (ON) by the output potential of the comparison unit D1 (S5), and conversely, when the comparison signal is smaller than the image signal, T2 is turned off (OFF). )
(S6).

At this time, the image signal is held constant, and the potential of the comparison signal gradually changes, so that the comparison unit D1
Outputs a pulse width corresponding to the potential of the image signal and can perform time modulation. When the magnitude relationship between the image signal and the comparison signal is exchanged, the comparison unit D1 inverts the output signal to end the pulse width and bring the drive transistor T2 into the cutoff state. This cutoff state is maintained until the end of one frame. Then, when the comparison signal completes one cycle, all are returned to the initial state (S7).

By repeating the above operation for each pixel and each frame, an image can be displayed on the display.

FIG. 3 is an example of a timing chart when driving the display device of the present invention. Scanning is performed over all scanning lines, an image signal is written in each pixel, and then a comparison signal common to all pixels is input. In the case of this method, since only one type of comparison signal needs to be generated, the circuit for generating the comparison signal can be downsized.

FIG. 4 is another example of a timing chart for driving the display device of the present invention. In this example, since the comparison signal is sequentially input from the pixel in which the image signal is written, the light emission time in one frame can be utilized to the maximum, and the display current of the display can be maintained and the driving current of the pixel can be suppressed low. Therefore, low power consumption and long life can be achieved.

In the present embodiment, the display pixel has been described as being composed of an organic EL element, but the display device and its driving method of the present invention are not limited to an organic EL display, but an inorganic EL or a liquid crystal. It is obvious that the present invention can be widely applied to display devices using light emitting elements as display pixels.

[0042]

As described above, according to the present invention, a plurality of comparison lines that extend in the X direction and supply comparison signals to pixels are connected to the scanning line drive circuit of the display device, and the pixel configuration is A writing transistor that takes in an image signal via a data line when a scanning line to which this pixel is connected is selected,
A storage capacitor that holds the image signal captured in the writing transistor, a comparison unit that compares the voltage level of the comparison signal supplied from the comparison line with the image signal held in the storage capacitor, and the voltage level of this comparison unit. Since it is provided with a drive transistor which is controlled by an output signal generated according to the comparison result of 1 to control the brightness of a display pixel, an active matrix type of high definition, low drive voltage, low power consumption, and long life is provided. It is possible to provide a display device and a driving method thereof.

[Brief description of drawings]

1A and 1B are diagrams illustrating a structural example of an active matrix display device of the present invention, FIG. 1A is an example of a circuit diagram of the display device, and FIG. 1B is an inside of a pixel included in the display device. 2 is an example of a circuit diagram of FIG.

FIG. 2 is a diagram for explaining a sequence when driving an arbitrary pixel included in the display device of the present invention.

FIG. 3 is an example of a timing chart when driving the display device of the present invention.

FIG. 4 is another example of a timing chart when driving the display device of the present invention.

5A and 5B are views for explaining a configuration example of a conventional active matrix type organic EL display, FIG. 5A is an example of a circuit diagram of this display, and FIG. It is an example of a circuit diagram.

FIG. 6 is a timing chart when gradation is performed by the subfield method.

[Explanation of symbols]

11,51 Data line drive circuit 12, 52 Scan line drive circuit 13, 53 scan lines 14 comparison line 15, 54 data lines 16,55 pixels T1, Tr1 write transistor T2, Tr2 drive transistor C1, C holding capacity D1 comparison section

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Claims (6)

[Claims] 1. A scanning line drive circuit connected to a plurality of scanning lines extending in the X direction and sequentially supplying a scanning signal to the scanning lines, and a scanning line driving circuit connected to a plurality of data lines extending in the Y direction according to luminance information. A display device comprising a data line driving circuit for sequentially supplying a voltage level image signal to the data line, and pixels provided at each intersection of the scanning line and the data line, wherein the scanning line driving circuit comprises: Is connected to a plurality of comparison lines that extend in the X direction and supply a comparison signal to the pixel. The pixel is connected to the scan line and the data line, and when the scan line is selected, the data A first active element that captures the image signal via a line, a holding unit that retains the image signal captured by the first active element, and a comparison signal that is connected to the comparison line and supplied from the comparison line. And to the holding means Comparing means for comparing the voltage level with the held image signal, a second active element controlled by an output signal generated according to the comparison result of the voltage level by the comparing means, and the second active element. A display device comprising a display pixel whose brightness is controlled by an element. 2. The comparison signal is a step voltage or a triangular wave voltage that gradually increases or decreases, and the output signal generated by the comparison means is a signal obtained by converting the image signal into a time width signal. The display device according to claim 1. 3. The display device according to claim 1, wherein the comparison signal is supplied to the holding unit after the image signals are sequentially stored from the scanning line. 4. The display device according to claim 1, wherein an analog voltage is applied to the data line drive circuit, and the number of addresses of the pixel in one frame is one. . 5. A scan line drive circuit connected to a plurality of scan lines extending in the X direction and sequentially supplying a scan signal to the scan lines, and a scan line driving circuit connected to a plurality of data lines extending in the Y direction according to luminance information. A data line driving circuit that sequentially supplies an image signal of a voltage level to the data line and a pixel provided at each intersection of the scanning line and the data line are provided, and the scanning line driving circuit is provided in the X direction. A driving method of a display device, wherein a plurality of comparison lines that are expanded and supply a comparison signal to the pixel are connected, wherein the pixel is configured to connect the data line when a scanning line to which the pixel is connected is selected. The image signal is taken in through, the image signal is held, the voltage levels of the comparison signal supplied from the comparison line and the held image signal are compared, and an output signal is generated according to the result of the comparison. The output signal The driving method of a display device and controls the brightness of the display pixel Zui. 6. The comparison signal is a step voltage or a triangular wave voltage that gradually increases or decreases, and the output signal is a conversion of the image signal into a time width signal. A method for driving the described display device.