JP2002358059A - Liquid crystal display device and its driving method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a driving method capable of preventing an afterimage and burning caused by shifting of characteristics of nonlinear elements in an active matrix type liquid crystal display device using the nonlinear elements. SOLUTION: The shifting of the characteristic of the nonlinear elements depends upon the voltage applied to the element. Then, in a selection period, a plurality of selection pulses are given to a signal and the signal having level opposite to that of an original signal is applied to the elements just before the original signal is written. As a result, effective values of voltages applied to the nonlinear elements become equal to each other regardless of image data. Thus, the afterimage and the persistence caused by the shifting of the characteristics of the nonlinear elements can be eliminated.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to an active matrix type liquid crystal display device using a non-linear element and a driving method thereof.

[0002]

2. Description of the Related Art An example of a conventional driving method of an active matrix type liquid crystal display device using a non-linear element is disclosed in "Nikkei Micro Devices, July 1987, No. 2".
5, p. 121-126, Oguchi et al. " FIG. 2 is an example of the timing chart, in which 21 is a scanning signal waveform Yi, 22 is a data signal waveform Xi, and 23 is a waveform Yi-Xi applied to one pixel. This is an example of gradation display by the pulse width modulation method. However, if the height of the data signal waveform 22 is changed according to data, the pulse height modulation method is used. FIG. 3 is an example of an equivalent circuit of a liquid crystal display device. 31 is a non-linear element, and 32 is a liquid crystal capacitance.

[0003]

However, the above-mentioned prior art has the following problems.

[0004] In general, the stability and reliability of a thin film nonlinear element is extremely poor as compared with a normal diode formed on a single crystal semiconductor substrate. This is presumably because there are many levels due to poor crystallinity of the thin film, and the state changes by continuing to supply current. Since the change in the characteristics of the element has a bias dependence, for example, when the same pattern is displayed for a long time, the distribution of the element characteristics similar to the display pattern can be generated, and the pattern remains for a long time after the pattern is erased. Is generated. This is called an afterimage or burn-in, and is a fatal defect in data display.

[0005] The driving method of the liquid crystal display device of the present invention solves such a problem, and an object thereof is to optimize the driving method of the liquid crystal display device and to reduce the characteristics of the nonlinear element in the display screen. An object of the present invention is to realize uniform image quality and high image quality without image sticking and image sticking.

[0006]

In order to solve the above-mentioned problems, a method for driving a liquid crystal display device according to the present invention includes a method for selecting a pixel included in a liquid crystal display device during a selection period.
A pulse width, which is a difference between the scanning signal for selecting the pixel and the scanning signal having one of two polarities with respect to a predetermined potential and a data signal defining an image to be displayed by the pixel; A method for driving a liquid crystal display device in which the image is displayed on the pixel by applying the modulated difference signal to the pixel, wherein one polarity is set in a first period of the selection period. Applying a first difference signal, which is a difference between the first scanning signal and the data signal, to the pixel, and a second period immediately before the first period of the selection period, Applying a second difference signal that is a difference between a second scanning signal having another polarity different from one polarity and the data signal to the pixel, wherein the first scanning with respect to the first period is performed. Comparison of difference signal pulse application period Wherein the sum of the ratio of the application period of the pulse of the second difference signal for the second period is 1.

[0007]

This embodiment will be described below with reference to the drawings. FIG. 1 is an example of a timing chart showing driving waveforms of the liquid crystal display device of the present invention. 11 is a scanning signal waveform Yi, 12 is a data signal waveform Xi, and 13 is a waveform Yi- applied to one pixel.
Xi. FIG. 3 is an equivalent circuit of a pixel portion of an active matrix type LCD using a non-linear element.
The non-linear element 31 and the liquid crystal 32 arranged in series between i and the scanning electrode Yi constitute one pixel, and a voltage Yi-Xi in FIG. 1 is applied to this pixel. In this example, gradation display is performed by the pulse width modulation method, and when the duty ratio is large, the voltage applied to the liquid crystal increases.
For example, in the case of a TN liquid crystal in a normally white mode, a brighter image is obtained as the duty ratio is smaller, and a darker image is obtained as the duty ratio is larger. A feature of the present invention resides in that a signal obtained by inverting the image signal is applied immediately before the original image signal is applied to the pixel. In FIG. 1, the selection period is divided into three periods T1, T2, and T3, and signals having positive, negative, and positive polarities are respectively applied to the pixels. Here, the signal of the duty ratio d3 applied at T3 is the original signal. The signal with the duty ratio d2 applied at T2 is an inverted signal of d3, and the relationship of d2 + d3 = 1 holds. In general, a current sufficient to rewrite data flows by applying a pulse whose polarity is inverted to a nonlinear element. Therefore, when writing an inverted signal and an original signal twice consecutively, the polarity is inverted in advance. Need to be kept. This is the duty ratio d1 applied at T1, which is kept constant regardless of the image data.

When such a driving method is used, the current flowing through the nonlinear element becomes constant irrespective of the image data. Therefore, it is possible to avoid image sticking and afterimage caused by changing the characteristics of the element according to the difference in the applied voltage. Here, it is desirable to set the period of T2 and T3 to be the same, but the period of T1 does not need to be the same as T2 and T3. Rather, by shortening T1 and lengthening the original signal writing time T2, T3, there is a margin in the writing operation of the nonlinear element.

FIG. 4 is a timing chart showing driving waveforms according to the second embodiment of the present invention. 41 is a scanning signal waveform Yi, 42 is a data signal waveform Xi, and 13 is a waveform Yi-Xi applied to one pixel. In this example, gradation display is performed by the pulse height modulation method. When the amplitude of the data pulse Xi is large, the voltage applied to the liquid crystal increases. For example, in the case of a TN liquid crystal in a normally white mode, an image having a smaller amplitude is brighter and an image having a larger amplitude is darker. Also in this example, the selection period is divided into three periods T1, T2, and T3, and signals having positive, negative, and positive polarities are respectively applied to the pixels. Here, the signal applied at T3 is the original signal. The signal applied at T2 is an inverted signal of the original signal, and the sum of the two voltages is constant.
Note that the signal applied at T1 has a constant magnitude regardless of the image data.

As in the example shown in FIG. 1, when such a driving method is used, the current flowing through the non-linear element becomes constant irrespective of the image data. Therefore, it is possible to avoid image sticking and afterimage caused by changing the characteristics of the element according to the difference in the applied voltage.

Next, a specific configuration of a liquid crystal display device that realizes these driving methods will be described. FIG. 5 is a circuit block diagram of the liquid crystal display device. In this example, four line memories for storing image data for one horizontal scanning period are used. First, the image signal amplified for driving the liquid crystal from the data input unit is sequentially input to the line memory 1 during the horizontal selection period. At the same time, the image data of which the black and white and the polarity are inverted by the data inversion circuit are sequentially stored in the line memory
Is stored. Next, during the blanking period, the line memories 1 and 2 are synchronized with the pulse of the timing signal generation circuit.
Are simultaneously sent to the line memories 3 and 4 respectively. The data driver refreshes all selected non-linear elements during one horizontal scanning period, then outputs inverted data fetched from the line memory 4, and finally outputs original image data fetched from the line memory 3. I do. At this time, it goes without saying that the data driver and the scanning driver are synchronized.

In this example, the storage circuits are provided outside the data driver. However, it is easy to incorporate these circuits in the data driver. For example, a two-stage two-stage latch and a buffer may be arranged for each output of the driver.

According to the present invention, a nonlinear element is turned on by applying a selection pulse to a scanning line, such as a thin-film diode comprising a non-single-crystal Si thin film or a compound semiconductor thin film, an MIM comprising an insulator thin film and a metal thin film, a varistor or a thyristor. -Applicable to all active matrix type liquid crystal display devices to be turned off.

[0014]

As described above, the liquid crystal display device and the driving method of the present invention can make the voltage applied to the non-linear element constant regardless of the image signal. Therefore, afterimages and image sticking due to a change in element characteristics can be completely eliminated. According to this method, since only the drive waveform is changed without changing the structure of the element, a high image quality and high reliability of the liquid crystal display can be realized with almost no increase in cost.

[Brief description of the drawings]

FIG. 1 is a timing chart showing driving waveforms of a liquid crystal display device.

FIG. 2 is a timing chart showing driving waveforms of a conventional liquid crystal display device.

FIG. 3 is an equivalent circuit diagram of a pixel portion of a liquid crystal display device.

FIG. 4 is a timing chart showing driving waveforms of the liquid crystal display device.

FIG. 5 is a circuit block diagram of a liquid crystal display device.

[Explanation of symbols]

 11, 21, 41 Scanning signal waveform 12, 22, 42 Data signal waveform 13, 23, 43 1 Applied waveform to one pixel 31, 51 Non-linear element 32, 52 Liquid crystal

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) G09G 3/20 670 G09G 3/20 670K F term (Reference) 2H093 NA16 NA31 NA41 NA51 NC37 ND12 5C006 AA01 AA15 AA16 AC24 AF73 BB17 BF05 FA34 FA56 GA03 5C080 AA10 BB05 CC03 DD29 EE29 FF11 GG08 GG12 JJ02 JJ04

Claims (1)

[Claims] 1. A scanning signal for selecting a pixel included in a liquid crystal display device during a selection period, the scanning signal having one of two polarities based on a predetermined potential. And driving a liquid crystal display device that causes the pixel to display the image by applying a pulse width modulated difference signal, which is a difference between data signals defining an image to be displayed by the pixel, to the pixel. Applying a first difference signal, which is a difference between a first scanning signal having one polarity and the data signal, to the pixel during a first period of the selection period; A second difference signal that is a difference between a second scanning signal having another polarity different from the one polarity and the data signal in a second period immediately before the first period in the selection period. Applying to the pixel The sum of the ratio of the application period of the pulse of the first difference signal to the first period and the ratio of the application period of the pulse of the second difference signal to the second period is one. Driving method for a liquid crystal display device.