KR20000001238A - Method of driving tft liquid crystal element - Google Patents

Abstract

PURPOSE: Method of driving TFT liquid crystal element is provided to reduce horizontal and vertical crosstalk by controlling polarity of voltage of signal line for maintaining '0'. CONSTITUTION: The method comprises the steps of: dividing signal voltage of identical gray scale into two parts according to order of input on the basis of gate pad and supplying voltage of opposite polarity to the parts, respectively. Thereby interference is reduced, quality of picture is improved.

Description

How to drive TFT LCD

According to the present invention, the amount of charge accumulated in the liquid crystal layer of the pixels connected to the same scan line is spatially " 0 " to reduce signal interference, thereby preventing flicker and improving the image quality by reducing the load of the circuit driving the common electrode. .

The present invention relates to a method of driving a TFT liquid crystal display device. 1 is a cross-sectional view of a TFT liquid crystal display device. The gate electrode 1 of the TFT in each pixel is connected to the scan line, the source electrode 6 to the signal line, and the drain electrode 5 to the pixel electrode 7. There is a liquid crystal layer 9 between the common electrode 7 and the pixel electrode 7. Usually, the a-Si layer 3 of the TFT is n-type, and when the gate electrode connected to the scan line is subjected to a voltage higher than the signal line in the selection period, the resistance of the channel between the drain electrode and the source electrode becomes small, so that the voltage across the signal line is reduced. It is caught by the liquid crystal layer through this pixel electrode. In the non-selection period, a voltage lower than that of the signal line is applied to the gate electrode connected to the scan line, and the drain electrode and the source electrode are electrically disconnected to maintain the charge accumulated in the liquid crystal layer during the selection period. While scanning the scan line, each pixel electrode is charged through the signal line to apply voltage to the liquid crystal layer. When the rms voltage applied to the liquid crystal layer between the pixel electrode and the common electrode is adjusted, the polarized state changes as the linearly polarized light passes through the liquid crystal layer and passes through the liquid crystal layer, and the spectrometer passes through the light to display information as the brightness of the pixel. By controlling the waveform of the voltage across the signal line and the common electrode, the polarity of the voltage across the liquid crystal layer is changed every frame to prevent electrochemical reactions of the liquid crystal molecules. One period is the selection period plus the non-selection period. When sending the screen at 60 Hz, one period is about 16.7 msec, and the selection period is 21.7 μsec. A storage capacitor is also made in parallel with the liquid crystal layer so that the voltage applied to the pixel electrode in the selection period is maintained for the non-selection period.

3 shows an example of a drive block diagram of a digital TFT liquid crystal display element. The data control circuit receives the R, G, and B signals from the video RAM and sends them to the source driver. Each source driver has a shift register connected in series to control the information of the signal line according to the horizontal synchronization and other control signals from the time control circuit. Gate drivers are sequentially selected in accordance with the vertical synchronization of the time control circuit.

Parasitic capacitances at portions where TFT electrodes overlap each other distort the signal and cause signal delay. The parasitic capacitance between the gate electrode and the source electrode The parasitic capacitance between the gate and drain electrodes The parasitic capacitance between the source and drain electrodes Represented by The parasitic capacitance between TFT electrodes varies from product to product, but the TFT liquid crystal display device used in notebook computers end About 1/40 of Wow Is about 1/20 to 1/5 of C _LC . There are three major signal distortions that degrade the image quality of TFT liquid crystal displays. The first signal distortion is that parasitic capacitance changes when the scan line changes from selection to non-selection. By lowering the pixel voltage. The second signal distortion is caused by lack of charge rate, especially when there are many driving scan lines and the selection period is short.

The third signal distortion is parasitic capacitance This is caused by the fact that the polarity of the signal voltage has the greatest effect on the entire screen.

In order to prevent the image quality from being degraded by the above signal distortion, driving methods for adjusting the voltage polarity of the signal line are proposed so that the signal distortion is frequently generated by 60 Hz or more. 2 shows charges charged in the liquid crystal layer in various driving methods. Flicker is the largest in the frame inversion that changes the polarity of the liquid crystal layer for each frame. A row inversion driving method in which polarities are reversed for each liquid crystal layer of a pixel of an adjacent scan line and a dot inversion driving method in which polarities are reversed for each liquid crystal layer of an adjacent pixel are most commonly used.

In the thermal inversion driving method of Fig. 2, since the polarities of the liquid crystal layers of the scanning lines are the same, there is a large amount of current flowing through the common electrode, so that the load of the circuit driving the common electrode is large. Therefore, horizontal crosstalk and vertical crosstalk are large. In the point inversion driving method, since the polarities of the voltages of the liquid crystal layer are opposite for each adjacent pixel of the same scan line, the amount of charge charged in the liquid crystal layer of the pixel varies depending on the pixel information, but is much smaller than that of the thermal inversion driving method. The dot inversion driving method determines the polarity of the pixel liquid crystal layer charge irrespective of the information of the pixel. The amount of charge of the same scan line or the sum of the charges of the liquid crystal layer of an arbitrary region of the same scan line is not completely "0", and the information of each pixel is different. Different. In the case of the point inversion driving method, when the amount of charges charged in the liquid crystal layer of the same scan line is large, the load of the circuit for driving the common electrode is large, and thus the image quality may be degraded due to crosstalk. In the present invention, the horizontal and vertical crosstalk is reduced by adjusting the voltage polarity of the signal line so that the amount of charge in the liquid crystal layer of the pixel of the scan line is always kept "0".

1 is a cross-sectional view of a TFT liquid crystal display device.

2 is a polarity of a liquid crystal layer according to various conventional driving methods.

Fig. 3 is a drive block diagram of a TFT liquid crystal display element.

4 is a flow chart of an example for determining the polarity of the signal voltage.

5 is an example flowchart for determining the polarity of the signal voltage.

6 is an example flowchart for adjusting the polarity of the initial gray scale voltage.

※ Explanation of codes for main parts of drawing

1 gate electrode 2 gate insulating film 3 amorphous silicon (a-Si) layer

4 n ⁺ 5 drain electrode 6 source electrode

7 pixel electrode 8 common electrode 9 liquid crystal

10 TFT substrate 11 Common electrode substrate

4 is an example of a flowchart of a data control circuit for adjusting the polarity of the signal voltage of the present invention. Initialize the comparator when the vertical sync signal is applied. The comparator contains information about the least significant bit of the input frequency for the various gray levels of the screen. If the gray level of a pixel of a signal line is the draft number of an arbitrary gray level based on the gate pad, the value of the comparator is 1 and the even number is 0. When the vertical synchronization signal is applied, all information of the comparator is initialized to [0,0,0, ... 0,0,0]. If you have 64 gradations like a laptop, you will need a 64-bit comparator. The meaning of [0,0,0, ... 0,0,0] represents the number of the least significant bits of the same gradation input to the same scan line by dividing each gradation with ",". In accordance with the horizontal synchronization signal, n-bit pixel information corresponding to pixel information of the same scan line is input from the video RAM. The processing of the input signal is divided into two parts. One part enters the block that finds the least significant bit of the number of inputs of the gradation, and the other part enters the data buffer to match the time difference required to determine the number of times that it is entered. Since the pixel information is input in parallel through n lines, the signal is decoded and input to the comparator. The comparator may be configured with a 1-bit counter corresponding to each gray level. After decoding, 1 is added to the data of the corresponding gray levels, and the changed value is transmitted to the polarity signal generating circuit. The data synthesizing unit adds the first n bits of information from the data buffer and 1 bit, the most significant bit from the polarity signal generation. There may be several blocks for decoding and generating a polarity signal. For example, if 4 horizontal sync signals are required to decode and generate polarity signals, 4 blocks and 4 data buffers to decode and generate polarity signals can be processed simultaneously to process horizontal synchronization. have.

When the data of the signal line is processed as above, the most significant bit of the n-bit signal gradation to be input in odd-numbered bits is 1, and the remaining bits become the same signals as those coming from the video RAM and are transmitted to the source driver. If the most significant bit is 1, a + voltage is applied to the liquid crystal layer, and if it is 0, a-voltage is applied. On the contrary, if the most significant bit is 1, a negative voltage may be applied. If the most significant bit is 1, a positive voltage may be applied. When the initial value of the comparator is set to [0,0,0, ... 0,0,0], the minimum and maximum absolute values of the sum Q of the charges of the liquid crystal layer of the same scan line pixel are as follows.

Set the comparator's initial value so that polarity is reversed between adjacent grayscales. In this case, the sum Q of the charges in the liquid crystal layer of the same scan line pixel is 0 when all grays are even, and smaller than the charges charged in the liquid crystal layer of one pixel when all grays are odd. In this case, the maximum value of the sum of the charges in the liquid crystal layer of the scan line pixel is an odd number of odd grays and an even number of grays, which is 0.5 times the maximum value in the equation (1). By controlling the polarity of the signal voltage by dividing the voltage of the same gray level into two classifications, the polarity of the signal voltage can be adjusted by changing each of the classifications between adjacent grayscales to make the amount of charge induced in the liquid crystal layer of the same scan line close to "0". .

The polarity of the voltage applied to the signal line is reversed every cycle based on the voltage induced or applied to the common electrode. The polarity of the comparator of FIG. 4 may be set differently for each frame. Rather than reversing the polarity of the voltage applied to the liquid crystal layer every frame, random numbers are generated for a certain period of time so that the polarity of the liquid crystal layer is irregularly determined from the characteristics of random number parity or most significant bit, and every cycle until the next random number is generated. By changing the polarity of the voltage of the liquid crystal layer can completely block the DC voltage of the liquid crystal layer that can occur in the signal processing. When the external electrode driving the common electrode is disconnected, the common electrode is floated. The voltage V _COM induced to the common electrode according to the voltage of the pixel electrode can be derived from Kirchoff's law as follows.

In the case of 64 gradations, 5V driving liquid crystal, and XGA class, the voltage induced to the common electrode of Eq. (2) is about 0.04V, which is lower than 0.06V, which is the difference between the average voltages of 1 gradation. Can be very low. If the sum of the charges applied to the liquid crystal layer of the scan line pixels is always close to "0" in any region as described above, the influence of the external interference signal is divided into the entire screen, thereby reducing vertical and horizontal crosstalk, and also the liquid crystal layer. Since the polarity of is disordered in the time axis or spatially, the effective frequency is increased, so that low frequency flicker is hardly generated.

Fig. 5 shows the signal voltages of the pixels of the same scan line using a data buffer to find even and odd numbers of grays, and separates odd and even numbers of pixels of the same scan line. Shows a signal processing method in which the sum of charges in the liquid crystal layer is kept close to " 0 ". By applying this method, the sum of charges of the pixels of the liquid crystal layer of the same scan line can be set to "0" regardless of the screen information. When the vertical synchronization signal is input, all the information of the comparator 1 is initialized to [0, 0, 0, ... 0, 0, 0]. When the vertical synchronization signal is applied, pixel information is input in parallel through n lines in the video RAM. The signal is decoded and input to the comparator 1. The comparator 1 may be configured with a 1-bit counter of the number corresponding to the number of gray levels of the screen. After decoding, 1 is added to data of a corresponding gray level to find the least significant bit of the number of grays of all the pixels connected to the scanning line. While finding the least significant bit input for each gray level, the signal from the video RAM is sequentially input to the data buffer 1, and when the initial value of the comparator 2 is set, the signal is output from the data buffer 1. The dotted line in FIG. 5 is the same as the function in FIG. The difference between FIG. 4 and FIG. 5 is that the polarity of FIG. 4 is determined according to the initial value of the comparator initially set regardless of the information of the pixel. In FIG. 5, the initial value of the comparator 2 is set according to the information of the pixel connected to the scan line. . An initial value of the comparator 2 should be determined by finding the least significant bits of the number input for each gray level of all the pixels connected to one scan line. An example of the procedure shown in FIG. 6 is shown.

The number of input grayscales is even and the number of odd grays are separated from each other, the odd grays are polarized, and the even grays are polarized. For example, the least significant bit of a number input for each gray level of all pixels connected to one scan line If the odd gradation is And even gradation is to be. * Indicates an empty gradation for convenience. Set the initial gradation to 1 with odd-numbered gradation set to 1, the even-numbered gradation set to 0, and the even-numbered gradation to 0, and the even-numbered gradation to 0 and the even-numbered gradation to 1, respectively. When the value is determined, the initial value of the comparator 2 is Becomes The actual calculation according to FIG. 6 is as follows. The signal that goes into the top shift register And the bottom is reversed to be. These signals go through a 1-bit counter and If we OR this, Becomes 5, the sum of the charges of the liquid crystal layer on the same scan line is smaller than the charge of the liquid crystal layer of one pixel. When the external electrode driving the common electrode is broken, the induced voltage can be obtained from the equation (2), ignoring the influence of the scanning line. In case of 256 gradations and 5V driving liquid crystal, and in case of XGA class, the induced voltage of common electrode of Eq. (2) is about 4 * 10 ^-5 V, which is much lower than 0.015V, which is the difference of average voltage of 1 gradation, to drive common electrode. The load on the circuit can be very low. Therefore, as shown in FIG. 5, when the driving voltage of the signal line is determined, the induced voltage becomes almost " 0 " V without connecting the common electrode to the external terminal. When the polarity is adjusted as shown in Fig. 5, the amount of charge in the liquid crystal layer in any space of the scanning line is always kept at " 0 ". According to the driving method of the present invention, the voltage induced to the common electrode becomes almost " 0 V "

According to the driving method of the TFT liquid crystal display device of the present invention, since the charge amount of the liquid crystal layer of all the pixels connected to the scan line has a value close to "0", the load of the circuit driving the common electrode can be greatly reduced, so that the vertical and horizontal cross Image quality is improved by reducing torque and signal distortion. In addition, even if signal distortion occurs, it is directed toward the high frequency in space or time, and thus there is almost no flicker below 60 Hz that the human eye feels, so the image quality is excellent. The driving method of the present invention can be applied to a TFT liquid crystal display device in which a large TFT liquid crystal display device and a common electrode are floated.

Claims (5)

Dividing the voltage of the signal lines of the same gray level to be applied to the liquid crystal layer of the pixels of the same scanning line into two categories and walking them so that their polarities are reversed; Alternating polarity of voltage divided by two classifications alternately between adjacent gradations; A method of driving a TFT liquid crystal display device for adjusting the polarity of a voltage applied to a signal line so that the sum of charges in the liquid crystal layer of pixels in an arbitrary space in the same scan line is kept close to " 0 ". According to claim 1, the signal voltage of the same gray level is divided into two categories inputted in the odd and even numbers on the basis of the gate pad, and the voltage polarity is reversed, and the polarity of the even and odd signal voltages in the adjacent gray scale is TFT liquid crystal display device driving method opposite to each other. The method according to claim 1, wherein the signal voltages of the same scan line pixels are searched to find even grays and odd grays, and to separate the odd grays and the even grays, respectively, in the liquid crystal layer of the pixels of the same scan line. A TFT liquid crystal display device driving method for adjusting the polarity of a voltage applied to a signal line so that the sum of charges is kept close to " 0 ". The method of driving a TFT liquid crystal display device according to claim 1, wherein the polarity of the voltage applied to the liquid crystal layer is irregularly determined every arbitrary period, and the polarity of the voltage applied to the liquid crystal layer is alternately changed during odd and even frames within an arbitrary period. The method of driving a TFT liquid crystal display device according to claim 1, wherein the common electrode (8) is divided by the number of scanning lines so as to correspond to the scanning line, and the divided common electrode (8) is floated.