JPH11109929A - Liquid crystal display device driving method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent off current from flowing through a switching element and to keep stable operation by high setting an offset voltage added to signal potential applied to the signal input side of the switching element so as not to be affected with the fluctuation in the threshold value of the switching ele ment. SOLUTION: A video signal is generated by adding a video component to grounded potential VSS in an odd numbered horizontal scan period HO, and subtracting the video component from source potential VDD in an even number-th horizontal scan period HE. At this time, the offset of a fixed level is imparted at every polarity. That is, a reference level set in a top of each horizontal scan period is set so as to become higher than the grounded potential VSS in a positive polarity, and to become lower than the source potential VDD in a negative polarity. In such a case, the offset voltage ΔV when the video signal is the positive polarity is set large so that the off current doesnot flow through a thin film transistor as the switching element even when operation characteristics are varied.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method of driving an active matrix type liquid crystal display device.

[0002]

2. Description of the Related Art FIG. 4 is a circuit diagram showing a configuration of an active matrix type liquid crystal display device. The switching elements 1 are, for example, thin film transistors, and are arranged in rows and columns on an insulating substrate in accordance with the number of display pixels. A pixel display electrode 2 arranged for each display pixel is connected to a signal output terminal (source) of the switching element 1. Row scan line 3
Are arranged corresponding to each row of the switching elements 1 and are commonly connected to the control terminal (gate) of each switching element 1 in the same row. The column signal lines 4 are arranged corresponding to the respective columns of the switching elements 1 and are commonly connected to a signal input terminal (drain) of each switching element 1 in the same column. The insulating substrate on which the switching element 1, the pixel display electrode 2, the row scanning line 3, and the column signal line 4 are formed, and the insulating substrate on which the counter electrode 5 is formed on one surface are arranged to face each other with the liquid crystal 6 interposed therebetween. Thus, a liquid crystal display device is formed. Such a liquid crystal display device is configured to control the orientation of the liquid crystal 6 according to a signal potential applied to each pixel display electrode 2 from the column signal line 4 via the switching element 1.

The row control circuit 7 is composed of, for example, a shift register that sequentially shifts a start pulse PX rising for one horizontal scanning period in response to a shift clock CX synchronized with a horizontal scanning period. Connected. As a result, the row scanning lines 3 are sequentially selected row by row at a constant period in synchronization with the horizontal scanning timing of the video signal, and the switching elements 1 connected to the row scanning lines 3 are sequentially turned on in row units. . The column control circuit 8 includes a shift register that sequentially shifts a start pulse PY that rises only for one pixel period in response to a shift clock CY synchronized with a pixel period, and an analog switch that turns on in response to an output of the shift register. And each column signal line 4
Connected to. Thereby, during the period when the row selection circuit 7 is selecting the row scanning line 3 of the specific row, that is, during the horizontal scanning period,
An input video signal is sequentially supplied to each column signal line 4. By the combination of the row control circuit 7 and the column control circuit 8 as described above, a signal potential corresponding to a video signal is applied to a plurality of pixel display electrodes 2 arranged in a matrix in a predetermined scanning order.

[0004]

In a conventional liquid crystal display device, the polarity of a video signal is inverted at a predetermined period in order to prevent the deterioration of the liquid crystal. For example, as shown in FIG. 5, the polarity of the video signal is inverted every horizontal scanning period so that it becomes positive during the odd-numbered horizontal scanning periods HO and becomes negative during the even-numbered horizontal scanning periods HE. You.

By the way, in a thin film transistor used as the switching element 1, an off-state current may flow in a non-selected state. That is, when the thin film transistor is turned off, the signal potential is held on the transparent electrode 2. Therefore, when the difference between the signal potential and the potential of the row scanning line 3 in the non-selected state increases, the thin film transistor tends to turn on. Current flows easily. Such off current is
When the polarity of the video signal is inverted every predetermined period, the video signal flows only in one polarity. For example, if the thin film transistor is an N-channel type, the off-state current flows only when the video signal shows a positive polarity. If the operation is continued for a long time while such an off-state current is flowing, electric charges are gradually accumulated in the pixel display electrodes 2, and a signal potential according to a video signal may not be correctly applied to the pixel display electrodes 2. In particular, when the operating environment of the device becomes high temperature and the threshold value of the thin film transistor fluctuates in the negative direction, the off-current flowing through the thin film transistor is likely to increase, so that the influence is large.

Accordingly, an object of the present invention is to prevent the off current from flowing through the switching element and to maintain a stable operation.

[0007]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and a plurality of row scanning lines and a plurality of column signal lines are arranged to cross each other, and a plurality of switching elements are provided. A first substrate in which a plurality of pixel display electrodes are connected to the respective switching elements and are arranged in a matrix, and a first substrate in which a plurality of pixel display electrodes are arranged in a matrix and connected to the intersection of each row scanning line and each column signal line, and a counter electrode arranged on one surface In a method for driving a liquid crystal display device in which two substrates are arranged to face each other with a liquid crystal interposed therebetween, the plurality of row scanning lines are sequentially selected one by one at a constant cycle, and a row selection in which each row scanning line is selected. A first step of supplying a video signal to the plurality of column signal lines during a period, and applying a signal potential corresponding to the video signal to a pixel display electrode connected to the selected row scanning line; From the first potential to the second potential And a second step of applying a third potential that is selected between a third step of reversing the polarity of the signal potential every predetermined period, in the third
The step of, when the signal potential indicates one polarity,
It is characterized in that an offset voltage that is widened in accordance with the variation width of the threshold potential of the switching element is added.

According to the present invention, the offset voltage added to the signal potential applied to the signal input side of the switching element (transistor) is set high so as not to be affected by the fluctuation of the threshold value of the switching element. You. Therefore, even if the threshold value of the switching element fluctuates,
The off current flowing through the switching element does not increase.

[0009]

FIG. 1 is a timing chart for explaining a first embodiment of the present invention. This figure shows a case where a thin film transistor as a switching element used in a liquid crystal display device is an N-channel type.
The structure of the liquid crystal display device to which the driving method of the present invention is applied is the same as that shown in FIG.

The row selection signal X is generated by a row control circuit 7 connected to the liquid crystal display and supplied to the row scanning lines 3, and the rising periods thereof are set to be shifted by one horizontal scanning period. Is done. For example, in the k-th horizontal scanning period, a row selection signal Xk for selecting the k-th row of the row scanning line 3 is raised, and in the subsequent (k + 1) -th horizontal scanning period, the (k + 1) -th row of the row scanning line 3 is changed. The row selection signal Xk + 1 to be selected rises. Thus, the row scanning lines 3 are sequentially selected one by one from the first row to the last row in synchronization with the horizontal scanning timing.

The column selection signal Y is generated by a column control circuit 8 connected to the liquid crystal display device, and converts a video signal into a column signal line 4.
, And the rising periods of the respective signals are set to be shifted by one pixel period. Here, one pixel period indicates a period representing information of one pixel in each horizontal scanning period of a video signal representing a continuous screen in units of one pixel. This column selection signal Y is
In one pixel period at the beginning of each horizontal scanning period, a column selection signal Y1 for selecting the first column of the column signal line 4 is raised, and the columns for sequentially selecting the second and subsequent columns of the column signal line 4 are successively provided. The selection signals Y2, Y3,... Are sequentially activated in units of one pixel period. Thus, the video signal is sequentially supplied from the first column to the last column of the column signal line 4 for each horizontal scanning period.

The video signal is generated such that the polarity is inverted every horizontal scanning period. That is, a video signal is generated by adding a video component to the ground potential VSS in the odd-numbered horizontal scanning periods H0, and subtracting a video component from the power supply potential VDD in the even-numbered horizontal scanning periods HE. At this time, a certain level of offset is provided for each polarity. That is, the reference level set at the beginning of each horizontal scanning period is set so as to be higher than the ground potential VSS for the positive polarity and lower than the power supply potential VDD for the negative polarity. For example, when the ground potential VSS is set to 0 V and the power supply potential VDD is set to 12 V, 2 V is used as a reference in the odd-numbered horizontal scanning periods HO having a positive polarity so that the video signal changes in a range from 2 V to 10 V. In the even-numbered horizontal scanning period HE, which is characteristic, 10 V is used as a reference. At this time, the offset voltage ΔV set in the odd-numbered horizontal scanning periods HO is set broadly in accordance with a change in threshold potential due to a temperature change in the operating environment of the thin film transistor used as the switching element 1.

The potential VCM applied to the counter electrode 5 is usually an intermediate potential, but does not necessarily have to coincide with the center of the amplitude of the video signal, and is a value that can maintain the balance of operation in the liquid crystal display device. And it is sufficient. In the N-channel type thin film transistor, as shown in FIG. 2, when the gate potential VG exceeds a predetermined threshold potential, a drain current ID starts to flow, and becomes higher than the threshold potential by a certain potential. At this point, the drain current ID is saturated. The operating characteristic shown by the solid line a shifts in the negative direction as shown by the broken line b due to the temperature rise of the operating environment. Due to such a change in the operation characteristics, the gate potential VG
Is a specific potential V0, the drain current ID is the current I1
To the current I2. Even when such a change in the operating characteristics occurs, the offset voltage ΔV for the positive polarity of the video signal is set large so that the off current does not flow through the thin film transistor as the switching element 1. Thereby, even if the threshold value of the thin film transistor as the switching element 1 is lowered, it is possible to prevent the off current from flowing when the video signal shows a positive polarity.

According to the driving method described above, since the off current of the switching element 1 is almost eliminated, no charge is accumulated on the display electrode 2 even when the operation is continued for a long time, and the image quality of the display screen is reduced. Deterioration can be prevented. FIG.
FIG. 6 is a timing chart for explaining a second embodiment of the present invention. This figure also shows a case where the thin film transistor as a switching element used in the liquid crystal display device is an N-channel type, as in FIG.

The row selection signal X and the column selection signal Y are the same as in the first embodiment shown in FIG. As in FIG. 1, the video signal is inverted in polarity every horizontal scanning period, has a positive polarity in odd-numbered horizontal scanning periods H0, and has a negative polarity in even-numbered horizontal scanning periods HE. Further, in both polarities, constant offset voltages ΔVS and ΔVD are given in addition to the video component, respectively. At this time, the offset voltage ΔVS added to the ground potential VSS is set higher than the offset voltage ΔVD subtracted from the power supply potential VDD. That is, the center of the amplitude of the video signal is set higher than the intermediate potential between the ground potential VSS and the power supply potential VDD, and the difference between the reference level for the positive polarity and the ground potential VSS is the reference for the negative polarity. It is set to be larger than the difference between the level and the power supply potential VDD. For example, the ground potential VSS is 0 V, and the power supply potential VDD is 12
V, 2 V is added for positive polarity, and 1 V for negative polarity.
By subtracting V, the video signal changes in the range from 2V to 11V. Note that the potential VCM applied to the counter electrode 5 does not necessarily have to coincide with the center of the amplitude of the video signal, as in the first embodiment. Thereby, even if the threshold value of the thin film transistor as the switching element 1 is lowered, it is possible to prevent the off current from flowing when the video signal shows the positive polarity, and to increase the amplitude of the video signal. become.

In the above embodiment, the switching element 1 is an N-channel type thin film transistor. However, a P-channel type thin film transistor can be used. However, in the case of a P-channel thin film transistor, since the polarity is opposite to that of an N-channel thin film transistor, the offset voltage for a video signal needs to be set wider on the negative polarity side. That is,
In the case of the first embodiment, the offset voltage to be subtracted from the power supply potential VDD is set to be large according to the variation of the threshold value of the thin film transistor. Further, in the case of corresponding to the second embodiment, the offset voltage subtracted from the power supply potential VDD may be made higher than the offset voltage added to the ground potential VSS.

[0017]

According to the present invention, the off current flowing through the switching element can be almost eliminated, and the accumulation of electric charge in the pixel display electrode can be prevented. Therefore, even when the operating environment becomes high temperature, the operation can be continuously performed without deteriorating the image quality of the display screen. Also,
Even if the offset voltage of one polarity is increased in the video signal whose polarity is inverted, by reducing the offset voltage of the other polarity, the possible amplitude of the video signal can be prevented from becoming narrow.

[Brief description of the drawings]

FIG. 1 is a timing chart illustrating a first embodiment of the present invention.

FIG. 2 is a characteristic diagram showing a state of a change in operating characteristics of a thin film transistor.

FIG. 3 is a timing chart illustrating a second embodiment of the present invention.

FIG. 4 is a circuit diagram showing a configuration of an active matrix type liquid crystal display device.

FIG. 5 is a waveform diagram of a video signal supplied to a liquid crystal display device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Switching element 2 Pixel display electrode 3 Row scanning line 4 Column signal line 5 Counter electrode 6 Liquid crystal 7 Row control circuit 8 Column control circuit

Claims (2)

[Claims] 1. A plurality of row scanning lines and a plurality of column signal lines are arranged so as to cross each other, a plurality of switching elements are respectively connected to intersections of each row scanning line and each column signal line, and a plurality of pixels are provided. In a driving method of a liquid crystal display device, a first substrate in which display electrodes are connected to each switching element and arranged in a matrix, and a second substrate in which counter electrodes are arranged on one surface are opposed to each other with a liquid crystal interposed therebetween. And sequentially selecting the plurality of row scanning lines one by one at a constant period, supplying video signals to the plurality of column signal lines during a row selection period in which each row scanning line is selected, and selecting the selected row scanning line. A first step of applying a signal potential corresponding to a video signal to a pixel display electrode connected to a line, and a third potential selected from a first potential to a second potential on the counter electrode. A second step of applying and a predetermined period And a third step of inverting the polarity of the signal potential every time, wherein the third step includes, when the signal potential indicates one polarity, a variation width of the threshold potential of the switching element. A method for driving a liquid crystal display device, characterized by adding an offset voltage that is widened in response to the addition. 2. The method according to claim 1, wherein, when the signal potential indicates one polarity, the third step includes adding a first offset voltage that is widened according to a variation width of a threshold potential of the switching element, and 2. The method according to claim 1, wherein when the polarity is indicated, a second offset voltage having an absolute value smaller than the first offset voltage is added.