JP2002072974A - Method for driving liquid crystal display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce screen-display unevenness caused in the first and last rows of a liquid crystal panel. SOLUTION: When a DISPOFF signal is at a low level, all the outputs of the row driver and column driver become V0 level (level at deletion). After a 1st LP signal (latch pulse) is outputted, the DISPOFF signal becomes high level. When the succeeding LP signal is inputted, an FR signal (polarity instruction signal) is changed from the high level to the low level. A counter counts the LP signals up to a count number 20, and then changes the FR signal from the low level to the high level. Therefore, when the counter selects the 20th line, the polarity is inverted.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a driving method for driving a matrix type liquid crystal display device having scanning electrodes and signal electrodes, and more particularly to a driving method for a liquid crystal display device capable of improving non-uniform contrast. About.

[0002]

2. Description of the Related Art As a driving method of a liquid crystal display device, there is a time division driving method (matrix driving method). The matrix driving method is a liquid crystal display in which a plurality of linear scanning electrodes (common electrodes) extending in the row direction are provided, and a plurality of signal electrodes (segment electrodes) extending in the column direction so as to intersect with the common electrode group. In the element, each common electrode is sequentially selected and driven, and a signal corresponding to a display image is applied to a segment electrode group in synchronization with a selection period of each common electrode.

In accordance with a sequence in which a signal is applied to the segment electrode group while selecting each common electrode, a voltage is applied to each of the common electrode and the segment electrode for one frame. Is applied with a voltage waveform as shown in FIG. In general, a liquid crystal display element deteriorates when driven by a direct current for a long time, so that it is necessary to perform an AC drive. In the example shown in FIG. 8, in the second frame, a voltage having a polarity inverted from the polarity of the first frame is applied to each of the common electrode and the segment electrodes.

[0004] Such a drive waveform is generally called a B waveform. The driving method using the B waveform is advantageous in that it is simple and can perform low-frequency driving, so that power consumption is reduced.

[0005]

However, since the driving method using the B waveform has many low frequency components, there is a problem that non-uniform contrast (uneven display) is likely to occur. In particular, when low-voltage liquid crystal is used, display unevenness tends to occur. One cause of display unevenness is that ionic impurities are biased in the liquid crystal when a voltage is applied to the liquid crystal display element. A phenomenon occurs in which the threshold voltage Vth changes due to the bias of the ionic impurities.
Then, display unevenness occurs due to such a phenomenon. Display unevenness is more likely to occur as the duty ratio increases, as the threshold voltage of the liquid crystal decreases, and as the temperature increases.

Further, display unevenness occurs due to the distortion of the driving waveform. When the driving method using the waveform B shown in FIG. 8 is used, the voltage applied to the liquid crystal display element greatly changes immediately before and immediately after the polarity reversal of the applied voltage. Therefore, at that timing, the degree of waveform distortion also increases, the effective value voltage tends to fluctuate, and display unevenness tends to occur. That is, display irregularities are likely to occur in the first line and the last line of the active area (actual display area).

To reduce display unevenness due to the duty ratio, bias ratio, drive waveform distortion, etc., the duty ratio, bias ratio, drive waveform distortion, etc. may be changed to an appropriate state. If it is changed, it affects power consumption, drive voltage, contrast ratio, response speed, and the like, and it is difficult to actually change it. Also, changing them does not improve the situation in which display irregularities are likely to occur in the first line and the last line.

SUMMARY OF THE INVENTION The present invention is to solve the above-mentioned problem, and reduces display unevenness which occurs during energization of a liquid crystal display element, particularly, display unevenness which occurs in a first row and a last row. It is an object of the present invention to provide a driving method of a liquid crystal driving device that can perform the driving.

[0009]

According to a method of driving a liquid crystal driving device according to the present invention, in a method of driving a liquid crystal driving device that performs an AC drive using a time-division driving method, a timing for inverting the polarity of a liquid crystal driving voltage is determined. The time is at least three selection periods (a period during which three common electrodes are selected: three lines) away from the selection of the first common electrode (line) and the last common electrode (line) in the liquid crystal display element. A dummy driving period (a period during which the common electrode is not scanned) is set before and after the inversion.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As shown in FIG. 1 illustrating an embodiment of the present invention with reference to the drawings, in a liquid crystal display panel (liquid crystal display element) 1, a large number of transparent electrodes each serving as a common electrode are provided. 2 (hereinafter simply referred to as a common electrode 2) and a transparent substrate formed with a number of transparent electrodes 3 (hereinafter simply referred to as “segment electrodes 3”) each serving as a segment electrode. 2
And the segment electrode 3 are superimposed so as to be substantially orthogonal to each other. In this embodiment, the number of the common electrodes 2 is, for example, 120.

Each common electrode 2 is driven line-sequentially by a row driver 21, and each segment electrode 3 is driven by a column driver 22.
Driven by The row driver 21 sequentially drives each common electrode 2 according to a latch pulse (LP signal) from the control unit 23. The column driver 22 has a shift register that inputs a display data signal for one row via the control unit 23, and a data signal voltage corresponding to data in the shift register is supplied by the LP signal from the control unit 23. It is applied to each segment electrode.

The row driver 21, the column driver 22, and the control unit 23 are implemented as LSIs, and are TAB (Tape Automat).
ed Bonding), COG (Chip on Glass), COF (C
A conductive material such as an anisotropic conductive film, a flexible cable, or a zebra rubber is mounted between the electrode terminal of the liquid crystal panel 1 and the mounted LSI terminal by a technology such as hipon film) and COB (Chip on Board). Is conducted.

FIG. 2 is a schematic diagram showing a connection example between the output terminal of the row driver 21 and the common electrode 2 of the liquid crystal panel 1. In this example, the output terminals # 19 to # 21 of the row driver 21 are not connected to the common electrode 2. And #
Output terminals 22 and thereafter are connected to the common electrode 2 after # 19. Therefore, # 19 to # 19 by the row driver 21
The driving of # 21 is a dummy driving.

FIG. 3 is a block diagram showing an example of the configuration of the column driver 22 and a portion related to the polarity inversion of the control unit 23. In the example shown in FIG. 3, the column driver 22 shifts the display data signal in accordance with the clock signal (CP signal), and
A shift register 221 that outputs a display data signal in parallel according to the signal, and a level shifter 2 that converts an output value of the shift register 221 into a liquid crystal driving voltage (data signal voltage).
22 and a buffer 223 for converting the polarity of the liquid crystal drive voltage according to the polarity instruction signal (FR signal) and applying the converted voltage to the segment electrode 3.

The control unit 23 includes a timing control unit 231 for outputting signals applied to the row driver 21 and the column driver 22, a counter 232 for counting the LP signal, and an FR applied to the row driver 21 and the column driver 22. An FR polarity control unit 233 for generating a signal is provided. Note that the counter 232
Initialization is performed when the non-display instruction signal (DISPOFF signal) is at a low level, and the count value becomes 0 each time the level of the FR signal is inverted.

Next, the operation will be described. In a simple matrix type liquid crystal display device, line-sequential driving is generally performed. For example, in the multiple common electrodes 2 shown in FIG. 1, each scanning line, that is, the common electrode is sequentially scanned by the row driver 21 from top to bottom. Then, a voltage corresponding to the display data signal corresponding to the selected scanning line is applied from the column driver 22 to each segment electrode 3. Usually, when scanning reaches the bottom scanning line, the top scanning line is scanned again.

Then, the polarity inversion of the liquid crystal drive voltage is performed at a predetermined timing for the purpose of AC conversion. In this embodiment, the polarity inversion timing is set at the first line or the last line in the active area. Instead, it is performed during the time-division driving. In particular,
In this embodiment, the polarity inversion is performed when the row driver 21 scans the line # 20.

Dummy driving is performed at the timing when the polarity is inverted and during the scanning period before and after the timing. FIG.
5 is a flowchart showing the operation of the control unit 23 regarding the polarity inversion. FIG. 5 shows F based on the operation of the control unit 23.
FIG. 4 is a timing chart showing a state of an R signal and the like.

When the DISPOFF signal is at a low level, all outputs of the row driver 21 and the column driver 22 are at the V0 level (erasing level) regardless of other input signals (step S1). After outputting the first LP signal, the timing control section 231
The OFF signal is set to a high level (step S2). DI
The row driver 21 and the column driver 22 output a significant liquid crystal drive voltage to the liquid crystal panel 1 when the SPOFF signal goes high.

As shown in FIG. 5, when the next LP signal is input, the FR signal changes from the high level to the low level, and the counter value of the counter 232 is cleared to 0 (step S3). And the counter 232 is LP
Each time one signal is input, the count value is incremented by 1 (step S4), and when the count value of the counter 232 becomes 20 (step S5), the FR signal is changed from low level to high level (step S6). At this time, the count value of the counter 232 is cleared to zero.

The row driver 21 and the column driver 22
When the level of the FR signal is inverted, the polarity of the liquid crystal drive voltage is inverted, and the polarity is inverted at this point. That is, in this example, the polarity inversion is performed when the row driver 21 attempts to drive the line # 20. Note that FIG.
As shown in (2), since the output of # 20 of the row driver 21 is not connected to the liquid crystal panel 1, the driving of the # 20 line by the row driver 21 is a dummy drive. Driving of each one line before and after that is also dummy driving.

When the dummy drive is being performed, the signal voltage applied to the segment electrode 3 is changed from # 19 line to # 2 line.
In order to avoid the signal voltage of one line, for example, CP
The input of the signal and the display data signal to the column driver 22 is interrupted. Therefore, in this example, the counter 2
32 outputs a signal indicating that the counter values are “19” and “21”. Then, for example, during the period from when the signal indicating “19” is output to when the signal indicating “21” is output, for example, data reading from a memory or the like in the preceding stage of the column driver 22 is prohibited.

Thereafter, the counter 232 increments the count value by one each time one LP signal is input (step S).
7) When the count value of the counter 232 becomes X (step S8), the level of the FR signal is inverted (step S9). At this time, the count value of the counter 232 is cleared to zero. Note that the value of X is a value for determining the next polarity inversion timing. When the polarity inversion is performed once per frame, X = (number of lines−20).

With the above control, the timing of the polarity inversion of the voltage applied to the liquid crystal can be set not during the first line or the last line but during the time-division driving. Therefore, the occurrence of display unevenness in the first and last lines of the active area is suppressed.

FIG. 6 shows an IAPT (Improved Alt Pleshko).
FIG. 9 is a timing chart showing an example of all ON waveforms when a (Technique) waveform is used. As shown in FIG. 6, when the polarity inversion is performed during the time-division driving, immediately before the polarity inversion,
Immediately after that, the drive voltage (common waveform) applied to the common electrode 2 is different from the others. Therefore, the shape of the waveform distortion is different, and linear Vth unevenness may occur.
Therefore, in the present embodiment, Vth
In order to prevent unevenness, a dummy drive period of one line is provided before and after the polarity inversion is performed.

As described above, in this embodiment, the timing of inverting the polarity of the liquid crystal driving voltage is at least three lines away from the selection of the first and last lines in the liquid crystal panel 1. In addition, a dummy driving period is set before and after the polarity inversion. Therefore, display unevenness that occurs in the first row and the last row generated during energization of the liquid crystal panel 1 is reduced.
Further, since a dummy drive period of one line is set before and after the polarity inversion is performed, the linear Vth
The occurrence of unevenness is suppressed.

In this embodiment, the polarity inversion is performed at a timing 20 lines away from the first line, but the inversion timing is at least 3 lines away from the first line and the last line. Preferably, the timing is more than 5 lines apart. Further, the dummy driving periods set before and after the polarity inversion is performed are not limited to the period for one line, but may be the period for the number of lines longer than that.

Furthermore, in this embodiment, the timing of the polarity inversion is set in the middle of time-division driving by shifting the switching timing of the FR signal. However, other methods, for example, FLM (first line marker) May be shifted by a required number of bits by a shift register or the like, so that the polarity inversion timing may be in the middle of time-division driving.

[Example 1] As the liquid crystal panel 1, an STN element subjected to a rubbing process so that the twist angle becomes 240 ° was used.
7A. The first line selection pulse (pulse for driving the common electrode) immediately after the polarity inversion as shown in FIG. 7A and the first line in the middle of the polarity inversion as shown in FIG. Using a waveform that allows the selection pulse to enter, 60 ° C
Continuous energization was performed in the environment described above. Then, when the waveform shown in FIG. 7A is used, display unevenness occurs at a corner of the active area, whereas when the waveform shown in FIG. 7B is used, display unevenness does not occur. Or reduced.

Example 2 For the same liquid crystal panel 1 as used in Example 1, a waveform in which the selection pulse of the first line is input several lines after the polarity inversion as shown in FIG. And continuous energization was performed. Several lines were three or more lines. Then, display unevenness was reduced.

[Example 3] For the same liquid crystal panel 1 as used in Example 1, a waveform in which the selection pulse of the first line is inserted several lines before the polarity inversion as shown in FIG. Continuous energization was performed. Several lines were three or more lines. Then, after all, the display unevenness was reduced.

[0032]

As described above, according to the present invention, the driving method of the liquid crystal driving device is described by setting the timing for inverting the polarity of the liquid crystal driving voltage to the time when the first line and the last line in the liquid crystal display element are selected. And at least three lines apart from each other, and a dummy driving period is set before and after the polarity inversion. Therefore, display unevenness occurring during energization of the liquid crystal display element, particularly the first row and the last row Display unevenness can be reduced,
In addition, there is an effect that Vth unevenness that tends to occur at the time of polarity inversion can be reduced.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a common electrode and a segment electrode together with a drive circuit.

FIG. 2 is a schematic diagram showing a connection example between an output terminal of a row driver and a common electrode.

FIG. 3 is a block diagram showing a configuration example of a column driver and a part related to polarity reversal of a control unit.

FIG. 4 is a flowchart illustrating an operation of a control unit regarding polarity inversion.

FIG. 5 is a timing chart showing a state of an FR signal and the like based on an operation of a control unit.

FIG. 6 is a timing chart showing an example of all ON waveforms when an IAPT waveform is used.

FIG. 7 is a timing chart showing an example of a combined waveform applied to a liquid crystal panel.

FIG. 8 is a timing chart showing a B waveform.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Liquid crystal panel (liquid crystal display element) 2 Common electrode 3 Segment electrode 21 Row driver 22 Column driver 23 Control part

Claims (1)

[Claims] 1. A liquid crystal display device having a plurality of common electrodes and a plurality of segment electrodes, wherein each common electrode is line-sequentially selected, and a data signal voltage corresponding to the selected common electrode is applied to each segment electrode. A driving method of a liquid crystal driving device in which the polarity of a liquid crystal driving voltage is inverted at a predetermined timing, wherein the predetermined timing is separated from a time point of selection of a first common electrode and a last common electrode in the liquid crystal display element by at least three selection periods. And a dummy driving period is set before and after the polarity inversion, respectively.