JPH02216121A - Liquid crystal display device - Google Patents

Abstract

PURPOSE:To prevent the visual persistence even after display of the same image for a long time by dividing a common electrode into two and giving different voltages to divided parts and arranging each picture element electrode and two kinds of common electrode so that they face each other. CONSTITUTION:A common electrode 7 consists of two divided common electrodes 7a and 7b, and different voltages V1 and V2 are applied to them, and each picture element electrode 5 and these common electrodes 7a and 7b are so arranged that they face each other. One of these voltages is most suitable for black display of a liquid crystal cell, and the other is most suitable for white display. Consequently, the voltage of the common electrode is not most suitable as the whole of one picture element, and the asymmetry of a driving waveform is fixed in any gradation display with respect to one picture element. Thus, the visual persistence does not occur even after the same pattern is displayed for a long time, and a satisfactory visual recognizability is obtained.

Description

[Detailed Description of the Invention] [Summary] Regarding an active matrix liquid crystal display device with no afterimage (and good visibility), a liquid crystal display device that can provide good display without afterimage without changing transmittance characteristics. A transparent substrate having a plurality of drain buses and a plurality of gate buses and having at least one active element and a pixel electrode connected thereto near the intersection of each, and a transparent opposing substrate having a common electrode. In a liquid crystal display device in which a liquid crystal is sealed between a substrate and a substrate, the common electrode is divided into two parts and different voltages are applied to each part, so that each pixel electrode faces at least two types of the common electrode. A common electrode is placed between the electrodes.

[Industrial application field]

The present invention relates to a liquid crystal display device, and more particularly to a driving device for a liquid crystal display device that has no afterimage and has good visibility.

In recent years, active matrix drive type flat displays have been actively researched, and in particular, liquid crystal display devices have been actively developed. Since this active matrix drive type liquid crystal display device can have a smaller depth than a cathode ray tube, it has also been commercialized as a display device for pocket televisions, laptop computers, and the like.

However, in active matrix drive type liquid crystal display devices, visibility may deteriorate when the voltage applied to the liquid crystal cell changes due to the parasitic capacitance of the TPT (thin film transistor) used as an active element (switching element). This improvement is desired.

[Conventional technology]

Figure 8 shows a conventional active matrix liquid crystal display device 8.
This shows the configuration of 0. Generally, a conventional active matrix type liquid crystal display device 80 is constructed by sandwiching a display layer in which a liquid crystal cell 83 is arranged between two glass substrates 81 and 82, and a switching element is mounted on one glass substrate 81. A scanning bus (gate bus) 85 connected to the gate of a certain TFT 84, a data bus (drain bus) 86 connected to the drain of the TFT 84, and a pixel electrode 87 of the liquid crystal cell are provided. This TFT 84 is provided near the intersection of the scan bus 85 and the data bus 86, and a pixel electrode 87 is connected to the source of the TFT 84. Further, a common electrode 88 is provided on the other glass substrate 82.
is formed. Each data bus 86 is driven by a data driver (not shown), and each scan bus 86 is driven by a data driver (not shown).
5 is driven by a scan driver (not shown).

FIG. 9 shows an equivalent circuit of one liquid crystal cell in the liquid crystal display device 80 of FIG. The gate of TFT 84 is connected to scan bus 85, and the drain is connected to data bus 86.
The source is connected to one electrode of the liquid crystal cell 83. Reference numeral 88 indicates a common electrode of the liquid crystal cell 83. In the above configuration, as shown in FIG. 10, when the gate voltage vG is at a high level "11", the TF
T84 is turned on and the drain voltage VD is written into the liquid crystal cell 83 as data. The data written to the liquid crystal cell 83 is at the gate voltage ■. is low level “L”
Even when the TFT 84 is turned off, it is retained in the liquid crystal cell 83, and due to this property, a writing operation is performed on all liquid crystal cells 83 of the panel to perform display.

As shown by the dotted line in the equivalent circuit of Figure 9, the TF
There is a parasitic capacitance C6° between the gate and source of T84, and a parasitic capacitance CDs between the drain and source, and the liquid crystal cell 8
It is known that 3 itself has a liquid crystal cell capacitance CLC. Due to the influence of these parasitic capacitances, the 10th
As shown in the figure, the voltage (T
When the gate voltage V falls, ΔVs − (Ccs * ΔVc ) / (Cas
c) changes by the data voltage ■. When ΔVS changes, ΔVS
It is also known that it changes by -CCn5*Δvo)/(Cns+etc). Therefore, the liquid crystal cell 84
The data waveform written to the terminal became asymmetrical with respect to the voltage of the common electrode, causing display unevenness. Therefore, the present applicant has already proposed a device in which symmetrical voltages are applied to liquid crystal cells by changing the voltage of the common electrode 88 of the counter substrate 82 (see Japanese Patent Application No. 63-G33736).

[Problem to be solved by the invention]

However, even with devices that have been improved to apply symmetrical voltages to the liquid crystal cells as described above, if the same pattern is displayed for a long time, it may become burned into the screen and become visible as an afterimage. It has been found that there is a problem in that the display quality after displaying the pattern is significantly degraded.

It was also found that this problem is caused by the fact that liquid crystal has dielectric anisotropy and the dielectric constant of liquid crystal changes depending on the voltage applied to the liquid crystal, resulting in data voltage dependence of the liquid crystal cell capacitance.

According to the present invention, even when the same pattern is displayed on a liquid crystal display device for a long time, image sticking does not occur on the screen.
It is an object of the present invention to provide a liquid crystal display device that improves the quality of display after displaying the same pattern by removing a phenomenon visually recognized as an afterimage.

[Means to solve the problem]

The basic structure of a liquid crystal display device of the present invention that achieves the above object is shown in FIG.

In FIG. 1, a transparent substrate 1 has a plurality of drain buses 2 and a plurality of gate buses 3, and has at least one active element 4 and a pixel electrode 5 connected thereto near the intersection of each. Reference numeral 6 denotes a transparent counter substrate having a common electrode 7, and a liquid crystal 8 is sealed between the counter substrate 7 and the substrate 1.

The common electrode 7 is a common electrode 7 a divided into at least two parts.
The pixel electrodes 5 each have common electrodes 7a and 7b arranged so as to face at least two types of common electrodes 7a and 7b. ing.

[Effect]

According to the liquid crystal display device of the present invention, a common electrode having at least two different voltages faces the liquid crystal cell of one pixel, and one of these voltages is optimal when the liquid crystal cell displays black. One of the remaining voltages is given so that it is optimal when the liquid crystal cell displays white, so the voltage of the common electrode is not optimal for one pixel as a whole, and one pixel The asymmetry of the drive waveform is constant no matter what gradation is displayed, and as a result, the occurrence of afterimages is suppressed.

〔Example〕

Embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

FIG. 2 shows the structure of an embodiment of the liquid crystal display device of the present invention. In FIG. 2, a liquid crystal display device 20 includes a display layer on which liquid crystal cells 23 are arranged, and two glass substrates 21.
．． A scanning bus (gate bus) 25 connected to the gate of the TPT 24, which is a switching element, and a data bus (drain bus) 26 connected to the drain of the TFT 24 are placed on the glass substrate 21 on the other side. ,
A pixel electrode 27 of a liquid crystal cell is provided. This TP
T24 is provided near the intersection of the scan bus 25 and data bus 26, and the pixel electrode 27 is connected to the source of TPT24.

Further, in this embodiment, two comb-shaped common electrodes 28 and 29 are formed on the other glass substrate 22 in a state in which they are interlocked with each other. And each data bus 2
6 is driven by a data driver (not shown), and each scan bus 25 is driven by a scan driver (not shown).

a comb-shaped common electrode 28 formed on the glass substrate 22;
29 and the pixel electrode 27 formed on the overlapping glass substrate 21 are shown in FIGS. 3 and 4.
As shown in the figure. In the example shown in FIG. 3, the common electrodes 28 and 29 have independent branch rods 28a and 2 that are half the width of the pixel electrode 27.
9a, respectively, and these branch rods 28a.

Each of the electrodes 29a covers half of one pixel electrode 27. In the figure, 24 is a TPT, 25 is a scan bus, and 26 is a data bus. In the example shown in FIG. 4, thick independent branch rods 28b and 29b, each having a width approximately the same as that of the pixel electrode 27, are formed on the common electrodes 28 and 29, and each of these branch poles 28b and 29b is Both halves of two adjacent pixel electrodes 27 are covered. In this figure as well, 24 represents a TPT, 25 represents a scan bus, and 26 represents a data bus.

FIG. 5 shows an equivalent circuit of one pixel of the liquid crystal display device of the embodiment configured as shown in FIG. One pixel in the above embodiment is equivalent to two pixels in terms of an equivalent circuit.
The liquid crystal cells 23A and 23B are each supplied with different voltages V, V2 from two independent common electrodes 28 and 29, respectively. Here, the voltage V applied to the common electrodes 28 and 29
,,V, is the data voltage V of the common voltage vc of the liquid crystal panel
For example, as shown in Figure 7, which shows the dependence on ,
Set the voltage ■2 as the optimum value (e.g. -1, OV) when the voltage is displayed in white (displayed when Vo is on at high level "H"), and the voltage ■2 is displayed in black (displayed when Vo is at the low level "H"). The optimum value (example: -1,4V) when the display is off at "L"
).

Next, we will compare the case where the liquid crystal cell 23 configured as above displays black or white and the case where it displays an intermediate tone such as gray, and examine the changes in the characteristics of transmittance T and voltage ■. The explanation will be given using FIG. 6 assuming that the initial TV characteristics are the same.

First, when displaying black or white, the common voltage is optimized for at least one part, and even when displaying for a long time, the TV characteristics change as shown by the solid line in Figure 6 (1). do not. However, in the other part, since the common voltage deviates greatly, the TV characteristic shifts to the lower voltage side with time, as shown by the dashed line in FIG. 6(1). This shift amount increases in proportion to the DC voltage and time, and the DC voltage is determined by the difference in common voltage, that is, the optimum common voltage for black display (VC black) and the optimum common voltage for white display. (V
C white), it is proportional to (VC black) - (Vc white). As a result, since the two common electrodes 28 and 29 that originally divided one common electrode are very small, the human eye recognizes the average value of the brightness of these two common electrode parts as the brightness of the liquid crystal cell 23. , the liquid crystal cell 23 is shown in FIG.
The appearance shown by the dotted line in 1) has the above TV characteristic.

In addition, when displaying halftones such as gray display, 2
In either part of the two common electrodes 28, 29 (V
, black") - (vc white), half of the shift amount of the T-■ characteristic in the part where the common voltage differs when the TV characteristic becomes black or white over time is applied. 6 (2) as shown by the dashed line in Figure 6 (2).This means that when displaying black or white, the appearance is the upper T.
- Matches the V characteristic.

Table 1 below summarizes the operation of the liquid crystal display device as described above in comparison with the operation of a conventional liquid crystal display device.

As is clear from this table, in the conventional device, the voltage of the common electrode is set to the optimum value when the data voltage VD is between the high level "H" and the low level 1L". Therefore, during white display and black display, the voltage of the common electrode does not reach the optimum value, and an afterimage remains when the same pattern is displayed for a long time.

(Table 1) “When taking voltage, the common voltage is set to the optimum value, so when displaying white or black, the voltage of one of the common electrodes will be the optimum value, and it will last for a long time. No afterimage remains even when the same pattern is displayed.

Furthermore, setting of a plurality of common voltages will be explained.

The average value VCAV of the plurality of common voltages must satisfy the following formula in order to equalize the shift amounts of the TV characteristics.

On the other hand, in the above-mentioned device, the data voltage VD is applied to the two common electrodes at a high level "11" and a low level, however, Δ
■G = Gate voltage variation C6,: Gate-source capacitance CLCon: Liquid crystal cell capacitance when the liquid crystal is on, CLCof:
The liquid crystal cell capacity when the liquid crystal is off, however, is the average value of the common voltage■. Even if , slightly deviates from this, the effect will be expected although it will be incomplete, so if the average value of the common voltage V CAV is set within the following range, at worst an afterimage equivalent to that of the conventional one will appear.

Also, if the display pattern does not change at all, the liquid crystal cell 2
3, where the common voltage is not optimal, a DC voltage with the same polarity continues to be applied. As described above, if the DC voltage continues to be applied, the amount of shift in the TV characteristics increases, and the brightness within the liquid crystal cell 23 becomes insufficiently averaged, causing not only defects in which bright areas become visible, but also There are effects such as decomposition of liquid crystal molecules due to voltage. In order to prevent this, the relationship between the common voltage and the common electrode to which it is applied is changed over time. In other words, (VC black) is now applied to the common electrode that was previously set to (VC white). , conversely, the common electrode that was previously set to (V, black) is now set to (VC white).
so that it is applied. In this way, by changing the polarity of the DC voltage applied to the liquid crystal cell 23, it is possible to eliminate defects in which bright areas become visible and the effects of decomposition of liquid crystal molecules due to the DC voltage.

As described above, in the present invention, no matter what display is performed,
Since the amount of change in the TV characteristics above is apparently constant,
By dividing the common electrode into multiple parts and applying different common voltages to each common electrode, it is possible to eliminate the situation where a DC voltage is applied locally, and it is possible to prevent afterimages and reduce the deterioration of display quality. It can be prevented.

〔Effect of the invention〕

As described above, according to the present invention, even if the same pattern is displayed for a long time, the previous pattern will not be burned in in the subsequent display, and a deterioration in display quality can be prevented.

[Brief explanation of the drawing]

FIG. 1 is a basic configuration diagram of a liquid crystal display device of the present invention, and FIG. 2 is a perspective view showing the configuration of an embodiment of a liquid crystal display device of the present invention.
3 and 4 are layout diagrams showing the positional relationship between one pixel electrode and a plurality of common electrodes, FIG. 5 is the equivalent circuit head of one pixel of the liquid crystal display device of the present invention, and FIG. 6 is the main A TV characteristic diagram showing the operation of the liquid crystal display device of the invention, FIG. 7 is a characteristic diagram showing the data voltage dependence of the common voltage, FIG. 8 is a perspective view showing the configuration of the conventional liquid crystal display device, and FIG. 9 is an equivalent circuit diagram of one pixel in FIG. 8, and FIG. 1O is a waveform diagram showing operating waveforms of the liquid crystal display device in FIG. 8. l...Transparent substrate, 2...Data bus, 3...Scanning bus, 4...Active element, 5...Pixel electrode, 6...
Counter substrate, 7, 7a, 7b... common electrode, 8...
Liquid crystal cell, 20... Liquid crystal display device of one embodiment of the present invention, 24... TFT, 25... Scanning bus, 26...
Data bus, 27...pixel electrode, 28, 29...common electrode.

Claims (1)

[Claims] 1. A plurality of drain buses (2) and a plurality of gate buses (3)
), a transparent substrate (1) having at least one active element (4) and a pixel electrode (5) connected thereto near each intersection, and a transparent counter substrate (1) having a common electrode (7).
6), in which the common electrode (7) is divided into at least two parts and different voltages are applied to each part, and each pixel electrode (5) is connected to the common electrode (7). A liquid crystal display device characterized in that a common electrode (7) is arranged to face at least two types of electrodes. 2. The liquid crystal display device according to claim 1, wherein the different voltages applied to the at least two divided common electrodes (7) are switched at predetermined time intervals.