JPH0335217A - Driving system for liquid crystal display device - Google Patents

Abstract

PURPOSE:To form high-contrast images without degrading the steepness of transmittance and voltage characteristics even when a screen display is executed by a relatively low frame frequency by determining the ratio of the crest value VP of the voltage to be impressed to selected picture elements and the crest value Vb of a bias voltage so as to satisfy a specific range. CONSTITUTION:The good display images are obtd. when the ratio a ( =Vp/Vb) of the crest value Vp of the voltage to be impressed to the selected picture elements and the crest value Vb of the bias voltage has the relation of equation I. In the equation, N denotes the number of scanning lines. Since a=Vp/Vb can be set in the adequate range in this way, the steepness of the transmittance and voltage characteristics is not degraded even if the frame frequency is changed to a low-frequency region by using the liquid crystal element of a high response speed.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to an improvement in the driving method of a liquid crystal display element, particularly a twisted nematic liquid crystal display device driven by a simple matrix, using a voltage averaging method.

<Prior Art> A simple matrix drive type LCD has a simpler structure than an active drive type LCD in which a switching element is provided for each picture element, so it has the advantage that it can easily be made to have a large screen. Taking advantage of this advantage, applications are expanding as displays for word processors and laptop computers.

However, since they are inferior to active drive type LCDs in terms of display characteristics such as contrast and response speed, improvements have been made in these respects.

1. Super Twisted Nematic, in which the twist angle/degree of molecular orientation in the liquid crystal layer is set to 1800 or more in order to improve contrast.
c) LCDs have been proposed and currently simple matrix driven LCDs
It has become the mainstream. In order to improve the response speed, efforts have been made to make the liquid crystal layer thinner and to lower the viscosity of the liquid crystal material, and the goal is to achieve a response speed of 150 m5ec or less, which is capable of displaying moving images.

A simple matrix drive type L that does not have a switching element for each picture element like an active matrix drive type LCD.
As shown in FIG. 5, the CD drive circuit shares the same electrode for several picture elements, so voltage is applied to picture elements other than the display picture elements. For this reason, a cross-effect phenomenon in which contrast decreases is unavoidable.

In order to alleviate this problem, a driving method called a voltage averaging method is generally used to average the voltages at selected points and non-selected points. In particular, the ratio a between the peak value Vp of the voltage applied to the selected picture element and the peak value Vb of the bias voltage for the number N of scanning lines is a-fi+1, and the peak value of the voltage applied to the non-selected picture element is The relationship between vo and the peak value Vp of the voltage applied to the selected picture element is Vo-(1-7) Vp. Optimized bias method is used.

<Problems to be Solved by the Invention> However, twisted nematic liquid crystals have characteristics that depend on the effective value of the voltage applied for driving.
As the response speed of the liquid crystal display element becomes faster and the pulse width of the drive voltage becomes relatively wider, good display characteristics cannot be obtained by applying the voltage waveform determined by the above-mentioned optimization bias method to the picture element. As an example of this, see Figure 8 (al and fb
The response speed is 400 m5ec and 150 m5 respectively.
The transmittance voltage characteristics are shown when a driving waveform determined by the above-mentioned optimization bias method is applied to a 5TN-LCD that is an ec. As shown in figure (a), the response speed is 400.
If it is as slow as m5ec, changing the frame frequency, which is the screen rewriting frequency, will only cause a change in the threshold voltage, but there will be no large difference in the steepness of the transmittance versus voltage, which is a measure of display contrast. do not have. However, when the response speed is as fast as 150 m5ec, the same figure (
As shown in b), as the threshold voltage changes, the steepness becomes extremely poor in a low frequency region where the frame frequency is 60 Hz or less. Therefore, in the conventional optimization bias method, the TV screen display (frame frequency of about 80Hz)
When displaying a moving image by increasing the response speed of a liquid crystal display element in a low frequency region where the problem of electrode resistance value is alleviated, there is a problem that a high contrast image cannot be formed.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a liquid crystal display device having a fast response speed.

A driving method using a voltage averaging method for liquid crystal display devices that does not deteriorate the steepness of the transmittance voltage characteristics even when screen display is performed at a relatively low frame frequency, and therefore can form high contrast images. Our goal is to provide the following.

Means for Solving the Problems> To achieve the above object, the present invention provides a method for driving a liquid crystal display device using a voltage averaging method, in which the peak value Vp of the voltage applied to a selected picture element and the bias voltage are The ratio a (=Vp/Vb) to the peak value Vb of (, /
This is a driving method for a liquid crystal display device characterized by satisfying the following range: W+t)/1.1>a>c4+t)/2.

Effects> According to the present invention, unlike the conventional optimization bias method, the ratio a −V p /V b of the peak value Vp of the voltage applied to the selected picture element to the peak value Vb of the bias voltage is determined appropriately. Therefore, even if the frame frequency is changed to a low frequency range using a liquid crystal element with a fast response speed, the steepness ratio of the transmittance-voltage characteristic will not deteriorate.

<Embodiment> A liquid crystal display device to which the driving method of the present invention is applied has a configuration similar to that of a conventional example, and has a driving circuit shown in FIG. The liquid crystal panel is a 2-layer 5TN with NXM wooden electrodes.
-Using an LCD. A method for manufacturing this two-layer type 5TN-LCD is shown below (see FIG. 6). For the drive cell 4, ITO is deposited on glass using a sputtering device to approximately xoooA, and then etched and notched using a photo process to form the scanning electrode 2 and signal electrode 8 shown in FIG. 5iO on top of that
After depositing an inorganic insulating film such as No. 1, a polyimide alignment film is applied and rubbed. The upper and lower substrates treated as described above are sandwiched with a spacer such as glass fiber to give a thickness of about 5 μm. A liquid crystal 8 which is a mixture of a phenyl-chlorohexane-based nematic liquid crystal and a left-handed optically active substance is injected here. The twist angle of the liquid crystal molecular alignment is 2400, and the viscosity of the liquid crystal is about 20 cp.

The optical compensation cell 5 attached to the upper section of the drive cell does not have an electrode part, and the liquid crystal is the same nematic liquid crystal as the drive cell, but has an optically active material with a right-handed twist opposite to that of the drive cell. Inject the mixed liquid crystal 9.

The cell thickness is about 5 μm, similar to the drive cell. Drive cell 4
FIG. 6 shows the relationship between the position of the optical compensation cell 5 and the twist angle of each liquid crystal molecule orientation. Note that 6a, 6
b is a polarizing plate, and 7 is a driving power source.

Furthermore, the response speed of the liquid crystal display panel of this example is approximately 150
m5ec, and the twist angle of the molecular orientation of the liquid crystal is 2400.

The drive waveforms of selected picture elements and non-selected picture elements are shown in Figure 1! a
lb and fbl, and in this embodiment, the number of scanning lines N is 10.
It's 0.

Also, the peak value Vp of the voltage applied to the selected picture element, and the peak value V of the voltage applied to the non-selected picture element. Figure 3 shows the measurement results of the contrast and 1/a-Vb/v when the relationship with the peak value Vb of the bias voltage is 0.05 Vp × Vb (0.2Vp V = V -2Vb p From the same figure, it is clear that Vb is 0 and I
It can be seen that high contrast can be obtained by setting the range from Vp to 0.18 Vp. That is, in this embodiment, good results can be obtained if the number of scanning lines N is set within 100.

FIG. 4 1a) shows the transmittance voltage characteristics when the frame frequency is changed by pressing the button in this embodiment. Here, Vb-Q,
It is 15 Vp. It is clearly seen that there is no change in steepness even when the frame frequency changes.

For comparison, FIG. 4 (bl) shows the transmittance voltage characteristics when the same liquid crystal display device is driven using the optimized bias method.

Furthermore, as a result of various experiments conducted on panels with different numbers of scanning lines N, we found that there is a difference between the peak value Vb of the bias voltage, the peak value V of the voltage applied to the selected picture element, and the number N of scanning lines. It has been found that good contrast can be obtained when there is a relationship. Furthermore, even better results were obtained by setting it within this range.

In addition, the twist angle of molecular orientation of liquid crystal is 180' to 270'.
A similar effect can be obtained by setting it to 0.

Furthermore, the effect was remarkable when the response speed was as high as 150 m5ec or less. Further, even if the waveform of the voltage applied to the picture element is shown in FIG. 2, the effect will not change.

In the above examples, a two-layer 5TN-LCD was shown, but similar results were obtained with a normal STN-L CD and a case where a uniaxial film was used instead of an optical compensation cell. .

From the above results, it is clear that when using the driving method using the voltage averaging method of the liquid crystal display device, the peak value V of the voltage applied to the selected picture element,
A good condition is obtained when the relationship of v5 - upper P is satisfied for a that satisfies the range between , the peak value vo of the voltage applied to the non-selected picture element, the peak value Vb of the bias voltage, and the number N of scanning lines. It became clear that a display image could be obtained.

<Effects of the Invention> As described above, according to the present invention, even when screen display is performed at a relatively low frame frequency, the steepness of the transmittance voltage characteristic can be maintained in a liquid crystal device having a fast response speed. Therefore, the response speed of the liquid crystal element can be increased as shown in FIG. 9 without deterioration of contrast.

Further, the driving method according to the present invention has the advantage that the driving circuit configuration and the liquid crystal display panel can be implemented without greatly changing the normally used ones, so that the manufacturing cost does not increase.

[Brief explanation of drawings]

FIG. 1 is a diagram showing a driving voltage waveform of a picture element according to the present invention, FIG. 2 is a diagram showing an example of a driving voltage waveform of a picture element according to the present invention, and FIG. 8 is a diagram showing display characteristics according to the driving method of the present invention. 4 is a diagram for comparing the display characteristics of the present invention and the conventional example, FIG. 5 is a schematic diagram of the drive circuit, FIG. 6 is a schematic diagram of the liquid crystal display device, and FIG. 7 is the conventional example. FIG. 8 is a diagram showing drive voltage waveforms of picture elements in an example, and FIG. 8 is a diagram showing display characteristics of a conventional example. l...Liquid crystal cell drive unit, 2...Scanning electrode, 8...
Signal electrode, 4... Drive cell, 5... Optical compensation cell. 6a, sea lion polarizing plate, 7... drive power supply, 8.9
···liquid crystal.

Claims (1)

[Claims of Claims] 1. In a driving method using a voltage averaging method for a liquid crystal display device, a ratio a (=Vp/Vb) between a peak value Vp of a voltage applied to a selected picture element and a peak value Vb of a bias voltage. But, (√(N)+1/1.1)>a>(√(N)+1/2)
However, N is a driving method for a liquid crystal display device that satisfies a range of the number of scanning lines.