JPS5849873B2 - liquid crystal display device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a display method using the electro-optical effect of liquid crystal, and particularly to a liquid crystal display device that has a large number of display points and displays them at a substantially high speed in a time-division manner. It is.

Liquid crystals have the disadvantage of slow response and the need to take a long time to apply voltage (hereinafter referred to as address time) for display, making it difficult to display multiple display points in a time-sharing manner. Met.

The present invention has been made in view of these drawbacks of conventional liquid crystal display devices.

In a display device using a nematic liquid crystal, when trying to select a desired display point from a large number of display points on the display surface and display an arbitrary pattern, the simplest device is a matrix line as shown in Figure 1. It is a device that has an electrode structure and can display information in a time-division manner.

That is, in an element formed by sandwiching a liquid crystal 5 between substrates 1 and 2, horizontal electrode groups 3a, 3b,
3c and the vertical electrode groups 4a, 4b, 4c are sequentially scanned at fixed time intervals, and the display points formed by the intersections of both electrode groups are sequentially excited in accordance with the given signal, and moreover, it is visible to the viewer's eyes. is a device in which all points appear to be excited at the same time and the desired pattern is displayed.

The reason why many points that are excited at different times appear to be excited at the same time is due to the afterimage effect of the eye.

In order to utilize the afterimage effect of the eye, the period of excitation of one point needs to be at most 33 ms (30 Hz).

If the period becomes longer than this, flickering becomes visible and the excitation does not appear to be continuing.

In other words, after exciting a point, the point is at least 3
Reexcitation within 3 ms is the minimum condition for complete display on a time-division basis.

However, since the liquid crystal does not respond immediately to an excitation signal, it is necessary to take a long time for excitation (address time), and thus such a scanning type display has been considered inappropriate.

The address time of a normal liquid crystal is about 10ms, and if this is to be scanned in time division, the minimum is 1.0771. 4 from the first point since we have to take the address time of s.
The scanning time is already 40 mB at the th display point, and it is impossible to display any more points without flickering.

In order to solve this problem, a method has been proposed in which nematic liquid crystals have a memory effect.

That is, the idea is to maintain the light scattering of the nematic liquid crystal due to the electro-optical effect that occurs when a voltage is applied, even after the voltage is removed.

In this way, until the voltage is applied to the fourth display point and it is excited, the first display point will continue to be in a light scattering state, so the fourth display point will light up at the same time. It is recognized that

This method can be easily realized by mixing a cholesteric liquid crystal substance or a phenol compound with nematic liquid crystal.

Due to the action of these additives, the light scattering caused by the application of voltage persists for a long time even after the voltage is removed.

However, at present, this mixture has the disadvantage that the response time becomes slower, the addressing time has to be longer, and the time required to display all the display points becomes longer as a whole.

Another disadvantage is that in order to return the scattering state to the non-lighting state before voltage application, a high alternating current voltage of several tens of kHz or more must be applied, which complicates the electrical circuit.

Another proposal is to connect an electrical element such as a diode or a capacitor in series to each display point of the liquid crystal device.

As a result, the voltage is continuously applied to the liquid crystal for a time sufficient for the liquid crystal to respond with an address time that is shorter than the time for the liquid crystal to respond.

However, as the number of display points increases, this method requires an extremely large number of electrical elements, making it complicated and expensive.

Furthermore, the method of connecting the above-mentioned electrical elements to each display point does not easily constitute a matrix electrode, and is difficult to implement in reality.

As described above, the present invention provides a method for driving a liquid crystal display device that compensates for the conventional drawbacks of the liquid crystal display device caused by the slow response time of the liquid crystal, and enables display at substantially high display speed. It is a purpose.

Another object of the present invention is to provide a liquid crystal display device that is capable of displaying a large number of characters or numbers and arbitrary graphs, etc., using a structure consisting of a liquid crystal panel having simple matrix electrodes and a simple time division operation circuit. The object of the present invention is to provide a driving method.

The liquid crystal display device of the present invention is characterized in that the pulse has a width that allows a plurality of pulses to be applied during the time required to excite the liquid crystal.

The present invention is based on the discovery that light scattering of nematic liquid crystals due to applied voltage exhibits a unique hysteresis phenomenon.

In other words, apparently no light scattering occurs, and the voltage responsiveness of the liquid crystal in an unexcited state may differ significantly depending on what kind of voltage the liquid crystal has been subjected to up to that point. It's cold.

This phenomenon will be specifically explained with reference to FIG.

■ in the diagram indicates a pulse voltage waveform applied to the liquid crystal, with a pulse width of 1 ms and a pulse interval of 1 ms.

(2) is the response waveform of light scattering of the liquid crystal. As shown in this figure, no light scattering occurs when voltage is first applied.

However, if the application is repeated several times at the same interval, light scattering will occur and the saturation value will soon be reached.

In other words, the liquid crystal at point A, where no voltage has ever been applied, and the liquid crystal at point B, after the lms pulse voltage has been applied several times, are apparently the same in that there is no light scattering, but after that, There is a significant difference in the responsiveness of

The reason for this is not clear at present, but the applied voltage changes the alignment state within the liquid crystal, and the applied voltage is removed before it reaches the point of scattering light, and then it is completely relaxed. It is presumed that since the next pulse voltage is applied before the next pulse voltage is applied, the change in orientation progresses, eventually leading to light scattering.

This is considered to be because the electro-optic effect of the liquid crystal does not depend on the height of the pulse voltage, but rather on the total amount of electrical energy (that is, the effective voltage) over a certain period of time.

゛One proof of this is that if a high frequency voltage higher than the upper limit frequency (cutoff frequency fc) at which liquid crystal scattering occurs during the rest time between pulses is applied,
Even if the voltage level is high, no light scattering occurs no matter how many times the pulse is applied.

That is, this is because the accumulation of changes in orientation due to pulses is canceled out by the high frequency voltage.

On the other hand, if a pulse voltage with a frequency equal to or lower than fc is applied during the rest period, the cumulative effect will not be hindered even if the voltage level is lower than the voltage used for display.

The above-mentioned hysteresis phenomenon is characterized in that it appears even if the pulse interval is considerably long.

For example, if the pulse width is 1 ms, the pulse interval is 200
Even at ms, this phenomenon was observed and light scattering occurred.

By utilizing the above-mentioned phenomenon, which the inventors call the cumulative effect of dynamic scattering of liquid crystals, it is possible to shorten the addressing time. It has been discovered that a method for driving a liquid crystal display device that can display images at a substantially high speed can be realized.

Another important feature of the present invention is that the voltage value at which light scattering due to this cumulative effect begins is constant and does not depend on the thickness of the liquid crystal layer.

This is a great advantage in device fabrication because no special measures are required for the thickness of the liquid crystal layer in order to obtain this effect.

A more specific example of the present invention will be explained.

Now, consider the case where a certain character is displayed on a display device as shown in FIG. 1 having matrix electrodes of 5 columns x 7 rows.

Five rows of electrodes are selectively turned on simultaneously. A line sequential scanning method is used in which seven rows of electrodes are sequentially scanned while an off signal is applied.

In this case, in order to avoid flickering to the human eye, it is necessary to excite the first line and then excite the first line again within 30 ms at the latest.

Therefore, the address time for one row is about 4 m3.

In a normal liquid crystal, it is difficult to excite it at 4 mB unless the thickness of the liquid crystal layer is made extremely thin or a high voltage is not applied.

However, if we continue to apply voltage repeatedly for 4 ms to each row, it will gradually become excited (cumulative effect) due to the above-mentioned hysteresis phenomenon.
light scattering occurs.

Even if it is necessary to repeat the application three times before light scattering occurs, it takes 60 to 7o1T to cause light scattering in all 7 rows.
In Ls, this degree of delay is practically no problem.

The intensity of light scattering repeatedly increases and decreases as shown in Figure 2 (2), but due to the afterimage effect of the eye, the intensity is not felt.

Also, the 7th row appears to be excited at the same time.

In order to increase the number of display lines, it is necessary to further shorten the addressing time per line, but according to the method of the present invention, it is possible to excite the liquid crystal with one voltage application. 0. 2
Since excitation can be achieved by repeated application with an address time of about ms to 2 ms, it is possible to display up to about 100 lines without flickering.

Furthermore, by performing line sequential scanning using an external memory as shown in FIG. 3, it is easy to increase the number of characters in one line, resulting in a display device that can simultaneously display a large number of characters over the entire screen.

The period at which the pulse voltage is applied can be arbitrarily set to 30 Hz or more, which is the lower limit for displaying without flickering as described above, but if this frequency is increased too much,
The most desirable range is about 50 to 100 Hz because the applied pulse width is shorter and the time required for lighting is longer.

According to research conducted by the inventors, a material with a short rise delay time (td) for a single pulse as shown in FIG. 4 is desirable.

td is. It also depends on the applied voltage, liquid crystal film thickness, etc.

The higher the applied voltage and the thinner the film thickness, the shorter td becomes, and the number of characters displayed using the cumulative effect can be increased accordingly.

When viewed from the characteristics of liquid crystal, the cutoff frequency (fc
) is higher and td is shorter, the cumulative effect becomes more pronounced and a larger number of characters can be displayed.

The relationship between fc and td and the number of lines that can be displayed without flickering is as shown in Figure 5, where fc and td are shown by the dashed curve.
The number of lines displayed increases for materials with a higher td, and the number of lines displayed increases for materials with a shorter td.

In order to increase fc or shorten td, it is effective to dope the liquid crystal compound with about 0.05 to 1% of a quaternary ammonium salt such as 1-hexadecylpyridinium bromide.

In the structure shown in Fig. 3, the thickness of the liquid crystal layer was determined using a material in which a mixture of equal weights of methoxybenzylidene parabutylaniline and ethoxybenzylidene parabutylaniline was doped with 0.2% of the above 1-hexadecylpyridinium bromide. When a display panel was made with a length of 9 m and characters were displayed with an applied voltage of 60 V and a pulse width of 1.0 ms,
It was possible to display up to 30 lines with high contrast and no flickering.

The time required for lighting at this time was less than Looms.

As described above, the display device using the cumulative effect as in the present invention imparts new functions to a liquid crystal display device that can be easily enlarged.

In addition to displaying characters and graphs in a fixed pattern as in the past, it is now possible to display a large number of characters, numbers, etc., and can be applied to computer terminal display devices, educational display devices, etc. .

Although the matrix display device has been described so far, the present invention can also be applied effectively to a multi-digit time-division display of numeric displays in desktop electronic computers.

In this case, it can be expected that reliability will be improved by reducing the number of terminals and manufacturing costs will be reduced by reducing the number of circuit components.

The explanation so far has been about display devices that utilize dynamic scattering of nematic liquid crystals, but we have also discussed display devices that utilize changes in the transmission, reflection, and polarization characteristics of incident light due to changes in the arrangement of liquid crystal molecules caused by electric fields. The present invention also brings about similar effects.

[Brief explanation of the drawing]

Fig. 1 is a configuration diagram of a liquid crystal display panel having matrix electrodes, Fig. 2 is a curve diagram showing the relationship between the voltage waveform applied to the liquid crystal display device according to the present invention and the corresponding light scattering output, and Fig. 3 is a block diagram showing a specific embodiment of the present invention, FIG. 4 is an explanatory diagram for defining the rise delay time (td), and FIG.
td), a curve diagram showing the relationship between the cutoff frequency (tc) and the number of displays. Explanation of symbols, 1, 2...Substrate, 3a, 3b
, 3c... horizontal electrode, 4a, 4b, 4c...
... Vertical electrode, P ... Voltage waveform, L ...
...Light scattering response waveform, 6...Liquid crystal display element,
7... Row electrode group, 8... Column electrode group, 9
-...Scanning circuit, 10...Character generator,
11... Code memory, 12... Serial to parallel conversion circuit, 13... Serial input signal.

Claims (1)

[Claims] 1. In a method for driving a liquid crystal display device in which a pulse voltage is applied to a liquid crystal layer in a time-division manner and a desired display point is selected from a large number of display points to display an arbitrary pattern, the frequency of the pulse voltage is 30 Hz or more. The method includes applying a plurality of pulse voltages to the liquid crystal layer before light scattering occurs, and then applying a pulse voltage to the liquid crystal layer to cause light scattering and subsequently to reach saturation. A method for driving a liquid crystal display device.