JPH04249216A - Method for driving liquid crystal display device - Google Patents

Abstract

PURPOSE:To efficiently erase a display and to improve the display contrast by eliminating a temporary increase in the light transmissivity of picture elements in an erasure period. CONSTITUTION:A matrix liquid crystal display device which uses a storable liquid crystal layer made of a mixture of high-molecular-weight liquid crystal and low-molecular-weight liquid crystal is driven. Display operation includes an erasing operation (erasure periods T31 and T41) for placing the whole screen in a light scattering state and a writing operation (writing periods T32 and T42) for inverting the optical states of respective picture elements selectively into a light transmission state while the residual picture elements are held in their right before states. The erasing operation is performed by applying a DC voltage which has a constant voltage value aE (e.g. a=3 and E=30V) and does not vary throughout the erasure periods. For AC driving, the polarity of the applied DC voltage is inverted in the erasure periods T31 and T41.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for driving a liquid crystal display device, which is applied to a liquid crystal display device using a liquid crystal layer having a so-called memory property, which is composed of a mixture of a high molecular weight liquid crystal and a low molecular weight liquid crystal.

0002

[Prior Art] In a TN (Twisted Nematic) type or STN (Super Twisted Nematic) type liquid crystal display device, a plurality of substrates are arranged on each surface of a pair of substrates that sandwich a liquid crystal so as to cross each other and face each other. A scanning electrode and a signal electrode are formed, and a voltage is applied to the scanning electrode in a time-sharing manner, so that the voltage is selectively applied to the liquid crystal of each pixel formed at the intersection of the scanning electrode and the signal electrode. The voltage averaging method has been widely applied.

However, in this driving method, if the number of scan electrodes to which voltage is applied in a time-division manner is increased in an attempt to increase display capacity, the time interval at which one scan electrode is selected becomes longer. , the unselected time becomes longer, and as a result, the difference in average applied voltage for one frame between the on pixel and the off pixel becomes smaller. Therefore, the contrast will be reduced.

On the other hand, the above-mentioned problem can be solved in principle by using a liquid crystal that has so-called memory, which maintains its previous state (for example, a light-transmitting state or a light-scattering state) even when the applied voltage is removed. be done. In other words, if a simple matrix type liquid crystal display device is constructed using liquid crystals with the above-mentioned memory property, the previous state is maintained in each pixel even when no voltage is applied, so even if the number of scanning electrodes is increased, Even if the display duty is reduced, the contrast does not deteriorate, and therefore a liquid crystal display device with a large display capacity can be realized. That is, since each pixel is reliably turned on/off even during a period when no voltage is applied, it is possible to increase the display capacity without causing a decrease in contrast.

[0005] A simple matrix liquid crystal display device using such a liquid crystal with memory properties is disclosed in, for example, ``Japanese Patent Laid-Open No. 61-10
3124 Publication” and “W.A. Crossland,
S. Canter, '85 Society for
Information Display
National Symposium Diges
to of Technical Papers, pp.
．． 124-127, (1985), session:8
．． 2,” etc. These documents disclose techniques that use ferroelectric liquid crystals and smectic dynamic scattering liquid crystals and perform display by simple matrix drive by utilizing the memory properties of these liquid crystals.

However, in the above liquid crystal display device,
When using a ferroelectric type liquid crystal, manufacturing is difficult as it requires cell gap adjustment with submicron precision, and when using a smectic dynamic scattering type liquid crystal, it is difficult to manufacture.
There are problems such as the need to apply a voltage exceeding 0V, so it has not been put into practical use. Incidentally, recently, a liquid crystal display device that achieves the above-mentioned memory by applying a mixture of high molecular weight liquid crystal and low molecular weight liquid crystal to the liquid crystal layer has been proposed (for example, "T. Kajiyama et al.,
Chemistry Letters, pp817-
820, 1989'', ``Japanese Unexamined Patent Publication No. 2-127494'', and ``Japanese Unexamined Patent Publication No. 2-193115''. ). The above liquid crystal layer has a high frequency (e.g. 1
When a relatively high voltage of kHz) is applied, a light transmitting state occurs where incident light is transmitted as is, and when a relatively high voltage of a low frequency (for example, 1 Hz to DC) is applied, a light scattering state occurs where incident light is scattered. Furthermore, when a relatively low voltage is applied, the previous state (light transmitting state or light scattering state) is maintained.

In this way, when constructing a simple matrix type liquid crystal display device in which the above mixture is applied to the liquid crystal layer,
Since the three states of the light transmission state, the light scattering state and the storage state can be selected, it is expected that a liquid crystal display device with a large display capacity and high contrast as described above can be realized. However, when applying the above mixture to a liquid crystal layer to construct a simple matrix type liquid crystal display device and attempting to utilize the memory properties of the liquid crystal layer, it is necessary to select the light transmitting state and light scattering state of each pixel, and Since it is necessary to put the pixels corresponding to the scan electrodes that are not selected among the plurality of scan electrodes into the above memory state, the two types of frequencies are switched, and the pixels corresponding to the scan electrodes that are selected and It is necessary to apply different voltages to pixels corresponding to non-selected scan electrodes.

A simple matrix driving method using signals having two types of frequencies is described, for example, in "M. Nagata,
H. Nakamura: Mol. Cryst. Liq
．． Cryst, vol. 139 (1986), pp14
3", etc., but this method is not a method for controlling the three states of pixel on, off, and storage, but rather it is not a method for controlling the three states of pixel on, off, and storage, but also forcibly turning on or off the pixel corresponding to the unselected scan electrode. Therefore, it was not possible to utilize the memory properties of liquid crystals as described above.

[0009] In a liquid crystal display device in which the above-mentioned mixture is applied to the liquid crystal layer, a driving method utilizing the memory property thereof has not been known, and therefore, a matrix type liquid crystal display in which the above-mentioned mixture is applied to the liquid crystal layer It was difficult to realize the device. Therefore, in Japanese Patent Application No. 2-403913 previously filed by the applicant, the inventors of the present invention have improved the memory properties of the above-mentioned mixture in order to realize a matrix type liquid crystal display device in which the above-mentioned mixture is applied to the liquid crystal layer. A driving method for the liquid crystal display device using this method is proposed.

This driving method includes an erasing operation that initializes the entire display screen to a light transmitting state or a light scattering state, and selectively changing the optical state (light transmitting state or light scattering state) of each pixel of the erased screen. This includes a write operation in which the display is written in reverse. In the write operation in this proposed example, for example, a plurality of scan electrodes are selected line-sequentially, and the selected scan electrodes are applied with an effective voltage (a-1)E (for example, a=3, E
A bipolar square wave (=30V) is applied, and no voltage is applied to the unselected scan electrodes that are not selected. In addition, in a plurality of signal electrodes arranged to cross and oppose a plurality of scanning electrodes with a liquid crystal layer in between, a scanning waveform is generated for a signal electrode corresponding to a pixel whose optical state is to be changed in a selected scanning electrode. A bipolar square wave of an effective voltage E is applied with the phase opposite to that of the scanning waveform, and a bipolar square wave of an effective voltage E is applied with the same phase as the scanning waveform to the signal electrode corresponding to the pixel whose optical state is to be maintained. applied. The voltage applied to each pixel is the difference between the voltages applied to the scanning electrode and the signal electrode. Therefore, during the write operation, the selected scan electrode
The pixel whose optical state is to be changed has an effective voltage aE (a
A bipolar square wave (90 V when E = 3 and E = 30) is applied to the pixel that is to maintain its optical state.
Effective voltage (a-2) E (3 when a=3, E=30
0V) is applied. In addition, in the non-selected scanning electrode, the effective voltage E( For example, 30V
) will be applied.

As a result, in the selected scanning electrode,
The bipolar square wave of the effective voltage aE is applied only to the pixel corresponding to the signal electrode to which a signal with a waveform having a phase opposite to that of the scanning waveform is applied, and the optical state of this pixel changes. On the other hand, to pixels with other signal combinations, an effective voltage E or (a-2)E (both 30V when a=3 and E=30V) is applied, and the optical state of such pixels is It is kept in its previous state.

For example, assume a display mode in which the above erasing operation is performed with the entire display screen in a light scattering state, and the writing operation is performed by selectively inverting the optical state of each pixel to a light transmitting state. In this case, for example, in the erasing operation, a low frequency signal (for example, DC to 1 Hz) having the waveform at the time of selection, that is, a bipolar square wave of effective voltage (a-1)E, is applied to all scanning electrodes. This is done by applying a bipolar square wave of effective voltage E to all signal electrodes, which is in opposite phase to the waveform applied to the scanning electrodes. The write operation is performed using a signal having the above-mentioned waveform and having a high frequency (for example, 1 kHz).

The operation in this display mode is shown in FIG.
3. FIG. 13(a) shows changes in the voltage applied to a pixel to be in a light transmitting state, and FIG. 13(b) shows the light transmittance of the pixel. The period T1 is an erase period in which a bipolar square wave of a low frequency effective voltage aE is applied to the pixel. As a result, the light transmittance is as shown in Figure 13(b).
, it decreases as indicated by reference numeral a1. Further, period T2 is a writing period, and a high frequency square wave is applied to the pixels during this period. Period T3 within period T2 is a period in which the scanning electrode corresponding to the pixel is selected, and during this period, the light transmittance of the pixel is increased with reference symbol a2 by applying a bipolar square wave of effective voltage aE. Rise as shown. The remaining period in period T2 is a period in which the scanning electrode corresponding to the pixel is not selected, and the effective voltage E (or (a-2)E. FIG. 13 shows the case where a=3,
a-2) E=E. ), the state of the pixel is maintained at the previous state.

A display mode in which the erase operation is performed by initializing the entire display screen to a light-transmitting state, and the write operation is performed by selectively inverting each pixel to a light-scattering state, is used for both erasing and writing. This is achieved by exchanging frequencies. The operation in this display mode is shown in FIG. 14, which has a similar illustration to FIG. 13 described above. In this case, period T11 is an erase period, period T12 is a write period, and the state of the pixel is reversed to a light scattering state in period T13 within period T12.

[0015] By the above method, a simple matrix type liquid crystal display device can be realized while utilizing the memory properties of a liquid crystal layer composed of a mixture of high molecular weight liquid crystal and low molecular weight liquid crystal. Thereby, even if the number of pixels increases, there is no decrease in contrast, and a high-resolution display device can be provided. Moreover, since the above-mentioned mixture itself has a film-forming ability and is self-supporting, there is no need to spray spacers for adjusting the cell gap, and therefore manufacturing is easy. Furthermore, since it can be driven at a lower voltage than when using a smectic dynamic scattering liquid crystal, it is easier to put it into practical use.

[0016]

By the way, in liquid crystal display devices, AC driving is generally performed in which an AC voltage is applied to the liquid crystal layer. This is to prevent electrolysis that occurs when a DC electric field is applied to the liquid crystal material. This also applies to the case of a mixture of the above-mentioned high molecular weight liquid crystal and low molecular weight liquid crystal, and it is necessary to perform AC driving.

On the other hand, a square wave is generally used for the drive signal in consideration of simplifying the configuration of the drive circuit.
If AC driving is performed using a square wave, a bipolar square wave will be applied to the liquid crystal layer. By the way, in order to bring the liquid crystal layer to which the above mixture is applied into a light scattering state, it is necessary to apply a voltage at a low frequency of 100 Hz or less. However, even when the frequency is low, as long as a square wave is used, a sudden voltage change occurs when the polarity is reversed. Therefore, when the polarity of the applied voltage is reversed, especially during a period in which a square wave with a large effective voltage aE is applied to change the optical state of the liquid crystal layer to a light scattering state, due to its high frequency component,
b), respectively reference numeral a3 in Fig. 14(b).
, a4, a temporary increase in light transmittance occurs.

This temporary increase in light transmittance reduces the efficiency of reversing the optical state of the liquid crystal layer, which leads to a decrease in display contrast. SUMMARY OF THE INVENTION Therefore, an object of the present invention is to solve the above-mentioned technical problems and drive a matrix type liquid crystal display device in which a mixture of high molecular weight liquid crystal and low molecular weight liquid crystal is applied to the liquid crystal layer while utilizing the memory property of the liquid crystal layer. It is an object of the present invention to provide a method for driving a liquid crystal display device that can improve the display contrast.

[0019]

[Means for Solving the Problems] A method for driving a liquid crystal display device according to claim 1 to achieve the above object is a method for driving a liquid crystal display device comprising a mixture of a high molecular weight liquid crystal and a low molecular weight liquid crystal, and having an effective voltage of at least a predetermined value. A light transmitting state is achieved by applying an alternating current voltage having a predetermined frequency or more, and a light scattering state is achieved by applying an alternating current voltage having an effective voltage greater than or equal to the predetermined value and less than the predetermined frequency, or a direct current voltage having an effective voltage greater than or equal to the predetermined value. and a liquid crystal layer that maintains its previous optical state for at least a certain period of time when a voltage lower than the predetermined value is applied; a signal electrode;
A method for driving a liquid crystal display device that performs display by applying a voltage signal to the scanning electrode and the signal electrode, and setting the intersection of the scanning electrode and the signal electrode to a pixel in a light transmitting state or a light scattering state, the method comprising: An AC voltage having an effective voltage equal to or higher than the predetermined value or a DC voltage equal to or higher than the predetermined value between the scanning electrode and the signal electrode in order to erase the display of the entire display screen by putting the pixels in a light transmitting state or a light scattering state. and applying a scanning signal of a predetermined waveform by selecting the plurality of scanning electrodes line-sequentially, each signal electrode having a scanning electrode corresponding to a pixel whose optical state is to be changed selected. During this period, a signal having a waveform is applied to the liquid crystal layer in relation to the scanning signal, and an AC voltage with an effective voltage of the predetermined value or more or a DC voltage of the predetermined value or more is applied to the liquid crystal layer;
During a period when a scanning electrode corresponding to a pixel whose optical state is to be maintained is selected, an AC voltage having an effective voltage less than the predetermined value or a DC voltage less than the predetermined value is applied to the liquid crystal layer in relation to the scanning signal. a writing operation in which the display is written by selectively changing the optical state of the pixels constituting the display screen by applying a waveform signal applied to each pixel during the erasing operation. and the voltage waveform applied to the pixel whose optical state is to be changed during the write operation, one of which has a waveform of the predetermined frequency or higher, and the other one which changes at least the optical state of the pixel. A method for driving a liquid crystal display device, characterized in that the voltage is a direct current whose voltage value does not change over a period of time, and a waveform whose polarity is inverted every time one screen is displayed.

[0020] A voltage applied to the pixel during the erasing operation,
The voltage applied to the pixel whose optical state is changed during the write operation is preferably two to three times the voltage applied to the pixel whose optical state is maintained during the write operation.

[0021]

[Operation] According to the above driving method, by combining the erasing operation for erasing the entire display screen and the writing operation for writing an image on the erased display screen, the matrix type Driving of the liquid crystal display device is achieved. For example, assume that an image is written by setting the entire screen to a light-scattering state, erasing the display, and selectively inverting each pixel to a light-transmitting state. At this time, the erase operation is performed by applying to all pixels a DC voltage of a predetermined value or higher, the voltage value of which does not change during the erase operation period. As a result, all the pixels of the liquid crystal layer made of a mixture of high molecular weight liquid crystal and low molecular weight liquid crystal become in a light scattering state. During the erase operation, the polarity of the voltage applied to the pixel is not reversed, so there is no temporary increase in the light transmittance of the liquid crystal layer during the transition from the light transmitting state to the light scattering state, which improves efficiency. You can erase the display automatically.

Furthermore, the polarity of the DC voltage applied to the pixels during the erasing operation is reversed for each display screen. This results in
Since alternating current driving is achieved when viewing through multiple display screens, electrolysis of liquid crystal molecules does not occur. In the write operation, a plurality of scan electrodes are selected line-sequentially. In each signal electrode, when a scanning electrode corresponding to a pixel to be in a light transmitting state is selected, the pixel has an effective voltage equal to or higher than the predetermined value in relation to the signal applied to the scanning electrode. A signal with a predetermined waveform is applied so that an alternating voltage of a predetermined frequency or higher is applied. moreover,
When a scanning electrode corresponding to a pixel whose light scattering state is to be maintained is selected, an AC voltage having an effective voltage of less than the above-mentioned predetermined value or less than the above-mentioned predetermined value is applied to the pixel in relation to the signal applied to the scanning electrode. A signal with a predetermined waveform is applied such that a DC voltage of . This selectively inverts the optical state of each pixel to achieve writing of an image.

[0023] When display is erased with the entire display screen in a light transmitting state and display is performed by selectively setting each pixel in a light scattering state, the voltage waveform applied to the pixel during the erasing operation is The frequency is set to be equal to or higher than the predetermined frequency. The voltage waveform applied to the pixel whose optical state is to be inverted during a write operation is applied at least to a period during which the optical state of the pixel is changed (for example, a period during which a scanning electrode corresponding to the pixel is selected). A direct current voltage whose voltage value does not change may be used. The polarity of the DC voltage applied to the pixels during this writing operation is reversed for each screen, thereby preventing electrolysis of liquid crystal molecules.

Note that the voltage applied to the pixel during the erase operation and the voltage applied to the pixel whose optical state is changed during the write operation are applied to the pixel whose optical state is maintained during the write operation. By setting the voltage to be two to three times the voltage applied to the pixel, the applied voltage when maintaining the optical state of the pixel is kept below the predetermined value, and when changing the optical state, a high voltage is applied to change the optical state of the pixel. It can be changed in a short time.

[0025]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Examples will be explained in detail below with reference to the accompanying drawings showing examples. FIG. 2 is a sectional view showing the basic structure of a liquid crystal display device to which a method for driving a liquid crystal display device according to an embodiment of the present invention is applied. A pair of transparent substrates 1 and 2 facing each other
A plurality of scanning electrodes 3 and a plurality of signal electrodes 4 each formed of a band-shaped transparent electrode film are formed on the electrodes 3.
, 4, a liquid crystal layer 5 is formed between them. The scanning electrode 3 and the signal electrode 4 are arranged on the transparent substrate 1, so as to cross each other and face each other.
A pattern is formed on each surface of the electrode 2, and the intersection of each electrode 3 and 4 becomes a pixel.

For example, a mixture of a high molecular weight liquid crystal having siloxane as a main chain and a group exhibiting liquid crystallinity as a side chain and a low molecular weight liquid crystal exhibiting nematic properties is applied to the liquid crystal layer 5. Examples of such a mixture include a mixture of poly(4-cyanophenyl 4'hexyloxybenzoate methylsiloxane) and E63 (manufactured by Merck Japan).

The liquid crystal layer 5 has a frequency (eg, 1 kHz) greater than or equal to a predetermined frequency (eg, 100 Hz) between the scanning electrode 3 and the signal electrode 4, and an effective voltage (eg, 1 kHz) greater than or equal to a predetermined value (eg, 50 V). For example, by applying an alternating current voltage of 90 V), a light transmitting state (transmittance = 80% or more) can be achieved in which the light incident on the liquid crystal layer 5 is transmitted as it is. Furthermore, when a high voltage having a relatively low frequency (1 Hz to DC) and higher than the above-mentioned predetermined value is applied, this liquid crystal layer 5 enters a light scattering state (transmittance = 5) in which incident light is scattered.
% or less). Further, when a voltage lower than the predetermined value (for example, 30 V) is applied, the liquid crystal layer 5 maintains its previous state (light transmitting state or light scattering state). That is, this liquid crystal layer 5 has memory properties. The above predetermined value varies depending on the mixture constituting the liquid crystal layer.

The liquid crystal layer 5 itself has a film-forming ability, and can be formed into a film by applying a liquid crystal material onto the substrate 1 on which the transparent electrodes 3 are formed, for example. Therefore, since the liquid crystal layer 5 has self-supporting properties, the cell gap can be strictly controlled to form large-area cells without having to scatter spacers on the substrate as in the case of using a liquid crystal layer. can be easily manufactured. Further, there is no need for cell gap adjustment with submicron precision or alignment films for monodomain formation, which are required in cells using ferroelectric liquid crystals. Moreover, since it contains liquid nematic liquid crystal (low molecular weight liquid crystal), the local viscosity of the liquid crystal layer is low, and it can be used at lower voltages than a smectic dynamic scattering cell that uses smectic liquid crystal, which has relatively high crystallinity. It is possible to drive.

The transparent substrates 1 and 2 can be made of, for example, glass or a resin film, but since the liquid crystal layer 5 itself has a film-forming ability as described above, it is preferable to use a resin film. This is advantageous from the viewpoint of making the device thinner and lighter, and furthermore, a flexible display device can be easily constructed. Suitable materials for this resin film include polyethylene terephthalate, polyethylene naphthalate, and polyether sulfone. Further, as the transparent electrode film constituting the scanning electrode 3 and the signal electrode 4, for example, indium tin oxide (ITO) is used.
) film or tin oxide film can be applied.

FIG. 1 is a simplified plan view showing the scanning electrode 3 and signal electrode 4 in the above liquid crystal display device.
In order to explain the driving method of the present invention, one scanning electrode 3
signal waveforms Ss and Sd applied to one signal electrode 4a and one signal electrode 4a, as well as scanning electrode 3a and signal electrode 4a.
The voltage waveform Sp applied to the pixel P formed at the intersection of
are shown at the same time. In FIG. 1, five scanning electrodes 3 and five signal electrodes 4 are shown for ease of explanation, but in reality, for example, there are about 50 to 200 electrodes. Generally, the limit on the number of electrodes varies depending on the characteristics of the liquid crystal layer.

A signal with a predetermined waveform corresponding to the display content is applied to the scanning electrode 3 and the signal electrode 4 from a drive circuit (not shown), so that each pixel formed at the intersection of the scanning electrode 3 and the signal electrode 4 This state is considered to be a light transmitting state or a light scattering state, and display is thereby achieved. In this embodiment, such a display operation includes an erasing operation in which the entire display screen is initialized to a light-scattering state and the display is erased, and the state of necessary pixels on the initialized display screen is reversed to a light-transmitting state. This also includes a write operation for writing an image to be displayed while maintaining the state of the remaining pixels in a light scattering state.

A pixel P to which a scanning signal having a signal waveform Ss is applied from the scanning electrode 3a and a signal waveform Sd from the signal electrode 4a senses a voltage waveform Sp corresponding to the difference between the waveforms Ss and Sd. This voltage waveform Sp includes a high frequency portion (for example, 1 kHz) higher than a predetermined frequency (for example, 100 Hz) and a DC portion where the voltage value is constant and does not change. Period T3 during which DC voltage is applied to pixel P
1, T41 is a period for erasing the entire display screen, and during this period, the voltage value aE or -aE (for example, a=3,
A DC voltage of E=30V) is applied, which causes the pixel P to
The state becomes a light scattering state. That is, the application of a sufficiently high DC voltage causes turbulent flow due to the movement of ions in the liquid crystal layer 5, and as a result, the light incident on the liquid crystal layer 5 is scattered, resulting in a light scattering state (cloudy state). however,
The DC voltage applied during period T31 and the DC voltage applied during period T41 have different polarities. On the other hand, periods T32 and T4 during which a high frequency AC voltage is applied to the pixel P
2, during periods T33 and T43 during which an AC voltage with a high effective voltage aE is applied, the liquid crystal molecules constituting the liquid crystal layer 5 enter a homeotropic alignment state due to the application of the high frequency voltage, thereby changing the state of the pixel P. The light scattering state is reversed to the light transmitting state. Further, during the periods T32 and T42 when an AC voltage with a relatively low effective voltage E is applied, the optical state of the pixel P is maintained at the previous state.

FIG. 3 shows the waveforms of each signal applied to the scanning electrode 3a and the signal electrode 4a during the erasing operation, and
3 is a waveform diagram showing voltage waveforms to be applied to a pixel P at the intersection of electrodes 3a and 4a to which signals of each waveform are applied. FIG. Applying a signal with the waveform shown in column X to the scanning electrode 3a,
When a signal having the waveform shown in the Y column is applied to the signal electrode 4a, a voltage having the waveform shown in the intersection of the X column and the Y column is applied to the pixel P. That is, a DC voltage that does not change at a voltage value (a-1)E is applied from the scanning electrode 3a over the erasing period T31, and from the signal electrode 4a, the polarity is opposite to that of the signal applied to the scanning electrode 3a. A DC voltage of voltage value E is applied. For example, if the display in the display operation for one screen is erased by applying the signal shown by the solid line in FIG. 3, the display in the display operation for the next screen is erased by applying the signal shown by the broken line in FIG. 3. This is done by That is, a signal whose polarity is inverted every screen is applied to the scanning electrode 3a and the signal electrode 4a, and as a result, a DC voltage aE whose polarity is inverted every screen is applied to the pixel P. .

Note that during the erasing period T31, etc., signals having the waveforms shown in column X in FIG. Waveform signals are applied equally. As a result, the optical states of all pixels constituting the entire display screen are simultaneously reversed from a light transmitting state to a light scattering state. FIG. 4 shows the waveforms of each signal applied to the scanning electrode 3 and the signal electrode 4 during the write operation, and the fact that the signal of each waveform is applied to the pixel P at the intersection of the applied electrodes 3 and 4. FIG. 2 is a waveform diagram collectively showing waveforms of voltages. This FIG. 4 shows a diagram similar to FIG. 3, and when the signals of the waveforms shown in the X column and the Y column are applied to the electrodes 3 and 4, respectively, the waveforms shown at the positions where each column intersects are shown. voltage will be applied to the pixel at the intersection of electrodes 3 and 4.

A plurality of scanning electrodes 3 are selected in line order, and a scanning signal XON is applied to each scanning electrode 3 when selected, and a scanning signal XOFF is applied when not selected. Further, the signal YON is applied to the signal electrode 4 during a period in which the scanning electrode 3 corresponding to the pixel to be in the light transmitting state (on state) is selected, and the signal electrode 4 is in the light scattering state (off state). The signal YOFF is applied during a period in which the scanning electrode 3 corresponding to the pixel to be in the state) is selected.

In this embodiment, the scanning signal XON applied to the scanning electrodes 3 line-sequentially selected has a sufficiently high frequency (for example, 1 kHz) and has an effective voltage (a-1)E.
(for example, 60V when a=3 and E=30V) is a bipolar square wave. Also, the scan electrode that is not selected (
The scanning signal XOFF applied to 3 (hereinafter referred to as "non-selected scanning electrode") is a DC signal that does not change at 0V. That is, no voltage is applied to the non-selected scan electrodes 3. Focusing on one scanning electrode 3a, the signal XON is applied during the period T33 of the write period T31 (see FIG. 1), and the signal XOFF is applied during the remaining period of the write period T32.
is applied.

On the other hand, each signal electrode 4 is in a light transmitting state (
The signal YON applied during the period when the scanning electrode 3 corresponding to the pixel to be turned on (on state) is selected, is an effective voltage E, and is a bipolar square wave having an opposite phase to the scanning signal XON. In addition, a signal YOFF is applied during a period when the scanning electrode 3 corresponding to a pixel that should maintain a light transmitting state is selected.
is an effective voltage E, which is a bipolar square wave in phase with the scanning signal XON.

Since the voltage between the scanning electrode 3 and the signal electrode 4 is applied to the pixel at the intersection of the scanning electrode 3 and the signal electrode 4, the selected scanning electrode 3 (to which the scanning signal XON is applied) Regarding the scanning electrode 3), the pixel corresponding to the signal electrode 4 to which the signal YON is applied has the reference symbol A1.
A voltage with a waveform shown in will be applied. That is,
The applied voltage is a bipolar square wave of effective voltage aE. Furthermore, regarding the selected scanning electrode 3, the signal YOFF
In the pixel corresponding to the signal electrode 4 to which is applied, a bipolar square wave of an effective voltage (a-2)E ((a-2)E=E when a=3) is generated, as indicated by reference symbol A2. is applied.

Furthermore, the unselected scanning electrode 3 (scanning signal XO
Regarding the scanning electrode 3) to which FF is applied, the signal Y
A waveform indicated by reference symbol A3, that is, a bipolar square wave of effective voltage E is applied to the pixel corresponding to the signal electrode 4 to which ON is applied. Further, to the pixel corresponding to the signal electrode 4 to which the signal YOFF is applied, a waveform indicated by reference numeral A4, that is, a bipolar square wave of the effective voltage E is applied.

When the signal YON is applied from the signal electrode 4 to the selected scanning electrode 3 in this way,
An AC voltage of an effective voltage aE (for example, 90 V) is applied to the pixel, and in other cases, an effective voltage E or (a
-2) An AC voltage of E (30 V in both cases when a=3) is applied. As mentioned above, when an alternating current voltage of the effective voltage aE is applied, the optical state of the liquid crystal layer 5 changes, and when an alternating current voltage of the effective voltage E or (a-2)E is applied, the liquid crystal layer 5 changes. Since the state before application of the voltage is maintained, the optical state of the liquid crystal layer 5 changes from the light scattering state (off state) to the light transmitting state (on state) only in the pixels to which the signals XON and YON are applied. For other pixels, the previous state is the light scattering state (off state)
As mentioned above, is held.

Note that in this embodiment, the polarity of the signal applied to each signal electrode during the above writing operation is also reversed for each screen, similar to the signal during the above erasing operation. It is not necessary to invert the polarity of the signal when the signal is input. FIG. 5 is a diagram for explaining the display operation, and FIG. 5(a) shows the voltage waveform applied to the pixel P in FIG. 1, and FIG. 5(b) shows the voltage waveform applied to the pixel P in FIG.
indicates a change in the light transmittance of the pixel P. In FIG. 5, it is assumed that the state of the pixel P is inverted. During the erasing period T31, a DC voltage with a large voltage value aE is applied, so during this period, the light transmittance rapidly decreases as indicated by reference symbol b1, and the pixel becomes in a light scattering state (cloudy state). becomes. During this period T31, the polarity of the applied voltage does not change, so a temporary increase in light transmittance does not occur during the clouding process.

Write period T32 following erase period T31
When entering, the period T34 before the scanning electrode 3a is selected
During this period, a bipolar square wave of an effective voltage E is applied to the pixel P, and during this period, the pixel P is maintained in a light scattering state. Furthermore, during the period T33 when the scanning electrode 3a is selected, a bipolar square wave with a large effective voltage aE is applied to the pixel P. As a result, the light transmittance of the pixel P increases as indicated by reference numeral b2 in FIG. 5(b), and the state of the pixel P is reversed to the light transmitting state. A period T35 after the period T33 is a period in which another scanning electrode 3 is selected, and during this period,
A bipolar square wave of an effective voltage E is applied to the pixel P, so that its optical state is maintained at its previous state, the light transmitting state.

The display of one screen is completed in the period F1 of one frame consisting of the periods T31 and T32, and the display of one screen is completed in the period T32.
A period F2 of one frame following 2 becomes a display period of the next screen. This display period F2 consists of an erase period T41 and a write period T42, and during the erase period T41, a signal having a waveform shown by a broken line in FIG. 3 is applied to the scanning electrode 3a and the signal electrode 4a. That is, signals whose polarities are inverted from those in the period T31 are applied to the scanning electrode 3a and the signal electrode 4a, respectively, and as a result, the polarity of the voltage applied to the pixel P is also inverted. During this period T41, the DC voltage (
Since the voltage value −aE) is applied, the period T31
As in the case of , the light transmittance does not temporarily increase during the clouding process.

Further, during the writing period T42, a signal whose polarity is inverted from that of the writing period T32 of the previous screen is applied to the scanning electrode 3a.
and the signal electrode 4a, and as a result, an AC voltage having a polarity opposite to that of the period T32 is applied to the pixel P during the write period T42. Of course, when the state of the pixel P is set to the light scattering state, the voltage waveform applied to the pixel P during the period T43 in which the scanning electrode 3a is selected becomes a bipolar square wave of the effective voltage E, and the state of the pixel P is is retained. The period following period T42 includes periods T31 and T42.
A voltage having a waveform similar to 32 is applied to the pixel P.

As described above, in this embodiment, the display screen erasing operation by initializing all the pixels constituting the display screen to the light scattering state applies a direct current voltage that does not change over the period during which this erasing operation is performed to the pixels. This is done by applying . Therefore, a temporary increase in light transmittance during the erasing period does not occur, and the transition to the light scattering state can be efficiently performed. This makes it possible to significantly improve display contrast. On the other hand, since the signal waveform applied to the pixel is inverted every time the display operation of one screen is performed, AC driving can be achieved as a whole through the display operation of multiple screens, and this makes up the liquid crystal layer 5. can prevent electrolysis of liquid crystal molecules. Further, since a square wave can be used for the drive signal, the drive circuit can also be configured with a simple circuit configuration and at low cost.

By selecting the value of the constant a close to 3, the voltage applied to the pixel whose optical state is changed is 2 to 3 times higher than the voltage applied to the pixel whose optical state is maintained.
It will be doubled. As a result, a voltage below the threshold voltage for causing a state transition is applied to the pixels whose optical state is to be maintained, and a high voltage is applied to the pixels for which the state transition is to occur, so that the state transition occurs in a short time. can be completed.

FIGS. 6 and 7 are waveform diagrams for explaining the operation of another embodiment of the present invention. In the description of this embodiment, reference will be made to FIG. 1 described above again. In this embodiment, the display screen is erased by initializing all pixels constituting the display screen to a light-transmitting state, and the state of each pixel of this initialized screen is selectively reversed to a light-scattering state. The display is written by doing this. That is, in this embodiment, the inverted display in the case of the first embodiment described above is achieved. In the description of this embodiment, the light transmitting state is defined as the off state, and the light scattering state is defined as the on state.

FIG. 6 shows the scan electrode 3 during the erase operation.
and signal waveforms applied to the signal electrodes 4, respectively, and are illustrated similarly to FIG. 3 above. That is, as shown in column X, a bipolar square wave of an effective voltage (a-1)E having a sufficiently high frequency (for example, 1 kHz) is equally applied to the plurality of scanning electrodes 3. In addition, as shown in the Y column, the plurality of signal electrodes 4 include scanning electrodes 3
A bipolar square wave of effective voltage E is equally applied, with opposite polarity to the signal applied to E. As a result, a bipolar square wave of the effective voltage aE having a sufficiently high frequency is applied to all pixels making up the display screen, as shown at the intersection of the X column and the Y column. It turns out. As a result, all pixels enter a light transmitting state (off state). The waveform of each signal may be inverted for each screen, and the signal waveform shown in FIG. 6 or a waveform obtained by inverting this signal waveform may be used when erasing any screen. FIG. 7 shows signal waveforms applied to the scanning electrodes 3 and the signal electrodes 4 during a write operation, and is illustrated similarly to FIG. 4. A plurality of scan electrodes 3 are selected line-sequentially, and a signal that is a DC voltage having a constant voltage value (a-1)E or -(a-1)E is applied to the selected scan electrode 3 over the selection period. XON is applied. Then, a signal XOFF which does not change at 0V is applied to the non-selected scan electrodes.

On the other hand, focusing on the scanning electrode 3 to which the signal XON is applied, among the pixels formed by this scanning electrode 3, the signal electrode 4 corresponding to the pixel to be turned on has a
A signal YON is applied, and a signal YOFF is applied to the signal electrodes 4 corresponding to the remaining pixels. The signal YON is a DC voltage having a voltage value -E or E and has a polarity opposite to that of the signal XON. Further, the signal YOFF is a DC voltage having the same polarity as the signal XON and having a voltage value of E or -E.

In the pixel corresponding to the selected scanning electrode 3, the pixel to which the signal YON is applied from the signal electrode 4 has a voltage value aE or -aE, as indicated by reference numeral B1.
DC voltage is applied, and the state of the pixel changes to a light scattering state (on state). In addition, in the pixel to which the signal YOFF is applied from the signal electrode 4, reference symbol B2
As shown in the voltage value (a-2)E or -(a-2)E
(When a=3, a DC voltage of (a-2)E=E) is applied, thereby maintaining the state of the pixel in the previous state (light transmission state: off state).

On the other hand, in the pixels corresponding to the non-selected scanning electrodes 3, the voltage value E or - as indicated by reference symbols B3 and B4 is applied.
Since a DC voltage of E is applied, the state of each pixel is maintained at the state immediately before it. Each signal
ON, XOFF, YON, YOFF have waveforms as shown by the solid line in FIG. 7 during the period when one screen is displayed, and during the period when the next one screen is displayed, as shown by the broken line in FIG. The waveform has the polarity reversed.

FIG. 8 is a diagram for explaining the operation focusing on a certain pixel. In the write operation, the pixel P at the intersection of the scanning electrode 3a and the signal electrode 4a in FIG. 1 is turned on ( The waveform of the voltage applied to the pixel P and the change in light transmittance when the pixel is in a light scattering state are shown. Display of one screen is performed in one frame period F11 including an erasing period T51 and a writing period T52. During the erasing period T51, a bipolar square wave of an effective voltage aE having a high frequency is applied,
The light transmittance increases as indicated by reference numeral c1 and enters a light transmitting state (off state). During the writing period T52, the scanning electrodes 3 are selected line-sequentially, and the signal waveform used at this time is a waveform in which the voltage value does not change during the period when one scanning electrode 3 is selected (i.e., The period is twice the selection period of one scanning electrode). Therefore, for pixel P,
Period T during which scan electrode 3a is selected in writing period T52
A direct current voltage that does not change at a voltage value aE is applied over the period of 53. Therefore, during this period T53, the light transmittance sharply decreases as indicated by reference numeral c2, and no temporary increase in the light transmittance occurs during this period.

Period F of one frame following period T52
12, the next screen is displayed, and as described above, during this period, a signal whose polarity is inverted from that of the signal waveform in periods T51 and T52 is used. As described above, in this embodiment, when writing a display by selectively setting a pixel in a light scattering state, a voltage value is applied to the pixel whose optical state is to be inverted during the selection period of the scanning electrode corresponding to the pixel. A DC voltage that does not change at aE or -aE is applied. Therefore, a temporary increase in light transmittance does not occur during the transition from a light transmitting state to a light scattering state, and therefore display contrast can be improved. Furthermore, since a square wave is used, it is possible to use a drive circuit with a simple configuration.

Furthermore, since the signal waveform is inverted for each display screen, alternating current driving can be achieved when viewed through multiple display screens, thus preventing electrolysis of the liquid crystal molecules constituting the liquid crystal layer 5. be able to. 9 and 10 are waveform diagrams showing signal waveforms used in still another embodiment of the present invention. FIG. 9 shows waveforms during the erasing operation, and is illustrated similarly to FIG. 3 above. Further, FIG. 10 shows signal waveforms during a write operation, and the illustration is similar to that of FIG. 4 described above.

Comparison between FIG. 3 and FIG. 9, and FIG. 4 and FIG.
As is clear from the comparison with FIGS.
The waveforms of the signal applied to the scanning electrode 3 and the signal applied to the signal electrode 4 are exchanged with those of the first embodiment shown in FIG. Similar actions and effects are achieved. 11 and 12 are waveform diagrams showing signal waveforms used in still another embodiment of the present invention. FIG. 11 shows waveforms during the erasing operation, and is illustrated similarly to FIG. 6 above. Further, FIG. 12 shows signal waveforms during a write operation, and is similar to FIG. 7 described above.

Comparison between FIG. 6 and FIG. 11, and FIG. 7 and FIG.
As is clear from the comparison with 2, this example
The waveforms of the signal applied to the scanning electrode 3 and the signal applied to the signal electrode 4 are exchanged with those of the second embodiment shown in FIG. Similar actions and effects are achieved. Note that the present invention is not limited to the above embodiments. For example, in each of the above embodiments, both the scanning electrode 3 and the signal electrode 4 have zero
Although it is assumed that a bipolar square wave whose voltage value changes around V is applied, the signals applied to each electrode 3 and 4 do not need to be bipolar; , 4, 6, 7, and 9 to 12 may be applied to each electrode 3, 4. Even in this case, a bipolar square wave is ultimately applied to the liquid crystal layer. Further, the signal waveform does not need to be a square wave, and any waveform such as a sine wave that satisfies the conditions can be selected.

Furthermore, in the above embodiment, the display period of one screen is made up of an erasing period and a writing period, but after the writing period, an alternating current voltage of a relatively low effective voltage is applied to all pixels. A holding period may be provided in which the state of each pixel is maintained at its previous state by applying a voltage or a relatively low DC voltage. Furthermore, the mixture of high molecular weight liquid crystal and low molecular weight liquid crystal applied to the liquid crystal layer 5 can cause a state change by switching the frequency, and can maintain the previous state for at least a certain period of time by adjusting the applied voltage. Any device having a memorability that can be used can be applied. Various other changes can be made without departing from the gist of the invention.

Test examples conducted by the inventors of the present invention are shown below. [Test Example 1] 60 parts by weight of poly(4-cyanophenyl 4'-hexyloxybenzoate methylsiloxane), E6
A mixture consisting of 40 parts by weight of No. 3 (manufactured by Merck Japan) and a very small amount of tetraethylammonium bromide was used. This mixture was sandwiched between a pair of transparent electrodes patterned by etching to construct a simple matrix type liquid crystal display device with a film thickness of 10 microns and 16×16 dots. This liquid crystal display device is driven by the driving method shown in the second embodiment (Figs. 6 to 6).
Figure 8)).

To erase the display, apply an effective voltage of 90V to all pixels,
Performed by applying an alternating current voltage with a frequency of 1 kHz,
The display was written by applying a DC voltage of 90 V (a=3, E=30). Furthermore, the display screen after writing was maintained at an AC voltage of 30 V effective voltage. As a result, writing of one scanning line was completed in 2 seconds, and writing of one screen was completed in 32 seconds. In the holding operation, it was confirmed that the state at the time the writing was completed was maintained.

[Test Example 2] Poly(4-cyanophenyl 4)
'-hexyloxybenzoate methylsiloxane) 60
A mixture consisting of parts by weight, 40 parts by weight of E63 (manufactured by Merck Japan), and a very small amount of tetraethylammonium bromide was used. This mixture was sandwiched between a pair of transparent electrodes patterned by etching, and the film thickness was 10 microns.
A simple matrix type liquid crystal display device of ×16 dots was constructed. This liquid crystal display device was driven by the driving method shown in the first example (FIGS. 1 to 5).

Erasing the display is performed by applying a DC voltage of 90 V to all pixels, and writing to the display is performed by applying an effective voltage of 90 V (a=3, E=30 V) and a frequency of 1 kHz to the necessary pixels.
This was done by applying an AC voltage of z, and the display screen after writing was maintained at an AC voltage of 30 V effective voltage. As a result, writing of one scanning line was completed in 2 seconds, and writing of one screen was completed in 32 seconds. In the holding operation, it was confirmed that the state at the time the writing was completed was maintained. In this example, display with higher contrast than in Test Example 1 was possible.

[0062]

As described above, according to the method for driving a liquid crystal display device of the present invention, at least the optical A DC voltage whose voltage value does not change is applied over a period during which the state is to be reversed. Therefore, during the transition from the light transmitting state to the light scattering state, the polarity of the voltage applied to the pixel does not change, thereby preventing a temporary increase in light transmittance during the state transition process. In this way, erasing of the screen or writing of the display can be performed efficiently, so that the display contrast can be improved and the display quality can be improved.

On the other hand, since the polarity of the above-mentioned DC voltage is reversed for each display screen, AC driving is also achieved at the same time. This can prevent electrolysis of liquid crystal molecules.

[Brief explanation of the drawing]

FIG. 1 is a conceptual diagram for explaining a method for driving a liquid crystal display device according to an embodiment of the present invention.

FIG. 2 is a sectional view showing the basic configuration of a liquid crystal display device to which the above method is applied.

FIG. 3 is a waveform diagram showing signal waveforms applied to scanning electrodes 3 and signal electrodes 4 and waveforms of voltages applied to pixels.

FIG. 4 is a waveform diagram showing signal waveforms applied to scanning electrodes 3 and signal electrodes 4 and waveforms of voltages applied to pixels.

FIG. 5 is a diagram for explaining display operation.

FIG. 6 is a waveform diagram showing signal waveforms applied to scanning electrodes 3 and signal electrodes 4 and voltage waveforms applied to pixels in a driving method according to another embodiment of the present invention.

FIG. 7 is a waveform diagram showing signal waveforms applied to scanning electrodes 3 and signal electrodes 4 and waveforms of voltages applied to pixels.

FIG. 8 is a diagram for explaining display operation.

FIG. 9 is a waveform diagram showing signal waveforms applied to scanning electrodes 3 and signal electrodes 4 and waveforms of voltages applied to pixels in a driving method according to still another embodiment of the present invention.

FIG. 10 is a waveform diagram showing signal waveforms applied to scanning electrodes 3 and signal electrodes 4 and waveforms of voltages applied to pixels.

FIG. 11 is a waveform diagram showing signal waveforms applied to scanning electrodes 3 and signal electrodes 4 and waveforms of voltages applied to pixels in a driving method according to still another embodiment of the present invention.

FIG. 12 is a waveform diagram showing signal waveforms applied to scanning electrodes 3 and signal electrodes 4 and waveforms of voltages applied to pixels.

FIG. 13 is a diagram for explaining the operation of the example proposed earlier by the inventors of the present invention.

FIG. 14 is a diagram for explaining the operation of the previously proposed example by the inventors of the present invention.

[Explanation of symbols]

3 Scanning electrode 4 Signal electrode 5 Liquid crystal layer

Claims (2)

[Claims] [Claim 1] A liquid crystal that is composed of a mixture of a high molecular weight liquid crystal and a low molecular weight liquid crystal, becomes a light transmitting state by applying an alternating current voltage of a predetermined frequency or more and has an effective voltage of a predetermined value or more, and has an effective voltage of the predetermined value or more. A light scattering state is achieved by applying an AC voltage below the predetermined frequency or a DC voltage above the predetermined value, and when a voltage below the predetermined value is further applied, the previous optical state is maintained for at least a certain period of time. It has a liquid crystal layer, and a plurality of scanning electrodes and a plurality of signal electrodes arranged cross-opposed with the liquid crystal layer in between, and a voltage signal is applied to the scanning electrode and the signal electrode, and a voltage signal is applied to the scanning electrode and the signal electrode. A method of driving a liquid crystal display device that performs display by setting pixels in a light-transmitting state or light-scattering state at intersections with a pixel, and erasing the display of the entire display screen by setting all pixels in a light-transmitting state or a light-scattering state. In order to do this, an erasing operation is performed in which an AC voltage having an effective voltage equal to or higher than the predetermined value or a DC voltage equal to or higher than the predetermined value is applied between the scan electrode and the signal electrode, and the plurality of scan electrodes are selected line-sequentially. A scanning signal of a predetermined waveform is applied to each signal electrode, and during the period when the scanning electrode corresponding to the pixel whose optical state is to be changed is selected, the liquid crystal layer is applied with the above-mentioned predetermined value or more in relation to the scanning signal. A waveform signal is applied to which an AC voltage with an effective voltage of selectively changes the optical state of the pixels constituting the display screen by applying a waveform signal to the liquid crystal layer in which an AC voltage with an effective voltage lower than the above predetermined value or a DC voltage lower than the above predetermined value is applied. A voltage waveform applied to each pixel during the erasing operation, and a voltage waveform applied to the pixel whose optical state is to be changed during the writing operation. , one of them is a waveform having a frequency equal to or higher than the above-mentioned predetermined frequency, and the other is a DC waveform whose voltage value does not change at least over a period of time during which the optical state of the pixel is changed, and whose polarity is reversed every time one screen is displayed. A method for driving a liquid crystal display device characterized by: 2. A voltage applied to the pixel during the erasing operation;
and a claim characterized in that the voltage applied to the pixel whose optical state is changed during the write operation is two to three times the voltage applied to the pixel whose optical state is maintained during the write operation. Item 1. A method for driving a liquid crystal display device according to item 1.