JPS63281135A - Method for driving liquid crystal display device - Google Patents

Abstract

PURPOSE:To prevent the occurrence of a black change of a transparent electrode, the discoloration of a dichroic pigment, and the deterioration of a liquid crystal, even if the device is driven for many hours, by setting an average voltage level of a pulse group applied to a display element, to '0'. CONSTITUTION:After a pulse group P1 or P2 has been applied, a high frequency AC pulse group P3 or P4 is applied by a non-selecting signal NS, and a response state is held by an AC stabilizing effect. In this case,the pulse groups P1, P3 and P4 are AC pulses being equal in both the waveform and the number of pulses whose polarities are different, and with regard to the pulse group P2, as well, its average voltage level applied to a display element is '0'. In such a way, the black change of a transparent electrode, the deterioration of a liquid crystal, the discoloration of a dichroic pigment, etc., are hardly occurred.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to a method for driving a ferroelectric liquid crystal.

[Prior Art] Recently, ferroelectric liquid crystals have been attracting attention in place of TN type liquid crystals, and the development of display devices using them is progressing.

The display modes of ferroelectric liquid crystals include a birefringent display mode and a guest-host display mode.

When driving these, unlike conventional TN-type liquid crystals, the display state (brightness and darkness) is controlled by the direction of electric field application, so the driving method used for TN-type liquid crystals cannot be used.
This requires a special driving method.

Furthermore, considering the lifespan of the display device, it is not preferable that a direct current component be applied to the display element for a long period of time, and a driving method that takes this point into consideration is required.

One of the driving methods that does not apply this DC component to the display element for a long time is rsID-85DigestJ (
1985) (P, 131-P, 134). Furthermore, Japanese Patent Application Laid-open No. 176097/1984 also discloses a driving method that uses a ferroelectric liquid crystal having an AC stabilizing effect and can achieve display bistability with a driving electric signal.

[Problems to be solved by the invention] However, in the latter driving method, a direct current component may be applied to the display element for a long time, and the transparent electrode for display may be reduced and darkened, or two-color There were problems such as discoloration of the color pigment and deterioration of the liquid crystal. On the other hand, with the former driving method, there is no problem of deterioration, but the time T required to rewrite one screen is T-4XtXN (N is the number of scanning lines/screen), where t is the time required to write one pixel. Therefore, there are problems in that the rewriting time T is long, the number of scanning lines cannot be increased much, and it is not suitable for displaying moving images. In addition, it was not possible to display halftones (the present invention) does not cause blackening of the transparent electrode, discoloration of the dichroic dye, or deterioration of the liquid crystal even when driven for a long time, and moreover, the rewriting time T for one screen can be reduced to 1 /2(
In addition, within the same rewriting time, the number of scanning lines can be doubled, and it is also possible to display halftones.

[Means for Solving the Problems] The present invention is based on a DC pulse for bringing a ferroelectric liquid crystal into a desired response state, and a pulse in which a high-frequency AC pulse is superimposed on a DC pulse of opposite polarity that is symmetrical to the DC pulse. A group of pulses having an average voltage level of The line selection time is shortened and the average voltage level applied to the display element can be reduced to zero.

In addition, by controlling the voltage value or duty (the ratio of the time when the high-frequency AC pulse is applied and the time when the high-frequency AC pulse is not applied) of the high-frequency AC pulse superimposed on the DC pulse according to the gradation of the display, halftones can be produced. This is what I did.

[In the embodiment shown in FIGS. 1 and 2, the selection circuit SE generates a selection signal S (FIG. 2) that sequentially and time-divisionally selects the scanning electrode group L'' L7. When no selection signal is supplied, a non-selection signal NS is generated.

The selection signal S consists of a voltage regulator V, and the non-selection signal NS consists of a voltage ±H.

On the other hand, the drive control circuit DR generates a response signal or a reverse response signal RD shown in FIG. 2, and is supplied to the control electrode group R-R5. That is, a response signal RI is supplied to the control electrodes for which a response display is desired, and a reverse response signal RD is supplied to control electrodes for which an appropriate response display is desired.

By supplying the above signals, the pulse group PI is applied to the response display element, and the pulse group P is applied to the appropriate response display element. In the pulse group P1, first the DC pulse of voltage -■ once enters a saturated reverse response state, but then the voltage V
Since a DC pulse of 1 is applied, the liquid crystal enters a saturated response state. On the other hand, in the pulse group P2, a DC pulse of voltage -■ temporarily enters a saturated reverse response state, and then a pulse in which a high-frequency AC pulse of voltage ±2H is superimposed on voltage ■ is applied, so the AC stabilizing effect of this high-frequency AC pulse Therefore, the saturated response state does not occur, and the saturated response state is maintained.

After applying the above pulse group P or P2, the high frequency AC pulse group P3 or P4 is activated by the non-operation selection signal NS.
is applied, and the response state is maintained due to the AC stabilization effect. Here, the pulse groups P , P ,
P is an AC pulse with the same waveform and number of pulses with different polarities, and since the average voltage level applied to the display element in pulse group P2 is 0, blackening of the transparent electrode, deterioration of the liquid crystal, and dichroism occur. This eliminates the possibility of discoloration of the pigment.

Furthermore, the scan time (time for applying the selection signal) for the - line is short, and since the response and reverse response are written simultaneously within the same line, the time required to rewrite one screen can be shortened.

Note that the pulse width and pulse height H of the response pulse P1 are appropriately determined in relation to the magnitude of spontaneous polarization of the ferroelectric liquid crystal and the display cell thickness so as to obtain a saturated reverse response state and a saturated response state.

Further, the frequency of the high-frequency AC pulse is preferably twice or more (preferably four times or more, an integral multiple) of that of the response pulse P1,
The pulse height H is appropriately determined in relation to the magnitude of dielectric anisotropy of the ferroelectric liquid crystal so that the response state is maintained stably.

Next, an example of displaying halftones will be explained.

FIG. 3 shows an example in which halftones can be displayed by applying the example in FIG. 2. In FIG. 3, the selection signal S is
As in the figure, the voltage ±h of the control signal C supplied to the control electrode groups R1 to R5 is controlled according to the gradation. In FIG. 3, due to the potential difference P5 between the selection signal S and the control signal C, a DC pulse of -V is first applied to initialize the saturated reverse response state, and then a DC pulse V
An unsaturated response pulse on which a high frequency alternating current pulse of ±h is superimposed is applied to display halftones. In other words, the display will be in a saturated response state with only a DC pulse of voltage V, but an unsaturated response state can be obtained by controlling the AC stabilizing effect of the high frequency AC pulse. Thereafter, a high frequency AC pulse P6 is applied by the non-selection signal NS- and the control signal C, and the above response state is maintained. In addition,
The non-selection signal NS- is different from the non-selection signal NS in FIG.
This is a signal with a phase change of 80".

The pulse for displaying halftones is not limited to modulation of the voltage ±h of the control signal mentioned above, but may also be produced by pulse width modulation. It is important to initialize to a responsive state. If a pulse is simply applied to produce halftones, the response state will change depending on the display state before the pulse is applied.
However, in the example shown in Figure 3, the display is initialized to a saturated reverse response state before rewriting the display, so stable halftones can be displayed regardless of the previous response state. be.

In the above explanation, we referred to the positive side voltage as a response and the negative side voltage as a reverse response, but since the response and reverse response are two sides of the same coin, conversely, the positive side voltage causes a reverse response, and the negative side voltage causes a reverse response. It may also be said that it responds at a voltage of .

By the way, the signals supplied to each electrode are not limited to those described above, and various changes can be made, and a bias voltage may be applied as appropriate.

Furthermore, in the above embodiment, a matrix display as shown in FIG. Needless to say, the present invention can also be applied to driving a liquid crystal shutter array.

In this case, high contrast can be achieved by setting the reverse response state to a dark display state.

[Effects of the Invention] According to the present invention, since the average voltage level of the pulse group applied to the display element is 0, even if the display element is driven for a long time, the transparent electrode will not darken, the dichroic dye will change color, etc. The LCD will not deteriorate. Moreover, since the application time of the selection signal to the scanning electrode group is short, and the display response and the reverse response are written within the same selection time, the time required to rewrite one screen can be shortened. In other words, the number of digits scanned within the same time can be increased, and high-definition display becomes possible.

In addition, since only high-frequency AC pulses are applied when not selected, stable holding force can be obtained due to the AC stabilization effect.
A high-contrast display can be obtained.

Furthermore, by controlling the high-frequency AC pulse applied to the DC pulse, it is possible to display halftones, and before the pulse that displays the halftone, a stable halftone can be displayed by once entering a saturated reverse response state. The effects of this are enormous.

[Brief explanation of drawings]

Figure 1 is an explanatory diagram showing an example of a display device, Figures 2 and 3 are
The figure is an explanatory diagram showing voltage waveforms for realizing the present invention. L-L...Scanning electrode group R°~R...Control electrode group S...Selection signal NS, NS-...Non-selection signal D...Response signal RD...Reverse response signal C. ... Control signal P ~ P ... Pulse group or more (1 other person) Fig. 1 Fig. 2

Claims (2)

[Claims] (1) In a matrix liquid crystal display device in which a ferroelectric liquid crystal having an AC stabilizing effect is interposed between a scanning electrode group and a control electrode group, a selection signal is sequentially supplied to the scanning electrode group, and when this selection signal is not supplied, A non-selection signal is supplied, a desired signal is supplied to the control electrode group, and a DC pulse is applied to bring the ferroelectric liquid crystal into a saturated reverse response state by a potential difference between the desired signal and the selection signal, followed by a DC pulse. A pulse in which a high-frequency AC pulse is superimposed on a DC pulse of opposite polarity that is symmetrical to the DC pulse is applied, and by controlling the high-frequency AC pulse, the ferroelectric liquid crystal is brought into a desired response state, and the desired signal and the non-selection signal are combined. Due to the potential difference of
A method for driving a liquid crystal display device, comprising applying a high frequency alternating current pulse that maintains the desired response state of the ferroelectric liquid crystal. (2) The method for driving a liquid crystal display device according to claim 1, wherein the ferroelectric liquid crystal is a ferroelectric liquid crystal exhibiting negative dielectric anisotropy in the frequency range of the high-frequency AC pulse.