JP4290679B2 - Driving method of nematic liquid crystal - Google Patents

Description

  The present invention relates to a liquid crystal driving method, and more particularly to a nematic liquid crystal driving method.

  When a nematic liquid crystal is sandwiched between two transparent flat plates having transparent electrodes and placed between two polarizing plates, light passing through the two polarizing plates depends on the voltage applied to the two transparent electrodes. It is known that the transmittance changes.

  A liquid crystal display device using this principle has features such as a small thickness and low power consumption, and is widely used for watches and calculators.

  In recent years, it has been used in color display dot matrix type display devices such as notebook personal computers and small liquid crystal televisions in combination with color filters.

  As a dot matrix driving method, a simple matrix driving method with a simple structure and an active matrix method represented by a TFT method in which an active element is added to each pixel to realize high image quality are known.

  In the active matrix method, it is very difficult to form active elements and the cost is high, and a large amount of investment is required for production equipment. However, because the highest quality TN type nematic liquid crystal can be used, a high contrast ratio is required. High viewing angle and multi-gradation can be realized.

  In the simple matrix driving method, the formation of the electrodes of the liquid crystal panel is very simple, but there is a problem that the contrast ratio decreases as the duty ratio, which is the ratio of the repetition period to the selection time, increases, and the duty ratio is high. A large-scale matrix liquid crystal panel uses STN type nematic liquid crystal which is characteristically disadvantageous in terms of contrast ratio, viewing angle, response speed, and multi-gradation.

  Further, in a liquid crystal display device capable of color display in combination with a color filter, color display is performed by combining dots of three colors of red, green, and blue.

  This color filter is very expensive, and high accuracy is required for the work of attaching to the panel.

  Furthermore, in order to obtain the same resolution as that of a monochrome liquid crystal display panel, the number of dots is three times that of a normal liquid crystal panel, so that the number of horizontal driving circuits is tripled in a normal liquid crystal panel, resulting in a cost reduction. In addition, since the number of connection points between the panel and the drive circuit is tripled, the connection work becomes difficult.

  Therefore, as a method of performing color display using a liquid crystal panel, a method using a color filter has many elements that are expensive in terms of cost, and is difficult to manufacture at a low cost.

  As a color liquid crystal display device that does not use a color filter, a method of performing color display by combining a monochrome liquid crystal panel and a three-color backlight has been proposed as disclosed in JP-A-1-179914, which is less expensive than the color filter method. Although there is a possibility that high-definition color display can be realized, it is difficult to drive the liquid crystal at high speed with the conventional liquid crystal driving method, and it has not been put into practical use.

  In addition, since the response speed of liquid crystals is slow in conventional liquid crystal display devices, the performance of video playback on a TV or when the mouse cursor of a personal computer is moved at a high speed is higher than that of a display using a cathode ray tube. It was inferior.

  The problem to be solved by the present invention is that a high-contrast ratio can be obtained in a large-scale matrix liquid crystal panel having a high duty ratio by a simple matrix driving method even if a TN type nematic liquid crystal is used by changing the driving method. This makes it possible to obtain the same or better performance as a display using a cathode ray tube in color reproduction by the above-mentioned three-color backlight and video reproduction, that is, the response speed is high with a high contrast ratio. A method for driving a matrix of a fast nematic liquid crystal is provided.

  The present invention, which has been made to solve the above problems, applies a voltage to the liquid crystal at a timing different from that of a conventional liquid crystal driving circuit, so that the contrast ratio is high even when the duty ratio is high, and the response speed of the liquid crystal is high. It is characterized by doing.

  The electro-optical characteristics of a normal nematic liquid crystal are as shown in FIG. 1, and the effective value of the applied voltage in FIG. 1 becomes a problem regardless of the polarity.

  In recent years, an active driving method for simultaneously selecting a plurality of scanning lines has been proposed as a driving method for realizing an image quality equivalent to that of a TFT liquid crystal panel in an STN liquid crystal panel.

  This active drive method improves the contrast ratio and response speed by selecting multiple scanning lines at the same time, increasing the number of scanning lines selected in one frame period, and applying the light transmittance of nematic liquid crystal. It was not different from the conventional driving method in that the characteristic determined by the effective value of the voltage was used.

  Conventionally, the response speed of a nematic liquid crystal takes several tens of milliseconds to several hundred milliseconds, and it has been considered difficult to obtain a response speed that can realize colorization with a three-color backlight.

In order to develop a liquid crystal panel having a response speed capable of realizing colorization with a three-color backlight, the present inventor measured the dynamic characteristics of the applied voltage waveform and light transmittance of the nematic liquid crystal. It was found that depending on the voltage waveform, there is a state in which the light transmittance changes at high speed when the applied voltage changes.
Therefore, the present invention provides a nematic liquid crystal, a plurality of common electrodes and a plurality of segment electrodes sandwiching the liquid crystal, and a liquid crystal sandwiched between the electrodes between two polarizing plates. In a liquid crystal display device in which the transmittance is determined by the applied voltage regardless of polarity, means for applying a selection pulse to the common electrode, and a voltage corresponding to the image data to be displayed is applied to the segment electrode according to the selection pulse Means for applying a voltage different from a voltage corresponding to the image data to the segment electrode during a period when no selection pulse is applied to any of the common electrodes, and to the plurality of segment electrodes. Average voltage of the segment electrode in one period from the application of the selection pulse to the application of the next selection pulse In the nematic liquid crystal driving method which is characterized in that the pressure at a constant value.

  As described above, in the present invention, since an image is drawn on the liquid crystal panel and the image disappears completely during one frame period, a very fast response speed can be obtained, which is an optimum method for moving image reproduction. is there.

  The present invention can be applied not only to a simple matrix type liquid crystal panel but also to achieve a response speed much higher than that of a TFT type liquid crystal panel using a simple matrix type liquid crystal panel. Contrast ratio can be realized equally, viewing angle is good, and performance equivalent to or better than TFT liquid crystal panel can be realized.

  Furthermore, the conventional active driving method requires many types of voltages necessary for driving, and the controller becomes complicated, so that the driving circuit becomes expensive. In the present invention, the types of voltages necessary for driving are high. Since the drive timing is simple and the drive timing is simple, it can be realized at a cost equivalent to that of a conventional simple matrix drive system drive circuit.

  Furthermore, since the present invention is a method in which an image is drawn on a liquid crystal panel until the image disappears completely during one frame period, it is an optimum method for the above-described color display method using a three-color backlight. A high-performance, low-cost color display can be realized.

  By repeatedly generating the state in which the light transmittance changes at high speed, it is possible to obtain characteristics with a much faster response speed and a better contrast ratio than the conventional driving method.

  FIG. 2 shows one preferred embodiment of the present invention, in which a voltage waveform applied to a segment electrode and a common electrode for one dot of a simple matrix nematic liquid crystal panel, and light transmission of the one dot. Represents the rate.

  Here, the voltage applied to the common electrode outputs a pulse only during the period for selecting the common electrode, and the voltage applied to the segment electrode is Vseg1 during the period when the pulse for the selected common electrode is output. When the light transmittance of the dot to be changed momentarily changes and the voltage applied to the segment electrode is Vseg0, the light transmittance of the corresponding dot does not change.

  Therefore, an image can be displayed by applying a voltage corresponding to image data to be displayed to the segment electrode in accordance with the timing of the pulse applied to the common electrode.

  The feature of the drive timing of the embodiment of the present invention is that when the segment voltage in the period in which the common electrode is selected within one frame period is Vseg1, the segment voltage is set to Vseg0 in the period in which the common electrode is not selected. It is to be.

  3 and 4 show a comparison with the embodiment of the present invention when the conventional voltage application method is performed. The difference in the waveform of the applied voltage is shown in FIG. 3 and FIG. It is only that the voltage applied to is a constant value.

  The liquid crystal material used in FIG. 2, FIG. 3 and FIG. 4 is a general TN type liquid crystal that exhibits the characteristics as shown in FIG. ing.

  Therefore, according to the concept of the prior art, the light transmittance of the liquid crystal is determined by the effective value of the applied voltage when the common electrode is selected. Therefore, as shown in FIGS. 3 and 4, the segment voltage is one of Vseg0 and Vseg1. Even in a fixed case, if the light transmittance is constant in a low state, the light transmittance should not change even if the segment voltage is switched between Vseg0 and Vseg1 as shown in FIG.

However, even if a very common TN liquid crystal material is used and a panel with a gap as thin as 5 to 6 μm is used, the light transmittance changes as shown in FIG. The time required to start the change according to the change and return to the original light transmittance is 15 to 20 mS, which is operating at a very high speed.

  Here, the characteristic that the light transmittance changes at high speed as shown in FIG. 2 is most noticeable when Vcom0 is lower than Vseg0 and Vcom1 is higher than Vseg1, that is, the period during which the common electrode is selected. This is a case where the polarity of the applied voltage is reversed with respect to a period in which the common electrode is not selected.

  FIG. 5 shows how the light transmittance changes when only the segment voltage change period is changed in the embodiment of the present invention. When the segment voltage is changed every frame period, FIG. It can be seen that the rate of change in light transmittance is considerably slower than when the segment voltage is changed within one frame period.

  Therefore, it can be seen that the light transmittance of the liquid crystal changes at a high speed by changing the segment voltage at an early cycle.

  The problem with the simple matrix drive method is the crosstalk phenomenon in which the liquid crystal responds due to the influence of the segment voltage applied during non-selection. To prevent this, the effective voltage of the applied voltage waveform during non-selection is kept constant. A method called a voltage averaging method is generally used.

  Also in the embodiment of the present invention, when the liquid crystal is driven by simple matrix driving, the light transmittance of the liquid crystal changes when the applied voltage waveform at the time of non-selection changes.

  In the driving method according to the embodiment of the present invention, if the average value is constant, not the effective value of the applied voltage at the time of non-selection, the drive voltage waveform at the time of non-selection is not affected.

  Therefore, the influence of the applied voltage waveform at the time of non-selection can be eliminated with a simple circuit as compared with the conventional driving method.

  FIG. 6 is a drive circuit diagram of an embodiment of the present invention, in which 1, 2, 3 and 4 are D flip-flops, 5 is an XOR gate, 6, 7 and 8 are AND gates, 9 is a segment drive buffer, Reference numerals 10, 11 and 12 denote common drive buffers.

  In the circuit diagram of FIG. 6, only one segment drive circuit and three common drive circuits are shown for the sake of simplicity. However, by adding the same circuit as the number of segments and common electrodes, any number of segment drive circuits can be obtained. A number of dots can be driven in a matrix.

  FIG. 7 is a timing chart showing the operation of the drive circuit diagram of the embodiment of the present invention in FIG.

  6 and 7, the clock signal is a clock having a duty ratio of 1: 1, and the segment data signal is latched by the clock signal by the D flip-flop 1 and then exclusive-ORed with the clock signal by the XOR gate 5. Is output through the segment drive buffer 9.

  The common drive signals 1, 2 and 3 are shifted by the D flip-flops 2, 3 and 4 so that the common synchronizing signal is shifted at the rising edge of the clock signal, and logically ANDed by the clock signal and the AND gates 6, 7 and 8. And output through the buffers 10, 11 and 12.

  Therefore, in the embodiment of the present invention shown in FIGS. 6 and 7, during the period when the common electrode is selected, the voltage corresponding to the segment data signal is output to the segment electrode and the period when the common electrode is not selected. Since the voltage of the segment electrode can be changed at a fast cycle to a voltage different from the period during which the common electrode is selected, the liquid crystal can be operated at high speed.

  In addition, since the average value of the segment drive signal in one cycle from the rising edge of the clock signal to the next rising edge can always be made constant, it can be crossed with a simple circuit without using the conventional voltage averaging method. The talk phenomenon can be eliminated.

  In the embodiment of the present invention, in order to perform display with a high contrast ratio, a pulse is applied to the common electrode, and after the light transmittance of the liquid crystal changes instantaneously, the light transmittance returns to the original value. Therefore, it is better to apply the next pulse.

  Therefore, in the embodiment of the present invention, if the frame period is made faster, the contrast ratio becomes lower. On the other hand, if the frame period is made slower, flickering occurs.

  In the embodiment of the present invention, it has been shown that the period of change in the segment voltage at the time of non-selection greatly affects the rate of change in light transmittance, but the time for the light transmittance to return to the original value is Greatly changes depending on the characteristics of the liquid crystal material, particularly the viscosity of the liquid crystal material.

  Accordingly, by selecting a liquid crystal material that has a short time for the light transmittance to return to the original value, it is possible to perform display with a high contrast ratio while suppressing the occurrence of flicker.

  In addition, since the time for the light transmittance to return to the original value is greatly affected by the viscosity of the liquid crystal material, display with a high contrast ratio can be performed without changing the liquid crystal material by increasing the temperature of the liquid crystal panel. It is also possible.

It is a relationship figure which shows the electro-optical characteristic of a nematic liquid crystal. It is explanatory drawing which shows the time change of the light transmittance with respect to the change of the applied voltage of the nematic liquid crystal of this invention. It is explanatory drawing which shows the time change of the light transmittance with respect to the change of the applied voltage of a nematic liquid crystal when not changing a segment voltage. It is explanatory drawing which shows the time change of the light transmittance with respect to the change of the applied voltage of a nematic liquid crystal when not changing a segment voltage. It is explanatory drawing which shows the time change of the light transmittance with respect to the change of the applied voltage of a nematic liquid crystal when the period of the change of a segment voltage is doubled. It is a circuit diagram in an embodiment of the present invention. FIG. 7 is a timing chart showing the operation of the circuit in the embodiment of the present invention in FIG. 6.

Claims (3)

And nematic liquid crystal, a plurality of common electrodes and a plurality of segment electrodes sandwiching the liquid crystal, Place the liquid crystal sandwiched between the electrodes between two polarizing plates, the voltage applied between the segment electrode and the common electrode In the liquid crystal display device where the transmittance is determined regardless of the polarity ,
Means for applying a selection pulse to the common electrode, and means for applying a voltage corresponding to the image data to be displayed to the segment electrode in response to the selection pulse;
Means for applying a voltage different from a voltage corresponding to the image data to the segment electrode during a period in which a selection pulse is not applied to any of the common electrodes;
The nematic liquid crystal is characterized in that an average voltage of the segment electrode voltage in one period from when the selection pulse is applied to the plurality of segment electrodes to when the next selection pulse is applied is set to a constant value. Driving method.   The driving of the nematic liquid crystal according to claim 1, wherein the voltage applied to the common electrode and the segment electrode is set so that the polarity of the voltage applied to the liquid crystal is reversed when a selection pulse is applied to the common electrode. Method.   The driving method of the nematic liquid crystal according to claim 1, further comprising heating means for raising the temperature of the nematic liquid crystal to a predetermined temperature.