JPH02267523A - Driving method for liquid crystal element - Google Patents

Abstract

PURPOSE:To stabilize a memory in a non-selection period at the time of time division driving by applying a set pulse corresponding to the display contents, and thereafter, applying a pulse in which the product of a crest value and pulse width is smaller than that of the set pulse, in a ferroelectric liquid crystal. CONSTITUTION:In a scanning electrode waveform 101, a crest value Ve of a reset pulse of a selection period 1 is set to a value by which a liquid crystal molecule is inverted enough by some pulse width Pw. Also, a crest value Vw of a set pulse is set to an average value of a crest value Vth whose inversion is started and a crest value Vsat whose inversion is ended, when a pulse whose polarity is opposite to that of the reset pulse is applied. Subsequently, in accordance with the polarity which a panel has, a positive or negative pulse is applied, and in accordance with magnitude of its polarity, a memory stabilizing pulse of a crest value VC2 is applied. On the other hand, in a signal electrode waveform 102, a crest value of pulse width Pw is set to >= (Vpat - Vth/2) and <=Vw. In such a way, in the course of a non-selection period, the polarity of the panel is negated, and instability of the memory can be avoided.

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a time-division driving method for a liquid crystal element. [Prior art] In conventional devices using ferroelectric liquid crystals, if there is a charge bias in the drive waveform, charge bias occurs in the upper and lower substrate directions during driving, and this charge bias causes the spontaneous activation of the ferroelectric liquid crystal. Polarization was reversed, memory properties were lost, and the display often became unclear. Here, the term "memory property" refers to the effect that the display state selected during the selection period is maintained even when the display state is not selected. Moreover, when such charge bias occurs, the liquid crystal itself also deteriorates. As a driving method without such charge bias, there is a patent application filed in 1983.
1-179346. [Problems to be Solved by the Invention] However, in the conventional method, when driving a liquid crystal element, the operating margin for the thickness of the liquid crystal layer of the liquid crystal element, the operating margin for the pulse width and peak value of the driving pulse, and the operation for the operating temperature are Margins are narrow. One reason for this is that the liquid crystal element is driven by a composite waveform of a scanning electrode signal and a selection electrode signal (see FIG. 3). That is, in the scanning electrode signal, the average pulse (peak value (Vth+Vsat)/2) of set pulse 1 (peak value vth) and set pulse 2 (peak value Vsat) described earlier is applied during the selection period, In synchronization with this, if state 1 is selected, a pulse with a peak value of (Vs at - V th )/2 is applied to the selection electrode, and if state 2 is selected, (Vt
A pulse with a peak value of h-VSat)/2 is applied. Therefore, selection electrode signals for pixels on other scanning lines are applied even outside the selection period, making the memory state unstable. That is, even if the state is reset to state 1 and state R2 is written, the liquid crystal responds to the selection electrode signal and a portion of the pixels return to state B1. Alternatively, even if state 1 is reset once and state 1 is written, the liquid crystal responds to the selection electrode signal and a portion of the pixels are inverted to state 2. These two phenomena usually do not occur at the same time, and one of the phenomena is more noticeable depending on the polarity in the direction of the liquid crystal layer of the panel. It would be better to reduce the amplitude of the signal electrode signal, but that would result in state 1 and state 2.
There are not enough choices. SUMMARY OF THE INVENTION The present invention is intended to solve these problems, and its purpose is to provide a driving method that makes it difficult for the liquid crystal to respond to a selection electrode signal during a non-selection period.

[Means to solve the problem]

(1) An element having a ferroelectric liquid crystal or a ferroelectric substance sandwiched between at least a substrate on which a scanning electrode is formed and a substrate on which a signal electrode is formed is placed in the first state during or before the selection period. A pulse (reset pulse) is applied to set the element to the second state during the selection period, and a pulse (set pulse 2) having an effective value that puts the element into the second state or a pulse below the threshold value that puts the element into the second state is applied depending on the display content during the selection period. In a driving method that applies a pulse (set pulse 1), after applying the set pulse, the product of the peak value and the pulse width is set pulse 10 with a polarity that cancels out the charge asymmetry that exists in the thickness direction of the element.
It is characterized by applying a pulse smaller than the product of the peak value and the pulse width. (2) Means for Solving the Problems Before applying the set pulse described in item 1, the present invention is characterized by applying a pulse that neutralizes the entire waveform in terms of charge within one frame. [Operation] According to the above configuration of the present invention, as described in the problem to be solved by the invention, the polarity of the panel is cited as a factor that makes the memory unstable. Such memory instability can be avoided by applying pulses in the canceling direction. In addition, devices using ferroelectric liquid crystals are sensitive to electrical bias, so applying a pulse to neutralize the charge over the entire waveform before or during the selection period will improve the memory. characteristics, and the lifespan of the liquid crystal is extended. Hereinafter, the details of the present invention will be shown by examples. [Example] For convenience, in this example, the case where a pulse with a negative polarity and a voltage higher than the saturation voltage is applied is defined as a dark state, and the case where a pulse with a positive polarity and a voltage higher than the saturation voltage is applied is defined as a bright state. Embodiment 1 FIG. 1 shows a timing chart of drive waveforms in this embodiment. A method for setting the peak value of each waveform will be explained. First, the scanning electrode waveform 101 will be explained. The peak value Ve of the reset pulse is set to a value sufficient to sufficiently invert the liquid crystal molecules with a certain pulse width Pw. A certain pulse width Pw is a value slightly longer than the response speed of liquid crystal molecules. The wave height value Vw of the set pulse is the wave height value vth at which the display is inverted when a set pulse with a pulse width Pw and the opposite polarity to the reset pulse is applied after the reset pulse is applied, and the wave height value at which the display is inverted when an even higher voltage is applied. Vs
Let it be the average value of at. The memory stabilizing pulse is positive or negative depending on the polarity of the panel, and the peak value VC2 is determined depending on the magnitude of the polarity. The peak value Vcl of the charge correction pulse is set to Ov in this embodiment. Next, the signal electrode waveform 102 will be explained. The pulse width is Pw, and the peak value V d 4t (V s a t - V th
) / 2 or more and Vw or less. If the signal waveform shown in the selection period 102 in FIG. 1 is output, the dark state is selected by the combined waveform 103 with 101, and if the signal waveform shown in the selection period 2 is output, the bright state is selected. The optical response of the element at this time is shown in 104. In this example, Dainippon Ink DOFOOO4 is used as the liquid crystal layer 2. Um,
Pw=150μsec, Ve=-25V, Vw=10V
, Vc2=-1,5V. Vcl=OV% Vd=1.5V. ■c2 to O
v, the contrast was 10:1, but Vc
When 2 was set to -1.5V, the contrast was 15:1. In this embodiment, the charge correction pulse is set outside the selection period, but it may be set within the selection period. Further, the first pulse and the third pulse may be interchanged in the selection period of the signal electrode waveform 102. This is because the pulse that is actually involved in selection is the second pulse, and the other pulses are only for correcting the charge. In this embodiment, it goes without saying that if the liquid crystal used is changed to another type, the set values of the pulse width and peak value must be changed. Embodiment 2 In this embodiment, a case will be described in which a charge correction pulse is applied in Embodiment 1. The peak value Vc1 of the charge correction pulse in the scanning electrode waveform 101 in FIG. 1 is set so as to neutralize the total charge amount of the reset pulse, set pulse, and memory stabilization pulse. Here, Vcl is 16.5
It was set to V. This corrects the imbalance in charge during driving and increases the stability of the memory state. In fact, in the first embodiment, the device operates for a while after setting the voltage, but after that it was necessary to frequently adjust VC2, but in this embodiment, there is almost no need for this. Example 3 In Examples 1 and 2, the selection period was required to be at least three times the response speed of the liquid crystal, but here, the selection period was required to be twice the response speed of the liquid crystal.
We will explain a method that allows you to select at twice the speed. Second
The figure shows a timing chart of drive waveforms in this embodiment. A method for setting the peak value of each waveform will be explained. First, the scanning electrode waveform 201 will be explained. The peak value Ve of the reset pulse and the set pulse Vw are determined in the same manner as in the first embodiment. The memory stabilizing pulse is positive or negative depending on the polarity of the panel, and the peak value Vc2 is determined depending on the magnitude of the polarity. However, the pulse width shall be half of the set pulse. The peak value Vcl of the charge correction pulse was determined in the same manner as in Example 2. Next, the signal electrode waveform 202 will be explained. The pulse width is half that of Example 1, and the peak value Vd is set to be greater than (Vsat-Vth) and less than Vw*2. If the signal waveform shown in selection period 1 in FIG. 2 202 is output, the combined waveform 203 with 201
The dark state is selected at , the signal waveform shown in selection period 2 is output, and the bright state is selected. The optical response of the element at this time is shown in 204. In this example, Hoechst@Fe1ixool was used as the liquid crystal as a base, and the liquid crystal layer was 2 μm thick, Pw
=120. usec, Ve=-25V, Vw=12V,
Vc2=-3, OV, Vcl=15.5V, Vd=3.
It was 0V. When Vc2 was set to OV, the contrast was 12:1, but when Vc2 was set to -3.0■, the contrast was 17:1. In this embodiment, the charge correction pulse is set outside the selection period, but it may be set within the selection period. However, the selection period will be longer. In this embodiment, it goes without saying that if the liquid crystal used is changed to another type, the set values of the pulse width and peak value must be changed. Although the embodiments have been described above, the present invention can be applied not only to the waveforms of the above embodiments but also to any drive waveform of the ten-set pulse type with reset pulses. It can also be applied to elements having threshold characteristics and memory properties, such as ferroelectric display elements. [Effects of the Invention] As described above, according to the present invention, the memory state is stabilized during the non-selection period during time-division driving of an electro-optical element using a ferroelectric liquid crystal, and furthermore, the effect of long life can be achieved. have

[Brief explanation of drawings]

FIG. 1 is a timing chart of drive waveforms in Embodiment 1 of the present invention. FIG. 3 is a drive waveform diagram in a conventional example. 301...Scanning electrode signal waveform 302...Signal electrode signal waveform 303...Synthetic waveform 304...Optical response and above Applicant Seiko Epson Corporation Agent Patent attorney Yoshi Suzuki Part 3 (1 other person) 101...
...Scanning electrode signal waveform 102...Signal electrode signal waveform 103...Synthetic waveform 104...Optical response FIG. 2 is a timing chart diagram of the form according to the third embodiment of the present invention. 201...Scanning electrode signal waveform 202...Signal electrode signal waveform 203...Synthesized waveform 204...Optical response drive wave

Claims (2)

[Claims] (1) An element having a ferroelectric liquid crystal or a ferroelectric substance sandwiched between at least a substrate on which a scanning electrode is formed and a substrate on which a signal electrode is formed is placed in the first state during or before the selection period. A pulse (reset pulse) is applied to set the element to the second state during the selection period, and a pulse (set pulse 2) having an effective value that puts the element into the second state or a pulse below the threshold value that puts the element into the second state is applied depending on the display content during the selection period. In a driving method that applies a pulse (set pulse 1), after applying the set pulse, the product of the peak value and the pulse width is the wave of set pulse 1, with a polarity that cancels out the charge asymmetry that exists in the thickness direction of the element. A method for driving a liquid crystal element, characterized by applying a pulse smaller than the product of a high value and a pulse width. (2) The method for driving an electro-optical element according to claim 1, characterized in that, before applying the reset pulse, a pulse is applied that charges neutralizes the entire waveform within one frame.