JPH0833535B2 - Ferroelectric liquid crystal electro-optical device - Google Patents

Description

TECHNICAL FIELD The present invention relates to a driving method of an electro-optical conversion device utilizing spontaneous polarization and negative dielectric anisotropy of a ferroelectric liquid crystal.

[Outline of Invention]

The present invention utilizes the spontaneous polarization of a ferroelectric liquid crystal to selectively align liquid crystal molecules in a first stable state or a second stable state, and to reduce the negative dielectric anisotropy of the ferroelectric liquid crystal. In an electro-optical conversion device that performs line-sequential scanning while maintaining each stable state, a DC pulse is applied to the selected dot and a chopping pulse is applied to the non-selected dot during the selection period on the scanning electrode side. It was possible to obtain an electro-optical conversion device having a high contrast by applying an AC pulse having a high frequency to all the dots on the scanning electrode side during the non-selection period.

[Conventional technology]

2. Description of the Related Art An electro-optical converter using spontaneous polarization and negative dielectric anisotropy of a ferroelectric liquid crystal has been known. For example, it is disclosed in JP-A-60-176097.

FIG. 2 shows a perspective view of a conventional ferroelectric liquid crystal cell (hereinafter, simply referred to as a liquid crystal cell). Reference numerals 1 and 1 denote a pair of transparent glass substrates which are arranged to face each other. Numerals 2 and 2 are uniaxial horizontal alignment films arranged on the inner plane of the substrate 1, for example, a polyimide rubbing film is used. The rubbing directions of the pair of alignment films are substantially parallel. 3 is a ferroelectric liquid crystal, for example, a chiral smectic liquid crystal (hereinafter referred to as SmC ^* ), which has spontaneous polarization in a direction orthogonal to the long axis of the liquid crystal molecule (hereinafter referred to as molecular axis), and particularly Then, a material having a negative dielectric anisotropy Δε at least above a certain frequency is selected.

Δε <0 means that an external electric field in a constant frequency region causes induced polarization in a direction orthogonal to the long axis of the molecule and a dielectric torque acts, so that it is perpendicular to the electric field direction, specifically, parallel to the glass substrate. The molecule moves so that SmC ^* 3
Molecules are sandwiched between the substrates 1 and 1, and are horizontally aligned as shown in the figure due to the influence of the alignment films 2 and 2 to form a layer.
Reference numerals 4 and 5 denote a pair of electrodes which sandwich the SmC ^* 3 thin film and which are opposed to each other for applying a driving voltage.

FIG. 3 shows a conventional drive waveform diagram of a liquid crystal cell. A first DC pulse having a positive polarity is applied between the electrodes 4 and 5.
However, the electrode 4 is set to the ground potential. Then, the molecules are oriented so that the spontaneous polarization 6 of the liquid crystal molecules is aligned in a vertical position toward the electrode 4 (see FIG. 2). This is the first stable state 7
Which has a molecular axis tilted by + θ with respect to the normal 8 of the SmC ^* layer. When an AC pulse is applied next, dielectric polarization occurs in the direction orthogonal to the molecular axis due to the negative dielectric anisotropy in this frequency region, and the first stable state is maintained and fixed by the dielectric torque. When a second DC pulse having a negative polarity is further applied between the electrodes 4 and 5, the liquid crystal molecules respond and the spontaneous polarization 6 is realigned so that the spontaneous polarization 6 is vertically oriented to the electrode 5.
This is the second stable state 9, which is the position where the molecular axis is tilted by −θ with respect to the normal line 8 of the SmC ^* layer (see FIG. 2). After that, the second stable state is maintained by the application of the AC pulse. That is, the positive DC pulse writes the first stable state, the negative DC pulse writes the second stable state, and the AC pulse holds the stable state.

Returning to FIG. 2 again, 10 and 10 are a pair of polarizing plates whose polarization axes are orthogonal to each other, sandwiching the SmC ^* thin film 3 and utilizing the birefringence to the liquid crystal domain in the first stable state and the liquid crystal domain. The two stable liquid crystal domains are optically distinguished. For example, the first stable state 7 is the light blocking state (hereinafter referred to as black) the second
Stable state 9 is identified as a light transmitting state (hereinafter referred to as white).

[Problems to be solved by the invention]

Now, in the above-mentioned prior art document, the electrode arrangement of the above-mentioned liquid crystal cell has a matrix structure as shown in FIG. 4, and the scanning electrode group 4 (hereinafter referred to as common) and the signal electrode group 5 (hereinafter referred to as segment). It is disclosed that the two are opposed to each other. However, there is no disclosure about a drive waveform or a drive circuit for actually performing the line-sequential drive. It is impossible to perform matrix driving with the waveforms shown in FIG.

[Means for solving problems]

An object of the present invention is to provide a driving circuit for matrix driving a conventional liquid crystal cell.

The present invention will be described with reference to FIG. FIG. 1 (A) is a matrix electrode configuration diagram of a liquid crystal cell. 2 segments
S ₁ and S ₂ and two commons C ₁ and C ₂ are arranged so as to form _four matrix pixels (hereinafter referred to as dots) D ₁ , D ₂ , D ₃ and D ₄ . Other configurations of the liquid crystal cell are as shown in FIGS. 2 and 4.

FIG. 1 (B) shows the waveform applied to each dot. The potential reference is the segment potential. In this example, common C ₁ is selected by line-sequential scanning, and common C ₁
Only the upper dot D ₁ is written in the first stable state, that is, black is written, the dot D ₂ remains in the previous state, and the dots D ₃ and D ₄ on the non-selected common C ₂ also retain the previous state. Waveform is applied.

A DC pulse having a peak value of ± V ₁ is applied to the dot D ₁ during the selection period, while a chopping pulse having a peak value of ± V ₂ is applied to the dot D ₂ . Comparing the two pulses, it can be seen that the DC component is exactly the same ± V ₁ , but the chopping pulse is weighted by a high-frequency AC pulse of ± V _{1 on} this DC component. The response of the liquid crystal molecules is suppressed by this weighted high frequency AC pulse,
Dot D ₂ will retain its previous state. This detailed description will be given in the section on action.

Now the D ₃ and D ₄ of the non-selected dot AC pulse of high frequency is applied. The frequency of this high-frequency AC pulse is selected to be larger than the maximum response frequency due to spontaneous polarization. Therefore, only the dielectric torque based on Δε <0 acts on the liquid crystal molecules, and the liquid crystal molecules are held so as to be horizontal with the glass substrate.

Therefore, the stable state is not inverted and the previous state is maintained. This frequency is selected to be equal to the frequency of the weighted high frequency AC pulse applied to the dot D ₂ .

If the scanning described above is performed line-sequentially for many commons and segments (that is, if commons are scanned)
Black is written in a dot that should be black when one frame is completed.

Next, in order to write white on a dot that should be white, the waveform and phase applied to D ₁ in FIG.
Apply 180 ° inverted waveform. Hereinafter, applies the D _2, D _3, waveform inverted by 180 ° every also applied waveform phase of D _4. If the common is scanned using such a waveform, white is written in the dots at the time when one frame is completed. That is,
One screen is formed by writing black dots in the first frame and white dots in the next frame.

FIG. 1 (C) shows a waveform applied to the segment and the common in order to create a drive waveform applied to the dots D _{1 to} D ₄ shown in FIG. 1 (B). a is a common selection signal applied to the common C ₁ , b is a common non-selection signal applied to the common C ₂ , C is a black write signal applied to the segment S ₁ , and d is a non-selection signal applied to the segment S _2. It is a write signal. In addition, this waveform is the first frame (hereinafter,
The waveform applied in the first frame) and the waveform in the next frame (hereinafter, referred to as the second frame) are all 180 ° in phase inversion as described above, and In the frame, a non-write signal is applied to the segment S ₁ to which the black write signal is applied, and a white write signal (the phase of the black write signal is 180 ° inverted) is applied to the segment S ₂ to which the non-write signal is applied. Is applied.

Specific circuits for producing these common and segment signals will be described in the embodiment section.

By the way, the non-selection period, that is, the peak values of the waveform applied to the dots D ₃ and D ₄ are ± 2V ₁ and ± V ₁ in FIG. ₁ (B).
The role of this voltage waveform is to hold the liquid crystal molecules horizontally with the glass substrate by the dielectric torque. Therefore, if the value of the dielectric anisotropy of the liquid crystal material is different, the peak value of the voltage waveform is also changed accordingly. Come on.

In such a case, if the peak value of the waveform applied to the common C ₂ is arbitrarily changed, the peak value of the waveform applied to the dots D ₃ and D ₄ also changes. For example, in FIG. 1 (C), if the peak value applied to the common C ₂ is now ± 2V ₁ ,
The waveform applied to the dot D _3, D ₄ of FIG. 1 (B), a dot D ₃ ± 3V _1, a waveform dot D ₃ with a peak value of ± 2V _1.

[Action]

We explain that the SmC ^* molecule does not respond to the chopping pulse, but does respond to the DC pulse. FIG. 5 shows a test pulse applied to a dot of the liquid crystal cell shown in FIGS. 2 and 4. a is a pulse in which a DC pulse 2 having a peak value of ± V having a positive polarity and a DC pulse having a peak value of −V having a negative polarity are continuous during the selection period (3 msec). The display state changes from black to white. Similarly, b is a waveform in which a chopping pulse having a peak value of + 2V is applied in the first half and a chopping pulse having a peak value of -2V is applied in the latter half of the selection period.

FIG. 6 shows the contrast ratio when the waveforms of FIGS. 5A and 5B are applied while changing V and the voltage changes from black to white during the selection period at each voltage level. In the case of the DC pulse a, a large contrast ratio can be obtained at about V = 30V or more. That is, the SmC ^* molecule makes a complete transition from the first stable state to the second stable state at a threshold value of 30 V or higher.

However, in the case of the chopping pulse b, it can be seen that even if a pulse having an amplitude of 60 V is applied, there is little change in the transmitted light intensity and the SmC ^* molecule does not completely transition from the first stable state to the second stable state. This can be explained as follows. SmC ^*
The physical properties that contribute to the inversion mechanism of the molecule are considered to be due to spontaneous polarization and dielectric torque. The former torque due to spontaneous polarization always works so that the electric field and the direction of spontaneous polarization are parallel to each other regardless of whether Δε is positive or negative.

However, in the latter dielectric torque, Sm with Δε> 0
In C ^* liquid crystal, molecules are parallel to the electric field and SmC ^* with Δε <0 ^.
In liquid crystal, it works to make molecules perpendicular to the electric field. That is, in the system of Δε <0, the torque due to spontaneous polarization (the molecule moves in the direction parallel to the electric field in the initial stage when the molecule transitions from the first stable state to the second stable state) and the dielectric torque Will work in the opposite direction. Therefore, it is considered that the system with Δε <0 has a slower response than the system with Δε> 0. This dielectric torque is proportional to the effective value of the voltage and the value of the dielectric anisotropy Δε, but in the chopping pulse and the DC pulse shown in FIG. 1 (B), the former is 2V ₁ and the latter is the effective value of V _1. Because it has, the former works twice as strongly as the latter. Therefore, the response of the chopping pulse becomes slower than that of the DC pulse, and when the pulse width is inverted with a constant pulse width as shown in FIG. 6, the first state cannot be completely inverted to the second state.

The SmC ^* liquid crystal used was a Merck 3234.
ε = −2.4.

〔Example〕

FIG. 7 shows a common electrode drive circuit for producing the common selection signal a and the common non-selection signal b shown in FIG. 1 (C). As can be seen from FIG. 1 (C), the required voltage levels are + V ₁ and −V ₁ , and the required AC signal is 2 in the first half and the second half of the selection period.
DF1 for dividing and DF2 for making high frequency necessary for maintaining stable state (see time chart in FIG. 9). DF2 is also used for chopping. Reference numeral 11 is a shift register, which is a common shift pulse CL for line-sequentially distributing the signals FLM and FLM designating the selection period to each common.
Enter 1. The output of the shift register 11 is connected to the gate group 12. The gate group 12 receives DF1 and DF2 and a signal obtained by dividing the signal FLM by a JK flip-flop, and its output controls the transmission gates 13 and 14.
The input of the transmission gate 13 is -V ₁ potential,
Its output is applied to each common. The input of transmission gate 14 is at + V ₁ potential and its output is applied to each common. Now, when the output of the shift register 11 is HIGH, the gate group 12 receives DF1 and makes the transmission gate 14 conductive in the first half and makes the transmission gate 13 conductive in the second half.

As a result, the common selection signal shown in FIG. 1 (a) is output to the common C ₁ . Also, the output of the shift register 11 is LO
When W, gate group 12 accepts DF2 and synchronizes with DF2 + V
_1, and outputs the AC pulse oscillating between -V ₁ to the common C _2. This is the common non-selection signal shown in b of FIG. 1 (C). The above description is for the first frame and the second
Since it is necessary to invert the phases of all waveforms in the frame by 180 °, a signal obtained by dividing the FLM is applied to the exclusive OR of the gate group 12.

FIG. 8 is applied to the segment shown in FIG. 1 (C),
It is a segment electrode drive circuit which makes c and d.

As can be seen from FIG. 1 (C), the required voltage level is +
There are three transmission gates, V ₁ , 0, -V ₁ ,
Delivered to the segment via 16, 17 and 18. The signals that control ON-OFF of each gate are the alternating signal DF2 and the latch output.

A shift register 19 reads and stores serial video data DATA at a high speed clock CL2. A latch circuit 20 latches the video data parallelized by the shift register 19 in synchronization with the clock CL1 and outputs HIGH or LOW information according to the line sequential timing CL1.

Now, the data that appears at the output terminal 0 ₁ of the latch circuit 20 is HIGH.
At the time, the transmission gate 17 is turned on and the zero level potential is output to the segment S ₁ . Therefore, the waveform shown in c of FIG. 1 (C) is obtained at S ₁ . Further, when the data appearing at the output terminal 0 ₂ of the latch circuit 20 is LOW, the transmission gate 18 is ON, + V _1, the voltage of -V ₁ is output to the segment S _2. The waveform at this time is that the transmission gates 15 and 16 are alternately turned on and off by the alternating signal DF2.
As a result, a waveform such as d in FIG. 1 (C) is obtained. The above description is for the first frame, and for the second frame, the phases of all waveforms must be inverted by 180 °, so the FLM signal is divided by a JK flip-flop and the exclusive logic of the DF2 signal is used. The sum is applied and applied to the transmission gates 15 and 16. Further, as described above, in the first frame and the second frame, since the DATA signal of the segment has to be inverted, the exclusive OR of the output of the JK flip-flop and the DATA signal is taken and input to the shift register 19. are doing.

〔The invention's effect〕

As described above, according to the present invention, in the electro-optical device for writing the two optical states of white and black by utilizing the spontaneous polarization and the negative dielectric anisotropy of the SmC ^* molecule, the selection during the scanning period is performed. By applying a DC pulse to the dots and a chopping pulse to the non-selected dots, while applying a high frequency symmetrical AC pulse to all the dots during the non-scanning period, electro-optical conversion with high contrast is achieved. The effect is that the device can be obtained.

[Brief description of drawings]

FIG. 1 (A) is a matrix electrode configuration diagram, FIG. 1 (B) is a waveform diagram applied to a matrix dot of the present invention, and FIG. 1 (C) is applied to a common electrode and a segment electrode of the present invention. Waveform diagram, FIG. 2 is a perspective view of a conventional liquid crystal cell, FIG.
FIG. 4 is an operation waveform diagram of a conventional liquid crystal cell, FIG. 4 is an electrode arrangement diagram of the liquid crystal cell, FIG. 5 is a test waveform diagram for explaining the action, and FIG. 6 is a contrast ratio application for explaining the action. FIG. 7 is a common electrode drive circuit diagram, FIG. 8 is a segment electrode drive circuit diagram, and FIG. 9 is a timing chart for common and segment drive circuits. 1,1 …… Substrate 2,2 …… Alignment film 3 …… SmC ^* Thin film 11,19 …… Shift register 12 …… Gate group 13,14,15,16,17,18 …… Transmission gate 20 …… Latch circuit

Claims (4)

[Claims] 1. A ferroelectric liquid crystal thin film having spontaneous polarization in a direction perpendicular to the long axis of liquid crystal molecules and having a negative dielectric anisotropy in at least a high frequency region, and a glass substrate sandwiching the thin film. A pair of electrodes for applying a voltage in the thickness direction of the thin film, a pulse for aligning liquid crystal molecules in a first stable state or a second stable state, and a high-frequency AC pulse for maintaining the stable state to the electrodes Circuit and first of liquid crystal molecules
In the ferroelectric liquid crystal electro-optical device including a conversion member that optically distinguishes between the second stable state and the second stable state, the pulse applied to the selected dot during the selection period on the scan electrode side supplied by the drive circuit is a direct current. The pulse, which is composed of pulses and is applied to the non-selected dots during the selection period on the scan electrode side, is formed by superposing a high-frequency AC pulse on the DC pulse, and further during the non-selection period on the scan electrode side. The ferroelectric liquid crystal electro-optical device characterized in that the pulse applied to all the dots in (1) is composed of a high frequency AC pulse. 2. The frequency of a high-frequency AC pulse superposed on a DC pulse that is a pulse applied to a non-selected dot during the selection period on the scan electrode side is all within the non-selection period on the scan electrode side. 2. The ferroelectric liquid crystal electro-optical device according to claim 1, wherein the frequency is the same as the frequency of a high-frequency AC pulse that is a pulse applied to the dot. 3. The ferroelectric liquid crystal electrical device according to claim 1, wherein the frequency according to claim 2 is higher than the maximum frequency at which the ferroelectric liquid crystal thin film responds by spontaneous polarization. Optical device. 4. The peak value of a high frequency AC pulse applied to all the dots in the non-scanning period is such that the dielectric torque generated by the AC pulse causes molecules in the ferroelectric liquid crystal thin film to move to the glass substrate. 2. The ferroelectric liquid crystal electro-optical device according to claim 1, wherein the ferroelectric liquid crystal electro-optical device has a voltage value equal to or higher than a level at which the liquid crystal is leveled substantially or completely.