JP2881303B2 - Ferroelectric liquid crystal electro-optical device - Google Patents

Description

The present invention relates to a ferroelectric liquid crystal electro-optical device. In particular, the present invention relates to an improvement in a driving circuit for compensating for temperature dependence of a ferroelectric liquid crystal. SUMMARY OF THE INVENTION In the present invention, in a ferroelectric liquid crystal electro-optical device, a driving circuit is controlled by dividing a temperature-responsive frequency variable oscillation circuit and an output reference clock signal from the oscillation circuit at a constant rate and shaping a waveform. The signal generating circuit includes a timing generating circuit for outputting a signal and a driver for supplying a driving pulse having a predetermined pulse width in response to the control signal. Over, a drive pulse having a pulse width that allows the liquid crystal molecules to respond electrically can be applied. [Prior art] Conventionally, a ferroelectric liquid crystal cell (hereinafter simply referred to as “switching”) that drives a ferroelectric liquid crystal such as a chiral mectic liquid crystal (hereinafter referred to as “SmC ^* ”) molecule by mutually electrically switching the bistable state thereof (hereinafter referred to as switching) The liquid crystal cell) and its driving circuit are known. It has also been known to adjust the pulse width of an applied pulse required for switching (hereinafter referred to as an inversion pulse) in order to compensate for the operation of a liquid crystal cell having temperature dependence (see, for example, Japanese Patent Application Laid-Open No. 61-18931). ). FIG. 2 shows a perspective view of a conventional liquid crystal cell. Reference numeral 1.1 denotes a pair of substrates arranged to face each other. Reference numeral 2.2 denotes a uniaxial or random horizontal alignment film provided on the inner surface of the substrate. Reference numeral 3 denotes an SmC ^* thin film sandwiched between the alignment films 2.2. SmC ^* originally has a helical layer structure, but when formed into a thin film sandwiched between alignment films, liquid crystal molecules are layered and horizontally aligned as shown in the figure.
However, when the SmC ^* thin film 3 is viewed from above, the molecular axis is inclined θ with respect to the normal 4 of the layer. There are two ways in this position: a first stable state 5 and a second stable state 6. Incidentally, the head of the SmC ^* molecule has a spontaneous polarization 7 in a direction orthogonal to the molecular axis. The direction of the spontaneous polarization 7 is perpendicular to the substrate 1 and has the opposite polarity between the bistable states. Therefore, the stable state can be mutually switched by applying a pulse of a predetermined polarity. 8.8 is a pair of polarizing plates having polarization axes orthogonal to each other, and is an optical conversion member that identifies a first stable state domain and a second stable state domain of liquid crystal molecules as light passing and light blocking by birefringence. . Electrodes 9 and 10 are opposed to each other and apply an inversion pulse to SmC ^* . FIG. 3 shows the electrode configuration. 9 is a scanning electrode and 10 is a signal electrode. A matrix is formed by both electrodes, and a well-known time-division driving is performed. FIG. 4 is an example of a driving waveform applied to one of the matrix pixels shown in FIG. First, an AC pulse having a peak value V _OP and a pulse width τ that are equal to or larger than a threshold during the first selection period is added for one cycle. Now the polarity of the first half pulse is the first in the liquid crystal molecules
Assuming that the current state is in the direction of switching from the stable state to the second stable state, the polarity of the second half pulse switches in the opposite direction (second stable state → first stable state).
As a result, the switching of the first half pulse is not preserved, and the switching by the second half pulse is always effective. Therefore, the latter half pulse is the inversion pulse. Next, during the non-selection period, an AC pulse having a peak value equal to or less than the threshold value is applied, and the previously obtained first stable state is stored. Further, during the second selection period, a pulse having a polarity opposite to that of the pulse applied during the first selection period is applied. Therefore, switching from the first stable state to the second stable state is performed by the second half pulse. As described above, the switching between the bistable states is performed by scanning one frame. By the way, the switching speed of the bistable state has temperature dependence, and the response speed becomes lower as the temperature becomes lower.
Therefore, a technique for compensating temperature by increasing the inversion pulse width τ as the temperature decreases as shown in FIG. 4 is disclosed in the above-mentioned prior art. [Problems to be Solved by the Invention] However, the prior art document does not disclose any specific driving circuit for temperature correction of τ. Further, it has been clarified that there is not only a lower limit but also an upper limit at each temperature as an appropriate value of the inversion pulse width τ. Therefore, in order to realize complete temperature compensation, it is necessary to always keep the value τ between the upper limit value and the lower limit value. However, there has been a problem that temperature compensation taking both of these into consideration has not been conventionally performed. [Means for Solving the Problems] The present invention aims to solve the problems of the related art, and over the entire operating temperature range of the liquid crystal cell, the inversion pulse width τ is set to an upper limit value for each temperature and It is set between the lower limit values. In particular, it utilizes the reference clock pulse frequency control of the drive circuit. That is, in a ferroelectric liquid crystal electro-optical device including a liquid crystal cell that operates by switching a bistable state of SmC ^* having a temperature dependence in response characteristics and a driving circuit thereof,
The drive circuit includes a variable oscillation circuit that generates a reference clock present in a predetermined frequency range according to an external temperature, a timing generation circuit that divides the reference clock at a constant rate to generate a control signal, The driver is configured to apply an inversion pulse having a pulse width to which the liquid crystal can respond in response to the electrode. [Operation] FIG. 1 shows a liquid crystal cell (the cell shown in FIG. 2 has the following SmC ^*
2 shows the dependence of the inversion pulse width on the temperature of the mixture (with the mixture enclosed). The driving condition is a 1/3 bias method of V _OP = 5V. As is clear from the figure, in a region below the straight line indicating the lower limit, τ is too small, and the liquid crystal molecules do not respond. The response becomes slower as the temperature decreases. In addition, in the region above the straight line indicating the upper limit value, the inversion pulse width τ is too large and an excessive torque is applied to the liquid crystal molecules, so that switching becomes unstable, and the liquid crystal molecules once switched to the stable state return to the original stable state. And as a result, switching is not performed. Therefore, in the present invention, temperature compensation is performed so that the inversion pulse width τ is always set in a region sandwiched between the upper limit line and the lower limit line. The present inventors have proposed a drive circuit with linear temperature compensation shown in FIG. 5 as a means for solving the above problem. This will be described in conjunction with the time chart of FIG. Reference numeral 11 denotes a variable frequency oscillation circuit having a positive temperature response. Details will be described later. Output of the oscillation circuit 11 provides a reference clock pulse CL ₂ is divided by the frequency divider 12 as desired. 13 the CL ₂ shaping frequency-divided in accordance with a predetermined frequency division ratio to enter CL ₂ a timing generation circuit, a control signal required for driving the scan electrodes FLM, CL ₁
And create a DF. FLM is a scanning electrode (hereinafter referred to as common) selection signal, and one common is selected during this period. CL ₁ is a common shift signal, and assigns FLMs to respective commons in line order. DF is an AC signal, and converts the signal applied to the common into AC for one cycle. As can be seen from FIG. 6, the inversion pulse τ matches the pulse width τ ′ of the DF. τ ′ is 1/2 of the period of DF. DF is the period of from the resulting DF by dividing the CL ₂ is determined by the number of periods of the CL _2. Thus the pulse width of the inverted pulse (see FIG. 4) by adjusting the frequency of CL ₂ tau
Can be adjusted. That is, the frequency of CL ₂ and the inversion pulse τ
Are inversely proportional. 14 is a common driver,
FLM, and outputs to the common of the LC cell by combining the common driving voltage to enter CL ₁ and DF. A ROM 15 stores display data. Read by CL _2. Reference numeral 16 denotes a segment driver for driving a signal electrode (hereinafter, referred to as a segment), which inputs DATA from the ROM 15 and, in accordance with this, applies a segment drive voltage CL to each segment in parallel.
Output in synchronization with ₁ . The LC cell is shown in FIGS.
Use the one shown in the figure. SmC ^{* The} material shown in the section of action is used. The driving waveform shown in FIG. 4 is used. Now, the variable frequency oscillator 11 will be described in detail. This oscillator is a multivibrator composed of two inverters, one capacitor C and one thermistor R17, and has an oscillation frequency of 1 / 2.2CR. Here, if a thermistor 17 having a negative temperature characteristic is used, R increases as the temperature decreases, and thus R decreases. Becomes smaller when
The frequency of ₂ also decreases, and τ of the inverted pulse, which is inversely proportional thereto, increases. Since the thermistor R changes exponentially, τ also changes exponentially. As can be seen from FIG. 1, since the temperature dependence of the switching speed is also exponential, the temperature compensation using a thermistor can set τ to a preferable region shown in FIG. Further, another drive circuit with linear temperature compensation shown in FIG. 7 was proposed. Except for the variable frequency oscillator 11, it is the same as FIG. The variable frequency oscillator 11 has a configuration in which a constant current is supplied from a constant current source 18 to a thermistor 17 having a positive temperature characteristic, and a voltage generated at both ends of the thermistor 17 is applied to a voltage controlled oscillator 19 (VCO). The VCO used here has an output frequency that increases in proportion to the input voltage (see FIG. 9).
Therefore, when the temperature decreases, the R of the thermistor 17 decreases because it has a positive temperature characteristic. Therefore, the voltage across the thermistor 17 decreases. Therefore, the frequency of the output of the VCO becomes smaller, and the inverse pulse τ is inversely proportional to this.
Becomes larger. Since this tendency coincides with the tendency of the response characteristics shown in FIG. 1, it is possible to perform temperature compensation in which τ is always set to a preferable region shown in FIG. FIG. 8 is a diagram showing an embodiment of the present invention showing a drive circuit with discrete temperature compensation. The difference from the proposed example shown in FIG. 7 is that the output voltage V across the thermistor 17 is digitally processed and then input to the VCO 19, and the other points are the same. In this digital processing, τ is set to a constant value of 40 msec in a range of 0 ° C. to 10 ° C. in FIG.
It is performed to set τ to 9 msec at ec to 21 ° C to 30 ° C, to 4 msec at 31 ° C to 40 ° C, and to 2 msec at 41 ° C to 50 ° C. That is, τ is set stepwise within a preferable region shown in FIG. Reference numeral 20 denotes an A / D converter, which converts the analog output voltage V of the thermistor 17 into five-stage digital data according to the above-described temperature division. Therefore, the output only needs to be at least 3-bit data. Reference numeral 21 denotes a ROM which stores the input voltage value of the VCO 19 as digital information. The 3-bit output of the A / D converter 20 is accepted, and this is used as an address to output predetermined voltage digital data. The voltage digital data is input to the D / A converter 22, and a predetermined voltage is applied to the VCO 19. The following table summarizes the digital processing during this time. First, 3-bit data of (0.0.0) to (1.0.0) is output to the output of the A / D converter 20 according to the temperature range of five stages. Using this as an address, the voltage value (0.26 to 0.70) is read from the conversion ROM table 21 and applied to the VCO 19 via the D / A converter 22. According to the characteristic curve of the input voltage and the oscillation output frequency of the VCO 19 shown in FIG. 9, the VCO output (this is supposed to be used as it is for the CL ₂ ) CL ₂ 300 Hz to 6000 Hz
Get. Now, the relationship between CL ₂ and τ is that the number of segment electrodes is now 2
If there are four, CL ₂ = (1 / τ × 2) × 24. In other words, since the common scan frequency is 1 / τ × 2 (doubled because τ corresponds to a half cycle), 24 serial segment data are processed for each scan, so that 1 / τ × 2 × reference clock CL ₂ of 24 times faster than 2 than is necessary. Now, the result of obtaining τ from the value of CL ₂ by this equation is shown on the right side of the table. As described above, it is understood from the table that discrete temperature compensation of τ is performed. [Effect of the Invention] As described above, according to the present invention, in order to compensate for the switching temperature dependence of the ferroelectric liquid crystal cell, the inversion pulse width is always adjusted to an appropriate value by adjusting the reference clock within a predetermined range. It can be set to a value, and there is an effect that stable driving can be performed in a wide operating temperature range. Since the reference clock is adjusted, the operation of the entire drive circuit is controlled collectively in accordance with each temperature condition, so that there is an effect that the efficiency is high.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing the temperature dependence of an inversion pulse width, FIG. 2 is a perspective view of a conventional liquid crystal cell, FIG. 3 is an electrode arrangement diagram of a conventional liquid crystal cell, and FIG. Is a driving waveform diagram of a conventional liquid crystal cell, FIGS. 5 and 7 are driving circuit diagrams with linear temperature compensation,
FIG. 6 is a time chart for explaining the circuit operation, FIG. 8 is a drive circuit diagram with discrete temperature compensation, and FIG. 9 is a VCO oscillation frequency characteristic diagram. 1.1 Substrate 2.2 Alignment film 3 Chiral smectic liquid crystal thin film 8.8 Polarizers 9, 10 Electrodes 11 Frequency variable oscillation circuit 13 Timing generation circuit 14 Common driver 16 Segment driver

   ────────────────────────────────────────────────── ─── Continuation of front page    (72) Inventor Masaaki Taguchi               6-31-1, Kameido, Koto-ku, Tokyo               Co., Ltd. (72) Inventor Kokichi Ito               6-31-1, Kameido, Koto-ku, Tokyo               Co., Ltd.                (56) References JP-A-60-123825 (JP, A)                 JP-A-50-46496 (JP, A)                 JP-A-56-125792 (JP, A)                 JP-A-49-48384 (JP, A)

Claims (1)

(57) [Claims] A ferroelectric liquid crystal thin film, a pair of substrates sandwiching the thin film, an alignment film in contact with the plane of the thin film to provide a bistable state of liquid crystal molecules, and a predetermined pulse width for sandwiching the thin film and switching the bistable state A liquid crystal cell comprising an electrode for applying a voltage and a conversion member for optically identifying a bistable state,
A ferroelectric liquid crystal electro-optical device comprising a drive circuit for applying a drive voltage to the electrode, wherein the drive circuit generates a reference clock present in a predetermined frequency range according to an external temperature; The variable oscillation circuit includes a timing oscillation circuit that divides a clock at a fixed rate to generate a control signal, and a driver that applies a drive pulse having a pulse width to which the liquid crystal can respond in response to the control signal to the electrodes. It consists of a thermistor, a constant current source, and a voltage-controlled oscillator whose output frequency increases in proportion to the input voltage. The output frequency of the variable oscillation circuit decreases as the external temperature decreases. A ferroelectric liquid crystal electro-optical device, wherein linear temperature compensation is performed so as to increase a pulse width.