JPH0448366B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a liquid crystal device, and particularly to a liquid crystal device using ferroelectric liquid crystal and having an improved drive circuit.

[Summary of the Disclosure] This specification and drawings describe the para state of liquid crystal molecules by applying an alternating current voltage to all pixels before applying a display signal by multiplexing drive in a liquid crystal device using ferroelectric liquid crystal. A liquid crystal device with good characteristics such as high contrast and low flicker can be constructed using a simple electrical circuit.

[Prior Art] In recent years, emphasis has been placed on the development of ferroelectric liquid crystal elements in place of nematic liquid crystals, which have been widely used in the past. Ferroelectric liquid crystal elements can be made to have bistability by changing the cell construction method, so it is expected that high time division liquid crystal display elements will be realized.

FIG. 6 schematically depicts an example of a ferroelectric liquid crystal cell. 21 and 21' are In ₂ O ₃ , SnO ₂
It is a substrate (glass plate) coated with transparent electrodes such as ITO (Indium-Tin-Oxide), etc., and an SmC ^* phase liquid crystal with liquid crystal molecules 22 oriented perpendicular to the glass surface is sealed between them. ing. A thick line 23 represents a liquid crystal molecule, and this liquid crystal molecule 23 has a dipole moment (P⊥) 24 in a direction perpendicular to the molecule. Board 2
When a voltage higher than a certain threshold is applied between the constant electrodes 1 and 21', the helical structure of the liquid crystal molecules 23 is unraveled, and the crystal molecules 23 are turned so that all dipole moments (P⊥) 24 are directed in the direction of the electric field. The orientation direction can be changed. The liquid crystal molecules 23 have an elongated shape and exhibit refractive index anisotropy in the long axis direction and the short axis direction. Therefore, for example, if polarizers are placed above and below the glass surface in a crossed nicol positional relationship, It is easily understood that this is a liquid crystal optical modulation element whose optical characteristics change depending on the polarity of applied voltage.
Furthermore, when the thickness of the liquid crystal cell is made sufficiently thin (for example, 1μ), the helical structure of the liquid crystal molecules unravels (non-helical structure) even when no electric field is applied, as shown in Figure 7. The dipole moment P or P' takes either an upward direction 24a or a downward direction 24b.

A liquid crystal cell in which the helical structure of liquid crystal molecules is unwound due to the interfacial effect is called a surface-stabilized cell (SSFLC cell). When an electric field E or E' with a different polarity above a certain threshold value is applied to the SSFLC cell for a predetermined time as shown in FIG.
4a or downward 24b, and accordingly the liquid crystal molecules are aligned in either the first alignment state 25 or the second alignment state 25'.

The advantage of using such a ferroelectric liquid crystal as an optical modulation element is that the response speed is extremely fast and the alignment of liquid crystal molecules has a bistable state. When the voltage is applied, the liquid crystal molecules are aligned in the first alignment state 25, but this state is stable even when the electric field is turned off. Furthermore, when applying an electric field E′ in the opposite direction, the liquid crystal molecules
The molecules are oriented to the orientation state 25' and change their orientation, but they remain in this state even when the electric field is turned off. Further, as long as the applied electric field E does not exceed a certain threshold value, each orientation state is still maintained. In order to effectively realize such fast response speed and bistability, it is preferable for the cell to be as thin as possible, generally 0.5μ to 20μ, especially
1μ to 5μ is suitable. A liquid crystal-electro-optical device having a matrix electrode structure using ferroelectric liquid crystals of this type has been proposed, for example, by Clark and Lagerwall in US Pat. No. 4,367,924.

[Problems to be Solved by the Invention] However, in SSFLC cells, rather than the state in which the liquid crystal molecules are arranged in parallel within the liquid crystal molecular layer (called the para state) as shown in Figure 7, the upper substrate in the molecular layer is A state in which the liquid crystal molecules are twisted toward the lower substrate (referred to as a "twisted state") is easily realized.
When the liquid crystal molecules are twisted in this way, the first
The angle (tilt angle) between the liquid crystal molecule axes in the first alignment state and the second alignment state appears to be smaller, leading to a decrease in contrast and amount of transmitted light, and also to the response of the liquid crystal molecules when switching the alignment state. As a display element, it has various disadvantages, such as fluctuations in the amount of transmitted light due to the appearance of overshoot. Therefore, for display elements, it is desirable that liquid crystal molecules be in a para state.

The present invention was made to solve the above-mentioned drawbacks, and an object of the present invention is to provide a liquid crystal device that stabilizes the para-state of liquid crystal molecules and improves display characteristics.

[Means for Solving the Problems] The present inventors have discovered that the above-mentioned twisted state can be achieved by applying an appropriate alternating voltage (hereinafter referred to as an alternating current voltage for para-alignment) to all pixels of a liquid crystal panel. , it was discovered that it is possible to transition to a para state. That is, the present invention provides a matrix electrode formed of scanning electrodes and information signal electrodes crossed at intervals, and a matrix electrode arranged between the scanning electrode and the information signal electrode, and having two different orientations in the absence of an electric field. express the condition,
A ferroelectric liquid crystal in a first state in which the angle between the average molecular axes of each of the two orientation states is 2θ,
By applying an alternating voltage that is higher than the threshold voltage of the liquid crystal, the angle formed by the average molecular axes of the two mutually different orientation states in the absence of an electric field after the application of the alternating current voltage is removed is made larger than the 2θ. a liquid crystal panel having a ferroelectric liquid crystal in a second state at an angle, b. a first means for sequentially scanning and selecting the electrodes to apply a scan selection signal to the scanning electrodes; c. applying the scanning selection signal to the information signal electrodes; In synchronization with the signal, one polarity voltage or the other orientation state that causes one orientation state of the ferroelectric liquid crystal in the second state is generated at the intersection of the scanning electrode selected for scanning and the information signal electrode. a second means for applying an information signal such that the other polarity voltage is selectively applied; and d) before said scan selection signal and information signal are applied to said scan electrode and said information signal electrode, respectively; The liquid crystal device is characterized by having a third means for applying an alternating voltage to the ferroelectric liquid crystal in the first state.

In the present invention, the application of the display signal and the application of the AC voltage for para alignment to all pixels are controlled by a common drive power supply circuit, and the method of applying the AC voltage for para alignment is to apply the AC voltage for para alignment to the scanning signal side. One method is to apply voltage from either the drive circuit or the information signal side drive circuit, and all the other signal lines are grounded during the period of AC voltage application. The method is selected from two methods of applying an alternating current voltage.

Furthermore, as the AC voltage for para alignment, for example, a bipolar rectangular wave can be applied, and the voltage peak value can be set higher than the voltage value of the display signal required for switching the liquid crystal in the para state. .

[Function] By connecting the liquid crystal drive circuits arranged on each of the scanning signal side and the information signal side to a common drive power supply circuit, the applied voltage for display signals and the AC voltage for para alignment to all pixels can be adjusted as described above. It will be applied from the drive power supply circuit. Specifically, first, prior to multiplexing drive using a display signal, a rectangular pulse having a desired peak value and pulse width is applied, and after the liquid crystal is brought into a para-aligned state in advance, liquid crystal display drive is started. .

[Example] Hereinafter, an example of the present invention will be described in detail based on the drawings.

FIG. 2 is a diagram showing an electrode configuration of a matrix display of scanning signal lines and information signal lines, and an example of display of pixels at each intersection.

In the figure, S ₁ to S ₅ indicate scanning signal lines, and I ₁ to _{I 5} indicate display signal lines. In addition, the pixels in the shaded area in Fig. 2 are "black",
Pixels shown in white correspond to each writing state of "white".

When writing the pixels shown in FIG. 2 using a matrix circuit, that is, scanning the scanning signal lines S ₁ to S ₅ line-sequentially, and writing the columns of information signal lines I ₁ and I ₂ alternately in white and black. The timing chart of
As shown in the figure. In Figure 1, ΔT indicates the write pulse width, and during writing, the electric field in the + direction causes white,
Assume that black is written using an electric field in the − direction. The write pulse (pulse exceeding the threshold) has a pulse width ΔT and a peak value ±3V ₀ .

The pixel writing shown in Fig. 2 is performed by first writing white on the scanning signal line, and then writing black on the selected pixel on the display signal line (line clear - line writing). An auxiliary signal of opposite polarity is applied following the write signal to prevent crosstalk caused by successive signals of the same polarity.

Also, immediately after turning on the drive circuit,
Before such a display signal is applied, an AC voltage for para alignment as shown in FIG. 1 (AC application period) is applied to at least one of all scanning signal lines and information signal lines. However, when the scanning signal line is applied to only one of the information signal lines,
It is assumed that all other signal lines are grounded.

As described above, this AC voltage for para alignment is used to transition the liquid crystal molecules from the twisted state to the para state, and both the voltage peak value and the pulse width can be set larger than the write pulse. In the example described later, the write pulse is 1ms.
10V, whereas the AC voltage is 50Hz and V _PP is approximately
A para state was achieved by applying a rectangular wave of about 30V for several seconds.

The liquid crystal material used here is p-n-octyloxybenzoic acid-p'-(2-methylbutyloxy).
A liquid crystal composition (ferroelectric liquid crystal) whose main components are phenyl ester and p-n-nonyloxybenzoic acid-p'-(2-methylbutyloxy) phenyl ester, and the liquid crystal cell is patterned on a glass substrate. After applying an alignment film such as PVA on a transparent electrode such as ITO, a rubbing process is performed.
In addition, SiO ₂ for insulation is placed between the transparent electrode and the alignment film.
can be formed, and the cell thickness is approximately 1 μm.

In addition, the voltage value, frequency, and application time of the alternating current applied to transition the liquid crystal molecules to the para state strongly depend on the liquid crystal material and alignment film used, and by selecting an appropriate frequency and alignment conditions, it is possible to It is possible to lower the voltage to a value of .

FIG. 3 is a diagram showing an example of a circuit configuration of a liquid crystal device for applying each signal voltage shown in FIG. 1.

In Figure 3, 5 is a shift register (S/
R) circuit, 6 is a latch circuit, 7 is an information signal side drive circuit, 8 is a scanning signal side drive circuit, 9 is a matrix panel, and 10 is a drive power supply circuit. The drive power supply circuit 10 includes a drive power supply main body 10a and a drive voltage control circuit 10b. 11 is an I/F (interface).

First, when the main switch (not shown) is turned on, the drive voltage control circuit 10b applies voltage to all scanning electrodes and information signal electrodes as shown in FIG.
Para-orientation alternating current voltages with pulse widths of ΔT′ and peak values of V S ′ and V _{I ′ are applied in opposite phases, so that a rectangular shape of V PP =} V _S ′+V _I ′≡V _AC is created between the upper and lower substrates. An alternating current voltage (peak-to-peak voltage is V _AC ) is applied. After this AC voltage is applied for a predetermined period of time and the liquid crystal molecules become para-state, the drive voltage control circuit 10b controls the drive voltage of the display signal, that is, the scanning signal side 3V ₀ , −2V ₀ ,
A pulse voltage having a voltage peak value of ±V ₀ on the information signal side and a pulse width of ΔT is set as a write pulse, and multiplexing drive is started in response to an input signal to the interface 11. Needless to say, the peak value of the AC voltage for para orientation V _S ′,
As shown in FIG. 5, V _I ' and pulse width ΔT' are larger than the peak value 3V ₀ , V ₀ and width pulse width ΔT of the write pulse, respectively.

FIG. 4 shows the final stage drive circuit of the drive circuits 7 and 8 in FIG. Tr1, Tr2
represents an output stage transistor. Here, in the drive waveform shown in FIG. 1, the breakdown voltages V _C of the two output stage transistors are set equally as follows.

That is, in the scanning signal side drive circuit 8 in FIG. 3, when the breakdown voltage is _VSC , V _SC >V _S ', and in the information signal side drive circuit 7, when the breakdown voltage is V _IC , V _IC >V _I '.

Furthermore, when applying the AC voltage for para orientation to either the scanning signal side drive circuit 8 or the information signal side drive circuit 7, for example, when applying it from the scanning signal side electrode, V _SC > 1/2V _AC , V _IC > V ₀ and the information side electrode may be grounded during the application period of the AC voltage for para orientation.

In this way, the AC voltage for para alignment, which is slightly lower than the breakdown voltages V _SC and V _IC of the output stage transistors Tr1 and Tr2 shown in FIG. V ₊
By applying power from the driving power supply circuit 10 shown in _FIG. 3 between .

Furthermore, in this embodiment, the driving power supply for providing the display signal and the power supply for providing the AC voltage for para-orientation are common, but separate power supplies are provided for each as shown in FIG. When ON, AC power supply 10c is connected,
It is also possible to switch to the drive power source 10a after a predetermined time.

[Effects of the Invention] As explained above, according to the present invention, by applying the para-alignment AC voltage before the display signal drive voltage, good display characteristics with high contrast and less flickering can be achieved. You can get it.

Further, by applying the AC voltage for para orientation and the drive voltage for display signals through a common drive circuit, the device can be configured with a simpler circuit, and it is possible to obtain a device at low cost.

[Brief explanation of the drawing]

FIG. 1 is a timing chart of an embodiment of the present invention, FIG. 2 is a pixel matrix configuration diagram of an embodiment of the invention, FIG. 3 is a circuit diagram of an embodiment of the invention, and FIG. 4 is a diagram of the pixel matrix configuration of an embodiment of the invention. FIG. 5 is a circuit configuration diagram of another embodiment of the present invention, and FIGS. 6 and 7 are schematic diagrams of a ferroelectric liquid crystal. _S1 to _S5 ...Scanning signal line, _I1 to _I5 ...Information signal line, 7...Information signal side drive circuit, 8...Scanning signal side drive circuit, 9...Matrix panel, 10...
Drive power circuit.

Claims (1)

[Claims] 1a A matrix electrode formed of scanning electrodes and information signal electrodes crossed at intervals, and a matrix electrode formed between the scanning electrodes and the information signal electrodes, and two different By applying an alternating current voltage that is higher than the threshold voltage of the liquid crystal to a ferroelectric liquid crystal in a first state that exhibits an oriented state and in which the angle between the average molecular axes of each of the two oriented states is 2θ, a liquid crystal panel having a ferroelectric liquid crystal in a second state in which the angle formed by the average molecular axes of each of the two mutually different alignment states in the absence of an electric field after the application of an alternating current voltage is canceled is larger than the 2θ; b. A first means for sequentially scanning and selecting the electrodes and applying a scanning selection signal to the scanning electrode; c. A first means for sequentially scanning and selecting the electrodes and applying a scanning selection signal to the scanning electrode; c. an information signal such that a voltage of one polarity that causes one alignment state of the ferroelectric liquid crystal in the second state or a voltage of the other polarity that causes the other alignment state of the ferroelectric liquid crystal in the second state is selectively applied to the intersection with the electrode; and d) a second means for applying the AC voltage to the ferroelectric liquid crystal in the first state before the scan selection signal and the information signal are applied to the scan electrode and the information signal electrode, respectively. A liquid crystal device characterized by having the means of 3.