JPH0535847B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a liquid crystal device, and particularly relates to a liquid crystal device having at least two
This invention relates to a ferroelectric liquid crystal device with two stable states. [Prior Art] Development of flat panel display devices is currently being actively conducted throughout the world. Among them, displays using liquid crystals are considered to have become completely established in society in small-scale fields, but displays with high resolution that can replace CRTs, and large screens, are far superior to conventional liquid crystal displays. It was extremely difficult to do so using methods such as TN and DSM. In order to improve the drawbacks of liquid crystal devices, Clark and Lagerwall proposed the use of bistable liquid crystal devices (Japanese Unexamined Patent Publication No. 107216/1983, U.S. Pat. Patent No. 4367924 specification, etc.). As a liquid crystal having bistability, chiral smectic C is generally used.
A ferroelectric liquid crystal having a phase (SmC ^* ) or an H phase (SmH ^* ) is used. This liquid crystal has a bistable state consisting of a first optically stable state and a second optically stable state with respect to an electric field, and is therefore different from the above-mentioned TN type liquid crystal element. The liquid crystal is aligned in a first optically stable state with respect to the other electric field spectrum, and the liquid crystal is aligned in a second optically stable state with respect to the other electric field spectrum. Furthermore, this type of liquid crystal has the property of very quickly taking one of the above two stable states in response to an applied electric field, and maintaining that state when no electric field is applied. By utilizing these properties, many of the problems of the conventional TN type devices mentioned above can be significantly improved. [Problems to be Solved by the Invention] However, problems arise when the number of display pixels is extremely large and high-speed driving is required.
That is, the threshold voltage for providing the first stable state in a ferroelectric liquid crystal cell having bistability for a predetermined voltage application time is -Vth ₁ , and the threshold voltage for providing the second stable state is -Vth 1. +Vth ₂ ,
Even if these threshold voltages are not exceeded, if the voltage continues to be applied for a long time, the display state written to the pixel (e.g., white state) will be reversed to another display state (e.g., black state) Sometimes. 1st
The figure represents the threshold characteristics of a bistable ferroelectric liquid crystal cell. Figure 1 plots the application time dependence of the threshold voltage (Vth) required for switching when DOBAMBC (12 in the figure) and HOBACPC (11 in the figure) are used as ferroelectric liquid crystals. As is clear from Fig. 1, the threshold value Vth is dependent on the application time, and the shorter the application time, the more
It is understood that the slope is steep. From this, when applied to an element that has an extremely large number of scanning lines and is driven at high speed, for example, even if a certain pixel is switched to the bright state during scanning, the information signal will always be below Vth from the next scanning onwards. It can be seen that if the voltage continues to be applied, there is a risk that the pixel will turn into a dark state during the completion of scanning one screen. [Means for Solving the Problems] and [Operation] An object of the present invention is to provide a liquid crystal device that solves the problems in the conventional liquid crystal devices as described above. The present invention provides a liquid crystal device having a plurality of intersecting scanning electrodes and a plurality of signal electrodes, and a ferroelectric liquid crystal disposed between the scanning electrodes and the signal electrodes, including: (a) among the plurality of scanning electrodes; A plurality of blocks are formed by a plurality of scan electrodes smaller than the number of scan electrodes, and for each of the plurality of blocks, a voltage applied to the scan electrodes in the block is applied based on the voltage applied to the scan electrodes when scan is not selected. , applying a voltage signal of one polarity to the plurality of signal electrodes, applying a voltage signal of the other polarity synchronized with the voltage signal of one polarity to the plurality of signal electrodes, and applying a voltage signal of the other polarity to the plurality of signal electrodes. a first means for applying a voltage signal of opposite polarity to the voltage signal of the other polarity before or after the plurality of blocks; and (b) for each of the plurality of blocks, when the scanning electrodes in the block are not selected for scanning. Applying a scan selection signal having a voltage signal of the other polarity based on the voltage applied to the scan electrode of the scan electrode, and applying a scan selection signal having a voltage signal of the other polarity to the selected signal electrode among the plurality of signal electrodes in synchronization with the voltage signal of the other polarity. applying one information signal of opposite polarity to the voltage signal of the other polarity,
The liquid crystal device is characterized in that it has second means for applying the other information signal of the same polarity to the voltage signal of the other polarity to the remaining signal electrodes in synchronization with the voltage signal of the other polarity. [Example] The optical modulating substance used in the driving method of the present invention is one that has at least two stable states, particularly one that has a first optically stable state and a second optically stable state depending on the applied electric field. Take one. That is, a substance having a bistable state with respect to an electric field, particularly a liquid crystal having such a property, is used. As the liquid crystal having bistability that can be used in the driving method of the present invention, a chiral smectic liquid crystal having ferroelectricity is most preferable, and among these, chiral smectic C phase (SmC ^* ) or
H-phase (SmH ^* ) liquid crystal is suitable. Regarding this ferroelectric liquid crystal, “Le Journal de Physique Lutheran” (“Le Journal de Physique Lutheran”)
``Physiove letter'') Volume 36 (L-69), 1975 ``Ferroelectric Liquid Crystals''; ``Applied Physics Letters''
Physics Letters” Volume 36 (Issue 11) 1980 “Submicron Second Electro-Optical Switching in Liquid Crystals”
Bistable Electrooptic Switching in Liquid
“Crystals”); “Solid State Physics” 16 (141) 1981 “Liquid Crystals”
The ferroelectric liquid crystal disclosed in these documents can be used in the present invention. More specifically, examples of ferroelectric liquid crystal compounds used in the method of the present invention include decyloxybenzylidene-P'-amino-2-methylbutylcinnamate (DOBAMBC), hexyloxybenzylidene-P'-amino- 2-chloropropyl cinnamate (HOBACPC) and 4-o-(2-methyl)
-butylresolcylidene-4'-octylaniline (MBRA8) and the like. When constructing an element using these materials, the element is supported by a copper block etc. with a heater embedded in it, as necessary, in order to maintain the temperature state such that the liquid crystal compound becomes the SmC ^* phase or SmH ^* phase. can do. Further, in the present invention, in addition to the above-mentioned SmC ^* and SmH ^*, it is also possible to use a ferroelectric liquid crystal that appears in a chiral smectate F phase, I phase, J phase, G phase, or K phase. FIG. 2 schematically depicts an example of a ferroelectric-containing cell. 21a and 21b are In ₂ O ₃ , SnO ₂ or
It is a substrate (glass plate) coated with a transparent electrode such as ITO (indium-tein-oxide).
In between, a liquid crystal of SmC ^* phase, which is oriented such that a liquid crystal molecular layer 22 is perpendicular to the glass surface, is sealed.
The thick line 23 represents liquid crystal molecules,
This liquid crystal molecule 23 has a dipole moment (P⊥) 14 in a direction perpendicular to the molecule. When a voltage higher than a certain threshold is applied between the electrodes on the substrates 21a and 21b, the helical structure of the liquid crystal molecules 23 is unraveled, and the alignment direction of the liquid crystal molecules 23 is changed so that all dipole moments (P⊥) 24 are directed in the direction of the electric field. can be changed. The liquid crystal molecules 23 have an elongated shape and exhibit refractive index anisotropy in the long axis direction and short axis direction. Therefore, for example, if polarizers are placed above and below the glass surface in crossed nicol positions, It is easily understood that this results in a liquid crystal optical element whose optical properties change depending on the polarity of applied voltage. Furthermore, if the thickness of the liquid crystal cell is made sufficiently thin (for example,
1μ), the helical structure of the liquid crystal molecules unwinds even when no electric field is applied, as shown in Figure 3.
Its dipole moment Pa or Pb is upward 34a
or downward 34b. When an electric field Ea or Eb of different polarity above a certain threshold value is applied to such a cell for a predetermined period of time as shown in FIG.
4b and the liquid crystal molecules change direction accordingly.
stable state 33a or second stable state 33
Orient in either direction b. There are two advantages to using such a ferroelectric liquid crystal as an optical modulation element. Firstly, the response speed is extremely fast, and secondly, the alignment of liquid crystal molecules has a bistable state. The second point will be explained with reference to FIG. 2, for example. When an electric field Ea is applied, the liquid crystal molecules are oriented in a first stable state 33a, and this state remains stable even when the electric field is turned off. or,
When an electric field Eb in the opposite direction is applied, the liquid crystal molecules
The molecules are oriented in a stable state 33b and change their orientation, but they remain in this state even after the electric field is turned off. Further, as long as the applied electric field Ea does not exceed a certain threshold value, each orientation state is maintained. In order to effectively achieve such fast response speed and bistability, it is preferable for the cell to be as thin as possible, and in general, it is 0.5μ to 20μ,
Particularly suitable is 1μ to 5μ. Preferred specific examples of the driving method of the present invention will be explained with reference to FIGS. 4 to 12. FIG. 4 is a schematic diagram of a cell 41 having a matrix electrode structure in which a ferroelectric liquid crystal compound is sandwiched between. 42 is a scanning electrode group, and 43 is a signal electrode group. Now, to simplify the explanation,
An example will be shown in which a black and white binary signal is displayed. In Figure 4, the pixels indicated by diagonal lines correspond to "black" based on the second stable state of the ferroelectric liquid crystal, and the others correspond to "white" based on the first stable state of the ferroelectric liquid crystal. shall be taken as a thing. FIGS. 5a to 5c show scanning signals applied to scanning electrodes _S1 to _S3 . FIGS. 5a to 5f show information signals applied to the signal electrodes I ₁ to I ₃ and correspond to the display state of FIG. 4. FIGS. 6 a to 6 f are time-series representations of the voltages applied to the pixels A to F shown in FIG. 4, respectively. The write period shown in FIGS. 5 and 6 is a period for writing to pixels in a block consisting of a predetermined number n of scanning lines, and this writing is performed sequentially for each of a plurality of (M) blocks so that the pixels can be written in one block. You can write on the screen. In the driving method shown in FIGS. 5 and 6, all pixels on a plurality of scanning lines within a block are simultaneously
Writing is performed by bringing the pixel into a stable state (hereinafter referred to as the "white" state), and then sequentially selecting those scanning lines to bring the desired pixel into the second stable state (hereinafter referred to as the "black" state). be able to. FIGS. 5 and 6 are explanatory diagrams of this driving method. At the erase signal application phase _t1 , a pulse voltage of positive polarity is applied to all scanning lines, and in synchronization with this, a pulse voltage of negative polarity is applied to all signal lines. This erases all pixels within the block to "white". Next, a scan selection signal in which a 2V ₀ positive polarity pulse and a -2V ₀ negative polarity pulse alternate is applied to the scanning lines in the block sequentially at write signal application phases t ₂ and t ₃ , and is synchronized with this scan selection signal. To maintain the "white" state, the signal line of the pixel receives a positive polarity pulse of V ₀ at phases t ₂ and t ₃ and −
“White” signal and “black” with alternating negative polarity pulses of V ₀
The phase t ₂ and the signal line of the pixel that you want to invert
Writing in the block is completed at _t3 by selectively applying a "black" signal consisting of alternating negative pulses of _-V0 and positive pulses of _V0 . In the driving method of the present invention, as shown in FIGS. 5 and 6, the write signal is applied to the pixels in the m+1 block in synchronization with the scanning selection signal at phases t ₂ and t ₃ , and the signal electrode I ₁ is applied to the pixels in the m+1 block. , I ₂ ..., the polarity is opposite to that of the voltage signal applied to the information signal during the erase signal application period t ₁ after the writing of pixels in the m-th block is completed (with respect to the reference potential). By applying the voltage signal as an auxiliary signal at the auxiliary signal application phase t ₀ (at this time, a voltage below the threshold voltage is applied to the pixel), it is applied to the pixels in the unselected block. The duration of the same polarity voltage is the maximum write pulse width (e.g. t ₁ = t ₂ = t ₃
= t ₀ ), and the above-mentioned problem of crosstalk generation can be solved. In the above, each voltage value is set to a desired value that satisfies the following relationship. 2V ₀ <Vth ₂ <3V ₀ −3V ₀ <−Vth ₁ <−2V ₀ That is, Vth ₂ is the threshold voltage of the ferroelectric liquid crystal in the second stable state (black), and −Vth ₁ is the threshold voltage of the ferroelectric liquid crystal in the second stable state (black). The threshold voltage in the first stable state (white) of , which can be substantially |−Vth ₁ |≡|Vth ₂ |. The above points will be explained in more detail below. FIG. 7 represents a liquid crystal device using the method of the invention. In FIG. 7, 71 is a scanning line drive circuit, 72 is a signal line drive circuit, 73 is a scanning electrode group, 74 is a signal electrode group, and 75 is a ferroelectric liquid crystal panel. The case where a binary signal is displayed is shown as an example, and the number of signal lines for each line is 16, and the pixels in the blacked part in both figures of Fig. 7 are "black",
Pixels in the white background correspond to "white". here,
If one character has 8×8 pixels, the panel 75 will display four characters. As shown in Fig. 7, “A” and “B” are placed in the m-th block _A1 .
The character is “C” in the m+mth block A ₂ .
and the letter “D” are displayed. FIG. 8 is a partial circuit diagram showing an example of the output stage of the above scanning line drive circuit 71. In FIG. 8, 81 is a gate element _gN , whose output level is controlled by a selection line 82, and when terminal _Q2 is selected, gate elements _g1 to _g8 are turned on all at once, and the terminal
If the levels of R ₁ to R ₈ are transmitted as they are and terminal Q ₁ is not selected, all output lines S ₁ to _{S 8} are at a predetermined constant level that makes panel 75 non-selected. The terminal _Q1 has the same function as the gate elements _g9 to _g16 . FIG. 9 is a timing chart of the output stage. At this time, in the example shown in FIG. 9, the pixels into the block are supplied with voltage signals in the order of erase signal signal phase t ₁ , auxiliary signal application phase t ₀ , and write signal application phases t ₂ and t ₃ . is applied, the mth block is written during write period _T1 , and then m+1
The write period T ₂ (for example, T ₁ = T ₂ )
Writing is performed in . FIG. 10 is a partial circuit diagram showing an example of the output stage of the signal line drive circuit 72, and FIG. 11 is a timing chart thereof. The D terminal is a data (image information) serial input terminal, and CLK is a shift clock input terminal. When all the data is in the shift register 101, the potential of the L terminal changes from 0 to 1.
The rising data is latched (latch 102).
Up to this point, there is no difference from a normal information side drive circuit. The characteristic feature of the driving method of this embodiment is that the latch output is followed by an OOR gate 103 that ORs with the CLR signal.
This is what we have set up. The CLR signal is normally 0, and the latch output remains unchanged for the next EOR (Exclusive
OR) is transmitted to gate 104. S is simply a repeating pulse of 1 and 0 as shown in Figure 11, and if the latch output = image signal is 0 (“black”), the pulse of 1 and 0 is directly output to the positive logic side output terminal, and the The inverted pulse is output to the negative logic side output terminal. If the latch output is 1 ("white"), the inverted pulse of S is output to the positive logic output terminal, and the pulse S as is is output to the negative logic output terminal. This enters the gate array 105 controlled by the G terminal,
The output goes to the level converter 106 at the final stage. The level converter 106 is connected to GND, - as shown in Table 1 according to the inputs A and B as shown in FIG.
It outputs three values, V ₀ and +V ₀ , but as you can see from the circuit in Figure 10, when G = 1 (gate open), if the image signal is "", +V ₀ → -V ₀ "white" -V ₀ →+V ₀ When G=0 (gate closed), GND is output regardless of the image signal. The above is a case where CLR=0, but as shown in FIG. 11, CLR=1 during the beginning time (t ₀ and t ₁ ) of block scanning. At this time, OR gate 10
All outputs of 3 become 1, and level converter 106
All outputs are −V ₀ → +V ₀ . Therefore,
−V ₀ → for all pixels before block scanning
Since a voltage of +V ₀ is applied, a desired driving method is realized. (However, a voltage of -3V ₀ → +V ₀ is applied to the pixels within the scanning block.)

〔Effect of the invention〕

According to the present invention, even if a display panel using a ferroelectric liquid crystal element is driven at high speed, the maximum pulse width of the voltage waveform that continues to be applied to pixels in an unselected block is 2 times the write pulse ΔT. In addition to this, the maximum pulse ΔT during writing is also twice the pulse ΔT for pixels to which no scan selection signal is applied, so it is effective for the phenomenon that the display state is reversed to another display state during the writing scan of one screen. can be prevented.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing the threshold characteristics of a ferroelectric liquid crystal. FIGS. 2 and 3 are perspective views schematically showing a ferroelectric liquid crystal element used in the present invention. FIG. 4 is a plan view of the matrix pixel structure used in the present invention. FIGS. 5a to 5f are explanatory diagrams showing voltage waveforms of signals applied to the electrodes, respectively. FIGS. 6a to 6f are explanatory diagrams showing voltage waveforms of signals applied to pixels, respectively. Figure 7 shows
1 is a plan view showing a liquid crystal device using the method of the present invention. FIG. 8 is an explanatory diagram showing the scanning line side drive circuit, and FIG. 9 is an explanatory diagram showing its timing chart. FIG. 10 is an explanatory diagram showing the signal line side drive circuit, and FIG. 11 is an explanatory diagram showing its timing chart. FIG. 12 is an explanatory diagram showing a method other than the present invention. FIG. 13 is an explanatory diagram showing another embodiment of the present invention.

Claims (1)

[Claims] 1. A plurality of intersecting scanning electrodes and a plurality of signal electrodes,
and a liquid crystal device having a ferroelectric liquid crystal disposed between the scanning electrode and the signal electrode, wherein (a) a plurality of blocks are formed by a plurality of scanning electrodes smaller than the number of the plurality of scanning electrodes; For each of the plurality of blocks, a voltage signal of one polarity is applied to the scanning electrodes in the block based on the voltage applied to the scanning electrode when scanning is not selected, and the voltage signal of one polarity is applied to the plurality of signal electrodes. , applying a voltage signal of the other polarity synchronized with the voltage signal of the one polarity, and applying a voltage signal of opposite polarity to the voltage signal of the other polarity to the plurality of signal electrodes before or after the voltage signal of the other polarity. (b) for each of the plurality of blocks, applying a voltage of the other polarity to the scan electrode in the block, based on the voltage applied to the scan electrode when scanning is not selected; A scan selection signal having a signal is applied to a selected signal electrode among the plurality of signal electrodes, and one of the voltage signals of opposite polarity to the voltage signal of the other polarity is applied to the selected signal electrode of the plurality of signal electrodes in synchronization with the voltage signal of the other polarity. apply an information signal;
A liquid crystal device comprising second means for applying the other information signal of the same polarity to the voltage signal of the other polarity to the remaining signal electrodes in synchronization with the voltage signal of the other polarity.