KR100898862B1 - Display element driving method - Google Patents

Abstract

In a display element using a cholesteric liquid crystal, in order to be able to rewrite a partial screen at a high speed by a low-cost general purpose driver, the area of the partial rewriting non-target is scanned in a high speed mode within the crosstalk tolerance of the liquid crystal. Then, the area to be partially rewritten is scanned at a normal speed within the rewritable range and partially rewritten. As a result, even if an inexpensive general-purpose driver IC is used, fast rewriting of any specific area can be realized.

Cholesteric liquid crystal, crosstalk, driver IC, segment mode, common mode

Description

Display method drive method {DISPLAY ELEMENT DRIVING METHOD}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of driving a display element using cholesteric liquid crystals, and more particularly to a method of driving a liquid crystal display element capable of rewriting a partial screen at high speed.

In recent years, the development of electronic paper has been actively conducted in each company and university. BACKGROUND ART As an application market in which electronic paper is expected, applications to various portable devices such as sub-displays of mobile terminals and display units of IC cards, including electronic books, have been proposed.

One of the promising methods of electronic paper is the use of cholesteric liquid crystals.

The cholesteric liquid crystal has excellent characteristics such as semipermanent display retention (memory), clear color display, high contrast, and high resolution. The cholesteric liquid crystal may also be referred to as a chiral nematic liquid crystal, and by adding a relatively large amount (hundreds of percent) of chiral additives (also called chiral materials) to the nematic liquid crystal, the molecules of the nematic liquid crystal have a spiral shape. It is a liquid crystal which forms a cholesteric phase.

Hereinafter, the display and driving principle of the cholesteric liquid crystal will be described.

The cholesteric liquid crystal controls the display by the alignment state of the liquid crystal molecules. As shown by the graph of the reflectance of FIG. 1, the cholesteric liquid crystal has a planar state P reflecting incident light and a focal unique state FC transmitting therethrough, and these are stable even under an electroless field. In the planar state, light of a wavelength corresponding to the spiral pitch of the liquid crystal molecules is reflected. The wavelength λ at which reflection is maximized is expressed by the following expression from the average refractive index n of the liquid crystal and the spiral pitch p.

λ = np

On the other hand, the reflection band Δλ increases with the refractive index anisotropy Δn of the liquid crystal.

Therefore, by selecting the average refractive index n and the spiral pitch p of the liquid crystal, the color of the wavelength? Can be displayed in the planar state.

In addition, by forming the light absorbing layer separately from the liquid crystal layer, black can be displayed in the focal unique state.

Next, a driving example of the cholesteric liquid crystal will be described below.

When a strong electric field is supplied to the liquid crystal, the helical structure of the liquid crystal molecules is completely pulled, and all molecules are homeotropic in the direction of the electric field. Next, if the electric field is rapidly made zero from the homeotropic state, the spiral axis of the liquid crystal becomes perpendicular to the electrode, and the planar state selectively reflects light according to the spiral pitch. On the other hand, when the electric field is removed after formation of a weak electric field such that the helical structure of the liquid crystal molecules is not solved, or when the electric field is gently removed by applying a strong electric field, the helix axis of the liquid crystal becomes parallel to the electrode and is in a focal unique state that transmits incident light. do. In addition, if an electric field of medium intensity is applied and abruptly removed, the planar state and the focal unique state are mixed to allow the display of the intermediate tone. Information can be displayed using the above phenomenon of a liquid crystal.

Referring to FIG. 1, which is a diagram illustrating the response characteristics of the cholesteric liquid crystal, the above voltage response characteristics are summarized as follows.

If the initial state (0 V) is a planar state (high reflectance), when the pulse voltage is raised to an arbitrary range (for example, 24 V), the driving band becomes the focal unique state (low reflectivity), and when the pulse voltage is increased (for example, 32V), the drive band is returned to the planar state (high reflectance) again. When the initial state (0 V) is a focal unique state, the driving band for the planar state gradually increases as the pulse voltage is raised.

2A to 2C, the voltage output of a commercially available STN driver of binary output applicable to cholesteric liquid crystal will be described.

First, since the AC drive is essential for the liquid crystal, it is necessary to set two driving voltages when the AC signal is High or Low. Correspondence between the voltages V0, V21, V34, V5 at that time and the high and low of the AC signal is, for the data signal and the scan signal, generally as shown in FIG. 2A, the magnitude of the voltage output is V0? V21. It is necessary to make ≥ V34 ≥ V5.

For example, as described in Fig. 2A, for data supplied in the segment mode, when the data signal is High, the corresponding AC signal is High is V0 and Low is V5.

Accordingly, when driving the cholesteric liquid crystal, as shown in FIG. 2B, for example, in the segment mode, the voltages V0, V21, V34, V5 are respectively set to (32, 24, 8, 0). Set to the voltage value. This yields a voltage waveform as described in FIG. 2C.

That is, a voltage is applied from 0 to 32V to the line selected by the scan as in " ON scan " In contrast, a voltage of 32 → 0V as "ON data" and 24 → 8V as "OFF data" is applied on the data side. When these are combined, the potential difference of "ON scan-ON data" is generated in the pixel which is ON display among the selected lines, and the potential difference of "ON scan-OFF data" is generated by ± 32V, and ± 24V is applied to pixel which is OFF display. do.

Similarly, a potential difference between " OFF scan-ON data " and " OFF scan-OFF data "

Therefore, as understood from the voltage response characteristic described in Fig. 1, the cross-talk does not occur in the unselected line, and the previous display state (planar state or focal unique state) is maintained.

By the way, in the electronic paper which is the application field of the display element using a cholesteric liquid crystal, the function (henceforth partial rewriting) which rewrites only a specific area | region in a display area is calculated | required. When performing partial rewriting, it is necessary to keep the display state previously written in other areas.

However, commercially available STN drivers have the following problems.

(1) Since the scan mode shifts (scans) all lines, voltages of ON and OFF are always applied to an area where partial rewriting is not performed. That is, only a specific area cannot be scanned. In the end, since all lines are scanned, the processing unit synthesizes the displayed image data and the image data used for partial rewriting, and rewrites all the screens.

(2) Scan side and data side Also in segment mode, it is possible to scan only a specific line. However, in this case, the magnitude relationship of voltages such as V0? V21? V34? The driving waveforms cannot be the same as in the combination of the mode and the segment mode, and crosstalk is always generated in the non-selected pixels, and the displayed image is damaged.

On the other hand, in the partial rewrite method of the known cholesteric liquid crystals described in Patent Documents 1 to 5, a voltage output is made bipolar (plus and minus), and a custom driver free of the above-described voltage constraints is used. have.

Here, FIG. 3 shows an example in which a bipolar and voltage constrained free driver is simply driven in accordance with the response characteristics of the liquid crystal shown in FIG. 1. In the case of Example 1 and also in Example 2, the applied voltages of the ON level and the OFF level shown in FIG. 2C can be applied to the pixels of the selected line. In this way, if there is no voltage restriction, it is easy to select and drive only the region of partial rewriting.

However, by making it bipolar, a high breakdown voltage and a complicated structure of the LSI in the driver are required, which causes cost up.

That is, conventionally, in order to partially rewrite only a specific region of a display element using a cholesteric liquid crystal, a mode (commonly known as segment mode) in which a scan line and a data driver can select an arbitrary line, and cross In order to suppress torque, it was necessary to make a specification of a special output voltage. Therefore, it became a big factor of cost up. In the general-purpose STN driver, as described above, since both the scan driver and the data driver generate large crosstalk in segment mode, partial rewriting is impossible, and it is necessary to update the entire image simply by rewriting part of the display. there was. Therefore, there existed a subject of taking rewriting time.

[Patent Document 1] Japanese Unexamined Patent Application Publication No. 2000-147465

[Patent Document 2] Japanese Patent Application Laid-Open No. 2000-147466

[Patent Document 3] Japanese Unexamined Patent Publication No. 2000-171837

[Patent Document 4] Japanese Patent Application Laid-Open No. 2000-180887

[Patent Document 5] Japanese Patent Application Laid-Open No. 2000-194005

<Start of invention>

An object of the present invention is to enable a partial screen rewriting at a high speed by a low cost general purpose driver in a display element using cholesteric liquid crystal.

For that purpose, the area of the partial rewrite non-object is scanned in a high speed mode within the crosstalk tolerance of the liquid crystal, and the area of the partial rewrite object is scanned at a normal speed within a rewritable range and partially rewritten.

As a result, even if an inexpensive general-purpose driver IC is used, fast rewriting of any specific area can be realized.

BRIEF DESCRIPTION OF THE DRAWINGS The figure explaining the response characteristic of a cholesteric liquid crystal.

Fig. 2A is a diagram showing a relationship between voltage output, signal output in a common mode and a segment mode of a commercially available general purpose STN driver;

Fig. 2B is a diagram showing a correspondence example of the voltage output of a commercially available general purpose STN driver and its voltage value when driving a cholesteric liquid crystal.

2C shows a data signal, a scan signal, and a voltage applied to a pixel.

Fig. 3 is a diagram showing a display element driving example in the bipolar driver.

4 illustrates the principle of the present invention.

5 is a diagram showing a shift of a threshold characteristic by a fast scan.

6 is a diagram illustrating a relationship between a driver clock and a scan pulse.

FIG. 7 is a diagram for explaining a method for rewriting only a part of a rewrite area. FIG.

8 is a diagram showing a block configuration example of a drive circuit according to the present invention;

9 is a diagram illustrating a cross section of an example of a display element.

<Best Mode for Carrying Out the Invention>

4 is a diagram for explaining the principle of the present invention.

When the partial area 220 is rewritten to the original image 100 and the image 200 after partial rewriting is displayed, the area 210 which does not perform partial rewriting is scanned in a high speed mode, and the partial rewriting is performed. When it reaches the area | region 220 which performs mouth, rewriting is performed at a normal scanning speed. After the scan of the rewrite area 220 is finished, the process returns to the high speed mode again, and the remaining area 230 is scanned at high speed.

Even in the high-speed mode in the region before and after rewriting, a voltage of ± 24 V or ± 32 V shown in FIG. 2C is applied to each pixel. However, as shown in FIG. 5, the operation threshold of the cholesteric liquid crystal due to the high-speed scan. Since the voltage rises significantly above 32 V, the alignment state (display state) of the liquid crystal does not change. Therefore, as shown in the image 200 after partial rewriting, the guidance of holding a meeting can be displayed on the original image 100.

4 shows an example in which a part of the original image 100 remains in the region 220 to be partially rewritten, but only a part of the original image 100 can be removed simply by keeping the scanning speed normal. I cannot leave it. The processing for leaving the part will be described later with reference to FIG.

6 is a diagram illustrating a relationship between a driver clock, which is a clock for data acquisition, and a scan pulse. When the bus width for inputting display data is 8 bits, for example, and the number of pixels in one line is 320 bits, for example, the scan pulse LP in the normal mode is generated every 40 clocks of the driver clock XSCL. In the high speed mode, for example, as shown in FIG. 6, a scan pulse LP having a high cycle of dividing the driver clock XSCL by two is used. In this case, scanning is performed at 20 times higher speed than the normal mode. The shorter the period of the scan pulse LP in the high speed mode can reliably prevent crosstalk.

In addition, although the scan pulse LP in the high speed mode is supplied irrespective of the data acquisition timing by the driver clock XSCL, data writing is not performed in the high speed mode, so there is no problem. In the so-called high speed mode, the image data provided to the driver IC is arbitrary and does not interfere.

Next, with reference to FIG. 7, the process of leaving an original image of a part of partial rewrite area is demonstrated. In that case, an image is synthesized in advance as shown in FIG. That is, although a partial area of the write completion display shown in FIG. 7 is rewritten, when a part of the rewritten area is left as it was before, the partial rewritten portion of the original area that is actually rewritten is rewritten. When a writing pattern is created and the partial rewriting area is scanned, the data signal of the created partial rewriting pattern is given, so that only a part of the area can be rewritten.

In the embodiment described above, the same voltage is applied to each pixel as in the high-speed scan mode in the region before and after the rewriting. On the other hand, while scanning an area not partially rewritten, it is conceivable to set all of the driver's output voltages to 0 V. In this case, however, when moving from the non-partial rewrite area to the partial rewrite area, It may take several hours to set the output voltage from 0V to a predetermined size. Doing so may result in edge unsharpness in the first part of partial rewriting when scanning as it is.

It is also conceivable to add a waiting time for several seconds until reaching a predetermined size, but this may also cause noise to enter the display.

Therefore, it is preferable that the area of the partial rewriting non-target is also made into a predetermined voltage output.

By processing the above flows, the general purpose driver of the binary output can be used as it is, and high-speed partial rewriting can be realized without cost up.

Next, FIG. 8 is a block diagram of an example of a driving circuit for implementing the method of driving the display element of the present invention. The driver IC 10 includes a scan driver and a data driver. The calculation unit 20 outputs the image data processed for display and supplied to the data driver to the driver IC 10, and also outputs various control data shown to the driver IC 10.

The data shift latch signal is a signal for controlling the shift of the scan line to the next line and the latch of the data signal. In the present invention, in order to use the high speed mode in the partial rewriting process, the scanning speed control unit 30 It is output to the driver IC 10 via. The polarity inversion signal is a signal for inverting the output of the unipolar driver IC 10. The frame start signal is a synchronization signal when the display screen starts to be written for one screen. The driver clock is a signal indicating the acquisition timing of the image data as described in the description of FIG. 6. The driver output off signal is a signal forcibly zeroing the driver output.

The driving voltage input to the driver IC 10 boosts the logic voltage of 3 to 5 V in the boosting section 40, and is formed at various voltage outputs in the voltage forming section 50. Based on the control data output from the calculating section 20, the voltage selecting section 60 selects a voltage input to the driver IC 10 from the voltage formed in the voltage forming section 40, and through the regulator 70 Input to IC 10.

Next, embodiment of the reflection type liquid crystal display element which concerns on this invention is described with reference to an accompanying drawing, and also the liquid crystal composition which concerns on this invention is demonstrated concretely.

It is a figure which shows the cross-sectional structure of embodiment of a liquid crystal display element which applies the drive method of this invention. This liquid crystal display element has memory characteristics, and the planar state and the focal unique state are maintained even after the application of the pulse voltage is stopped. The liquid crystal display element includes the liquid crystal composition 5 between the electrodes. The electrodes 3, 4 are oriented toward each other to intersect with each other as seen from the direction perpendicular to the substrate. An electrode driven by the scan driver described in FIG. 4 is a scan electrode, and an electrode driven by the data driver is a data electrode. It is preferable that an insulating thin film and an orientation stabilizer film are coated on the electrode. In addition, the visible light absorbing layer 8 is formed on the outer surface (rear surface) of the substrate on the side opposite to the side on which light is incident.

In the liquid crystal display element which concerns on this invention, the code | symbol 5 is a cholesteric liquid crystal composition which shows a cholesteric phase at room temperature, These materials and its combination are demonstrated concretely by the following experiment example.

6 and 7 are sealing materials, and are for sealing the liquid crystal composition 5 between each board | substrate 1 and 2. As shown in FIG. Reference numeral 9 is a drive circuit, which applies an arbitrary predetermined voltage in pulse shape to the electrodes 3 and 4.

Although the board | substrates 1 and 2 have all light transmittance, it is necessary that at least one of the pair of board | substrates which can be used for the liquid crystal display element which concerns on this invention has light transmittance. As a board | substrate which has translucency, there exists a glass substrate, but film substrates, such as PET and PC, can be used besides a glass substrate.

As the electrodes 3, 4, for example, Indium Tin Oxide (ITO: Indium Tin Oxide) is typical, but other transparent conductive films such as Indium Zic Oxide (IZO: Indium Zinc Oxide), aluminum, silicon, etc. A metal electrode or an optical conductive film such as amorphous silicon or BSO (Bismuth Silicon Oxide) can be used. In the liquid crystal display element shown in FIG. 9, as described above, a plurality of strip-shaped transparent electrodes 3, 4 parallel to each other are formed on the surfaces of the transparent substrates 1, 2, and these electrodes are formed of a substrate ( From the direction perpendicular to 1, 2, they are oriented to cross each other.

Next, although not shown in FIG. 9, preferred elements for use in the liquid crystal display device according to the present invention will be described.

(Insulating thin film)

As for the liquid crystal display element which concerns on this invention including the liquid crystal display element shown in FIG. 9, the insulating thin film which has the function which prevents the short circuit between electrodes or improves the reliability of a liquid crystal display element as a gas barrier layer may be formed.

(Orientation stabilizer film)

Examples of the orientation stabilizing film include organic films such as polyimide resin, polyamideimide resin, polyetherimide resin, polyvinyl butyral resin and acrylic resin, and inorganic materials such as silicon oxide and aluminum oxide. In this embodiment, the orientation stabilizing film is coated on the electrodes 3 and 4. In addition, the alignment stabilization film may be used as an insulating thin film.

(Spacer)

As for the liquid crystal display element which concerns on this invention including the liquid crystal display element shown in FIG. 9, the spacer for maintaining the gap | interval between board | substrates uniformly between a pair of board | substrate may be formed.

The spacer is inserted between the board | substrates 1 and 2 in the liquid crystal display element of this embodiment. As this spacer, the sphere made of resin or an inorganic oxide can be illustrated. In addition, a fixing spacer having a thermoplastic resin coated on its surface is also preferably used.

Next, a liquid crystal composition is demonstrated. The liquid crystal composition which comprises a liquid crystal layer is a cholesteric liquid crystal which added 10-40 weight% of chiral materials to a nematic liquid crystal mixture. Here, the addition amount of a chiral ash is a value when the total amount of a nematic liquid crystal component and a chiral ash is 100 wt%.

Although various conventionally well-known things can be used as a nematic liquid crystal, it is preferable for the driving voltage to have a dielectric constant anisotropy of 20 or more. If the dielectric anisotropy is 20 or more, the driving voltage can be made relatively low. It is preferable that the dielectric anisotropy ((DELTA) epsilon) as a cholesteric liquid crystal composition is 20-50.

In addition, the refractive index anisotropy (Δn) is preferably 0.18 to 0.24. When smaller than this range, the reflectance of a planar state becomes low, and when larger than this range, scattering reflection in a focal unique state becomes large, viscosity also becomes high under influence, and a response speed falls.

Moreover, as for the thickness of this liquid crystal, about 3-6 micrometers is preferable. If it is smaller than this, the reflectance of the planar state is low, and if it is larger than this, the driving voltage becomes too high.

Next, a display element of monochrome 8 gray scale and resolution Q-VGA described above is fabricated, and an experimental example of the present invention using the same will be described.

The liquid crystal shows green in the planar state and black in the focal unique state.

The driver IC used two S1D17A03 (160 outputs) and one S1D17A04 (240 outputs) made by EPSON, a general purpose STN driver. Then, the driving circuit was set with the 320 output side as the data side and the 240 output side as the scan side. At this time, in order to stabilize the voltage input to a driver, you may stabilize by the voltage follower of an op amp.

The input voltage to this driver IC is the same as that shown in FIG. 2B. As shown in Fig. 2C, a pulse voltage of ± 32 V is applied to the ON pixel and ± 24 V to the OFF pixel, and a pulse voltage of ± 4 V is applied to the non-selected pixel.

The area of partial rewriting is scanned at a speed of about 3 ms / line, and the area of non-object finishes scanning at a moment by the scanning speed of several to several tens of ms / line level. A voltage of ± 24 V is applied to all the pixels in the non-target area, but no crosstalk occurs due to the high speed, and the displayed image is maintained. In the area of partial rewriting, the display is newly updated due to the predetermined scan speed.

In addition to the cholesteric liquid crystal, the present invention can be applied in the same manner as long as it is a display material having memory property, and particularly preferably a kind of liquid crystal.

As described above, according to the driving method of the display element using the cholesteric liquid crystal of the present invention, since it is possible to rewrite the partial screen at a high speed by a low cost general purpose driver, it is possible to apply a lot of effects by applying it to electronic paper or the like. Can be imported.

Claims (14)

A driving method of a display element having a plurality of scan electrodes and a plurality of data electrodes that cross each other in an opposing state, and applying a pulsed driving voltage by a driver IC while selecting the scan electrodes in a predetermined order, comprising: The rewritable range is scanned at a normal speed for the area that is partially rewritten from the existing display state, and the area that preserves the existing display state is switched at a speed faster than the normal speed within the range where no crosstalk occurs. A method of driving a display element, characterized in that scanning. The method of claim 1, And a voltage output of the driver IC is unipolar. The method according to claim 1 or 2, And the driver IC uses a general-purpose STN driver with a binary output to set the scan electrode to the common mode and the data electrode to the segment mode. The method according to claim 1 or 2, A method of driving a display element, wherein the voltage output at the time of partially rewriting is maintained even during scanning of an area holding an existing display state. The method according to claim 1 or 2, A method of driving a display element characterized by synthesizing a rewrite image and a preservation image before inputting image data to the driver IC in the case of locally preserving an existing display state in a partially rewritten region. The method according to claim 1 or 2, And the display element is displayed using a reflective material having memory characteristics. A display element having a plurality of scan electrodes and a plurality of data electrodes that cross each other in an opposing state, and applying a pulsed driving voltage by a driver IC while selecting the scan electrodes in a predetermined order, Scan the area that is partially rewritten from the existing display state at the normal speed within the rewritable range, and switch the area that preserves the existing display state to a speed faster than the normal speed within the range where no crosstalk occurs. A display element characterized by scanning. The method of claim 7, wherein A display element, characterized in that the voltage output at the time of partially rewriting is maintained even during scanning of an area holding an existing display state. The method according to claim 7 or 8, And the display element is displayed using a reflective material having memory characteristics. An electronic terminal comprising the display element according to claim 7 or 8. delete delete delete delete

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