KR20070116024A - Method for driving liquid crystal display element - Google Patents

Abstract

In order to realize multilevel halftone display excellent in uniformity by a liquid crystal display element through use of an inexpensive general purpose driver having a low breakdown voltage, pulse application employing accumulation response (overwrite) of liquid crystal is performed a plurality of times, the drive voltage and the pulse width are made variable step by step, and the liquid crystal is controlled to a predetermined halftone state by using a region having a large margin from the initial stage of reflection state. Since increase in drive voltage can be avoided, an inexpensive binary output general purpose driver having a low breakdown voltage can be utilized. Furthermore, multilevel halftone display excellent in uniformity can be realized by high level conversion employing a region having the large margin.

Description

A method of driving a liquid crystal display device {METHOD FOR DRIVING LIQUID CRYSTAL DISPLAY ELEMENT}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of driving display elements using cholesteric liquid crystals, and more particularly to a method of driving display elements capable of realizing high quality multi-gradation display.

In recent years, the development of electronic paper is actively progressing 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 strongest methods of electronic paper has been to use cholesteric liquid crystals.

The cholesteric liquid crystal has excellent characteristics such as semipermanent display retention (memory), clear color display, high contrast, and high resolution. A cholesteric liquid crystal may also be called a chiral nematic liquid crystal, and by adding a relatively large amount (10%) 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 in the alignment state of the liquid crystal molecules. As shown in the graph of the reflectance of Fig. 1A, the cholesteric liquid crystal has a planar state P for reflecting incident light and a focal conic state FC for transmitting, and these are stable even under an electric 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.

Accordingly, by selecting the average refractive index n of the liquid crystal and the spiral pitch p, 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 conic state.

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

When a strong electric field is applied to the liquid crystal, the helical structure of the liquid crystal molecules is completely solved, and all molecules are in a homeotropic state along the direction of the electric field. Next, if the electric field is suddenly 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 does not loosen, or when the electric field is gently removed by applying a strong electric field, the spiral axis of the liquid crystal is parallel to the electrode, and the focal conic penetrates the incident light. It is in a state. In addition, if an electric field of medium intensity is applied and abruptly removed, the planar state and the focal conic state are mixed, and the display of the intermediate tone is possible.

Information is displayed using this phenomenon.

Referring to Fig. 1A, the above voltage response characteristics are summarized as follows.

If the initial state is the planar state P (solid line graph), raising the pulse voltage to an arbitrary range results in the drive band for the focal conic state FC, and further raises the pulse voltage to the drive band for the planar state again.

If the initial state is a focal conic state (dotted graph), the driving band to the planar state gradually increases as the pulse voltage is increased.

Applying the voltage of the area formed as the halftone area A and the halftone area B, the halftone which mixed the planar state and the focal conic state mentioned above is obtained.

In addition, as shown in Fig. 1B, the cholesteric liquid crystal is known to have a cumulative response, i.e., a transition from the planar state to the focal conic state or the focal conic state to the planar state by applying a plurality of weak pulses.

For example, when the initial state is a planar state, by applying a weak voltage pulse in the halftone region A continuously, as shown in Fig. 1B, the transition gradually to the focal conic state in accordance with the number of pulses applied. On the other hand, regardless of the initial state, by successively applying the weak voltage pulse in the halftone region B, as shown in Fig. 1B, it gradually transitions to the planar state in accordance with the number of pulses applied. Therefore, the desired gradation degree can be displayed by the number of pulses applied. In addition, as shown in an enlarged view in FIG. 1C, the scattered reflection of the focal conic state can be gradually reduced, and it is also possible to achieve a better black state.

Next, with reference to FIG. 1D, the structure of the electrode which drives a liquid crystal in a matrix type liquid crystal display element is demonstrated. Generally, the liquid crystal drive electrode of a liquid crystal display element is comprised from the some scan electrode 16 and the some data electrode 18 which cross | intersect in mutually opposing state, as shown to FIG. 1D. The portion where the scan electrode 16 and the data electrode 18 cross each other becomes a pixel. The scan electrode 16 is sequentially selected (common mode) by the scan electrode driver 12, and a pulse-shaped voltage is applied to each pixel by the data electrode driver 14 to the data electrode 18. A pulse-shaped voltage corresponding to the display state of is applied (segment mode) to drive the liquid crystal of the pixel. The voltage of the difference between the voltage applied to the data electrode 18 and the voltage applied to the scan electrode 16 is a voltage applied to the liquid crystal of the pixel, and is a voltage for driving the liquid crystal shown in FIG. 1A.

Hereinafter, although the main prior art about the driving method of the multi-gradation display of a cholesteric liquid crystal is introduced, each subject has a subject.

For example, among driving waveforms divided into three stages of a preparation section, a selection section, and an evolution section, as described in Patent Documents 1 and 2, the halftone is selected using the amplitude, pulse width, and phase difference of the selection section. There is a method called dynamic driving to display. However, these dynamic drives are high speed, but the granularity of halftone is high. In addition, dynamic driving generally requires a dedicated driver capable of generating a large number of voltages, which is a significant factor in cost-up due to the manufacture of the driver and the complexity of the driver control circuit.

On the other hand, although the improvement of this dynamic drive so that it can be applied to a low cost general purpose STN driver is disclosed by following Nonpatent Document 1, this also cannot solve the high granularity which is the subject of dynamic drive.

Moreover, as a prior art of another halftone driving method, immediately after applying the 1st pulse which makes liquid crystal a homeotropic state described in following patent document 3, a 2nd, 3rd pulse is applied, Although the desired gradation can be displayed by the potential difference of three pulses, in this driving method, since the granularity of the intermediate tone is concerned and the driving voltage is high, the problem that it is difficult to provide inexpensive remains.

Since the above-mentioned driving methods are all driving using the halftone region B irrespective of the initial state, the granularity is high but high, and the display quality remains a problem.

On the other hand, although the driving method using the halftone region A is described in the following non-patent document 2, this problem still remains.

In the method described in Non-Patent Document 2, by using a cumulative response peculiar to a liquid crystal, a short pulse is applied to gradually drive the planar state to the focal conic state, or the focal conic state to the planar state at a high speed of the quasi-video rate. It is explained.

However, in this method, due to the fast speed of the quasi-dynamic image rate, the driving voltage is increased to 50-70V, which is a factor of cost-up, and the two phase cumulative drive scheme described therein is used to prepare two stages of preparation phase and selection phase. Since the cumulative response in two directions (that is, halftone region A and halftone region B) of the cumulative response to the planar state and the cumulative response to the focal conic state is used, the problem of display quality also remains.

As described above, the multi-gradation display of electronic paper using the conventional cholesteric liquid crystal requires a driver IC of a special specification capable of generating a multi-level driving waveform, and the driving voltage is also high as 40 to 60V. In addition, high internal voltage of IC is needed. Therefore, it was a big cause of cost up. Moreover, although the prior art rewrites at high speed, the granularity of an intermediate | middle tank is high (low uniformity), and it was difficult to apply to the electronic paper application which requires high display quality.

Moreover, in the prior art, the gray level of the intermediate tone is controlled by switching the voltage value or pulse width of the voltage pulse for each selected pixel. For this reason, the construction of the driver IC or peripheral circuit which can switch voltage value or pulse width arbitrarily is needed, and it was a big factor of cost up. In addition, as described in Patent Literature 1, there is also a half-tone driving method using a driver with a small number of outputs. In this case, however, the gray level is controlled from an inconsistent initial state. 60V). Moreover, since the driving margin of an intermediate | middle tank was narrow, the granularity of an intermediate | middle tank became high also in the element with high uniformity of the cell gap like a glass element, and it was difficult to make high quality.

Patent Document 1: Japanese Patent Application Laid-Open No. 2001-228459

Patent Document 2: Japanese Patent Application Laid-Open No. 2003-228045

Patent Document 3: Japanese Patent Application Laid-Open No. 2000-2869

Non-Patent Document 1: Nam-Seok Lee, Hyun-Soo Shin, etc, A Novel Dynamic Drive Scheme for Reflective Cholesteric Displays, SID 02 DIGEST, pp 546-549, 2002.

Non-Patent Document 2: Y.-M.Zhu, D.-K.Yang, Cumulative Drive Schemes for Bistable Reflective Cholesteric LCDs, SID 98 DIGEST, pp798-801, 1998

<Start of invention>

An object of the present invention is to provide a method of driving a liquid crystal display element for realizing multi-gradation display excellent in uniformity using a low-cost general-purpose driver having a low breakdown voltage. To this end, a plurality of pulses are applied by applying the cumulative response (overwriting) of the liquid crystal, and the driving voltage and the pulse width are varied for each step, and the liquid crystal is determined using a region having a large margin from the initial state of the reflection state. Control in halftone state. As a result, the rise of the driving voltage can also be avoided, and therefore, a general-purpose driver having a cheap binary output with low breakdown voltage can be used. In addition, since the gray level conversion using the region having a large margin, multi-gradation display excellent in uniformity can be realized even in an element having poor cell gap accuracy such as a film element. Further, according to the present invention, even if the number of gradations increases, the increase in the number of overwrites can be suppressed.

1A is a diagram showing voltage response characteristics of cholesteric liquid crystals.

1B is a diagram showing cumulative response characteristics of a cholesteric liquid crystal.

1C is a diagram showing a response characteristic of a focal conic state.

1D is a diagram illustrating a configuration of a drive electrode of a matrix display element.

Fig. 2 is a diagram for explaining a display element driving method of the first embodiment.

3 is a view for explaining a display element driving method of the second embodiment;

FIG. 4A is an explanatory diagram illustrating a method of driving a display element when rewriting a display screen; FIG.

4B is a diagram showing a voltage applied to pixels on one line when the display screen is rewritten.

4C is a diagram for explaining an operation for rewriting a display screen.

5A is a diagram showing a voltage applied to a drive electrode in step 1;

5B is a diagram showing a voltage applied to each pixel in step 1;

FIG. 6 is a view showing driving of the display element in step 2 as compared with the case of step 1; FIG.

7A is a diagram showing waveforms of a normal ON pulse driving a display element.

Fig. 7B is a diagram showing waveforms of ON pulses in the embodiment of the present invention.

8 shows an example of voltage switching between step 1 and step 2. FIG.

9 is a diagram showing a voltage applied to each pixel in steps 1 and 2. FIG.

FIG. 10 is a view showing driving of a display element in each substep of step 2; FIG.

FIG. 11 is a diagram explaining execution of a plurality of substeps between scans of one line; FIG.

FIG. 12 is a diagram for explaining a process of generating sub image data for driving display elements from multi-gradation image data; FIG.

Fig. 13 is a diagram showing the stacked structure of display elements for full color display.

Fig. 14 is a diagram explaining a method of driving an ON pulse for full color display.

Fig. 15 is a diagram showing a block configuration example of a drive circuit according to the present invention.

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

Fig. 17 is a diagram showing multi-gradation display according to an embodiment of the present invention.

<Best Mode for Carrying Out the Invention>

First, with reference to FIG. 2, the first embodiment of the present invention will be described by taking a case where four gradation display is targeted. Now, since the example is four-gradation display, each pixel in the display area is finally driven to display one of the levels 0 to 3 shown in the completion pattern of FIG.

As shown in Fig. 2, first, in step 1, each pixel is driven in a planar state or a focal conic state. Only pixels of level 0 are driven in the focal conic state. As will be described later in detail, in step 1, the drive in the planar state, that is, the reflection state, is turned on at 32 V, and the drive in the focal conic state, that is, the non-reflective state, is turned off at 24 V as shown in FIG. Drive. Next, in substep 1 of step 2, an area other than the area of the level 3 gradation is selected, and an ON pulse 24V for transitioning in the direction of the focal conic state is applied. Then, the area to be set as level 1 and level 2 is driven in the state of gradation of level 2. The display area at level 3 to which the OFF pulse 12V is applied remains in the planar state.

Next, in the substep 2 of step 2, the ON pulse 24V which makes a transition to the direction of a focal conic state is applied except for the area | region which is set as level 2 from the area | region selected by the previous substep 1 ,. In this manner, in the halftone region A shown in FIG. 1A, the transition is sequentially performed in the direction of the planar state from the focal conic state in accordance with the gradation of the pixel.

As shown in Fig. 1B, the halftone area A is less responsive (because γ is gentle) than the planar to focal conic (area A of Fig. 1B) than the focal conic to planner (area B of Fig. 1B). Higher uniformity (lower granularity) is realized than the one using the halftone region B, and higher gradation number can be obtained.

In addition, since a pulse is repeatedly applied to the pixel which is set to a complete black state (level 0), high contrast display with better black density can be realized. This is because in the focal conic state taking a black state, weak scattering reflections remain in one pulse application and tend to become blurry black.

On the other hand, by repeatedly applying a pulse multiple times in Step 2 as in the present invention, as shown in Fig. 1C, the scattered reflection in the focal conic state can be gradually reduced, thereby making it possible to achieve a better black state. In addition, since the voltage value of the pulse is also low, crosstalk in the non-selected area can be avoided more stably.

Next, a second embodiment in which the number of driving is reduced will be described with reference to the eight-gradation display of FIG. 3 as an example.

The steps up to the portion driven in the planar state and the focal conic state in step 1 are the same as in the first embodiment. In Step 2, for example, half of the gray scale areas are selected for the ON group to be driven and the OFF group not to be driven, respectively, and the selected gray area is changed to the ON group of the sub step 1 of Step 2. Simultaneously apply the ON pulse.

Next, the half-tone areas are selected from among the ON group and the OFF group in substep 1, and ON pulses are applied to the ON group in substep 2. In the same manner as in the case of substep 3, the half-gradation area is selected from the ON group and the OFF group of substep 2, and the ON pulse is applied as the ON group in substep 3.

By doing in this way, each area | region is an area | region (white area | region) where ON pulse is not applied to any of substeps 1, 2, and 3 from the area | region (black area | region) to which ON pulse is applied in all the substeps 1, 2, and 3. Up to now, each sub-step is divided into eight areas by whether or not the ON pulse is applied. Therefore, by making the ON pulses applied in each sub step different from each other, eight regions having different gradations can be formed, and the number of driving in Step 2 can be made three times.

In the driving method of the first embodiment shown in Fig. 2, if eight gradations are required to drive eight times in total and seven times in step 2, the driving method of the second embodiment can significantly reduce the number of driving.

In addition, although the example of FIG. 3 has 8 gradations, it is clear that the same rule can be applied even if it is 16 gradations or more.

Next, an embodiment which can be commonly applied to the first embodiment and the second embodiment will be described.

The embodiment shown in FIGS. 4A to 4C relates to a method of driving a display element in the case of rewriting the display screen.

Conventionally, the method of collectively resetting the previous display screen at the time of screen rewriting was common. In this system, however, at least several tens of watts of power were consumed at the time of reset.

Therefore, in this embodiment, before forming an image in the 1st step of driving a display element, a liquid crystal is reset to a homeotropic state or a focal conic state sequentially by several lines. As shown in Fig. 4A, power consumption can be suppressed by rewriting the screen repeatedly by the number of lines, for example, by resetting each of four lines and simultaneously writing data of one line. .

Fig. 4B shows the voltage applied to the pixels on one line in the case of rewriting the display screen. As described later in Fig. 5B, an alternating current pulse is applied once. As shown in Fig. 4B, a reset pulse is applied to the liquid crystal of one pixel a plurality of times, for example, four times, and a write voltage is applied in the writing period after the rest period is interposed therebetween.

By using this reset driving method, the reflection state and the non-reflection state of Step 1 can be driven at low power consumption and at high speed. As the reset data, for example, the write data itself is used for the reset without using special reset data such that all pixels are made white.

In FIG. 4A, the lower half of the screen shows the screen of the previous display, and the upper half shows the screen of the new display. The common mode described in FIG. 4A is a mode for sequentially selecting lines, and the segment mode is a mode in which an applied voltage can be selected for each electrode. The scan side sequentially selects lines to apply ON scan pulses, and the data side applies pulses of ON data or 0FF data depending on the data to be displayed. 4A shows that the write start line starting from the top line, that is, the write lines by one line described above, is almost near the center of the screen, and data writing on this line is performed. At the same time, reset using the write data is performed on the reset lines, for example, four lines. This operation is further described with reference to FIG. 4C.

As shown in Fig. 4C, an operation of first setting four lines as a reset line is performed. In FIG. 4C, when the Eio signal, which is the start signal on the scan side, and the Lp signal for supplying the latch on the data side and the timing of the shift on the scan side are simultaneously input, first the first line from the top of the screen in FIG. 4A is selected. Data can be written to the line. Next, when the second pulses of the Eio and Lp signals are input together, the first line selected first is shifted by the Lp signal, the second line is selected, and the first line is simultaneously inputted by the Eio signal input at the same time. Selected, two lines of the first line and the second line are selected. This operation is repeated, and in the reset line setting section, the first to fourth lines are selected, and the data can be written to the four lines.

In the next idle line setting section, only the Lp signal is input, and the shift of one line is performed by this pulse, and the second to fifth lines on the screen are selected.

At the beginning of the next writing section, the Eio signal and the Lp signal are input simultaneously, and the second to fifth lines selected before are shifted by one line. As a result, while the third to sixth lines are selected, the first line on the screen, that is, the first line, is also selected by the input of the Eio signal. By supplying the data of the first line in this state, the data to be originally written is written in the first line, and the data of the first line from the third to the sixth line is supplied as data for resetting, and is displayed last time. The data is reset. At this time, the second line is the idle line set in the idle line setting section, and data is not written.

In response to the input of the next Lp pulse, the previously selected line is shifted so that the second line and the fourth to seventh lines are in a selected state. In this state, the data on the second line is supplied, the data originally written on the second line is written, and the previous display data from the fourth to seventh lines is reset.

Further, by the input of the next Lp pulse, the third line and the fifth to eighth lines are similarly selected, and the third line data is written. In the third line, the data of the first line is written at the input of the two previous Lp pulses, but in general, the response time of the cholesteric liquid crystal is in the order of tens of milliseconds, depending on the material properties. At the time of inputting the Lp pulse as the timing at which the second line of data is written, the third line is a resting section, and in this section (for example, 50 ms or less), the third line of pixels is in a focal conic state or planner. It is in the transient state during the transition to the state, and when the third line data is actually supplied, either the focal conic state or the planar state as the actual write state is determined. This operation is repeated, for example, until the 240th line, i.e., writing of the data of the lowest line on the screen is performed.

Next, the driving of the display element in Step 1 will be described with reference to FIGS. 5A and 5B. The voltages of the ON scan and the OFF scan described in FIG. 5A are respectively applied to the selected scan electrode and the other scan electrodes, and the voltage of the ON data described in FIG. 5A is applied to the data electrode for the pixel to which the ON pulse is applied on the line. The voltage of the OFF data is applied to the external data electrode.

In the example of FIG. 5A, a voltage having a first half of 32 V and a second half of 0 V is applied to the ON data, and a voltage of 24 V of the first half and 8 V of the second half is applied to the OFF data. For the ON scan, a voltage of OV in the first half and 32V in the second half is applied, and a voltage of 28V in the first half and 4V in the second half is applied for the OFF scan.

Since the difference between the applied voltage of ON data or OFF data and the applied voltage of ON scan or OFF scan is applied to each pixel, the ON level (first 32V, second -32V) or OFF level shown in FIG. 5B is applied to the pixels of the selected scan line. A voltage waveform of 24V in the first half and -24V in the second half is applied, and a voltage of 4V in the front and rear half is applied to the other non-selected pixels. The general purpose driver is typically a binary output of an ON waveform and an OFF waveform. In Step 1 of the present invention, as shown in Fig. 5A, the ON waveform is set to 32 V and the OFF waveform is set to 24 V, respectively, to drive in a planar state and a focal conic state. In addition, when driving a liquid crystal, using an alternating current pulse as mentioned above is normally performed for the purpose of preventing deterioration of a liquid crystal.

Next, the drive of the display element in Step 2 will be described with reference to FIG. 6.

In step 2 of the present invention, scanning is performed at a higher speed than step 1 or the pulse width is shortened. For example, as shown in FIG. 6, when the scan speed in step 1 is 2 Hz / line, the response characteristics are the same as in FIG. 6, and at 24V, the focal conic state is obtained. On the other hand, at 1 kHz / line in step 2, the response characteristic is shifted as shown in FIG. However, since the response characteristic with respect to speed | variety changes with a liquid crystal material or an element structure, it is not limited to this example.

By the ON waveform 24V of step 2, the reflectance of the part which made into the reflection state in step 1 is reduced (mixed focal conic state). At this time, the OFF waveform is, for example, about 12V, and the level is maintained even if applied to the liquid crystal in the reflected state.

Next, with reference to FIGS. 7A and 7B, a method of driving the display element in the case of applying the pulse of the ON signal will be described. Although the ON pulse described in FIG. 7A is a conventional normal waveform, in contrast, in the driving method of this embodiment, as shown in FIG. 7B, the front and rear of the ON pulse are forced to zero level.

By doing so, the present inventors have found that the following two merit occurs.

(1) Improvement of γ characteristic: The waveform of higher voltage and smaller pulse width, as shown in Fig. 7B, becomes smoother than the normal waveform as shown in Fig. 7A, and the γ characteristic becomes smoother, and more gray levels can be displayed. .

(2) Improvement of Crosstalk: In the normal waveform as shown in Fig. 7A, an unselected pulse is applied in succession to the ON pulse. That is, since the non-selective pulse is applied before the state of the liquid crystal is stabilized, the halftone is particularly susceptible to crosstalk. On the other hand, when the ON pulse is set to zero level as shown in Fig. 7B, the state of the liquid crystal changed by the ON pulse can be stabilized until the non-selection pulse is applied, making it difficult to be affected by crosstalk. .

Therefore, it is particularly preferable to employ the driving method in each substep of step 2.

Next, with reference to FIG. 8, an example of voltage switching between step 1 and step 2 will be described. As described above, the voltage values of the ON pulses and the OFF pulses of Step 1 and Step 2 are different from each other. It is easy to use an analog switch to switch this voltage.

In Fig. 8, the output switched to 32V in step 1 and 24V in step 2 is supplied as ON pulses in the segment mode and the common mode, and the waveforms are shown as waveforms of ON data and ON scan. Similarly, the waveform of the OFF pulse in the common mode is shown as the waveform of the OFF scan, and the waveform of the OFF pulse in the segment mode is shown as the waveform of the OFF data.

By switching in this way, each waveform of ON / OFF like FIG. 9 is applied to each pixel. For example, since the voltage of the difference between the waveform of the ON data and the waveform of the ON scan shown in FIG. 8 is applied to the pixel to which the ON level pulse is applied, ± 32 V is applied in Step 1 and ± 24 V is applied in Step 2.

Next, the drive of the display element in each substep of step 2 is demonstrated with reference to FIG. As described above in the second embodiment shown in FIG. 3, it is necessary to apply different ON pulses in each substep. Therefore, as illustrated in FIG. 10, in each sub-step of Step 2, the pulse width is set to an appropriate value, respectively. When driving at a higher black density, the scan is made slower or set to a wider pulse width. In order to increase the width of the ON pulse that is the output of the driver, the frequency of the clock driving the driver can be reduced to increase the output period. However, the switching of the pulse width can be performed by analogously switching the clock frequency itself. Rather, it is more stable to perform by changing the division ratio of the clock generation section logically input to the driver.

Next, with reference to FIG. 11, the drive method which can reduce the number of scans even when applying ON pulse to the same pixel multiple times is demonstrated.

FIG. 11 is a diagram showing the relationship between the scan pulse and the data-side latch pulse, showing execution of a plurality of substeps in one scan line. Although it can be set to 1 sub step per scan line, in such a method, for example, in the case of 8 gradations, a total of 5 scans are executed in combination with Step 1 and Step 2. However, the smaller the number of scans, the less flicker during writing, and the observer feels preferred. Therefore, in order to reduce the number of scans, a latch pulse of a plurality of substeps is applied per scan. By doing so, the number of scans can be reduced and writing with less flicker can be realized.

At this time, it is preferable to make step 1 and step 2 independent. That is, only one screen is written in step 1, and the remaining data is written in step 2. By doing this, the user can quickly grasp the overall feeling of the image by writing in Step 1.

FIG. 12 is a diagram for explaining a process of generating sub-image data for driving display elements from multi-gradation image data. With reference to Fig. 12, the processing of image data in which gradation is converted into eight gradations by, for example, an error diffusion method will be described. As described above, in the second embodiment, eight gray scales are displayed by applying four pulses in combination with Step 1 and Step 2. However, as shown in Fig. 12, as shown in FIG. The image is divided into four sub-images adapted to pulse application.

At this time, in the part corresponding to step 2, the part whose reflectance is lowered by the ON pulse becomes white (1) in the concept of the sub image data, and the part which maintains the reflectance by applying the OFF pulse is the part of the sub image data. The concept is black (0). That is, for each sub image, sub image data which is binary data of 0 and 1 indicating that an ON pulse or an OFF pulse is applied is generated. In addition, the error diffusion method and the blue noise mask method are preferable in terms of image quality.

Next, a driving method in full color display will be described with reference to FIGS. 13 and 14.

FIG. 13 is a diagram showing a laminated structure of display elements for full color display. As shown in FIG. 13, in the full-color display of cholesteric liquid crystal, the laminated structure of each RGB element is common, for example. And it is controlled by the control circuit corresponding to each layer. The display elements of each layer are driven by respective independent voltage waveforms, and perform full color display as a whole.

14 is a diagram for explaining a method of driving an ON pulse for full color display.

As shown in Fig. 7B, in the embodiment of the present invention, while the front and rear of the ON pulse are forced to zero level, a waveform having a small pulse width at a higher voltage is used as the ON pulse. As shown in FIG. 14, the position of a pulse is shifted so that it may not become the same timing. This is because if the stacked structure of the display elements is adopted and each RGB element is driven at the same timing, the spike current increases, the power supply voltage becomes unstable, and the display quality deteriorates.

In order to reduce this spike current, the application timing of the DSPOF signal indicating the timing of forcibly setting the applied voltage to 0 is shifted so as not to overlap the ON pulse positions at the time of driving each RGB element.

As a result, the driving circuit was stabilized, and it was confirmed that the display quality was obtained well.

As described above, when the driving method of the present invention is adopted, driving in a general-purpose driver component having a breakdown voltage of 40 V or less is possible.

Next, with reference to FIG. 15, the block structure example of the drive circuit which implements the drive method of the display element of this invention is demonstrated. The driver IC 10 includes a scan driver and a data driver. The arithmetic unit 20 performs an image processing for display consisting of the binary image for step 1 obtained from the original image and the binary image group for step 2 separated by the processing described with reference to FIG. The data is output to the driver IC 10 and various control data are output to the driver IC 10.

The data shift latch signal is a signal that controls the shift of the scan line to the next line and the latch of the data signal. 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. The driver output off signal is a signal forcibly zeroing the driver output.

The drive voltage input to the driver IC is boosted by the booster 40 by a logic voltage of 3 to 5V, and is formed at the voltage forming unit 50 at various voltage outputs. 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, an embodiment of the reflective liquid crystal display element according to the present invention will be described with reference to the accompanying drawings, and the liquid crystal composition according to the present invention will be described in detail.

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 conic 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 and 4 face each other to intersect with each other as seen from the direction perpendicular to the substrate. 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 from 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.

Reference numerals 6 and 7 denote sealing materials for encapsulating the liquid crystal composition 5 between the substrates 1 and 2. Reference numeral 9 is a driving circuit, and applies a pulse-shaped predetermined voltage to the electrode.

Although the board | substrates 1 and 2 have all the 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 and 4, for example, Indium Tin Oxide (ITO: Indium Tin Oxide) is typical, but in addition, transparent conductive films such as Indium Zic Oxide (IZO: Indium Zinc Oxide), aluminum, silicon, etc. A metal electrode or photoconductive film, such as amorphous silicon and BSO (Bismuth Silicon Oxide), can be used. In the liquid crystal display element shown in FIG. 16, as previously demonstrated, the several strip | belt-shaped transparent electrodes 3 and 4 parallel to each other are formed in the surface of the transparent substrates 1 and 2, and these electrodes are attached to the board | substrate. They face each other to intersect with each other as seen from the perpendicular direction.

Next, although not shown in FIG. 16, elements suitable for use in the liquid crystal display device according to the present invention will be described.

(Insulating thin film) Including the liquid crystal display element shown in FIG. 16, the liquid crystal display element which concerns on this invention forms the insulating thin film which has a function which prevents the short circuit between electrodes, or improves the reliability of a liquid crystal display element as a gas barrier layer. You may be.

(Orientation stabilization film) Examples of the orientation stabilization film include organic films such as polyimide resin, polyamideimide resin, polyether imide resin, polyvinyl butyral resin and acrylic resin, and inorganic materials such as silicon oxide and aluminum oxide. do. 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) The liquid crystal display element shown in FIG. 16 is included, and the liquid crystal display element which concerns on this invention may be provided with the spacer for maintaining the inter-substrate gap uniformly between a pair of board | substrate.

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. Moreover, the fixing spacer in which the thermoplastic resin is coated on the surface is also used suitably.

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 material is a value when the total amount of a nematic liquid crystal component and a chiral material is 100 wt%.

Although various conventionally well-known things can be used as a nematic liquid crystal, it is preferable on account of a drive voltage that dielectric constant anisotropy is 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 conic state becomes large, viscosity is also affected and becomes high, and response speed falls.

Moreover, as for the thickness of this liquid crystal, about 3-6 micrometers is preferable. When smaller than this, the reflectance of a planar state becomes low, and when larger than this, driving voltage will become high too much.

Next, Experimental Example 1 of the present invention using a monochromatic eight-gradation, resolution Q-VGA display element having the above-described content is fabricated and described therein.

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

The driver IC used two S1D17A03 (160 outputs) and one S1D17A04 (240 outputs) made by EPSON, a general purpose STN driver. The drive 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. Note that the driver IC is not limited to this, and it is clear that other driver may be used as long as it has the same function.

The input voltage to this driver IC was 32, 28, 24, 8, 4, 0V in step 1 (shown in FIG. 8), and was set to 24, 20, 12, 12, 4, 0V in step 2. In order to switch the voltage of this step 1 and step 2, the analog switch was used and it arrange | positioned in front of an op amp. As the analog switch, for example, Max4535 (36 V withstand voltage) manufactured by Maxim can be used.

As a result, in step 1, a pulse voltage of ± 32 V is applied to the ON pixel and ± 24 V to the OFF pixel stably, and a pulse voltage of ± 4 V is applied to the non-selected pixel.

On the other hand, in step 2, a pulse voltage of ± 24 V is applied to the ON pixel and ± 12 V to the OFF pixel, and a pulse voltage of ± 4 V or ± 8 V is applied to the non-selected pixel.

Step 1 was performed at a scanning speed of about 2 kHz / line. In step 2, the application time of sub step 1 was about 2 ms, the sub step 2 was about 1.5 ms, and the sub step 3 was about 1 ms / line, and it carried out at the scanning speed of 4.5 ms / line in total.

At this time, the insertion time of the voltage zero level DSPOF shown in Fig. 7B was set to 0.8 ms in total in sub-step 1, 0.6 ms in sub-step 2, and 0.4 ms in sub-step 3.

In other words, the effective time of the voltage pulse is 1.2 s in substep 1, 0.9 s in substep 2, and 0.6 s in substep 3.

In the image data input to the driver IC, 256 values of the original image were converted to eight values by the error diffusion method. After that, the image data in the sub step 1 and the sub step 2 was converted again by the method shown in FIG. As a result of driving under the above main conditions, high quality display with low granularity as shown in FIG. 17 was realized.

In order to demonstrate the level of this display quality, a test image was displayed and the granularity was compared with the display apparatus of the existing cholesteric liquid crystal. The step wedge from the back level to the black level was displayed on the display element of the present invention and the existing display device, and then it was imaged. After imaging, the variation in the reflectance (standard deviation) of the pixel value of each density pattern was calculated. As a result, the display according to the present invention is about half the granularity of the conventional display device, and the display quality of the present invention is high. Could confirm. In this experimental example, the comparison is carried out in 8 gradation display, but the same display quality can be realized even if there are more gradations than that, for example, 16 gradations or more.

As Experimental Example 2, an experimental example of 512-color display of the color element is introduced.

Three types of display elements of the Q-VGA described in Experimental Example 1 (Red, Green, Blue) were fabricated, and stacked in the order of Blue, Green, and Red from the observation surface. The drive circuit was set so that control of each color was performed separately. With respect to the stacked display elements, three layers were simultaneously driven under the same driving conditions as those in Experimental Example 1, and as a result, good 512 colors of display could be realized. At this time, the DSPOF timing is shifted as shown in Fig. 14 to reduce the spike current.

As described above, in the case of driving the display element using the cholesteric liquid crystal by the driving method of the present invention, a high-quality multi-gradation display that greatly exceeds the existing driving method even by a low-cost binary output general purpose driver Can be realized, and the maximum contrast of the liquid crystal can be extracted.

In addition, according to the present invention, the number of overwrites can be minimized even when the number of tones is increased.

In addition, since driving is performed in steps 1 and 2, the basic display contents can be quickly known as in the progressive display.

Claims (22)

A driving method of a liquid crystal display device in which a pulse-shaped driving voltage is applied to a reflective material while the scan electrodes are selected in a predetermined order from a plurality of scan electrodes and a plurality of data electrodes that cross each other in an opposite state, Step 1 in which each pixel is placed in a reflection state and a non-reflection state in the first scan, Step 2 of selecting a predetermined pixel in the reflection state and a pixel in the non-reflection state in the next scan to reduce the reflectance of the predetermined pixel in the reflection state and further reducing the reflectance of the pixel in the non-reflection state The drive method of the liquid crystal display element characterized by the above-mentioned. The method of claim 1, And said step 2 is constituted by at least one or more substeps for each pixel to have a reflectance corresponding to a predetermined gradation level, respectively. The method of claim 2, In step 2, the pixel group that is to be reduced in the reflectance in each pixel group in the current sub-step is simultaneously selected for the pixel group selected in the step 1 or the sub-step executed previously and the reflectance is simultaneously selected. A method of driving a liquid crystal display device characterized by reducing the pressure. As a driving method of a liquid crystal display element, a pulse-shaped driving voltage is applied to a liquid crystal forming a cholesteric phase while selecting the scan electrodes in a predetermined order from a plurality of scan electrodes and a plurality of data electrodes that cross each other in an opposite state. , Step 1 in which each pixel is placed in a reflection state and a non-reflection state in the first scan, Step 2 of selecting a predetermined pixel in the reflection state and a pixel in the non-reflection state in the next scan to reduce the reflectance of the predetermined pixel in the reflection state and further reducing the reflectance of the pixel in the non-reflection state The drive method of the liquid crystal display element characterized by the above-mentioned. The method of claim 4, wherein And said step 2 is constituted by at least one or more substeps for each pixel to have a reflectance corresponding to a predetermined gradation level, respectively. The method of claim 5, And the reflection state is a planar state or a state in which the planar state and the focal conic state are mixed, and the non-reflective state is a focal conic state. The method of claim 6, In step 2, the predetermined pixel in the reflection state and the pixel in the non-reflection state are selected, the reflectance of the predetermined pixel in the reflection state is reduced, and the reflectance of the pixel in the non-reflection state is further reduced so that each of the pixels is reduced. A method of driving a liquid crystal display device, characterized by comprising at least one or more substeps, each for achieving a reflectance corresponding to a predetermined gradation level. The method of claim 5, In step 2, the pixel group that is to be reduced in the reflectance in each pixel group in the current sub-step is simultaneously selected for the pixel group selected in the step 1 or the sub-step executed previously and the reflectance is simultaneously selected. A method of driving a liquid crystal display device, characterized by reducing the number of points. The method of claim 5, And said step 1 includes a step of resetting the liquid crystal to a homeotropic state or a focal conic state before forming an image. The method of claim 5, The liquid crystal display element has a means for bringing the potential to zero level before and after applying a pulse of an ON signal. The method of claim 5, And said voltage level for driving the liquid crystal forming said cholesteric phase is different from said step 1 and said step 2; The method of claim 5, And the pulse widths for driving the liquid crystals forming the cholesteric phase are different in each of the substeps of the step 2; The method of claim 12, The pulse width of the substep is controlled by changing the clock frequency of the driver. The method of claim 5, And said substep is executed within one line scanned. The method of claim 5, The display element has a structure in which a plurality of elements are stacked, and each of the stacked layers is driven by voltage pulses independent of each other, and each of the plurality of elements has a means for bringing the potential to zero level before and after applying a pulse of the ON signal, respectively. And shifting the timing of applying the pulses of the respective ON signals. The method of claim 5, Using the STN driver IC of the binary output, in step 1, the output of the ON level is used for bringing each pixel into the reflection state and the OFF level output is used for the non-reflective state. Way. The method of claim 5, A method of driving a liquid crystal display device using an STN driver IC having a binary output, wherein an output of an ON level is used for reducing the reflectance in Step 2 and an OFF level is used for maintaining the state. The method of claim 5, The display data used for driving in each step is formed by dividing and converting the gradation-converted original image data, The driving method of the liquid crystal display element characterized by the above-mentioned. The method of claim 18, The original image data is grayscale-converted by an error diffusion method or a blue noise mask method. The method of claim 5, The drive voltage is 40V or less, The driving method of the liquid crystal display element characterized by the above-mentioned. A liquid crystal display device for displaying an image by applying a pulse-shaped driving voltage to a reflective material while selecting the scan electrodes in a predetermined order from a plurality of scan electrodes and a plurality of data electrodes that cross each other in an opposite state, First means for bringing each pixel into a reflection state and a non-reflection state in the first scan; Second means for selecting a predetermined pixel in the reflective state and a pixel in the non-reflective state in the next scan, reducing the reflectance of the predetermined pixel in the reflected state, and further reducing the reflectance of the pixel in the non-reflective state It has a liquid crystal display element characterized by the above-mentioned. A liquid crystal display device for displaying an image by applying a pulse-shaped driving voltage to a liquid crystal forming a cholesteric phase while selecting the scan electrodes in a predetermined order from a plurality of scan electrodes and a plurality of data electrodes that cross each other in an opposite state. as, First means for bringing each pixel into a reflection state and a non-reflection state in the first scan; Second means for selecting a predetermined pixel in the reflective state and a pixel in the non-reflective state in the next scan, reducing the reflectance of the predetermined pixel in the reflected state, and further reducing the reflectance of the pixel in the non-reflective state It has a liquid crystal display element characterized by the above-mentioned.

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