JP2010145975A - Method for driving display element, and display device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To make a display part of cholesteric liquid crystal or the like faster in a rewriting speed and high in contrast. <P>SOLUTION: The method for driving a display element includes a process for applying a pulse-like driving voltage forming gradation corresponding to display information. The pulse-like driving voltage is an AC waveform of one period which includes a base part for determining a gradation level and a peak part obtained by adding a voltage to the base part, and the peak part is arranged on portions other than a starting end and a terminating end of a driving pulse. Since the peak part is arranged on a portion other than the terminating end of the driving pulse, the rewriting speed can be accelerated and high contrast can be achieved similarly to over-driving. Further since the peak part does not exist on the starting end of the driving pulse, a change from a state of no application of a driving voltage to a state of sudden application of a large voltage can be evaded, so that the loads of portions (a driver IC, a power supply, or the like) to which the voltage is applied can be reduced and a flicker or the like of the display part can be suppressed. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a display element driving method and a display device, and more particularly to a display element driving method and a display device by simple matrix driving.

  In recent years, development of electronic paper has been actively promoted in various companies and universities. As for electronic paper, various application methods such as a sub display of a mobile terminal device and a display unit of an IC card have been proposed, starting with an electronic book. One display method of electronic paper is one that uses a liquid crystal composition in which a cholesteric phase is formed (cholesteric liquid crystal). This cholesteric liquid crystal is also called chiral nematic liquid crystal, and by adding a relatively large amount (several tens of percent) of chiral additive (chiral material) to the nematic liquid crystal, the molecules of the nematic liquid crystal form a spiral cholesteric phase. It is a liquid crystal. Cholesteric liquid crystals have excellent characteristics such as a semipermanent display retention characteristic (memory property), vivid color display characteristics, high contrast characteristics, and high resolution characteristics.

  More specifically, the cholesteric liquid crystal has bistability (memory property), and is in an intermediate state in which the planar state, the focal conic state, or the planar state and the focal conic state are mixed by adjusting the electric field intensity applied to the liquid crystal. Take one of these states. In the cholesteric liquid crystal, once the planar state or the focal conic state is reached, the state is stably maintained even under no power thereafter.

  The planar state is obtained by applying a predetermined high voltage to apply a strong electric field to the liquid crystal and then suddenly reducing the electric field to zero. The focal conic state is obtained by applying a predetermined voltage lower than the high voltage, for example. After applying an electric field to the liquid crystal, it is obtained by making the electric field suddenly zero. An intermediate state in which the planar state and the focal conic state are mixed is, for example, by applying a voltage lower than the voltage at which the focal conic state is obtained to apply an electric field to the liquid crystal and then suddenly reducing the electric field to zero. can get.

  Here, driving methods for multi-tone display on cholesteric liquid crystal can be broadly classified into dynamic driving and conventional driving. Among these, since dynamic driving has a complicated driving waveform, a complicated configuration is required for the control circuit and the driver IC. Further, since the transparent electrode of the panel needs to have a low resistance, the power consumption is increased and the cost is increased. In recent years, there is an example in which dynamic driving is attempted with an inexpensive general-purpose driver, but there is a concern that a display with high contrast cannot be obtained.

  On the other hand, conventional driving has the advantage of reducing the cost of circuit components, etc., because the power consumption during rewriting is small compared to dynamic driving, and it has excellent advantages such as high uniformity and high contrast in halftones. Display quality can also be obtained.

JP 2006-309082 A Japanese translation of PCT publication No. 2001-510484 JP-T-2001-506376

  However, it is known that conventional driving has a slower rewriting speed than dynamic driving. For example, when the number of pixels is as large as XGA (1024 × 768 pixels), writing in a multicolor display of 4096 colors often requires around 10 seconds.

  In order to solve this problem, recently, it has been studied to use a material having a low liquid crystal viscosity as a material capable of increasing the speed. However, the reduction of the liquid crystal viscosity is not easy to realize because it tends to be in a trade-off relationship with other important factors such as electrical properties and optical properties.

  In addition, Patent Documents 1 to 3 disclose a technique (overdrive) that increases the response speed by adding an excessive voltage at the head of the signal. In this technique, the voltage is applied at the head of the signal. Since it is necessary to make it rise rapidly, there is a risk of applying a load to the driver IC or the like.

  In addition, for the liquid crystal display elements used for electronic paper such as cholesteric liquid crystal, the appearance of further high contrast technology is expected.

  Accordingly, the present invention has been made in view of the above problems, and an object of the present invention is to provide a display element driving method and a display device capable of improving the rewriting speed and increasing the contrast.

  As a result of earnest research on techniques for improving rewriting speed and increasing contrast in liquid crystal display elements used for electronic paper such as cholesteric liquid crystals, the present inventor applied as in the case of conventional overdrive. It has been found that the rewriting speed can be improved and the contrast can be increased without applying an excessive voltage at the head of the voltage signal. As a result of further studies by the present inventor, this phenomenon is caused by the fact that the ionic impurities contained in the display element form an electric field opposite to the electric field of the driving pulse, and the electric field of the driving pulse is generated by the reverse electric field. It was caused by the inhibition, and the idea that the inhibition of the electric field could be alleviated by placing an excessive voltage (peak voltage) in the drive pulse. Then, the inventor conducted an experiment under the assumption that the inhibition of the electric field could be alleviated if an excessive voltage (peak voltage) other than the end of the drive pulse was provided, and almost matched the estimation. To get the result. The display element driving method and the display device described in the present specification are based on the above-described novel findings and employ the following configurations.

  A driving method of a display element by simple matrix driving described in the present specification includes a step of applying a pulsed driving voltage for forming a gradation corresponding to display information to the display element, and the pulsed driving voltage Is a one-cycle AC waveform including a base part for determining a gradation level and a peak part obtained by adding a voltage to the base part, and the peak part is provided at a part excluding the start and end of the drive pulse. Only arranged.

  According to this, by arranging the peak portion in a portion excluding the end of the drive pulse, it is possible to improve the rewrite speed and increase the contrast as in the case of overdrive. In addition, since the peak portion is not at the start of the drive pulse, it is possible to avoid applying a large voltage suddenly from a state in which no drive voltage is applied. ), And flickering of the display element can be suppressed.

  The display device described in the present specification includes a display element, and a driver IC that applies a pulse-shaped drive voltage that forms a gradation corresponding to display information displayed on the display element to the display element, and the pulse The driving voltage is a one-cycle alternating current waveform including a base part that determines a gradation level and a peak part obtained by adding a voltage to the base part, and the peak part includes a starting end and a terminal end of the driving pulse. It is arranged only in the part except.

  According to this, the peak portion of the pulsed drive voltage applied to the display element from the driver IC is arranged in a portion excluding the end of the drive pulse, thereby improving the rewriting speed and increasing the contrast as in the case of overdrive. It becomes possible to plan. In addition, since the peak portion is not at the beginning of the drive pulse, it is possible to avoid applying a large voltage suddenly from the state where the drive voltage is not applied, thereby reducing the load on the driver IC and the power source, Flickering of the display element can be suppressed.

  The display element driving method and the display device described in this specification have an effect of improving the rewriting speed and increasing the contrast.

  Hereinafter, an embodiment of the present invention will be described in detail with reference to FIGS.

  FIG. 1 is a block diagram showing a schematic configuration of a liquid crystal display device 10 as a display device using a cholesteric liquid crystal capable of displaying an image in a memory without power. In this embodiment, for convenience of explanation, it is assumed that the liquid crystal display device 10 of FIG. 1 is a liquid crystal display device that displays a monochrome image of A4 size XGA (1024 × 768 pixels).

  The liquid crystal display device 10 includes a circuit block 10a and a display block 10b. The circuit block 10a includes a power supply 12, a boosting unit 14, a power switching unit 16, a power stabilization unit 18, a source clock 20, a frequency dividing unit 22, a display control unit 24, and an image data storage unit 26. ,have. On the other hand, the display block 10b includes a display unit 30, a scan electrode drive circuit (common driver) 32 on which a scan electrode driver IC is mounted, and a data electrode drive circuit (segment driver) 34 on which a data electrode driver IC is mounted. And have. A general-purpose binary output STN driver is used for the driver IC mounted on each driver.

  The power supply 12 outputs a direct current voltage of 3V to 5V. The boosting unit 14 includes, for example, a DC-DC converter, and boosts the DC voltage (3 V to 5 V) input from the power supply 12 to a voltage of about 10 V to 40 V DC necessary for driving the display unit 30. The boosting unit 14 preferably has high conversion efficiency with respect to the characteristics of the display unit 30. The power supply switching unit 16 uses the voltage boosted by the boosting unit 14 and the input voltage to generate a plurality of levels of necessary voltages depending on the gradation value of each pixel and selection / non-selection. Note that, for switching between the reset voltage for resetting the screen of the display element and the gradation writing voltage for writing gradation, a high-breakdown-voltage analog switch can be used and a switching circuit using a simple transistor can be used.

  The power supply stabilization unit 18 includes a Zener diode, an operational amplifier, and the like, stabilizes the voltage generated by the power supply switching unit 16 (if it is an operational amplifier, it is stabilized by a voltage follower), and the display block 10b has the voltage. The common driver 32 and the segment driver 34 are supplied. In the case where an operational amplifier is used as the power supply stabilizing unit 18, it is preferable to use a type of operational amplifier that is resistant to capacitive loads. The power supply 12 supplies predetermined power to the display controller 24, the source clock 20, and the frequency divider 22 in addition to the booster 14. The frequency divider 22 divides the clock input from the source oscillation clock 20 by a predetermined frequency division ratio and outputs it to the display controller 24 for switching the scanning speed.

  The display control unit 24 includes a processor and the like, and controls the entire liquid crystal display device 10. The display control unit 24 switches the scanning speed and driving voltage (driving pulse) of the display unit 30 via the common driver 32 and the segment driver 34 to display an image, and executes a display area reset process.

  Specifically, the display control unit 24 controls the display unit 30 by a line-sequential driving method in which the linear electrodes 43 and 44 (see FIG. 2) arranged on the display unit 30 at substantially equal intervals are sequentially scanned. The display control unit 24 controls the scanning speed of the common driver 32 to change the application time for applying the drive pulse voltage. At this time, the display control unit 24 controls the segment driver 34 so as to output a predetermined voltage based on the image data to the display unit 30 in synchronization with the scanning timing of the common driver 32.

  The display control unit 24 outputs the generated drive data to the common driver 32 and the segment driver 34 in synchronization with the data read clock signal. The display control unit 24 changes the scanning speed by outputting drive data to the common driver 32. In addition, the display control unit 24 sends control signals such as a scan / data mode signal, a data capture clock, a frame start signal, a pulse polarity control signal, a data latch / scan shift, and a driver output off to the common driver 32 and the segment driver 34. Output to.

  FIG. 2 schematically shows the configuration (cross-sectional structure) of the display unit 30. As shown in FIG. 2, the display unit 30 includes film substrates 41 and 42, ITO electrodes 43 and 44, a liquid crystal mixture 45, sealing materials 46 and 47, and an absorption layer 48.

  The film substrates 41 and 42 are both translucent. As a material of the film substrates 41 and 42, a glass substrate can be used, but is not limited thereto. For example, a film substrate such as PET (Polyethylene Terephthalate) or PC (Polycarbonate) can also be used.

  The ITO electrodes 43 and 44 are composed of a plurality of strip-like electrodes arranged in parallel, and the ITO electrode 43 and the ITO electrode 44 are from a direction perpendicular to the film substrates 41 and 42 (up and down direction in FIG. 2). They are arranged so as to cross each other at an angle of 90 °. The material of the ITO electrodes 43 and 44 is Indium Tin Oxide (ITO: indium tin oxide). However, instead of this, an electrode using a transparent conductive film such as Indium Zic Oxide (IZO) may be employed.

An insulating thin film is formed on the ITO electrodes 43 and 44. When this insulating thin film is thick, drive voltage rises and control with a general-purpose STN driver becomes difficult. On the other hand, if an insulating thin film is not provided, a leakage current flows, resulting in an increase in power consumption. Since this insulating thin film has a relative dielectric constant of around 5 and lower than that of liquid crystal, the thickness of the insulating thin film is generally about 0.3 μm or less. As the insulating thin film, an SiO ₂ thin film or an organic film such as a known polyimide resin or acrylic resin can be used as the orientation stabilizing film.

  The liquid crystal mixture 45 is a cholesteric liquid crystal obtained by adding 10 to 40 wt% of a chiral material to a nematic liquid crystal mixture. The addition amount of the chiral material is a value when the total amount of the nematic liquid crystal component and the chiral material is 100 wt%. As the nematic liquid crystal, a conventionally known liquid crystal can be used, but the dielectric anisotropy (Δε) is preferably in the range of 15 to 35. If the dielectric anisotropy is 15 or more, the drive voltage is relatively low, but if it exceeds 35, the drive voltage itself is low but the specific resistance is small, and the power consumption at high temperature is particularly increased. The refractive index anisotropy (Δn) is preferably about 0.18 to 0.24. If it is smaller than this range, the reflectivity in the planar state will be low, and if it is larger than this range, the scattering reflection in the focal conic state will increase, and this will also increase the viscosity and decrease the response speed. Become.

  The sealing materials 46 and 47 are for sealing the liquid crystal mixture 45 between the film substrates 41 and 42.

  The absorption layer 48 is provided on the back surface of the film substrate 42 on the side opposite to the light incident side (upper side of the drawing in FIG. 2) (lower side of the drawing in FIG. 2).

  The display unit 30 may be provided with a spacer for uniformly holding a gap between the pair of film substrates 41 and 42. As the spacer, a sphere made of resin or inorganic oxide can be used. As the spacer, a fixed spacer whose surface is coated with a thermoplastic resin can also be used. The gap formed by the spacer is preferably in the range of 3.5 to 6 μm, for example. This is because when the gap is smaller than this range, the reflectivity decreases and the display becomes dark, and when the gap is large, the drive voltage rises and it becomes difficult to drive with general-purpose components.

  Here, the display principle of the cholesteric liquid crystal will be described with reference to FIGS. 3 (a) and 3 (b). FIG. 3A shows the alignment state of the liquid crystal molecules 36 when the liquid crystal mixture 45 of the display unit 30 is in the planar state, and FIG. 3B shows the liquid crystal mixture 45 of the display unit 30 in the focal conic state. The alignment state of the liquid crystal molecules 36 in this case is shown.

As shown in FIG. 3A, the liquid crystal molecules 36 in the planar state are sequentially rotated in the thickness direction to form a spiral structure, and the spiral axis of the spiral structure is substantially perpendicular to the substrate surface. In the planar state, incident light L having a predetermined wavelength corresponding to the helical pitch of the liquid crystal molecules is selectively reflected by the liquid crystal layer. When the average refractive index of the liquid crystal layer is n and the helical pitch is p, the wavelength λ at which the reflection is maximum is expressed by the following equation (1).
λ = n · p (1)

  Therefore, in order to selectively reflect light in the planar state by the liquid crystal mixture 45 of the display unit 30, the average refractive index n and the helical pitch p are determined so that λ becomes a predetermined value. The average refractive index n can be adjusted by selecting a liquid crystal material and a chiral material, and the helical pitch p can be adjusted by adjusting the content of the chiral material.

  On the other hand, as shown in FIG. 3B, the liquid crystal molecules 36 in the focal conic state are sequentially rotated in the in-plane direction of the substrate to form a spiral structure, and the spiral axis is substantially parallel to the substrate surface. In this case, the display unit 30 loses the selectivity of the reflected wavelength and transmits most of the incident light L.

  As described above, in the cholesteric liquid crystal, the reflection and transmission of the incident light L can be controlled by the alignment state of the liquid crystal molecules 36 twisted in a spiral shape. Moreover, in this display part 30, when incident light permeate | transmits, since transmitted light is absorbed in the absorption layer 48 shown in FIG. 2, a dark display is realizable.

  4A to 4C show voltage response characteristics of the cholesteric liquid crystal. In addition, when driving a cholesteric liquid crystal with a simple matrix, in order to suppress deterioration of the liquid crystal material, the driving waveform is an alternating current as in general liquid crystals.

  As shown in FIG. 4A, when the initial state is the planar state, when the pulse voltage is increased to a certain range, the driving band is set to the focal conic state, and when the pulse voltage is further increased, the driving band is set to the planar state again. . When the initial state is the focal conic state, the driving band gradually becomes the planar state as the pulse voltage is increased. In this case, whether the initial state is the planar state or the focal conic state, the voltage at which the planar state is reached is ± 36V. Therefore, with these intermediate voltages, a halftone in which the planar state and the focal conic state are mixed is obtained.

  On the other hand, when a pulse having a lower voltage or a shorter cycle than that in FIG. 4A is applied, this responsiveness shifts. For example, if the applied voltage is a pulse of ± 20 V or ± 10 V, the cycle is 2 ms or 1 ms, and the initial state is the planar state, the cycle is 1 ms (FIG. 4 (FIG. 4 (b)). In the case of c)), when the voltage is ± 10 V, no response is shown and the planar state is maintained. On the other hand, when the voltage is ± 20 V, the response is shown in both the case where the period is 2 ms and the case where the period is 1 ms, and the halftone having a slightly reduced reflectance is obtained. As can be seen from a comparison between FIG. 4B and FIG. 4C, this decrease in reflectivity is greater when the period is 2 ms than when the period is 1 ms. The gradation becomes lower.

  Next, simple matrix driving of cholesteric liquid crystal will be described based on the schematic diagram of FIG. FIG. 5 shows a state in the middle of writing the alphabet “F” from the upper side to the lower side in the drawing. As shown in FIG. 5, the writing target line (selected line) is one line, while the remaining lines are all lines not to be written (non-selected lines). Further, the line above the selected line (upper side of the drawing) is a line for which writing has already been completed, and the line below the selected line (lower side of the drawing) is a line to be written from now.

  When attention is paid to the selection line among these, for example, a dot in which a part of “F” is written (dots surrounded by a broken line in FIG. 5) is a full selection dot, and other dots are half selection dots.

  A graph in which “full selection”, “half selection”, and “non-selection” are assigned to the pulse response characteristics of the cholesteric liquid crystal is schematically shown in FIG. In FIG. 6, the pulse voltage is V, the pulse width is T, and the response amount (brightness change amount) of the liquid crystal is dy.

As shown in FIG. 6, the liquid crystal does not respond until a predetermined value (threshold value) of V ² T. When the threshold value is exceeded, the liquid crystal responds almost linearly to the value of V ² T. In this case, if V ² T is a constant value, even if V and T are different values, almost the same responsiveness is exhibited. In this case, if the area where the liquid crystal does not respond is “area A” and the area where the liquid crystal responds is “area B”, non-selection / half-selection is assigned to area A and full selection is applied to area B in simple matrix drive Is assigned.

When a general-purpose simple matrix driver such as STN is used, the voltages in “full selection”, “half selection”, and “non-selection” cannot be freely set, and there are certain restrictions. For example, if the entire selection voltage (voltage for grading gradation) used for writing is constant, the half-selection voltage and non-selection voltage (voltage for maintaining gradation) that form a pair are reduced when the half-selection voltage is lowered. When the selection voltage increases and the half-selection voltage increases, the non-selection voltage decreases. This can also be derived from the fact that the voltages in full selection, half selection, and non-selection satisfy the relationship of the following equation (2).
Total selection voltage = half selection voltage + non-selection voltage × 2 (2)

  Here, in the case of a general-purpose driver such as STN, the total selection voltage is generally in the range of −40V to + 40V. For this reason, the voltage control needs to be performed within the voltage range in accordance with the characteristics of the display unit 30.

  In the present embodiment, as shown in FIG. 7A, a driving pulse in which a minute peak is added at a position immediately before and after the polarity inversion is used as a driving pulse for forming a gradation. The ratio of the peak portion to the entire drive pulse is set to 20%. The operation when such a drive pulse is used will be described below.

In the case of a liquid crystal that is written at a low speed such as a cholesteric liquid crystal, the polarity of the driving pulse is generally reversed within one line. Under such conditions, as shown in FIG. 7A, the relationship between V ² T and dy (brightness change amount) when the peak part is arranged immediately before and after the polarity inversion (sign inversion of the base part). It is shown in FIG. In addition, the graph of FIG. 8 has shown the actual measurement result corresponding to the graph (schematic diagram) of FIG.

As can be seen from FIG. 8, when the driving pulse as in the present embodiment is used, compared with the case of normal driving as shown in FIG. 7B (that is, when the driving pulse is not provided with a peak portion). , The change (response) in a region (corresponding to region B in FIG. 6) that changes almost linearly with respect to V ² T becomes steeper. This means that the writing speed is faster in this embodiment than in the case of normal driving. In the case of the present embodiment, the minimum value of dy (brightness change amount) can be reduced (bottomed down) as compared with the case of normal driving. This means that the contrast can be improved by using the drive pulse as shown in FIG.

  Here, in a general liquid crystal display (moving image use), an overdrive technology that can shorten the rise time of liquid crystal molecules by arranging a peak portion at the tip of a pulse is known (for example, patents). Documents 1 to 3 (see Japanese Patent Application Laid-Open No. 2006-309082, Japanese Patent Application Publication No. 2001-510484, Japanese Patent Application Publication No. 2001-506376). When the peak portion is arranged at the beginning of the drive waveform in this way, a very large power source capacity capable of outputting a large current is required to form a steep peak of a high potential from a level close to GND. . In addition, even if a peak is formed at the beginning using a general-purpose power-saving power source that can use a battery, there is a possibility that a peak portion having a desired shape may not be formed, which may cause flicker. There was sex. On the other hand, the drive pulse used in this embodiment does not require a large current output as described above because the peak portion is not arranged at the start of the drive waveform, and flickering can be suppressed. It becomes.

  Next, the reason why the driving pulse used in the present embodiment does not have a peak portion at the end thereof as in the driving pulse of FIG. 9A will be described.

FIG. 9B shows the relationship between V ² T and dy when the peak portion is arranged at the end of the drive pulse as shown in FIG. 9A and the ratio between the peak portion and the base portion is varied. It is shown. FIG. 9 (b) shows that the change in dy (brightness change amount) is the same as that in normal driving, regardless of whether the ratio of peak part: base part is set to 9: 1, 8: 2, 6: 4. It can be seen that this is steeper and the minimum value of dy is smaller in normal driving. That is, it can be determined that providing the peak portion at the end as shown in FIG. 9A does not contribute to the improvement of the writing speed and the increase in contrast. Therefore, in this embodiment, the peak portion is used as the end of the drive pulse. It was decided not to distribute it.

  Next, the relationship between the ratio (ratio) of the peak portion in the entire drive pulse and the steepness (write speed) will be described with reference to FIG. 10 schematically showing the relationship. As shown in FIG. 10, when the ratio of the peak portion (ratio of the peak portion in the entire drive pulse) is approximately 5 to 50%, the steepness is high, and when the ratio is higher or lower, the normal waveform It becomes almost the same as the steepness of time. Thus, the steepness increases in the above range (5 to 50%) is almost equivalent to the case where no peak portion is provided in the range of 0 to 5%, and it is considered that the peak portion does not function. This is because it is considered that when the voltage value is increased to 100%, the voltage value is not significantly changed, and the peak portion does not function as a peak portion.

Here, specific experimental results on the ratio (ratio) of the peak portion to the entire drive pulse will be described with reference to FIGS. 11 (a) and 11 (b). FIGS. 11A and 11B are graphs showing specific experimental results related to FIG. Among these, FIG. 11 (a) shows V ² T and dy when the ratio of the base part: peak part is different from 9: 1, 8: 2, 6: 4 when the pulse period is 2 ms. FIG. 11B shows V ² T when the ratio of the base part: peak part is 9: 1, 8: 2, 6: 4 when the pulse period is 5 ms. The relationship between and dy is shown.

  As can be seen from these figures, normal driving is possible regardless of whether the pulse period is 2 ms or 5 ms, regardless of whether the base part: peak part ratio is 9: 1, 8: 2, or 6: 4. The change of dy is steeper than that and is bottomed down. Based on the above experimental results, the ratio of the peak portion to the entire drive pulse should be set to 5 to 50%, preferably 10 to 40%. Therefore, also in the drive pulse of this embodiment, as described above, the ratio of the peak portion is set to 20%.

  Next, an effect when the driving method of the present embodiment is applied to the display unit 30 having a resolution of XGA (1024 × 768 pixels) will be described with reference to FIG. In this case, the write sequence of FIG. 13 is adopted as the write sequence. In this example, in the pulse cycle of SB1 to SB7 in the write sequence of FIG. 13, the ratio of the peak portion was set to 30%, and the potential difference between the base portion and the peak portion was set to 5V. As a result, as shown in FIG. 12, the contrast can be improved from about 4.4 to about 5.0 with almost no decrease in brightness as compared with the normal drive. Although not shown, the writing time can be shortened from about 11 seconds to about 9.4 seconds.

  Next, an example of voltage setting when the screen of the display unit 30 is rewritten will be described. In the present embodiment, as shown in FIG. 14, when rewriting from the display state (a), the range to be rewritten is first reset to the planar state (d).

  FIG. 15A and FIG. 15B show examples of STN driver voltage settings that enable resetting to the planar state. In resetting to the planar state, a pulse (selection ON) with a cycle of ± 36 V and 100 ms is applied to all pixels. In this case, since the entire screen (or the entire range to be rewritten) is reset at once, the above-described steepness or bottom-down does not become a problem. Therefore, the above-described peak portion is not provided in the drive pulse at reset.

  FIG. 16 is a diagram illustrating an example (first example) of voltage switching when the power supply switching unit 16 in FIG. 1 forms a drive pulse. In the first example, (20V, 15V, 10V, 10V, 5V) is set as the voltage (VDD, V21C, V21S, V34S, V34C) at the base portion in FIG. Moreover, (25V, 20V, 15V, 10V, 5V) is set as the voltage (VDD, V21C, V21S, V34S, V34C) at the peak portion in FIG.

  When such a voltage setting is performed, if the power supply switching unit 16 outputs in the order of (1) → (2) → (3) → (4) in FIGS. Then, a driving pulse having a peak portion (25 V) as shown in FIG. 16C can be formed (output). In this case, switching between ± 20V and ± 25V is performed in the full selection, and switching between ± 10V and ± 15V is performed in the half selection, but in the non-selected portion that occupies most of the screen of the display unit 30, the voltage is Is fixed at “± 5V”. As a result, an increase in power consumption can be suppressed and flickering of the screen can be suppressed.

  FIG. 17 is a diagram illustrating another example (second example) of voltage switching when the power supply switching unit 16 forms a drive pulse. In the second example, the voltages (VDD, V21C, V21S, V34S, V34C) at the base portion in FIG. 17A are (20V, 15V, 10V, 10V, 5V) as in FIG. 16A. Is set. Moreover, (27V, 22V, 17V, 10V, 5V) is set as the voltage (VDD, V21C, V21S, V34S, V34C) at the peak portion in FIG.

  When such a voltage setting is performed, when the power supply switching unit 16 outputs in the order of (1) → (2) → (3) → (4) in FIGS. Then, a driving pulse having a peak portion (27V) as shown in FIG. 17C can be formed (output). In this case, switching between ± 20V and ± 27V is performed in the full selection, and switching between ± 10V and ± 17V is performed in the half selection, but in the non-selection portion that occupies most of the screen of the display unit 30, As in the first example, the voltage is fixed at “± 5 V”. As a result, the increase in power consumption can be suppressed and flickering of the screen can be suppressed.

  On the other hand, FIG. 18 shows a comparative example of voltage switching. In this comparative example, as shown in FIG. 18A, the voltages (VDD, V21C, V21S, V34S, V34C) at the base portion are the same as those in FIGS. 16A and 17A. 20V, 15V, 10V, 10V, 5V) is set, but (25V, 19V, 13V, 12V, 6V) is set as the voltage (VDD, V21C, V21S, V34S, V34C) at the peak portion. Yes. When such voltage setting is performed, as shown in FIG. 18C, the drive pulses similar to those in FIG. 16C can be output in all selected portions. However, in this example, since the non-selection voltage varies between “± 5 V” and “± 6 V”, flickering occurs in the non-selected portion while drawing on the display unit 30, and power consumption May increase, which is not preferable. According to the experiments of the present inventors, in order to suppress flickering to such an extent that it does not cause a visual problem, the variation amount of the non-selection voltage is within a range of ± 10% (range of 0.9 times to 1.1 times). It was confirmed that it was preferable to do so. Therefore, in the present embodiment, by setting at least the non-selection voltage in the range of “4.5 V to 5.5 V”, flicker can be suppressed and increase in power consumption can be suppressed.

  As described above in detail, according to the present embodiment, the pulse-shaped drive voltage includes the step of applying to the display unit 30 a pulse-shaped drive voltage that forms a gradation corresponding to display information (display image). Is a one-cycle AC waveform including a base part for determining the gradation level and a peak part obtained by adding a voltage to the base part, and the peak part is arranged only in the part excluding the start and end of the drive pulse. Has been. In this case, by arranging the peak portion in a portion other than the end of the drive pulse, it is possible to improve the rewriting speed and increase the contrast as in the case of overdrive. In addition, since the peak portion is not at the beginning of the drive pulse, it is possible to avoid applying a large voltage suddenly from a state where no drive voltage is applied, so a portion to apply the drive voltage (driver IC or The load of the power source or the like can be reduced, and flickering of the display unit 30 can be suppressed.

  In this embodiment, the value of the non-selection voltage at the peak portion is in the range of 0.9 to 1.1 times the value of the non-selection voltage at the base portion (in this embodiment, as shown in FIGS. 16 and 17). Therefore, the increase in power consumption in the non-selected portion can be suppressed, and flickering of the screen of the display unit 30 can also be suppressed. In this embodiment, since the peak portion occupies a range of 5 to 50% of the entire pulsed driving voltage, the effect of providing the peak portion is appropriately exhibited, and the rewriting speed is improved. In addition, high contrast can be effectively realized.

  In the present embodiment, since the value of the pulsed drive voltage is in the range of −40 V to +40 V, a general-purpose driver can be used as the common driver 32 or the segment driver 34. Thereby, the cost of the liquid crystal display device 10 can be reduced.

  Further, in the present embodiment, as shown in FIG. 7A, as the drive pulse for forming the gradation, the drive pulse with a minute peak added at the position immediately before and immediately after the polarity inversion is used. The number of times of switching is three times (base voltage → peak voltage → base voltage). Thereby, the load of a DC-DC converter, an operational amplifier, etc. can be reduced and the waveform shaping degree can be increased. Therefore, excessive power consumption and flicker can be avoided.

  In the above embodiment, the case where the peak portion is arranged at the position immediately before and immediately after the polarity of the drive pulse is reversed has been described. However, the present invention is not limited to this. That is, as long as the peak portion is provided only in the portion excluding the start and end of the drive pulse, the peak portion may be arranged at other positions.

  In the above embodiment, the peak portion may be arranged in all of the write sequences SB1 to SB7 in FIG. 13, but when the display unit 30 is a large-sized panel, it is as short as SB3 and SB4. It is difficult to place a peak in the pulse. Therefore, it is also possible to determine whether or not to use the peak portion in consideration of the power supply capacity and the panel size, for example, a normal waveform is used for the short pulse and a peak is provided only for the middle to long pulse.

  In the above-described embodiment, the case where the liquid crystal display device 10 is a liquid crystal display device that displays a monochrome image has been described for convenience of explanation. However, the present invention is not limited to this, and the liquid crystal display device 10 can perform color display. A liquid crystal display device may be used. In this case, as shown in FIG. 19, the display unit 30 ′ of the liquid crystal display device is configured by laminating a blue (B) display unit 130B, a green (G) display unit 130G, and a red (R) display unit 130R. The In this case, as the image data input to the driver IC, a full-color original image is converted into 4096 colors of 16 gradations of RGB by the error diffusion method. Note that this gradation conversion is preferably performed using a blue noise mask method in addition to the error diffusion method from the viewpoint of display quality. Then, the image data converted into 4096 colors is converted into an image format (subframe, corresponding to SB1 to SB7 in FIG. 13) corresponding to the driver IC for binary writing. For example, in the case of writing using a cumulative response, image data of 4096 colors (16 tones for each RGB) is divided into binary bit planes corresponding to each halftone, and writing is performed under the driving conditions described above. To do.

  In the above-described embodiment, the case where the liquid crystal display device 10 is a display element using cholesteric liquid crystal has been described. However, the present invention is not limited to this, and any liquid crystal of simple matrix driving may be used as a display element using another liquid crystal. Also, it is possible to adopt the same driving method as in the above embodiment.

  The above-described embodiment is an example of a preferred embodiment of the present invention. However, the present invention is not limited to this, and various modifications can be made without departing from the scope of the present invention.

It is a block diagram which shows schematic structure of the liquid crystal display device which concerns on one Embodiment. It is a figure which shows roughly the structure (cross-sectional structure) of the display part of FIG. It is a figure which shows the display principle of a cholesteric liquid crystal. It is a figure which shows the voltage response characteristic of a cholesteric liquid crystal. It is a schematic diagram which shows the simple matrix drive of a cholesteric liquid crystal. It is a graph in which all selection, deselection, and non-selection are assigned to pulse response characteristics of cholesteric liquid crystal. FIG. 7A is a diagram showing drive pulses in one embodiment, and FIG. 7B is a diagram showing drive pulses in normal drive. It is a graph for demonstrating an effect | action at the time of using the drive pulse of Fig.7 (a). FIG. 9A is a diagram showing a state where the peak portion is arranged at the end of the drive pulse, and FIG. 9B shows the change in brightness when the drive pulse as shown in FIG. 9A is used. It is a graph to show. It is a figure which shows typically the change of the pulse response characteristic at the time of changing a peak ratio. It is a graph which shows the experimental result about the change of the pulse response characteristic at the time of changing a peak ratio. It is a figure which shows the measurement result of the change of brightness and contrast. It is a figure which shows a write-in sequence. It is a figure which shows an example of the drive sequence in one Embodiment. It is a figure which shows the voltage setting example of the STN driver which enables reset to a planar state. It is a figure which shows the 1st example of the voltage switching at the time of forming a drive waveform. It is a figure which shows the 2nd example of the voltage switching at the time of forming a drive waveform. It is a figure which shows the comparative example of FIG. 16, FIG. It is a figure which shows the liquid crystal display device in which a color display is possible.

Explanation of symbols

30 Display unit (display element)
32 Common Driver (Driver IC)
34 Segment driver (driver IC)
100 Liquid crystal display device (display device)

Claims (10)

Including a step of applying a pulsed drive voltage for forming a gradation corresponding to display information to the display element,
The pulse-shaped drive voltage is a one-cycle alternating current waveform including a base portion that determines a gradation level and a peak portion obtained by adding a voltage to the base portion.
The display element driving method by simple matrix driving, wherein the peak portion is arranged only in a portion excluding a start end and a termination end of the drive pulse.   2. The display element driving method according to claim 1, wherein the value of the non-selection voltage at the peak portion is 0.9 to 1.1 times the value of the non-selection voltage at the base portion.   The display element driving method according to claim 1, wherein the peak portion occupies a range of 5 to 50% of the entire pulsed driving voltage.   The display element driving method according to claim 1, wherein a value of the pulsed driving voltage is within ± 40V.   The method for driving a display element according to claim 1, wherein the display element includes a liquid crystal forming a cholesteric phase.   The display element driving method according to claim 5, further comprising an initialization step of setting the display element to a planar state before performing the step of applying the pulsed driving voltage.   The display element driving method according to claim 5, wherein the mixing ratio of the focal conic state of the display element is increased by the step of applying the pulsed driving voltage. A display element;
A driver IC that applies to the display element a pulsed driving voltage that forms a gradation corresponding to display information displayed on the display element;
The pulse-shaped drive voltage is a one-cycle alternating current waveform including a base portion that determines a gradation level and a peak portion obtained by adding a voltage to the base portion.
The display device according to claim 1, wherein the peak portion is disposed only in a portion excluding a start end and a termination end of the drive pulse.   The display device according to claim 8, wherein the value of the non-selection voltage at the peak portion is in a range of 0.9 to 1.1 times the value of the non-selection voltage at the base portion.   The display device according to claim 8, wherein the display element includes a liquid crystal that forms a cholesteric phase.

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