KR100563043B1 - Method for driving cholestric liquid crystal display panel by delayed homeotropic reset - Google Patents

Abstract

The present invention is a driving method sequentially performed for each scan electrode line in a unit display period for a cholesteric liquid crystal display panel, and includes reset, delay, and selection steps. In the reset step, a predetermined reset voltage is applied to the scan electrode line to be scanned so that the corresponding cholesteric liquid crystal cells are in a homeotropic state. In the delay step, after the reset step, a predetermined delay time elapses without applying a voltage to the cholesteric liquid crystal cells, thereby causing the cholesteric liquid crystal cells to be in an incomplete-planar state. In the selection step, a scan voltage equal to the reset voltage is applied to the scan electrode lines and corresponding data signals are applied to all the data electrode lines.

Description

Driving method of cholesteric liquid crystal display panel by delayed homeotropic reset {Method for driving cholestric liquid crystal display panel by delayed homeotropic reset}

1 is a view showing the basic characteristics of the cholesteric liquid crystal cell.

2 is a timing diagram showing waveforms of driving voltages applied to cholesteric liquid crystal cells in order to explain basic driving steps of the cholesteric liquid crystal display panel.

3 is a graph showing characteristics of reflectance with respect to a selection voltage in three conventional initialization schemes.

4 is a graph showing characteristics of reflectance with respect to a selection voltage in a conventional homeotropic initialization scheme and characteristics of reflectance with respect to a selection voltage when a predetermined delay step is added in a conventional homeotropic initialization scheme.

5 is a timing diagram showing driving signals according to a driving method of a cholesteric liquid crystal display panel according to a first embodiment of the present invention.

6 is a graph showing characteristics of reflectance with respect to time of a cholesteric liquid crystal cell selected at a selection time when various changes are made to the driving method of the initialization time of FIG. 5.

7 is a timing diagram illustrating driving signals according to a driving method of a cholesteric liquid crystal display panel according to a second exemplary embodiment of the present invention.

<Explanation of symbols for the main parts of the drawings>

H ... Homeotropic status, P ... Planar status,

F ... Focal conic state, S1, S2 ... selection time,

R1, R2, R2-1, R2-2 ... reset time, M1, M2 ... hold time,

D1, D2, D2-1, D2-2 ... delay time,

CH, CH1 ... Characteristic curves of conventional Homeotropic initialization methods,

CP ... characteristic curves of conventional Planar initialization methods,

CF ... characteristic curve of conventional focal conic initialization method,

CH2 ... Characteristic curve of homeotropic initialization method according to the present invention,

S _Rn ... drive signal applied to the nth scan electrode line,

S _Cm ... data signal applied to m- _th data electrode line,

S _LC ... voltage applied to the liquid crystal cell where the n th scan electrode line and the m th data electrode line meet,

C _US ... characteristic curve when the square wave pulse of the maximum voltage (R _H -C _L ) is applied to the liquid crystal cell while the reset time is equal to the initialization time,

C _MC ... characteristic curve when the initialization time is repeated continuously,

C _LU ... Characteristic curve when only one initialization time is applied.

The present invention relates to a driving method of a cholesteric liquid crystal display panel, and more particularly, to a driving method performed sequentially for each scan electrode line in a unit display period with respect to the cholesteric liquid crystal display panel. will be.

A cholesteric liquid crystal display panel includes transparent electrode lines arranged on two opposing transparent substrates, for example, glass substrates, for example, Indium-Tin. It is a reflective liquid crystal display panel in which a cholesteric liquid crystal is filled between -Oxide.

1 shows the basic characteristics of a cholesteric liquid crystal cell. Referring to FIG. 1, when a voltage E higher than the first threshold voltage E _th is applied to the cholesteric liquid crystal cell, the cholesteric liquid crystal cell is in a homeotropic state (H). In this homeotropic state H, molecules of the liquid crystal cell are arranged in a direction perpendicular to the surface of the liquid crystal cell.

When a voltage E lower than the first threshold voltage E _th and higher than the second threshold voltage E _F is applied to the cholesteric liquid crystal cell in the homeotropic state H, in other words, the call When the voltage E applied to the cholesteric liquid crystal cell in the meotropic state (H) is gradually lowered, the cholesteric liquid crystal cell switches from the homeotropic state (H) to the focal conic state (F). do. In this focal conic state F, the molecules of the liquid crystal cell have a helical structure and the helical axis is arranged in a direction substantially parallel to the surface of the liquid crystal cell. As a result, most of the light passes through without reflection and becomes almost transparent.

On the other hand, when a voltage E lower than the second threshold voltage E _F is applied to the cholesteric liquid crystal cell in the homeotropic state H, in other words, the cholesteric in the homeotropic state H When the voltage E applied to the liquid crystal cell is rapidly lowered, the cholesteric liquid crystal cell has a quasi-planar state and an incomplete-planar state in the homeotropic state (H). Transition to planar state (P) via. In this planner state P, the molecules of the liquid crystal cell have a periodic spiral structure while the helical axis has a direction perpendicular to the surface of the liquid crystal cell. Accordingly, only the wavelength corresponding to the product nP of the average refractive index n of the cholesteric liquid crystal cell and the helical pitch P may be reflected. On the other hand, the quasi-planar state is a state in which the spiral pitch of the liquid crystal is about twice that of the planar state while having a structure similar to that of the planar state P. FIG. Also, the incomplete planar state is a variable state that appears in the process of the transient-planar state relaxing to the planar state P. FIG.

The focal conic state F and the planar state P each have a memory effect of maintaining their state for a relatively long time even when the application of voltage is stopped in their state. As the bistable memory effect is present, the planar state P and the focal conic state F are used in the cholesteric liquid crystal display device according to whether one cholesteric liquid crystal cell is selected. The power consumption is lower. In addition, the cholesteric liquid crystal display panel has a relatively high luminance characteristic since the driving method of selective reflection is applied in view of its characteristics.

2 shows waveforms of driving voltages applied to cholesteric liquid crystal cells in order to explain basic driving steps of the cholesteric liquid crystal display panel. In Fig. 2, reference numeral R1 denotes a reset time, D1 denotes a delay time, S1 denotes a selection time, and M1 denotes a retention time. The reset time R1 and the delay time D1 constitute an initialization time for making the molecular arrangement state of all the cholesteric liquid crystal cells of the scan electrode line to be scanned uniform.

Referring to FIG. 2, at a reset time R1 in a unit frame for cholesteric liquid crystal cells of one scan electrode line, reset voltages V ₃ and − for initially determining an initialization state of the cholesteric liquid crystal cells. V ₃ ) is applied alternately. Here, the reason why the positive and negative voltages are alternately applied is to prevent the change in the physical properties of the cholesteric liquid crystal by removing the average DC voltage.

The subsequent delay time D1 is a time for finally determining the initialization state of the cholesteric liquid crystal cells. Therefore, in some initialization methods, this delay time D1 does not exist. In the delay time (D1), only ground voltage (V _G ) is applied to the corresponding cholesteric liquid crystal cells, but the positive voltage (V ₁ ) or the negative voltage (-V ₁ ) is added by crosstalk. do. Here, crosstalk refers to a phenomenon in which a voltage applied to liquid crystal cells of electrode lines that are not scanned is changed by data signals corresponding to the electrode lines to be scanned.

In the subsequent selection time S1, the selection voltages V ₂ and −V ₂ are alternately applied to the liquid crystal cells selected from among the liquid crystal cells of the corresponding scan electrode line. The absolute value of each selection voltage is a value in which the scan voltage applied to the scan electrode line and the data voltage applied to the selected data electrode line are added together. Accordingly, each cholesteric liquid crystal cell is in the planar state P or the focal conic state F depending on whether the cholesteric liquid crystal cell is selected.

In the holding time M1, a ground voltage V _G is applied to the liquid crystal cells in order to reduce power consumption of the liquid crystal cells of the corresponding scan electrode line. However, the positive voltage V ₁ or the negative voltage V− ₁ is added by the crosstalk. As described above, even when the ground voltage V _G is applied to the liquid crystal cells at the holding time M1, each liquid crystal cell made at the selection time S1 according to the memory characteristics of the planar state P or the focal conic state F. The conditions of

3 shows the characteristics of the reflectance with respect to the selection voltage in three conventional initialization schemes. In FIG. 3, reference numeral CH denotes a characteristic curve of a conventional Homeotropic initialization method, CP denotes a characteristic curve of a conventional Planar initialization method, and CF denotes a conventional focal conic initialization method. Point to the characteristic curve of.

In the homeotropic initialization method, the delay time (D1 of FIG. 2) does not exist. That is, at the reset time (R1 in FIG. 2), a high voltage is applied to the cholesteric liquid crystal cells of the scan electrode line to be scanned, so that the cholesteric liquid crystal cells are in a homeotropic state (H). According to the characteristic curve CH of the homeotropic initialization method, the selection voltage applied to the liquid crystal cells at the selection time S1 is relatively low, and thus has a large difference with respect to the reset voltage. Therefore, in the driving apparatus to which the homeotropic initialization method is applied, the scan-electrode driving element has a three-level output voltage of a reset voltage, a scan voltage lower than the reset voltage, and a reference voltage (for example, a ground voltage). I need it.

In the planar initialization scheme, at a reset time R1, a high voltage is applied to the cholesteric liquid crystal cells of the scan electrode line to be scanned so that the cholesteric liquid crystal cells are in a homeotropic state. In the subsequent delay time D1, the state of the cholesteric liquid crystal cells is switched to the planar state by passing over 100 milliseconds (ms). According to the characteristic curve CP of the planar initialization method, the selection voltage applied to the liquid crystal cells at the selection time S1 is relatively high, which is hardly different from the reset voltage. Therefore, in the driving apparatus to which the planar initialization method is applied, the scan-electrode driving element requires two levels of output voltages of the reset / scan voltage and the reference voltage (for example, the ground voltage).

The delay time D1 does not exist even in a focal conic initialization method. That is, at the reset time R1, a low voltage is applied to the cholesteric liquid crystal cells of the scan electrode line to be scanned so that the cholesteric liquid crystal cells are in a focal conic state. According to the characteristic curve CF of the focal conic initialization method, the selection voltage applied to the liquid crystal cells at the selection time S1 should be much higher than the reset voltage. Therefore, in the driving apparatus to which the focal conic initialization method is applied, the scan-electrode driving element includes three levels of output voltages of a reset voltage, a scan voltage higher than the reset voltage, and a reference voltage (for example, a ground voltage). need.

As described above, the homeotropic and focal conic initialization schemes have an advantage in that the driving speed increases because there is no delay time D1. However, since three levels of output voltages must be generated in the scan-electrode driving element, the conventional two-level scan-electrode driving elements, for example, the most used Super Twisted Nematic (STN) liquid crystals There is a problem that the scan-electrode driving elements of the display panel cannot be used interchangeably.

On the other hand, according to the planar initialization method, the conventional two-level scan-electrode driving elements can be used, while a relatively long delay time D1 exists. This delay time D1 may not only slow the driving speed but also cause a flicker on the screen.

Disclosure of Invention An object of the present invention is to provide a driving method for driving a cholesteric liquid crystal display panel in which a conventional two-level scan-electrode driving element can be used, while driving speed is increased and no flicker occurs on the screen. It is.

In order to achieve the above object, the present invention is a driving method sequentially performed for each scan electrode line in a unit display period for a cholesteric liquid crystal display panel, and includes reset, delay, and selection steps. In the reset step, a predetermined reset voltage is applied to the scan electrode line to be scanned so that the corresponding cholesteric liquid crystal cells are in a homeotropic state. In the delaying step, after the reset step, a predetermined delay time elapses without applying voltage to the cholesteric liquid crystal cells so that the cholesteric liquid crystal cells are in an incomplete-planar state. To become. In the selecting step, a scan voltage equal to the reset voltage is applied to the scan electrode lines and corresponding data signals are applied to all data electrode lines.

According to the driving method of the present invention, the selection voltage required by the liquid crystal cells in the selection step, even though a short delay time elapses so that the cholesteric liquid crystal cells are in an incomplete-planar state in the delay step. Higher characteristics can be obtained. Accordingly, in the selection step, the same scan voltage as the reset voltage is applied to the scan electrode line, so that the existing two-level scan-electrode driving elements can be used, while driving speed is increased and flickering on the screen does not occur. You can do that.

Preferably, the reset and delay steps are repeated one or more times in succession. Accordingly, even if the same reset and delay times are applied, the luminance performance of the cholesteric liquid crystal display panel may be increased.

Hereinafter, preferred embodiments according to the present invention will be described in detail.

4 is a characteristic curve CH1 of a reflectance with respect to a selection voltage in a conventional homeotropic initialization scheme, and a characteristic curve of reflectance with respect to a selection voltage when a predetermined delay step is added in a conventional homeotropic initialization scheme. CH2). The characteristic curve CH2 according to the present invention has a delay time of 5 milliseconds (ms) to 80 milliseconds (ms), that is, a quasi-planar state in the conventional homeotropic initialization method. There is a delay time longer than the time to be made and shorter than the time to become the Planar state, so that the liquid crystal cells of the scan electrode line to be scanned are incomplete-Planar state through the Transient-Planar state. This is the case. Comparing the characteristic curve CH2 with the characteristic curve of the planar initialization method of FIG. That is, despite a short delay time for the cholesteric liquid crystal cells to be in an incomplete-planar state, as the selection voltage required by the liquid crystal cells becomes higher in the selection step, The same scan voltage can be applied to the scan electrode line.

5 illustrates driving signals according to a driving method of a cholesteric liquid crystal display panel according to a first embodiment of the present invention. In FIG. 5, S _Rn denotes a driving signal applied to an nth scan electrode line, S _Cm denotes a data signal applied to an mth data electrode line, and S _LC denotes an nth scan electrode line and an mth data electrode line. It points to the waveform of the voltage applied to the liquid crystal cell of this encounter.

At the holding time M1 of the previous frame, only the low voltage R _L is applied to the nth scan electrode line. Nevertheless, crosstalk R _L -C _L <-> R _L -C is caused by the data signals C _L <-> C _H that are applied to the data electrode lines for scanning the other scan electrode lines. _H ) is generated. Here, when the low voltage R _L of the scan electrode line becomes half (C _H / 2) of the high voltage C _H of the data electrode line, the crosstalk R _L -C _L <-> R _L − C _H ) can be minimized.

At the reset time R2 of the current frame, only the reset voltage R _H is applied to the nth scan electrode line. Nevertheless, crosstalk R _H -C _L <-> R _H − is achieved by data signals C _L <-> C _H applied to all data electrode lines for scanning of other scan electrode lines. C _H ) is generated. As such, when the reset voltage R _H is applied to the n th scan electrode line, all the cholesteric liquid crystal cells of the scan electrode line to be scanned are in a homeotropic state.

In the subsequent delay time D2, a short time of 5 milli-seconds (ms) to 80 milli-seconds (ms) is applied while the low voltage R _L is applied to the n th scan electrode line. The liquid crystal cells of the n scan electrode line are brought into an incomplete-planar state through a transient-planar state. Although the short delay time has elapsed so as to become an incomplete-planar state, the voltage for resetting at the selection time S2 is increased as the selection voltage required by the liquid crystal cells becomes very high at the subsequent selection time S2. the same scan voltage (R _H) and (R _H) can be applied to the n-th scan electrode line.

In the subsequent selection time S2, the scan voltage R _H equal to the reset voltage R _H is applied to the n th scan electrode line and the data signals C _L <-> C _H are applied to all the data electrode lines. ) Is applied. More specifically, the ground voltage C _L is applied to the selected data electrode lines, and the high voltage C _H is applied to the non-selected data electrode lines. Accordingly, the selected liquid crystal cells are in a planar state by a high voltage of R _H -C _L is rapidly applied. In addition, non-selected liquid crystal cells are in a focal conic state by a relatively low voltage of R _H -C _H is gradually applied.

In the sustain time M2 that follows, only a low voltage R _L is applied to the nth scan electrode line. Nevertheless, crosstalk R _L -C _L <-> R _L -C is caused by the data signals C _L <-> C _H that are applied to the data electrode lines for scanning the other scan electrode lines. _H ) is generated. On the other hand, the state selected at the selection time S2 is maintained at the holding time M2 due to the memory effect of the planar state or the focal conic state.

FIG. 6 shows characteristics of reflectance with respect to time of a cholesteric liquid crystal cell selected at a selection time when various changes are made to the driving method of the initialization time of FIG. 5. In FIG. 6, reference numeral C _US denotes characteristic curves when a square wave pulse of the maximum voltage R _H -C _L is applied to the liquid crystal cell while the reset time is equal to the initialization time. That is, it indicates characteristic curves when the maximum voltage R _H -C _L is continuously applied to the cholesteric liquid crystal cell even in the delay time (D2 of FIG. 5). Reference numeral C _MC denotes characteristic curves when the initialization time is continuously repeated. Reference numeral C _LU denotes characteristic curves when only one initialization time is applied, respectively. Referring to FIG. 6, when the reset time (R2 of FIG. 5) is equal to the initialization time (R2 + D2 of FIG. 5), that is, a high voltage R _H -C _L , for example, a voltage of 30 volts (V). It can be seen that the luminance is the highest when the cholesteric liquid crystal cell is continuously applied even to this delay time (D2 in FIG. 5). However, in this case, not only is not included in the present invention, but inevitably crosstalk is not reflected in the simple matrix type liquid crystal display panel, and thus it cannot be actually employed for driving. Therefore, referring to the characteristic curves C _MC and C _LU , it was confirmed that higher luminance can be obtained when the initialization time (R 2 + D 2 in FIG. 5) is continuously repeated.

7 illustrates driving signals according to a driving method of a cholesteric liquid crystal display panel according to a second exemplary embodiment of the present invention. In FIG. 7, the same reference numerals as used in FIG. 5 indicate objects of the same function. In FIG. 7, reference numerals R2-1 and R2-2 denote reset times, and D2-1 and D2-2 denote delay times, respectively. Comparing FIG. 7 with FIG. 5, it can be seen that one initialization time (R2 + D2 of FIG. 5) is driven equally into two initialization times. The operation process related to this is as described above with reference to FIG. 5. However, according to the driving method of the second embodiment of FIG. 7, higher luminance performance can be obtained as compared with the driving method of the first embodiment of FIG. 5 (see FIG. 6 and the description thereof).

As described above, according to the driving method of the cholesteric liquid crystal display panel according to the present invention, in spite of the fact that a short delay time elapses so that the cholesteric liquid crystal cells become incomplete-planar state in the delay step, In the selection step, a characteristic that the selection voltage required by the liquid crystal cells becomes higher may be obtained. Accordingly, in the selection step, the same scan voltage as the reset voltage is applied to the scan electrode line, so that the existing two-level scan-electrode driving elements can be used, but the driving speed is increased and the flickering on the screen does not occur. have.

The present invention is not limited to the above embodiments, but may be modified and improved by those skilled in the art within the spirit and scope of the invention as defined in the claims.

Claims (3)

A driving method in which a cholesteric liquid crystal display panel is sequentially performed for each scan electrode line in a unit display period, A reset step of applying a predetermined reset voltage to the scan electrode line to be scanned to cause the corresponding cholesteric liquid crystal cells to be in a homeotropic state; After performing the reset step, a predetermined delay time elapses without applying a voltage to the cholesteric liquid crystal cells, thereby causing the cholesteric liquid crystal cells to be in an incomplete-planar state. ; And And applying a scan voltage equal to the reset voltage to the scan electrode lines and simultaneously applying corresponding data signals to all data electrode lines. The method of claim 1, The delay step is performed after the reset step is performed, After the delay step is performed, the reset step is performed again. After the reset step is performed again, the delay step is performed again, And the selection step is performed after the delay step is performed again. The method of claim 1, wherein in the delaying step, And a delay time that is longer than a time when the cholesteric liquid crystal cells become a quasi-planar state and shorter than a time when the cholesteric liquid crystal cells become a planar state.

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