JPH11133374A - Method for driving color liquid crystal display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable the use of uniform display without depending upon display patterns even of a liquid crystal panel having dependence on frequencies on a lower frequency side by executing AC inversion driving at the frequency specific times or higher of a frame frequency at a low duty ratio below a specific value. SOLUTION: A liquid crystal cell 1 is formed by holding a liquid crystal layer of a twist angle 160 to 300 deg. by nematic liquid crystals of positive dielectric anisotropy between substrates of transparent electrodes and alignment layers. The product of the refractive index anisotropy of the liquid crystal layer and the thickness of the liquid crystal layer is specified to 1.2 to 2.5 μm. The outer side of the liquid crystal layer is provided with polarizing plates 2, 3 and an optical isomer 5. In this method for driving the color liquid crystal display device, the parameter VF=|(VL-VH)/VH|×100 is >=0.5(%) when the threshold voltages of the frequencies 0l5 times and 1.5 times the frame frequency of the liquid crystal display element are defined as VL, VH. The AC inversion driving is executed a the frequency of >=5 times the frame frequency at the low duty ratio of <=1/99 D.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for driving a color liquid crystal display device using retardation.

[0002]

2. Description of the Related Art In recent years, liquid crystal display devices have been widely used in various fields as flat type displays. Liquid crystal display elements have begun to be commercialized from calculators, watches, and the like because of their thinness, light weight, low power consumption, easy colorization, and the like, and recently, large-sized displays for word processors and personal computers.

In recent years, these liquid crystal display devices have been remarkably colored, and a color system generally uses a color filter (CF). In this method, a vivid color image can be expressed by using CF, but on the other hand, since the CF itself realizes a color by absorption, the reflection type is dark and cannot be used. For this reason, in the case of a color liquid crystal display device, at present, it is usually used as a transmission type having a backlight on the back side of a liquid crystal display panel.

On the other hand, a liquid crystal display device is used in portable devices such as calculators and mobile phones because of little power consumption even in a driving state and low power consumption. However, in the case of color display, a backlight is required for the above-described reason. In this case, there is a problem that the power consumption of the backlight is extremely large and the battery cannot be driven for a long time.

As a means for solving this problem, a color liquid crystal display device utilizing the retardation of an optically anisotropic substance such as a liquid crystal and a retardation plate has been proposed (JP-A-8-2).
No. 92434). In this method, the retardation is changed by controlling the voltage applied to the liquid crystal display element, thereby realizing a color display. For this reason, a CF is unnecessary, and inexpensive and bright color display is possible, so that a low power consumption type color liquid crystal display element that does not require a backlight is possible.

[0006]

In this method, colorization is realized by using the retardation of the liquid crystal and the phase difference plate, and the color can be changed in the same pixel by changing the applied voltage.

This has the advantage that multi-color display is possible and bright display is possible while keeping the number of pixels small, as compared with a color liquid crystal display device using CF. Voltage control is very important. In fact, the allowable voltage width to be recognized as the same color is about 1% or less. For this reason, it is required to design a liquid crystal display element in consideration of the capacity and stability of a driving power supply circuit, waveform distortion, and the time constant of a liquid crystal panel.

However, if the threshold voltage of the liquid crystal panel is frequency-dependent even if the driving circuit and the driving waveform are idealized, the actual driving frequency differs depending on the display pattern, so that the effective applied voltage differs. , The color changes.

[0009]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and has a dielectric anisotropy between two substrates each having a transparent electrode and an alignment film and provided substantially in parallel. Has a positive nematic liquid crystal interposed therebetween, the twist angle of the liquid crystal according to the alignment direction of the liquid crystal molecules formed by the alignment film of each substrate is 160 to 300 °, the refractive index anisotropy (Δn ₁ ) of the liquid crystal layer and the liquid crystal The product (Δn ₁ ··) of the layer thickness (d ₁ )
d ₁ ) is set to 1.2 to 2.5, at least one polarizing plate and at least one optically anisotropic member are provided outside the liquid crystal layer, and the frame frequency of the liquid crystal display element is 0.5. times and 15 times the threshold voltage at a frequency when the V _L, V _H, respectively, the parameter V _F = representing the frequency dependence _{| (V L -V H) /} V H | × 100 is 0.5 (%) Or more, wherein the color liquid crystal display device is driven by AC inversion at a low duty ratio of 1/99 D or less and at a frequency of 5 times or more the frame frequency. Is provided.

[0010]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 2 schematically shows the configuration of a color liquid crystal display device to which the present invention is applied. In FIG. 2, 1 is a liquid crystal cell, 2 is a front polarizing plate, 3 is a back polarizing plate, 4 is a reflecting plate, and 5 is an optically anisotropic body.

The liquid crystal cell 1 is made of ITO (In ₂ O ₃ -SnO ₂ ), SnO ₂ patterned into a desired pattern.
A film of polyimide, polyamide or the like is provided on the surface of a substrate such as plastic or glass provided with a transparent electrode such as a transparent electrode, and the surface is rubbed, or SiO or the like is obliquely deposited to form an alignment film. A liquid crystal layer having a twist of 160 to 300 ° made of nematic liquid crystal having a positive dielectric anisotropy is sandwiched between substrates.

Here, when the twist angle is less than 160 °, the steepness of the transmittance change is small and the saturation is reduced.
If it exceeds 00 °, hysteresis and a domain that scatters light are likely to occur, which are both inconvenient.

The refractive index anisotropy (Δn) of the liquid crystal in the liquid crystal layer
Product [Delta] n ₁ · d _{1 1)} and the thickness of the liquid crystal layer (d ₁₎ is a 1.2～2.5Myuemu. If it is less than 1.2 μm, the change in the state of the liquid crystal when a voltage is applied is small, and
If it exceeds 5 μm, it will be difficult to display an achromatic color,
This is because the viewing angle and the response deteriorate. In particular, in order to enable achromatic color development and increase color change with voltage,
It is preferable that Δn ₁ · d ₁ of the liquid crystal layer is 1.25 to 1.8 μm.

Conventionally, this type of color liquid crystal display device has been applied to portable information devices such as mobile phones and pagers in order to provide an inexpensive color liquid crystal display device. In order to use an inexpensive IC and extend the life of the battery, these portable information devices are driven by a waveform having a simple driving sequence as shown in FIG. 3 (hereinafter referred to as a B waveform). The liquid crystal must be driven by an alternating current because various deteriorations occur when driven by a direct current. The B waveform is a driving method in which the liquid crystal is converted into an alternating current in one frame cycle. The B waveform has a drive waveform having the same electrical effective value as shown in FIGS. 3A and 3B depending on the display pattern, but having different frequency components. Here, low frequency driving is performed substantially in (a), and high frequency driving is performed in (b).

However, in general, the threshold voltage of a liquid crystal panel has frequency dependence as shown in FIG. The threshold voltage is a voltage at which an optical change starts to occur in the liquid crystal element when a static rectangular wave voltage is applied to the liquid crystal panel.

[0016] Here, defining a V _F as a parameter representing the frequency dependence of the threshold voltage. This parameter, the actual threshold voltage in the frequency of 0.5 times and 15 times the static rectangular wave having a frame frequency in the driving waveform of the liquid crystal display element is taken as V _L, V _H, respectively, V _F = | (V _L −V _H ) / V _H | × 100
(%). Here, the frame frequency in the actual drive waveform is a cycle frequency from when the selection voltage is applied to when the selection voltage is applied again, as shown in FIG.

The lower the frame frequency is, the lower the power consumption is, but flicker due to flicker is likely to occur. On the other hand, when the frame frequency is increased, the power consumption is increased and the overall frequency is increased, so that an increase in the threshold voltage on the high frequency side becomes a problem.

For this reason, in portable information equipment applications where power consumption is important, the frame frequency is usually 60 to 100H.
about z. Also, since the liquid crystal is driven by AC,
The lowest frequency applied to the liquid crystal panel was half of the frame frequency (V _L ), while the frequency characteristic became flat about 15 times the frame frequency (V _H ) as a result of various experiments.

The increase in the threshold voltage on the high frequency side is due to waveform distortion caused by the product CR of the capacitance (C) of the liquid crystal panel and the electrode resistance (R).
It is required to design as small as possible, and by setting a design standard for these, it is possible to control reliably.

On the other hand, on the low frequency side, the threshold voltage may decrease as shown in FIG. 4, or may increase in some cases. Although it is known that the cause is affected by the physical properties of the alignment film and the liquid crystal, it cannot be completely controlled because of the influence of the panel forming process. The as numbers of V _F actually varies in the range of 0%.

For this reason, if the driving frequency differs depending on the display pattern, such as the B waveform, the threshold voltage at that frequency differs, so that the voltage applied to the liquid crystal panel differs effectively. Conventionally, in a monochrome panel used in a portable information device, an ON or OFF voltage margin is sufficiently large as shown in FIG. 5A, so that a substantial driving voltage is reduced due to a frequency characteristic of a threshold voltage on a low frequency side. There was no particular problem even if it fluctuated a little.

On the other hand, since the color liquid crystal display element used in the present invention displays each color by a voltage and further performs a gradation display, the allowable voltage range of each color is extremely large as shown in FIG. Narrow, less than 1%. For this reason, if the conventional B waveform remains unchanged, the color will change due to the voltage fluctuation due to the display pattern, which is a serious problem.

As a method for solving this problem, a new driving method suitable for this color liquid crystal display device was applied. In this driving method, as shown in FIG. 1, the alternating frequency is forcibly changed during one frame to increase the effective frequency and minimize the frequency dependency of the display pattern.

Hereinafter, the number of scanning lines (the number of selection periods) corresponding to one AC inversion period (a period from one polarity inversion to the next polarity inversion) is used as an index of the AC inversion period. I will represent it. For example, the inversion of alternating current is
S = 1 for each book, and S = 5 for every book. When the drive duty number is expressed as 1 / D, 1 ≦ S ≦ D
, And S = D for the B waveform.

As the S value is smaller, the frequency dependence of the waveform by the display pattern can be reduced, and the effect of the present invention can be easily exerted. However, on the other hand, the frequency as a whole increases, the power consumption increases, and the waveform distortion increases. Problem may occur. Further, depending on the S value, it may interfere with the frame frequency or external light to cause flickering and may not be adopted.

Therefore, when setting the S value, it is necessary to take into account the size of the display panel, the capacity of the driving IC, the power consumption of the entire display device, and the above-mentioned flicker. It has been experimentally confirmed that the range of １５15, especially 2２〜8 is suitable.

This driving method itself is a driving method usually used in a monochrome or color liquid crystal display device having a high duty ratio of 1/100 D or more. However, 1/99
For low duty ratios below D, ON / OF as described above
The voltage margin of F is sufficient, and there is almost no application such as performing a gray scale display. Further, such a driving method requires additional circuit components, which increases the cost, and the like. Had not been applied.

Even if it is used, a monochrome liquid crystal display or a color liquid crystal display device using CF having a small number of display colors cannot sufficiently exhibit its effect because of a sufficiently large voltage margin.

[0029]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A case where the present invention is applied to a reflective liquid crystal panel having the structure shown in FIG. 2 will be described below by way of example. However, the present invention is not limited to this, and a semi-transmissive or transmissive type has the same effect. The present invention can also be applied to a single polarizing plate type in which the polarizing plate 3 on the rear side in FIG. 2 is omitted. Further, the timing of AC inversion does not necessarily need to be changed in units of one selection period, and the same effect can be achieved even if switching is performed in the middle of one selection period.

Example 1, Comparative Example 1 In the configuration of FIG. 2, a liquid crystal cell having a twist angle of 240 ° and a Δn ₁ · d ₁ of 1.27 μm at 25 ° C. was used.
The liquid crystal used was a nematic liquid crystal containing a rotatory substance, having a positive dielectric anisotropy, and having a transition temperature of 99.2 ° C. The retardation value of the phase difference plate is 1.39.
A μm polycarbonate film was used. By optimizing the polarization axis of the polarizing plate and the optical axis of the retardation plate and changing the voltage, a color liquid crystal panel capable of displaying four colors was prepared.

In this color liquid crystal panel, the hue changes from white to orange to blue to green as shown in FIG. In order to prevent the threshold voltage from increasing on the high frequency side, a sufficiently low sheet resistance of 30 Ω / □ was used as the ITO sheet resistance. Frequency dependency of the threshold voltage of the panel is decreased in the low-frequency side as shown in solid line in FIG. 4, the parameter V _F indicating a degree of the decrease was 1%. As a gradation method for performing four-color display, as shown in FIG.
A frame thinning method using a frame was adopted. Here, gradations 1 to 4 correspond to white, orange, blue, and green, respectively.

First, as Comparative Example 1, a color liquid crystal panel was driven with a B waveform having a duty ratio of 1 / 55D, a bias ratio of 1 / 5B, and a frame frequency of 70 Hz. In this case, FIG.
As shown in FIG. 7A, when the display was performed with the highlighting pattern having the most different frequency component, the hue of the region (the shaded portion in FIG. 7A) that should have the same color changed.

On the other hand, in the first embodiment, alternating inversion was performed for every two scanning lines (S = 2).
The hue of each color area did not change at all even when the display was performed with the display pattern as shown in FIG. Also, other display patterns other than those shown in FIG. 7A have a sufficient effect, and the hue of each color area is not changed at all by this driving method. In this case, the frequency of the AC signal is 55/2 of the frame frequency.
= 27.5 times.

Example 2 A change in hue due to a display pattern was examined using the same panel configuration and driving method as in Example 1 except that alternating inversion was performed every 10 scanning lines (S = 10). Although there was a slight change in hue in the display pattern as shown in FIG. 7A, the hue was sufficiently within the allowable range, and a remarkable progress was seen as compared with the B waveform drive. In this case, the frequency of the AC signal is 55/10 = 5.5 times the frame frequency.

Example 3 and Comparative Example 2 As a color liquid crystal display panel, the frequency characteristics of the threshold voltage were poor, V _F = 5.
%, And the same experiment as in Example 1 and Comparative Example 1 was performed. The effect in this case is enormous. In the B waveform drive (Comparative Example 2), although there was a clear change in hue even in a pattern other than the emphasis pattern as shown in FIG. In the performed driving method (S = 2, Example 3), the hue did not change in almost any display pattern as in Example 1.

[Embodiment 4 and Comparative Example 3] The same panel configuration as that of Embodiment 1 is adopted, except that a frame thinning method of 4 frames and a 9-level pulse width modulation (FIG. 8) are used in combination as gradation methods. The effect was confirmed by the same driving method.

In this case, the method of selecting four gradations is to select four gradation levels from among 32 gradation levels at which four colors can be identified most vividly. The gray level for each color is (0/9, 0/9, 0/9, 0/9) for white, (4/9, 4/9, 4/9, 4/9) for orange and (7/9) for blue. /
9,6 / 9,7 / 9,6 / 9), green color is (9 / 9,9 /
9, 9/9, 9/9). Here, the fractions in parentheses are the pulse width modulation levels for each frame, which means that they are completed in four frames.

In this case, since the pulse width modulation is performed, the influence of the display pattern on the frequency dependence is further increased, and the highlighted display pattern is as shown in FIG. 7B. In the case where the AC inversion is not performed even in this driving method (Comparative Example 3), since the white and green colors are not substantially subjected to pulse width modulation, the driving waveform is the low frequency driving waveform as in FIG. Becomes On the other hand, the orange and blue colors have a higher driving waveform than that of FIG. 3B due to the pulse width modulation. Therefore, in the case of the drive waveform in which the AC inversion was not performed, the change in the hue was larger than that in Comparative Example 1.

On the other hand, in the driving method (S = 2, Example 4) in which alternating current is performed every two scanning lines, as in the case of Example 1, FIG.
Hue did not change in almost any display pattern including the emphasis pattern as shown in FIG. In this case, the current consumption of the entire liquid crystal display device was 0.80 mA.

Example 5 The same experiment as in Example 4 was performed except that only pulse width modulation was performed as a driving waveform.
In the driving waveform of the present invention, a hue change due to the display pattern was hardly observed, and a sufficient effect was confirmed.

Example 6 and Comparative Example 4 Using the same panel as in Example 1, the duty ratio was set to 1 / 32D, the bias ratio was set to 1 / 5B, and the frame frequency was set to 100 Hz. The same experiment as in No. 3 was performed. Also in this case, a driving method without performing frame inversion (Comparative Example 4)
However, in the method (S = 2, Example 6) in which alternating current is performed every two scanning lines, uniform display was possible regardless of the display pattern.

[Embodiment 7, Comparative Example 5] A panel having a V _F = 5% similar to that of Embodiment 3 was used, and the duty ratio was 1/80.
D, the bias ratio was set to ８B, the frame frequency was set to 65 Hz, and the same experiment as in Example 4 and Comparative Example 3 was performed except for that. Also in this case, display failure occurred in the driving method without frame inversion (Comparative Example 5), whereas in the method of performing alternating current for every two scanning lines (S = 2, Example 7), the display failure occurred. Uniform display was possible regardless of the pattern.

Example 8 The same panel as in Example 1 was used, and the same driving as in Example 4 was performed except that the S value was set to 6. In this case, a slight hue change was recognized in the emphasis pattern of FIG. 7B, but it was at an allowable level, and the current consumption was reduced to 0.61 mA.

[Embodiment 9] The same driving as in Embodiment 8 was performed except that the S value was set to 8. Although the degree of the hue change in the emphasis test was slightly increased as compared with the case of Example 8, the degree was not a problem in practical use, and the current consumption was 0.5%.
It was further reduced to 7 mA.

[0045]

By using the method for driving a color liquid crystal display element of the present invention, a uniform display can be used without depending on a display pattern even in a liquid crystal panel having a frequency dependency on a low frequency side. Further, as described above, since the frequency characteristics on the low frequency side are easily affected by the process and fluctuate, this driving method also has the effect of improving the yield of the liquid crystal panel.

Further, in the present driving method, since the driving frequency is substantially increased, it is possible to greatly reduce the afterimage phenomenon correlated with the low-frequency driving and the display unevenness generated during the high-temperature energization.

[Brief description of the drawings]

FIG. 1 is a driving waveform diagram used in the present invention.

FIG. 2 is a schematic cross-sectional view illustrating a configuration of a color liquid crystal display element.

FIG. 3 is a conventional drive waveform diagram.

FIG. 4 is a graph showing frequency dependence of a threshold voltage of a color liquid crystal panel.

5A is a graph showing electro-optical characteristics of a monochrome liquid crystal display device, and FIG. 5B is a graph showing electro-optical characteristics of a color liquid crystal display device.

FIG. 6 is a gray scale driving waveform diagram by a frame thinning method.

FIG. 7 is a schematic diagram of a highlight display pattern for observing a hue change due to the display pattern.

FIG. 8 is a driving waveform diagram of pulse width modulation.

[Explanation of symbols]

 1: liquid crystal cell 2: front side polarizing plate 3: back side polarizing plate 4: reflecting plate 5: optically anisotropic body

Claims (4)

[Claims] A nematic liquid crystal having a positive dielectric anisotropy is sandwiched between two substrates each having a transparent electrode and an alignment film and provided substantially in parallel, and liquid crystal molecules formed by the alignment films on each substrate. Liquid crystal twist angle depending on the alignment direction of
300 °, and the product (Δn ₁ · d ₁ ) of the refractive index anisotropy (Δn ₁ ) of the liquid crystal layer and the thickness (d ₁ ) of the liquid crystal layer is 1.2 to 2.
2.5, at least one polarizing plate and at least one optically anisotropic member are provided outside the liquid crystal layer, and 0.5 times and 15 times the frame frequency of the liquid crystal display element.
Times the threshold voltage in the frequency respectively V _L, V _H
And when the parameter V _F representing the frequency dependence = |
(V _L −V _H ) / V _H | × 100 is a method of driving a color liquid crystal display device in which 0.5% or more, wherein 1/99
A method for driving a color liquid crystal display device, wherein AC inversion driving is performed at a low duty ratio of D or less and a frequency of 5 times or more of a frame frequency. 2. The driving method of a liquid crystal display device according to claim 1, wherein a pulse width modulation is used as a gradation display method of the liquid crystal display element. 3. The method of driving a liquid crystal display device according to claim 2, wherein pulse width modulation and frame thinning modulation are used together as a gradation display method of the liquid crystal display element. 4. The driving method of a liquid crystal display device according to claim 1, wherein the AC inversion is performed every 1 to 15 scanning lines.