Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
1997-11-07
2003-02-11
Ton, Toan (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C345S089000, C345S094000
Reexamination Certificate
active
06519013
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a novel driving method for a liquid crystal display (LCD) apparatus and a driving circuit for LCD and a LCD module which have been used widely for industrial use or domestic use.
2. Discussion of the Background
Needs for information displaying media have been increasing in a highly intelligent society. Since LCDs have advantages of a light weight, thinness, lower power consumption and so on, and it well matches with semiconductor technology, further widespread use is expected. In response to this widespread usage, there is a demand for a display surface with larger capacity and high precision. In response to this, technological innovation for forming a display screen having a large capacity has been progressing.
On the other hand, there have been proposed LCDs in which features of light weight and smallness are enhanced to the maximum. Namely, it is applicable to a hand-held data terminal device which can easily be carried. In particular, a passive driving type super-twisted nematic (STN) method is considered to be the mainstream approach to this type of display in the field of hand-held data terminal devices in comparison with an active driving type using active elements such as thin film transistors (TFTs), because it can be manufactured in a shorter time, has a simple element structure, and is produced at a low cost.
Performance and specifications for such hand-held data terminal devices are considered to have a stable and good production efficiency so as to respond to demands by individuals. Further, the terminal device is required to satisfy basic performance requirements for a display device (e.g., visibility, low power consumption and high preciseness).
In conventional techniques for the above-mentioned usage, a reflection type or a semi-transparent type LCD has been used. In order to have a good visibility, various improvements have been sought. Employment of a color display instead of a monochrome display is one of them. When a color filter is used for the color display, the picture for display is divided into three portions in corresponding to three colors of red (R), green (G), and blue (B), which reduces the aperture rate remarkably.
In the case of using a color filter of the reflection type or a semi-transparent type LCD, visibility becomes poor. As a newly proposed LCD to solve such problems, there is a super-reflective-color LCD (hereinbelow, referred to as SRC-LCD). It has a device structure that does not use any color filter and realizes a bright color display without reducing the aperture rate.
In “NIKKEI MICRODEVICES”, 1994, June, p. 34-39, there is introduction of a reflection type LCD having color development of white which is expected in markets, and a color developing sequence of white-red-blue-green is described in
FIG. 5
in p. 38.
Further, Japanese Unexamined Patent Publication JP-A8-15691 discloses an example capable of emitting white in an achromatic color at an off state of voltage, while emitting white, red, blue and green, and an example capable of emitting white, black, blue, yellowish green and pink. SRC-LC disclosed in this publication uses two birefringent plates. Further, there is description of using a passive matrix for driving.
Further, Japanese Unexamined Patent Publication JP-A-8-176547 discloses an example which employs the liquid crystal composition including 5-60 wt % of transdifluoroethylene derivatives.
In SRC-LCD, a color display is effected, without using a color filter, by utilizing the total birefringence of polarizing plates, a twisted nematic liquid crystal layer and light passing through a phase shifting plate. The proposed technique substantially increased visibility by realizing plural kinds of color development without losing the advantages of LCD such as reduced size, light weight and simple structure. In displaying a plurality of colors in LCDs, the birefringence of the liquid crystal layer is controlled by applying a voltage across the opposed electrodes, whereby a predetermined color is developed.
The inventors of this application proposed in Japanese Patent Application JP8-9422 (Japanese Unexamined Patent publication JP-A-8-292434, International Publication WO96/23244) is a device structure for SRC-LCD, which provides an achromatic (white) display at OFF of driving voltage, is easily driven and is capable of providing a multi-colored display.
In order to obtain three or more colors in a color display of SRC, it is necessary to apply intermediate voltages. As a passive matrix driving method for STN or the like, a successive line driving of APT (Alt-Pleshko Technique) or IAPT (Improved APT) is generally used.
This technique is very effective as a multiplexing driving method because ON/OFF levels can easily be generated.
On the other hand, in an active matrix method using active elements such as TFTs, an intermediate voltage can relatively easily be produced by using amplitude modulation. In the case of the passive driving system, however, when the amplitude modulation is implemented, there are variations of voltages at a non-selection time whereby an undesired voltage is applied to a non-display portion (or a non-selected region).
Accordingly, in evaluation of the display picture as a whole, it is not always to be a suitable driving method. Therefore, there have been proposed various techniques to produce an intermediate voltage.
In conventional multigradation driving methods for STN for a monochrome display, cross-talking in a displayed surface was a big problem. Further, since the number of colors to be displayed in the multigradation driving method for SRC-LCD is practically about 4 or 5, it is considered to be unnecessary to have a multivalued function as a personal computer of full color display type. However, the voltage-optical characteristics substantially change depending on the liquid crystal material used. Accordingly, it is necessary to determine precisely a driving voltage to obtain a predetermined color tone depending on a panel for SRC-LCD to be used.
In other words, a driving controller having high universality which can be adapted to various kinds of usage should have such function that various driving voltages can easily be set as desired and with high accuracy. Namely, a multivalue driving ability of high quality is needed although a display of large capacity is unnecessary in comparison with a desk-top type personal computer.
Accordingly, in order to construct a LCD at a reduced cost and without decreasing quality of display, a further effective multivalue driving ability should be provided. In conventional passive driving methods for STN, a frame modulation or a pulse width modulation has been generally used in order to produce an intermediate voltage. Further, an amplitude modulation method has recently been proposed. Hereinbelow, description will be made as to these methods, and problems caused when these driving methods are applied to SRC-LCD will be described.
(1) Frame Modulation (FRC Method)
This is a method for displaying gradation (i.e. producing an intermediate voltage) by using a plurality of display frames. Namely, an intermediate voltage (an intermediate tone) is produced in response to the number of ON states (a higher voltage) and OFF states (a lower voltage) as binary states.
In the FRC method, when a driving voltage is divided into many stages, a flicker may be produced (in the conventional STN, brightness is changed, i.e. a gray scale appears) since an increase in the number of frames takes a longer time to complete a display. This method is often combined with a spatial modulation in which phase is spatially shifted to thereby suppress the flicker. Even in such combination, however, use of about 16 gradations is considered to be critical.
(2) Pulse Width Modulation (PWM Method)
The technique is such that a selection time is divided into 2
n
portions to which a time of ON states and a time of OFF states are assigned. It can be considered to be a technique where FRC is effected in a frame. The
Kuwata Takeshi
Nagai Makoto
Ozeki Masao
Asahi Glass Company Ltd.
Duong Tai V.
Ton Toan
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