Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1999-03-15
2003-07-29
Hjerpe, Richard (Department: 2674)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S090000, C345S092000, C345S205000, C345S206000
Reexamination Certificate
active
06600472
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display device capable of halftone imaging display.
2. Description of the Related Art
FIG. 29
shows the configuration of a conventional liquid crystal display device. As shown in
FIG. 29
, a signal line
94
and a gate line
93
are provided and a thin-film transistor (TFT)
91
is provided in the vicinity of their crossing point. When rendered in a selected state (on-state), the thin-film transistor
91
supplies charge to an auxiliary capacitor (storage capacitor)
95
and a liquid crystal layer (liquid crystal capacitance)
92
via a pixel electrode
96
. One pixel is constituted of the pixel electrode
96
, a common electrode
98
confronting it, and the liquid crystal layer
92
.
As is well known, to prevent deterioration of the liquid crystal layer
92
, it is necessary to apply an AC voltage to the liquid crystal layer
92
.
In the above type of liquid crystal display device, since it is necessary to apply an AC voltage even during a period when there are no variations in display, the potential of the pixel electrode
96
is rewritten every time the pixel is selected, that is, once per frame period.
When an AC voltage, is applied to the capacitors including the liquid crystal layer (liquid crystal capacitance)
92
and the auxiliary capacitor (storage capacitor)
95
, the power consumption P is given by P=f×V
2
×C where f, V, and C represent the frequency, the voltage, and the total capacitance, respectively. Therefore, the power consumption increases as any of the frequency, voltage, and total capacitance increases.
In the AC driving of the liquid crystal display device, the driving frequency for each pixel is equal to the frame frequency, the driving frequency for each signal line is equal to the product of the frame frequency and the number of scanning lines, and the driving frequency of a signal line driver circuit (driver IC)
97
is equal to the product of the total number of pixels of the display screen and the frame frequency.
At present, where the liquid crystal display device is a color VGA (640×3 (RGB)×480 pixels) device having a 10.4-in. diagonal size, the power consumption of the signal line driver circuit
97
is about 1 W. Therefore, in the case of an A4-size, high-resolution (corresponding to 150 dpi) liquid crystal display device, the number of pixels amounts to 1,600×1,200, which is 6.25 times that of the VGA device, and hence the power consumption is as high as about 2-3 W or more.
Using a liquid crystal display device having such high power consumption as the display device of a portable information apparatus causes a problem that the usable time, which is limited by the battery performance, is shortened.
One method of solving this problem is use of a surface stabilized ferroelectric liquid crystal (SSFLC). In this case, the liquid crystal is given memorizing ability and hence the voltage supply can be stopped until a change occurs in display, which enables reduction in power consumption. However, because of the bistable nature, the liquid crystal display device basically performs binary imaging display. In this type of liquid crystal display device, it is difficult to perform halftone display and its power of expression is much lower than in a display mode capable of halftone imaging display.
Further, liquid crystals having memorizing ability are limited in display quality (contract, reflectance, etc.). For example, the display mode of the SSFLC necessarily requires polarizing plates, resulting in a small reflectance value of about 30%, which means a dark screen.
SUMMARY OF THE INVENTION
The present invention has been made to solve the above problems in the art, and an object of the invention is therefore to provide a liquid crystal display device using a novel driving method which can reduce the power consumption relating to the driving and can easily provide halftone and hence perform superior halftone imaging display.
According to a first aspect of the invention, there is provided a liquid crystal display device comprising a liquid crystal layer held between a first electrode and a second electrode; means for applying an AC voltage to the first electrode or the second electrode; means for supplying a display signal; means f or selecting the display signal; means for holding the selected display signal; and an impedance element connected in series to the first electrode, an impedance of the impedance element being varied in accordance with the display signal being held.
Alternatively, there is provided a liquid crystal display device comprising a liquid crystal layer held between first electrodes and a second electrode; holding means such as a capacitor provided for each of the first electrodes, for holding a display signal; an a an impedance element connected in series to each of the first electrode and including a variable resistance element whose impedance is varied in accordance with the display signal.
As a further alternative, there is provided a liquid crystal display device comprising a first substrate on which first electrodes are arranged in matrix form; a second substrate on which a second electrode is provided; a liquid crystal layer held between the first electrodes and the second electrode; means for applying an AC voltage to each of the first electrodes or the second electrode; means for supplying a display signal; means provided for each of the first electrodes, for selecting and holding the display signal; and an impedance element connected in series to each of the first electrodes, an impedance of the impedance element being varied in accordance with the display signal being held.
For example, the first electrodes a re pixel electrodes and the second electrode is an opposed electrode (common electrode) A pixel is constituted of each of the first electrodes, the second electrode, and the liquid crystal layer held in between.
An IPS (in-plane switching) mode liquid crystal display device may be constructed by disposing the first electrodes and the second electrode on the same substrate.
The pixel electrodes may be arranged in matrix form on a substrate that is made of glass, quartz, or the like and at least the surface of which is insulating. By arranging pixels two-dimensionally in this manner, incident light on the liquid crystal layer is modulated two-dimensionally to perform display.
Where the first electrodes are pixel electrodes, covering driving elements with the pixel electrodes is preferable for the purpose of increasing the aperture ratio. Where the pixel electrodes are reflection electrodes, the invention can be applied to a reflection-type liquid crystal display device. In this case, since the selecting means and the holding means can be formed under the reflection electrode, the degree of design freedom increases even if the driving element is increased in size.
The display signal is a signal for controlling the states of the pixels, that is, the states of the liquid crystal layer held between the first electrodes and the second electrode.
For example, the means for supplying such a display signal is signal supply lines. A plurality of display signal supplying means may be provided rather than a single one.
The means for selecting a display signal is means for performing, on apixel-by-pixelbasis, selection/sampling onthe display signal that is supplied in the above manner. For example, a nonlinear switching element such as a thin-film transistor whose source or drain is connected to a signal line may be used as the display signal selecting means. By controlling the gate electrode of the thin-film transistor by a scanning signal, a display signal can be captured independently for each of arbitrary pixels.
Where the display signal is supplied to the pixels as digital data, a sampling circuit may be constructed by combining, for example, logic gates, data latches, a shift register, etc.
For example, the impedance element may be a parallel connection of a plurality of series connec
Akiyama Masahiko
Nakai Yutaka
Hjerpe Richard
Kabushiki Kaisha Toshiba
Nguyen Kevin M.
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