Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
2001-10-04
2004-08-31
Eisen, Alexander (Department: 2674)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S094000
Reexamination Certificate
active
06784863
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an active matrix liquid crystal display and a method of driving the display, suitable for use in various data terminals and television sets, and in particular, relates to an active matrix liquid crystal display and a method of driving the display which allow for improvement of display quality and reduction of power consumption.
BACKGROUND OF THE INVENTION
The liquid crystal display of active matrix drive mode is an example of conventionally known image displays. As shown in
FIG. 16
, the liquid crystal display is composed of a liquid crystal panel
1
, a scan line drive circuit
2
, and a signal line drive circuit
3
.
The liquid crystal panel
1
includes a matrix substrate
7
, an opposite substrate
8
, and liquid crystal (not shown) injected between the substrates
7
,
8
. The opposite substrate
8
is disposed parallel to the matrix substrate
7
. On the matrix substrate
7
are there provided signal lines S(
1
) to S(I) and scan lines G(
1
) to G(J) that cross each other, as well as display cells P arranged in a matrix. On the opposite substrate
8
, an opposite electrode
13
shown in
FIG. 17
is provided commonly to all the display cells P.
As shown in
FIG. 17
, each display cell P has a thin film transistor (TFT)
11
, which is a switching element, and a liquid crystal capacitance C
LC
. As shown also in
FIG. 18
, the TFT
11
is connected at its source to the signal line S(i) and at its gate to the scan line G(j). The signal line drive circuit
3
supplies, to a signal line S(i), a source signal Vs which is then transmitted through the source and drain of the TFT
11
and applied as a drain voltage Vd(i,j) to a display electrode
12
which is one of the electrodes of the liquid crystal capacitance C
LC
. A common signal Vcom is applied to an opposite electrode
13
which is the other electrode of the liquid crystal capacitance C
LC
. Thus, a difference between the drain voltage Vd(i,j) and the common signal Vcom is applied to the liquid crystal capacitance C
LC
. As a result, the transmittance or reflectance of liquid crystal
14
sandwiched between the electrodes
12
,
13
so changes that an image is displayed by the display cells P in accordance with incoming image data. Switching off the TFT
11
does not cause the displayed image to change immediately, since in the display cell P the charge accumulated in the liquid crystal capacitance C
LC
is held for a specified period of time.
In liquid crystal displays, liquid crystal would deteriorate in terms of characteristics under continuous application of d.c. voltage to the liquid crystal; to avoid the inconvenience, the liquid crystal is driven by a voltage that changes from positive to negative and vice versa. The method of driving by means of ‘inversion drive voltage’ is generally termed inversion drive. Different forms of inversion include frame inversion, source line inversion, gate line inversion, and dot inversion.
Assume that the foregoing liquid crystal display is driven by frame inversion drive.
FIG. 19
describes, by means of waveform, development of drive voltages applied to display cells P in the liquid crystal panel
1
: namely, the display cell P(i,
1
) located in the i-th column, 1
st
row, the display cell P(i,j) centrally located in the i-th column, j-th row, and the display cell P(i,J) located in the i-th column, J-th row. For the purpose of simple description, the figure shows an example in which the source signal Vs is steady at 2 V and the common signal Vcom alternates by a 4 V amplitude to create ±2 V drive voltages for application to the display cells P.
Referring to the display cell P(i,
1
) in the top row, the source signal Vs is written as the TFT
11
is switched on by a gate pulse fed through the scan line G(
1
) at point A
1
. With the TFT
11
switched off later, the voltage across the liquid crystal
14
does not change because of the presence of the liquid crystal capacitance C
LC
. Subsequently, the common signal vcom goes negative at point R, varying by an amount equal to the aforementioned amplitude, and the drain voltage Vd varies by the same amount because of the principle of conservation of charge. The source signal Vs is written again as the TFT
11
is switched on by another gate pulse supplied to the scan line G(
1
) at point B
1
; the voltage across the liquid crystal
14
is retained. The writing and retaining recurs with a period T in this manner in the display cell P(i,
1
).
Referring to the central display cell P(i,j), the source signal Vs is written as the TFT
11
is switched on by a gate pulse fed through the scan line G(j) at point Aj; the voltage across the liquid crystal
14
is retained. Subsequently, the common signal Vcom goes negative at point R, varying by an amount equal to the aforementioned amplitude, and the drain voltage Vd varies by the same amount accordingly. The source signal Vs is written again as the TFT
11
is switched on at point Bj; the voltage across the liquid crystal
14
is retained. The writing and retaining recurs in this manner in the display cell P(i,j) similarly to the foregoing.
Referring to the display cell P(i,J) in the bottom row, the source signal Vs is written as the TFT
11
is switched on by a gate pulse fed through the scan line G(J) at point AJ; the voltage across the liquid crystal
14
is retained. Subsequently, the common signal Vcom varies by an amount equal to the aforementioned amplitude at point R, and the drain voltage Vd varies by the same amount. The source signal Vs is written again at point BJ; the voltage across the liquid crystal
14
is retained. The writing and retaining recurs in this manner in the display cell P(i,J) similarly to the foregoing.
As detailed above, the variation of the drain voltage Vd is equal to that of the common signal Vcom. Put it differently, the relative value of the drive voltage V
LC
(i,
1
) to the common signal Vcom is invariable, for example, in the display cell P(i,
1
). This makes it possible to drive the display cell (i,
1
) alternately with voltages ±2 V. The same description holds true with the other display cells P(i,j) and P(i,J).
Now, the following will describe a case where the common signal Vcom is a steady, d.c., voltage.
FIG. 20
describes, by means of waveform, development of drive voltages applied to display cells P in the liquid crystal panel
1
: namely, the display cell P(i,
1
) located in the i-th column, 1
st
row, the display cell P(i,j) centrally located in the i-th column, j-th row, and the display cell P(i,J) located in the i-th column, J-th row. For the purpose of simple description, the figure shows an example in which the common signal Vcom is steady at 2 V and the source signal Vs alternates by a 4 V amplitude to create ±2 V drive voltages for application to the display cell P.
According to this drive scheme, the common signal Vcom does not alternate between positive and negative voltage levels. Therefore, the common signal Vcom does not vary in amplitude, nor does the drain voltage Vd. Further, in a liquid crystal cell P depicted by means of an equivalent circuit in
FIG. 17
, a change in polarity of the source signal Vs does not lead to a change in polarity of the drain voltage Vd.
Generally, liquid crystal displays require a backlight as a light source, since liquid crystal itself does not emit light by nature. The lamp for the backlight is highly power consuming and makes it difficult to fabricate a power efficient crystal liquid display. In contrast, recently developed reflective displays do not require a backlight and are used in mobile data terminals and other like devices that are mostly used outdoors.
Electrodes used in some liquid crystal displays of this kind have a reflective electrode structure. Some liquid crystal displays employ an alternative structure in which pixel electrodes (reflective electrodes) and bus lines, such as signal lines, are provided in different layers separated by an interlayer insulating film.
In a reflective electrode structure, as shown i
Kumada Kouji
Mizukata Katsuya
Ohta Takashige
Yanagi Toshihiro
Conlin David G.
Edwards & Angell LLP
Eisen Alexander
Hartnell, III George W.
Sharp Kabushiki Kaisha
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