Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
2000-09-06
2004-01-13
Saras, Steven (Department: 2675)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S055000, C345S090000
Reexamination Certificate
active
06677925
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an active-matrix-type liquid crystal display device, such as a thin-film-transistor (TFT) liquid crystal display device, a data signal line driving circuit, and a liquid crystal display device driving method.
BACKGROUND OF THE INVENTION
Recently, liquid crystal display (LCD) devices have rapidly become in common use since consumed power is small and the size can be easily reduced, as compared with CRT (cathode-ray-tube) display devices. Among such LCD devices, active-matrix-type LCD devices that are characterized by quicker response and easier multiple-gray-level display are widely used.
In a conventional active-matrix-type LCD device
101
as above, for example, as shown in
FIG. 13
, when a scanning signal line driving circuit
104
selects a scanning signal line GL
j
, field-effect transistors SW shown in
FIG. 2
provide conduction at pixels PIX connected with the scanning signal line GL
j
, thereby connecting the pixels PIX
(i,j)
with data signal lines SL
i
corresponding to the pixels. On the other hand, a data signal line driving circuit
103
outputs display data D to data signal lines SL
1
through SL
n
based on video signals DAT so that the display data D are to be fed to the pixels PIX. Charges corresponding to respective differences between outputs of the data signal lines SL
1
through SL
n
and a common electrode potential Vcom are stored in pixel capacitors C
P
of the pixels PIX. At pixels PIX connected with the scanning signal lines GL that are not selected, switching elements SW thereof are opened, thereby holding charges in pixel capacitors C
P
thereof. Incidentally, transmittance of liquid crystal elements varies with a voltage applied. Therefore, while consecutively selecting the scanning signal lines GL
1
through GL
m
, the display data D are written in a pixel PIX
(i,j)
during a selection period of each scanning signal line GL
j
. By so doing, the LCD device
101
causes an image according to the foregoing video signal DAT to be displayed on the liquid crystal panel
102
.
In the foregoing active-matrix-type LCD device
101
, the data signal line SL
i
and the pixel capacitor C
P
are separated while the scanning signal line GL
i
is not selected, and a voltage according to the display data D that have been written in the pixel capacitor C
P
upon selection is continuously applied to the liquid crystal element. Therefore, as compared with a simple-matrix-type LCD device, the multiple-gray-level display can be relatively easily realized.
The foregoing arrangement, however, undergoes a problem such that, to realize a higher-definition active-matrix-type LCD device with a larger display screen particularly, horizontal shadow more easily occurs, impairing image quality.
More specifically, taking the case where polarities of outputs of the data signal lines SL
1
through SL
n
are reversed every horizontal scanning period, current flows to charge/discharge a capacitor between a source of each field-effect transistor SW and a common electrode T
com
every horizontal scanning period. Note that examples of such capacitors include, apart from the foregoing pixel capacitors C
P
, capacitors formed between the data signal lines SL
1
through SL
n
and the common electrodes T
com
, cross capacitors formed between the data signal lines SL
1
through SL
n
and bus lines, and cross capacitors formed between the data signal lines SL
1
through SL
n
and the scanning signal lines GL
1
through GL
m
.
Here, current charging/discharging the foregoing capacitors varies with output amplitudes of the data signal lines SL
1
through SL
n
. Therefore, in the case where resistances between supplementary capacitors C
S
of the pixel capacitors C
P
, common transfer resistances, output impedance of a common electrode driving circuit
105
, etc. cause resistance components to exist in the common electrode line COM connected with the common electrodes T
com
and C
S
bus lines connected with the supplementary capacitors C
S
, a voltage fall due to the foregoing resistance components varies with the output amplitudes of the data signal lines SL
1
through SL
n
. Consequently, a rising speed of a common electrode potential Vcom waveform varies with a display pattern that varies every horizontal scanning period.
For example, as shown in
FIG. 14
, comparing a portion A in which all the data signal lines SL
1
through SL
n
output a white level during one horizontal scanning period with a portion B that includes an output of a black level with a greater potential difference than that of the white level with respect to the common electrode potential Vcom, current flowing at a root part of the common electrode line COM and root parts of the C
S
bus lines is greater in the portion B than in the portion A. Therefore, the rising of the common electrode potential Vcom waveform is duller in the portion B as indicated by a broken line than in the portion A as indicated by a solid line in FIG.
15
.
Here, in the case where the charging period for the pixel capacitors C
P
is sufficient, charging voltage levels to the pixel capacitors C
P
are equal to each other in the portions A and B. In the case where, however, for example, the charging to the pixel capacitors C
P
is not completed during the charging period due to insufficient driving capacity or operating speed of the field-effect transistors SW, charges less than the value indicated by the display data D are provided to each pixel capacitor C
P
, and are maintained during a non-selection period as well. In this case, charging becomes insufficient in the portion B rather than in the portion A. Consequently, the brightness of a white part of the portion B becomes higher than the brightness of a white part of the portion A, resulting in that white horizontal shadow occurs. Incidentally, the explanation herein is made by using a normally-white-type LCD device, but the same applies in the case of a normally-black-type LCD device.
The foregoing horizontal shadow can be prevented by reducing the resistance components of the C
S
bus lines and the common electrode line COM and by ensuring sufficient charging period for charging the pixel capacitors C
P
. However, there are limits to reduction of the resistance components and improvement of characteristics of the field-effect transistors SW, while higher-definition LCD devices with larger display screens are demanded. Enlargement of the display screen will require longer C
S
bus lines and common electrode line COM, thereby making it difficult to reduce the resistance components. Besides, in a high-definition LCD device, data signal lines SL
1
through SL
n
and the scanning signal lines GL
1
through GL
m
increase in number, making it difficult to ensure sufficient charging time. Therefore, in such LCD devices in particular, horizontal shadow more often occur, and elimination of horizontal shadow is demanded.
Incidentally, the U.S. Pat. No. 2,960,268 (Date of Publication: Jul. 8, 1994) discloses an active-matrix liquid crystal panel including capacity-coupled sensing electrodes that cross data signal lines SL
1
through SL
n
with an insulating film provided between the same and the data signal lines SL
1
through SL
n
, and an inverter for applying to the common electrode a voltage that corresponds to a potential fluctuation occurring to the sensing electrode and that is obtained by reversing a polarity of the potential fluctuation. This arrangement is aimed to cancelling the potential fluctuation occurring to the common electrode with the voltage applied to the data signal lines SL
1
through SL
n
, so as to prevent occurrence of horizontal shadow. In the present arrangement however, application of the output signal of the inverter to the common electrode for driving the common electrode makes AC driving impossible, and causes the power consumed by the whole LCD device to drastically increase. On the other hand, as described above, it is preferable that power used for removing horizontal shadow is small, since the LCD devices are of
Kawaguchi Takafumi
Yanagi Toshihiro
Conlin David G.
Daley, Jr. William J.
Edwards & Angell LLP
Nelson Alecia D.
Saras Steven
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