Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
2000-11-09
2003-09-16
Saras, Steven (Department: 2675)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S090000, C345S098000
Reexamination Certificate
active
06621477
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display device, and in particular to a liquid crystal display device capable of gradation display.
2. Description of the Background Art
Conventionally, liquid crystal display devices for displaying a static or dynamic image have been utilized in personal computers, television receivers, portable telephones, personal digital assistants, and so forth.
FIG. 4
is a circuit diagram showing main parts of such a liquid crystal display device. In
FIG. 4
, the liquid crystal display includes a liquid crystal cell
30
, a vertical scanning line
31
, a common interconnection line
32
, a horizontal scanning line
33
and a liquid crystal driving circuit
34
, the liquid crystal driving circuit
34
including an N channel MOS transistor
35
and a capacitor
36
.
N channel MOS transistor
35
is connected between horizontal scanning line
33
and one electrode
30
a
of liquid crystal cell
30
, the gate thereof being connected to vertical scanning line
31
. Capacitor
36
is connected between electrode
30
a
of liquid crystal cell
30
and common interconnection line
32
. A power-supply potential VCC is applied to the other electrode of liquid crystal cell
30
, and a reference potential VSS is applied to common interconnection line
32
. Vertical scanning line
31
is driven by a vertical scanning circuit (not shown) and horizontal scanning line
33
is driven by a horizontal scanning circuit (not shown).
When vertical scanning line
31
is set to a level “H,” N channel MOS transistor
35
is conducting, and electrode
30
a
of liquid crystal cell
30
is charged to the level of horizontal scanning line
33
via N channel MOS transistor
35
. For example, the light transmittance of liquid crystal cell
30
will be minimum when electrode
30
a
is at the level “H,” while the light transmittance of liquid crystal cell
30
will be maximum when electrode
30
a is at a level “L.” A plurality of such liquid crystal cells
30
are arranged in a plurality of rows and columns to form a liquid crystal panel, the panel displaying an image.
A conventional liquid crystal display device has been configured as described above, so that, in order to perform gradation display in one liquid crystal cell
30
, an application of an analog potential corresponding to the gradation was required.
However, when an image is displayed in response to a digital image signal, a digital-to-analog conversion circuit will be required for converting a digital signal to an analog signal, leading to a problem of higher cost.
SUMMARY OF THE INVENTION
A main object of the present invention is, therefore, to provide an inexpensive liquid crystal display device capable of gradation display.
A liquid crystal display device according to the present invention includes a liquid crystal cell receiving a power-supply potential at one electrode thereof and having a light transmittance varied in accordance with a potential applied to the other electrode thereof, a variable capacitance circuit connected between a line of a first reference potential and the other electrode of the liquid crystal cell and having a capacitance value controllable in a plurality of steps, and a control circuit selectively setting the capacitance value of the variable capacitance circuit in response to an image signal to set a potential of the other electrode of the liquid crystal cell. Thus, the light transmittance of the liquid crystal cell can be varied by changing the capacitance value of the variable capacitance circuit, so that gradation display can be performed with one liquid crystal cell without adding a digital-to-analog conversion circuit, and hence the cost of the device will be reduced.
Preferably, the variable capacitance circuit includes a plurality of first capacitors each having one electrode connected to the other electrode of the liquid crystal cell, and a plurality of first switching elements connected, each at one electrode, to the other electrodes of the plurality of first capacitors, and receiving, each at the other electrode, the first reference voltage. The control circuit renders conductive or non-conductive each of the plurality of first switching elements to selectively set the capacitance value of the variable capacitance circuit. In this case, the light transmittance of the liquid crystal cell can be changed by the number of the first switching elements to be conducted.
Further, each of the plurality of the first capacitors preferably has a capacitance value different from each other. In this case, gradation display in a larger number of steps will be possible.
It is also preferable to provide a second capacitor having one electrode connected to the other electrode of the liquid crystal cell, and receiving, at the other electrode, a second reference potential. In this case, more accurate setting of the potential of the other electrode of the liquid crystal cell will be possible.
More preferably, a plurality of second switching elements connected, each at one electrode, to the other electrodes of the plurality of first capacitors and receiving, each at the other electrode, the second reference potential, and a third switching element having one electrode connected to the other electrode of the liquid crystal cell and receiving, at the other electrode, the second reference potential, are further provided. The control circuit renders conductive the plurality of second switching elements and the third switching element before setting a potential of the other electrode of the liquid crystal cell, to reset the potential of the other electrodes of the plurality of first capacitors and the other electrode of the liquid crystal cell to the second reference potential. In this case, residual charge in the first capacitors and the liquid crystal cell can be removed, so that the potential of the other electrode of the liquid crystal cell can more accurately be set.
It is also preferable to further provide a fourth switching element having one electrode connected to the other electrode of the plurality of first switching elements, and receiving, at the other electrode, the first reference potential. The control circuit renders non-conductive the fourth switching element after setting a potential of the other electrode of the liquid crystal cell to stop feeding of the first reference potential to the other electrodes of the plurality of first switching elements. This can prevent variation of the other electrode of the liquid crystal cell due to leakage current of the first switching elements.
More preferably, each of the plurality of first switching elements is a field effect transistor, and a plurality of third capacitors connected, each at one electrode, to respective input electrodes of the plurality of the field effect transistors and receiving, each at the other electrode, a third reference potential, is further provided. The control circuit charges or discharges one electrode of each of the plurality of third capacitors to renders conductive each of the plurality of field effect transistors.
Further, the liquid crystal display device is preferably installed in a portable electronic device. The present invention will be particularly advantageous in such a case.
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patent: 5671032 (1997-09-01), Lee
patent: 6020870 (2000-02-01), Takahashi et al.
patent: 6075505 (2000-06-01), Shiba et al.
patent: 6115018 (2000-09-01), Okumura et al.
patent: 6140990 (2000-10-01), Schlig
patent: 6266038 (2001-07-01), Yoshida et al.
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patent: 58-196582 (1983-11-01), None
patent: 5-119352 (1993-05-01), None
“Thin Film Transistor For Gray Scale LCD”, IBM Technical Disclosure Bulletin, vol. 33, No. 1A, Jun. 1990, p. 700.
Agari Masafumi
Inoue Mitsuo
Murai Hiroyuki
Tokioka Hidetada
Leydig , Voit & Mayer, Ltd.
Nelson Alecia D.
Saras Steven
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