Electricity: power supply or regulation systems – Including an impedance – Thermistor or resistor
Reexamination Certificate
2001-10-17
2002-11-05
Patel, Rajnikant B. (Department: 2838)
Electricity: power supply or regulation systems
Including an impedance
Thermistor or resistor
C345S211000, C345S095000
Reexamination Certificate
active
06476591
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a power supply device for driving liquid crystal, together with a liquid crystal device and electronic equipment which use that power supply device.
BACKGROUND ART
Conventional methods of reducing the required current for a power supply device which is used for driving liquid crystal have been disclosed in Japanese Patent Applications Laid-open No. 6-324640, No. 7-98577, No. 9-43568, and the like. An example of a conventional power supply device for driving liquid crystal is shown FIG.
7
.
A power supply device for driving liquid crystal
701
shown in
FIG. 7
has a voltage division circuit
702
, two first impedance conversion circuits
703
, and two second impedance conversion circuits
704
.
The voltage division circuit
702
contains resistors
706
to
710
and generates voltages V
1
to V
4
by dividing a voltage between a source voltage VDD and a reference voltage for driving liquid crystal VLCD.
When the source voltage VDD is a voltage V
0
and the reference voltage for driving liquid crystal VLCD is a voltage V
5
, voltages V
0
to V
5
correspond to voltage levels in the driving waveform for scan electrodes (or common electrodes) COM
0
, COM
1
, and COMX shown in FIG.
13
and also for signal electrodes (or segment electrodes) SEG
1
to SEG
4
shown in FIG.
14
.
The first impedance conversion circuit
703
is formed by voltage follower connection of an operational amplifier consisting of a constant current circuit
801
, P-type differential amplification circuit
802
, and output circuit
803
as shown in FIG.
8
. An N-type transistor
805
in the output circuit
803
forms a current source by receiving a constant bias voltage from the constant current circuit
801
, thereby providing a load for the P-type transistor
804
.
The characteristics of the first impedance conversion circuits
703
which generate the voltages V
1
and V
3
are determined by taking into account the direction of movement of electric charges in the scan electrodes (or common electrodes) or the signal electrodes (or segment electrodes) to which the voltage V
1
or V
2
is applied. Specifically, as indicated by
1102
in
FIGS. 13 and 14
, positive charges to be moved from the first impedance conversion circuits
703
to the electrodes is larger in amount than negative charges. For this reason, a P-type transistor
804
which causes a current to flow into the electrodes is used as an active element in the first impedance conversion circuits
703
.
The second impedance conversion circuit
704
is formed by voltage follower connection of an operational amplifier consisting of a constant current circuit
901
, N-type differential amplification circuit
902
, and output circuit
903
as shown in
FIG. 9. A
P-type transistor
904
in the output circuit
903
forms a current source by receiving a constant bias voltage from the constant current circuit
901
, thereby providing a load for the N-type transistor
905
.
The characteristics of the second impedance conversion circuits
704
which generate the voltages V
2
and V
4
are also determined by taking into account the direction of movement of electric charges in the scan electrodes (or common electrodes) or the signal electrodes (or segment electrodes) to which the voltage V
2
or V
4
is supplied. Specifically, as indicated by
1201
in
FIGS. 13 and 14
, negative charges to be moved from the second impedance conversion circuits
704
to the electrodes is larger in amount than positive charges. For this reason, an N-type transistor
905
which causes a current to be drawn from the electrodes is used as an active element in the second impedance conversion circuits
704
.
Among the divided voltages V
1
to V
4
in the voltage division circuit
702
, the voltages V
1
and V
3
are repectively input to the plus terminals of the first impedance conversion circuits
703
, and the voltages V
2
and V
4
are respectively input to the plus terminals of the second impedance conversion circuits
704
. The impedance conversion of the voltages V
1
to V
4
can be carried out in this manner, thereby generating voltages for driving liquid crystal V
1
to V
4
.
Conventional power supply devices for driving liquid crystal use an active load for the output circuit of an impedance conversion circuit to reduce current flowing through loading transistors, thereby reducing required current flowing through the impedance conversion circuit.
For maintaining display quality while limiting the amount of current flowing in the loading transistors through the impedance conversion circuits, the above-described load current must be supplemented. For this reason, it has been required to provide a capacitor element
705
between the output line for each of the voltages V
1
to V
4
and the output line for the voltage V
0
(VDD), as shown in FIG.
7
. The above load current can be supplemented by discharging the charges from the capacitor element
705
.
However, the capacitor element
705
has to be provided outside the power supply device for driving liquid crystal, because the capacitor element
705
has a large volume.
Downsizing and cost reduction are strongly demanded factors for electronic equipment, particularly for portable electronic equipment having a built-in liquid crystal device, so that the display quality is required to be maintained while reducing the number of parts such as capacitor elements.
The present invention has been devised to solve the above problems and has as an objective therof the provision of a power supply device for driving liquid crystal which enables low current comsumption, together with a liquid crystal device and electronic equipment using such a power supply device.
Another objective of the present invention is to provide a power supply device for driving liquid crystal which enables to omit parts such as a capacitor element while maintaining display quality, together with a liquid crystal device and electronic equipment using such a power supply device.
DISCLOSURE OF THE INVENTION
The power supply device for driving liquid crystal of the present invention which generates N numbers of liquid crystal drive voltages between first and second reference voltages, comprises: a voltage division circuit which divides a voltage between the first and second reference voltages to generate N pairs of first and second voltages comprising N numbers of first voltages each of which is equal to or higher than each of the N numbers of liquid crystal drive voltages, and N numbers of second voltages each of which is equal to or lower than each of N numbers of liquid crystal drive voltages, when the first voltage is not equal to the second voltage in each pair; and N numbers of impedance conversion circuits which generate N numbers of impedance transformed liquid crystal drive voltages based on the N pairs of the first and second voltages.
Each of the N numbers of impedance conversion circuits comprises: a voltage follower type of differential amplification circuit to which a pair of the first and second voltages among the N pairs of the first and second voltages is input; and an output circuit including a P-type transistor and N-type transistor connected in series between a first power supply line for the first reference voltage and a second power supply line for the second reference voltage, and having an output terminal which is connected between the P-type transistor and N-type transistor and outputs one of the N numbers of liquid crystal drive voltages.
On-and-off operation of the N-type transistor is controlled by the first output voltage from the differential amplification circuit, and on-and-off operation of the P-type transistor is controlled by the second output voltage from the differential amplification circuit.
In each impedance conversion circuit according to the present invention, a first and a second output voltage, each differing from the other, are output from a voltage follower type of differential amplification circuit to which a first and a second voltage, each differing from the other, a
Yamaguchi Hisashi
Yasue Tadashi
Oliff & Berridg,e PLC
Patel Rajnikant B.
Seiko Epson Corporation
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