Computer graphics processing and selective visual display system – Display driving control circuitry
Reexamination Certificate
1999-02-11
2001-05-08
Shalwala, Bipin (Department: 2673)
Computer graphics processing and selective visual display system
Display driving control circuitry
C345S094000
Reexamination Certificate
active
06229530
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention a driving circuit for a liquid crystal device. More particularly, the present invention relates to a liquid crystal driving circuit for driving a liquid crystal display screen in a personal digital assistant, etc.
2. Description of the Related Art
As a display means of a personal digital assistant such as a pager, a cellular phone, an electronic pocketbook, etc., a low power consumption liquid crystal element is employed. As a liquid crystal element driving system, there is a low power consumption driving system which employs a voltage step up/down circuit using capacitors and which is mainly employed in low duty display such as numerals, alphabets, etc. In contrast, there is employed a driving system which employs an operational amplifier and which is employed in high duty display such as Chinese characters, characters, etc. In this system, a large power is consumed because a large current flows through the operational amplifier. Today a larger display screen of the liquid crystal display, i.e., higher duty of the liquid crystal has been advanced with the progress of multi-function of the personal digital assistant. It is certain that such high duty display will become the mainstream of the liquid crystal display in the near future. Therefore, the low power consumption liquid crystal driving circuit is also earnestly desired in the field of the high duty display.
A liquid crystal driving circuit to enable the high duty display in the prior art will be explained hereunder.
FIG. 1
is a circuit diagram showing a configuration of the liquid crystal driving circuit employed for the high duty display in the prior art. In this liquid crystal driving circuit, voltage dividing resistors
103
to
105
, one of bias selection resistors
106
to
109
, and voltage dividing resistors
110
and
111
are connected in series between the supply voltage Vdd
101
for generating the liquid power supply and the reference voltage Vss
102
. Thus, intermediate potentials can be generated according to respective resistance values of the bias selection resistors
106
to
109
. The voltage dividing resistor
103
is a liquid crystal temperature compensating resistor whose resistance value RA can be varied by the software control.
In general, a proper value of a liquid crystal bias voltage VC
1
in the liquid crystal using the TN (Twisted Nematic) method or the STN (Super Twisted Nematic) method can be given by
VC
1
=1/((duty)
½
−1) to 1/((duty)
½
)+1 (1)
This liquid crystal bias voltage VC
1
can be decided by selecting any one of the bias selection resistors
106
to
109
. This selection of the bias selection resistors
106
to
109
is made by decoding 2-bit signals R
1
, R
2
by using a decoder
112
in the publicly known technology and then turning ON any one of analogue switches
113
to
116
selectively based on an output signal of the decoder
112
.
Normally the voltage dividing resistors
104
,
105
,
110
,
111
are set to have the same resistance value and the resistance values of the bias selection resistors
106
to
109
are set N times larger than that of the voltage dividing resistors
104
,
105
,
110
,
111
. Usually, 2 to 5 is used as the value N. For example, in case the resistance value of the voltage dividing resistors
104
,
105
,
110
,
111
is assumed as RB, the resistance value of the bias selection resistor
109
is selected as
2
RB, the resistance value of the bias selection resistor
108
is selected as
3
RB, the resistance value of the bias selection resistor
107
is selected as
4
RB, and the resistance value of the bias selection resistor
106
is selected as
5
RB. Accordingly, the liquid crystal bias voltage VC
1
becomes ⅙ bias if the bias selection resistor
109
is selected, the liquid crystal bias voltage VC
1
becomes {fraction (1/7)} bias if the bias selection resistor
108
is selected, the liquid crystal bias voltage VC
1
becomes ⅛ bias if the bias selection resistor
107
is selected, and the liquid crystal bias voltage VC
1
becomes {fraction (1/9)} bias if the bias selection resistor
106
is selected.
In this liquid crystal driving circuit, the resistors
103
to
111
are set to have high resistance such that the direct current flowing through them should be suppressed as small as possible. The intermediate potentials generated by using the resistors
103
to
111
are amplified by operational amplifiers
117
to
121
. As a result, sufficient current to drive the large size liquid crystal display screen can be generated. Thus, outputs of the operational amplifiers
117
to
121
are stored in the capacitors
122
to
126
to be stabilized.
FIG. 2
is a view showing behaviors of driving waveforms of a common bias voltage COM and a segment bias voltage SEG when the analogue switch
116
in
FIG. 1
is turned ON to select the resistor
109
and thus to set the liquid crystal bias voltage VC
1
to ⅙ bias. In
FIG. 2
, the liquid crystal element is brought into its energized state only in a period of time when potential difference between the segment bias voltage SEG and the common bias voltage COM is within ±VLC, and it is brought into its non-energized state in other periods of time. As shown in
FIG. 3
, the COM-based SEG becomes ±VLC in the energized state and becomes VLC
3
−VLC
4
(=+VLC/6) or VLC
2
−VLC
1
(=−VLC/6) in the non-energized state.
However, in the liquid crystal driving circuit in the prior art shown in
FIG. 1
, the direct current always flows through the resistors
103
to
111
and also the large current is consumed in the operational amplifiers
117
to
121
which are employed to amplify the generated intermediate potential. Since these currents always flow during the display operation, such currents have caused a serious problem to achieve lower power consumption of the personal digital assistant, etc.
SUMMARY OF THE INVENTION
The present invention has been made to overcome the above-mentioned problem in the prior art, and it is an object of the present invention to provide a low power consumption liquid crystal driving circuit for driving a liquid crystal device which enables high duty display.
In order to achieve the above object, according to a feature of the present invention, there is provided a liquid crystal driving circuit comprising a first capacitor, a plurality of external terminals, a plurality of second capacitors connected between the plurality of external terminals, a first regulator connected between a first power supply and a second power supply, a second regulator connected between an output of the first regulator and the second power supply, a circuit for generating a plurality of time division signals, and a switching means for connecting outputs of the first regulator and the second regulator to the first capacitor, or connecting the first capacitor to any one of the second capacitors, based on the time division signals.
In the feature of the present invention, preferably the first power supply may be a power supply which supplies a liquid crystal power supply generating voltage, and the second power supply may be a power supply which supplies a reference voltage. The reference voltage may be a ground voltage. The switching means may be composed of a plurality of analogue switches. This is because the switching means can execute switching of connection to either of the first capacitor and the second capacitor by using a simple circuit. The plurality of second capacitors may be composed of a plurality of third capacitors which are connected between the first power supply and the plurality of external terminals, and a plurality of fourth capacitors which are connected between the plurality of external terminals and the second power supply. This is because respective intermediate potentials being generated can be stored without fail.
According to the feature of the present invention, the low power consumption liquid
Kabushiki Kaisha Toshiba
Kovalick Vincent E.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Shalwala Bipin
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