Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1999-12-10
2003-11-25
Hjerpe, Richard (Department: 2674)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S084000, C345S085000, C345S087000, C345S089000, C345S090000, C345S095000, C345S098000, C345S099000, C345S211000, C345S212000, C345S204000, C327S540000, C315S169100, C315S169200, C315S169300, C315S169400
Reexamination Certificate
active
06653999
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an integrated circuit for driving liquid crystal capable of adjusting display contrast.
2. Description of the Related Arts
FIG. 1
is a circuit block diagram illustrating a method of adjusting display contrast using a conventional integrated circuit for driving liquid crystal.
Referring to
FIG. 1
, a liquid crystal panel
101
includes a plurality of segment electrodes and a plurality of common electrodes arranged in a matrix. A segment driving signal and a common driving signal are applied to the plurality of segment electrodes and the plurality of common electrodes of the liquid crystal panel
101
, respectively, and light is turned on only at the intersections of the matrix for which the potential difference between the segment driving signal and the common driving signal exceeds a prescribed value.
A liquid crystal driving integrated circuit
102
drives the liquid crystal panel
101
to present a display. In the liquid crystal driving integrated circuit
102
, respective connection points of four serially connected resistor elements R
1
forming a resistor are connected to terminals
103
-
107
. The terminal
103
receives a reference voltage VLCD
0
setting peak values of the segment and common driving signals, and the terminal
107
connects all components of the circuit
102
in common to ground. The potential difference between the reference voltage VLCD
0
and a ground voltage Vss is quartered by the four resistor elements R
1
. The voltages at the terminals
103
-
107
will be hereinafter denoted as VLCD
0
, VLCD
1
, VLCD
2
, VLCD
3
, and Vss, respectively. The common driving circuit
108
receives the voltages VLCD
0
, VLCD
1
, VLCD
3
, and Vss to generate the common driving signal. The common driving signal changes between the reference voltage VLCD
0
and the ground voltage Vss to turn on light at the liquid crystal panel
101
, and changes between the voltages VLCD
1
and VLCD
3
to turn off light at the panel
101
. Therefore, in this case, the common driving signal assumes a ¼ bias driving waveform. On the other hand, a segment driving circuit
109
receives the voltages VLCD
0
, VLCD
2
, and Vss to generate the segment driving signal. When a light is to be turned on at the liquid crystal panel
101
, the segment driving signal changes between the reference voltage VLCD
0
and the ground voltage Vss in a phase opposite to that of the common driving signal for turning on light. On the other hand, the segment driving signal remains unchanged at the voltage VLCD
2
when light is to be turned off at the panel
101
. The reference voltage VLCD
0
determines display contrast (difference in display between when light is on and off) of the liquid crystal panel
101
. Therefore, the display contrast of the liquid crystal panel
101
can be optimized by having a variable reference voltage VLCD
0
and changing the amplitudes of the common and segment driving signals.
A reference voltage generation circuit
110
applies the reference voltage VLCD
0
to the terminal
103
. In the circuit
110
, a resistor
111
and a variable resistor
112
are connected in series between a power supply voltage Vdd and a ground voltage Vss. An operational amplifier
113
outputs a voltage equal to that present at the connection point between the resistor
111
and the variable resistor
112
as the reference voltage VLCD
0
. When the impedance of the resistor formed by the four serially connected resistor elements R
1
exceeds the load impedance of the liquid crystal panel
101
and the like, the voltages VLCD
1
-
3
are likely to be unsettled. Therefore, the operational amplifier
113
having a small output impedance is used. A resistor may be externally connected between the terminals
103
-
107
to form a resistor member connected in parallel to the four serially connected resistor elements R
1
, to thereby reduce the impedance on the side of the serially connected resistor elements R
1
. The reference voltage generation circuit
110
receives a control signal for changing the value of the variable resistor
112
from an external controller. Thus, the reference voltage VLCD
0
is changed under the control of the external controller, to thereby adjust the display contrast of the liquid crystal panel
101
.
However, in the circuit arrangement of
FIG. 1
, the reference voltage generation circuit
110
must be externally connected to the liquid crystal driving integrated circuit
102
. Thus, as the circuit
110
includes a great number of elements, it would impede reduction of cost of electronic devices. In addition, ports of the external controller for specific use are dedicated for output of control signals, which would hinder the electronic devices from assuming higher functions.
FIG. 2
is another circuit block diagram illustrating a method of adjusting display contrast using a conventional liquid crystal driving integrated circuit, which attempts to solve the problems of the circuit in FIG.
1
. In
FIG. 2
, the liquid crystal panel
101
, the common driving circuit
108
, and the segment driving circuit
109
of
FIG. 1
are not shown.
In the integrated circuit
201
for driving liquid crystal, the respective connection points of the four serially connected resistor elements R
1
are connected to terminals
202
-
206
for a similar purpose to that described in connection with FIG.
1
. The terminal
202
is a power supply terminal receiving the power supply voltage Vdd. A regulator
207
outputs a constant voltage VRF based on the power supply voltage Vdd. An operational amplifier
208
has a positive terminal connected to the constant voltage VRF, a negative terminal connected to a terminal
209
, and an output terminal connected to the terminal
206
. The value of current IR flowing across the negative terminal of the operational amplifier
208
can be adjusted under the control of an internal controller.
Three serially connected external resistor elements R
2
, R
3
, and R
4
forming another resistor are connected between the terminals
202
and
206
, and an intermediate terminal of the external resistor element R
3
is connected to the terminal
209
. The serially connected resistor elements R
2
, R
3
, and R
4
are divided into two parts by the intermediate terminal of the resistor element R
3
. The resistance of the part consisting of the resistor element R
2
and a portion of the resistor element R
3
will be denoted as Ra, and that of the part consisting of the remaining portion of the resistor element R
3
and the resistor element R
4
as Rb.
A voltage VLCD
4
can be given by ((Ra+Rb)/Ra)VRF+IR·Rb. Thus, the value of current IR is controlled by the internal controller to change the voltage VLCD
4
, thereby adjusting the display contrast of the liquid crystal panel
101
.
However, while the liquid crystal driving integrated circuit
201
of
FIG. 2
requires only the resistor elements R
2
, R
3
, and R
4
as external elements, a ratio of the voltages Ra and Rb would deviate from the expected value because of variation in resistance of the resistor elements R
2
, R
3
, and R
4
, making it impossible to achieve appropriate display contrast. Consequently, the variation in resistance of the resistor elements R
2
-R
4
must be corrected under the control of the external controller, resulting in similar problems to those discussed in connection with FIG.
1
.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an integrated circuit for driving liquid crystal that requires no external elements and allows adjustment of display contrast.
The present invention has been conceived to solve the above problems. The present invention provides a liquid crystal driving integrated circuit for generating liquid crystal driving voltages that drive a liquid crystal panel to present a display from respective connection points of a plurality of serially connected resistor elements forming a first resistor. In the liquid crystal driving integrated circuit, a reference volt
Arai Hiroyuki
Motegi Shuji
Tokunaga Tetsuya
Abdulselam Abbas
Hjerpe Richard
Hogan & Hartson LLP
Sanyo Electric Co,. Ltd.
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