Computer graphics processing and selective visual display system – Display driving control circuitry – Waveform generator coupled to display elements
Reexamination Certificate
1999-06-04
2002-12-31
Chow, Dennis-Doon (Department: 2675)
Computer graphics processing and selective visual display system
Display driving control circuitry
Waveform generator coupled to display elements
C345S087000, C345S095000
Reexamination Certificate
active
06501467
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an liquid-crystal display panel drive power supply and to a method for reducing the power consumption of this liquid-crystal display panel drive power supply.
2. Description of the Related Art
In recent years, with the widespread use of liquid-crystal display panels in portable electronic equipment, there has been a demand for lower power consumption in a power supply for liquid-crystal displays and for an improvement in the output impedance of a power supply to accommodate a large liquid crystal panel for display of special characters.
FIG. 6
shows a block diagram that includes a liquid-crystal display panel and the peripheral drive circuitry therefore. The display panel M
4
is formed by sandwiching a liquid crystal between two glass electrodes that have a multitude of parallel wires such that the electrode lines are mutually perpendicular.
Of the two electrodes, a first electrode, the common (COM) electrodes, are usually taken from the lateral direction of the panel, and the second electrodes, the segment (SEG) or data electrodes, are usually taken from the vertical direction.
The points at which the common electrode intersects with the segment electrode with the liquid crystal therebetween form an equivalent capacitance (hereinafter referred to as a pixel capacitance), and by applying a prescribed potential difference between each of the common and segment electrodes, a potential is applied to corresponding pixel capacitance, resulting in display of that pixel. Therefore, by selecting the potential of the segment electrodes in accordance with display data while scanning (selecting) the common electrodes, it is possible to display data. The selection circuit M
2
, the common driver M
3
, and the segment driver M
5
are basically formed by analog MOS switches, a prescribed level of power supply circuit M
1
being selected in accordance with the scanning and data display timing, so as to apply voltages to the electrodes of the liquid-crystal panel.
FIG. 7
shows an example of output waveforms for the case in which the voltages V
1
to V
5
which are generated by the level power supply circuit =M
1
of FIG.
6
and VEE (ground) are output by the common driver M
3
and the segment driver M
5
. The segment driver M
5
outputs as a selected level (V
1
or ground) or non-selected level (V
3
or V
4
) in accordance with the existence or non-existence of data. Because when the voltages which is applied to a liquid crystal are applied in a DC manner, the deterioration of the liquid crystal is accelerated, in general the selected and non-selected levels are varied with a given period, so that they are applied as AC levels.
FIG. 7
is an example in which selected level and non-selected level are changed for each common scan, this being known as the frame reversal mode. For this reason, driving a liquid crystal requires the use of a multilevel power supply. However, with the use of liquid-crystal displays in portable equipment, it is also necessary for the liquid-crystal display power supply to have low power consumption. Because of this need, a circuit such as shown in
FIG. 9
was used in the past as a power supply circuit. In
FIG. 9
, to limit wasteful power consumption other than for driving the liquid-crystal, voltage-dividing resistors R
1
through R
5
are established with resistance values in the range from several tens of kilohms to several hundreds of kilohms, thereby limiting the current flowing in the idling condition.
However, if the output impedance is high, driving a liquid crystal, which represents a capacitive load, results in waveform distortion, this resulting in a deterioration of display quality. Because of this, the divided voltages are output via amplifiers (B
1
through B
5
), so that there is an improvement in the charging capacity and discharging capacity at the voltage levels required for liquid crystal drive. However, in order to limit the increase of current consumption caused by the use of amplifiers, an external bias is used with each amplifier to limit the bias current, thereby limiting internal current and unnecessary current. FIG.
10
(
a
) shows the charging capacity, while FIG.
10
(
b
) shows the discharging capacity of an amplifier, and in the prior art example of
FIG. 9
, the amplifiers B
1
, B
2
, and B
4
have the configuration of
FIG. 10
(
a
), while the amplifiers B
3
and B
5
of
FIG. 9
have the configuration of
FIG. 10
(
b
). The power supply voltages are the maximum potential within the circuit (VLCD) and the minimum potential (GND). FIG.
7
(
c
) is a specific example of segment output waveforms for display and non-display that are repeated. If the time when the common selection level is the maximum drive potential V
1
is frame
1
and the time when the common selection level is the minimum potential GND is frame
2
, during frame
1
the segment is selected between V
4
and GND, while during frame
2
the segment is selected between V
1
and V
3
. If we observe one segment, this segment has n intersections between n commons, meaning that it has n display pixels (capacitances) with respect to common. Because only a single common outputs a selection level during a given frame, only one terminal that is different from the above-noted pixel capacitance segment is shorted to common, with the remaining n−1 being shorted to the non-selected level. FIG.
8
(
a
) illustrates the condition of the current flow in the power supply that outputs the voltage levels V
1
and V
3
when the common and segment drivers operate, in the power supply that is shown in
FIG. 9
, when the frame
2
operation of
FIG. 7
(
c
) is done. Here, if the capacitances CL
1
and CL
2
per pixel are Cp, CL
1
=(n−1)×Cp and CL
2
=Cp. As the panel becomes larger (that is, as n increases), the load capacitance increases, this leading to an increase in the equivalent capacitance at each level, making it necessary to lower the output impedance sufficient so that it is possible to provide sufficient drive for the capacitive load. However, with the reduction of power consumption equipment using liquid-crystal displays in recent years, even the bias current becomes significant.
For example, in the case in which the resistors R
1
through R
5
are 500k&OHgr;, for VI
1
=10 V, the idling current flowing in the resistances can be limited to 10 V/(500k&OHgr;×5)=4&mgr;A. However, in the differential and output stages of the amplifiers of FIG.
10
(
a
) and FIG.
10
(
b
), in the bias current is 1 &mgr;A, the overall amplifier bias current in the power supply circuit is (1+1)×5=10 &mgr;A. This current flows even when a load is not being driven, and is thus wasteful, and this has represented a technological problem with the move to lower power consumption in drive power supplies in recent years.
In this type of circuit, because charging and discharging by the amplifier of the liquid crystal load is performed between the internal circuit maximum potential (VLCD) and minimum potential (GND), regardless of the voltage level to which charging and discharging is done, this is basically merely discharging via the MOS output stage of the amplifier to the maximum potential (VLCD) or the minimum potential (GND) and this circuit does not make re-use of load current. However, according to an example of prior art as disclosed in Japanese Unexamined Patent Publication (KOKAI) No.5-257121, as shown in
FIG. 11
, there is a circuit that takes each of the potentials that are divided by resistors as the power supply voltages of the amplifiers. In this circuit, the current from an amplifiers flows into divided resistances, this resulting in a deterioration of display quality according to level change. Because the amplifier power supply has an impedance of 5 k&OHgr;or greater (in the prior art example, R
1
is 5 k&OHgr;to 15 k&OHgr;), not only does the output impedance (sum of the power supply impedance and on resistance of the output
Anyaso Uchendu O.
Chow Dennis-Doon
NEC Corporation
Sughrue & Mion, PLLC
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