Computer graphics processing and selective visual display system – Display driving control circuitry – Display power source
Reexamination Certificate
1999-09-16
2001-11-13
Shalwala, Bipin (Department: 2673)
Computer graphics processing and selective visual display system
Display driving control circuitry
Display power source
C345S089000, C345S095000
Reexamination Certificate
active
06317122
ABSTRACT:
TECHNICAL FIELD
This invention relates to a power circuit, a liquid crystal display device that comprises this power circuit, and electronic equipment that comprises this liquid crystal display device.
BACKGROUND OF ART
A power circuit used in a liquid crystal display device driven by a one-line sequential drive method is described below as a first prior-art technique, with reference to FIG.
48
. This diagram is basically the same as FIG. 3 of Japanese Patent Application Laid-Open No. 2-150819. In this case, V
0
to V
5
are in the relationship VD=(V
0
−V
1
)=(V
1
−V
2
)=(V
3
−V
4
)=(V
4
−V
5
), where VD is on the order of 1.6 V when the duty ratio is 1/240, for example.
The power source inputs to the liquid crystal display device from the exterior comprises VCC for the logic portions of the driver IC and VEE for creating the liquid crystal panel drive voltages, with GND as a reference potential. VEE is considerably higher than VCC; for example, it is on the order of 20 V to 25 V when the duty ratio is 1/240. Among V
0
to V
5
, VEE is used without modification as V
0
and GND as V
5
. V
1
+0V
4
are obtained by division by resistances R
1
to R
5
between VEE and GND, the impedances of these outputs are lowered by operational amplifiers (op-amps) OP
1
to OP
4
, and the resultant outputs are used as the remaining V
1
to V
4
. OP
1
to OP
4
operate at VEE so that VCC is not directly used when the panel drive voltage is generated.
The description now turns to power consumption, with the scan line side being denoted by Y and the data line side being denoted by X. For instance, the scan line electrodes for the panel are called Y electrodes, the driver IC that drives these Y electrodes is called the Y driver, the data line electrodes of the panel are called X electrodes, and the driver IC that drives these X electrodes is called the X driver. The voltage applied to each non-selected Y electrode is V
1
or V
4
. If the non-selected Y electrodes are at V
1
, the voltage applied to the X electrodes is V
0
or V
2
; if the non-selected Y electrodes are at V
4
, the voltage applied to the X electrodes is V
3
or V
5
.
With a duty ratio of 1/240, the Y electrode for one line alone is in a selected state; the remaining
239
lines are all in a non-selected state. Therefore, the charging/discharging current that flows between each X electrode and the selected Y electrode is much smaller than the charging/discharging current that flows between each X electrode and non-selected Y electrodes. That is to say, the current consumption of the liquid crystal panel itself is largely due to the charging/discharging currents flowing between each X electrode and the non-selected Y electrodes. Thus the description here concentrates only on the charging/discharging currents flowing between the X electrodes and the non-selected Y electrodes.
Consider, as an example, a case in which the voltage at an X electrode changes from V
0
to V
2
when the voltage of the non-selected Y electrodes is V
1
. If the capacitance of the liquid crystal layer between the X and Y electrodes is assumed to be Cpn, a charge of Cpn×(V
0
−V
1
) flows from V
0
and into V
1
when the voltage at the X electrode changes from V
0
to V
1
(see D in FIG.
48
). When the voltage at the X electrode then changes from V
1
to V
2
, a charge of Cpn×(V
1
−V
2
) flows from V
1
and into V
2
(see E). Since V
0
−V
1
=V
1
−V
2
in this example, the charge flowing into V
1
and the charge flowing out of V
1
are equal. Therefore, the balance of the charges flowing into and out of V
1
is zero, so that a charge of Cpn×(V
0
−V
2
) effectively flows from V
0
and into V
2
(see F). This charge passes through the op-amp OP
2
and eventually flows to GND (see G). However, this charge migrates within OP
2
so that it does no effective work along the path to GND, so that thermal losses are generated and OP
2
simply becomes hotter. If it is assumed that the panel charging/discharging current in this case is Ipn and GND is 0 V, the power consumption due to this Ipn is: Ipn×VEE. As is clear from G in
FIG. 48
, the effective utilization factor of Ipn is: (V
0
−V
2
)/VEE. For a duty ratio of 1/240, VEE is 20 V to 25 V when (V
0
−V
2
) is on the order of (2×1.6) V, so that the effective utilization factor is no more than 16%.
The description now turns to a power circuit used in a liquid crystal display device driven by a four-line simultaneous selection drive method, as a second prior-art technique. The basic concept of the multiple lines selection (MLS) drive method in which a plurality of Y electrodes (row electrodes) are simultaneously selected is disclosed in Document 1 (A Generalized Addressing Technique for RMS Responding Matrix LCDS, Proceedings of the 1988 International Display Research Conf, pp. 80-85) and U.S. Pat. No. 5,262,881. A simple one-line sequential drive has a problem in that contrast is degraded if the response of the liquid crystal is fast, but use of the MLS drive method can solve this problem.
When L lines (where L is a positive integer greater than 1) are simultaneously selected by an MLS drive method, it is necessary to have potentials at a total of three levels for the Y electrodes thereof: VM, and VH and VL positioned with this VM as a center potential . In this case, VM is a non-selection potential and VH and VL are selection potentials. Similarly, potentials at (L+1) levels centered on VM are necessary for the X electrodes. As L increases, the voltage amplitude VH−HL for driving the Y electrodes decreases, but conversely a large voltage amplitude is necessary for driving the X electrodes.
An example of a power circuit that could be considered when using the four-line simultaneous selection drive method is shown in FIG.
49
. The voltages necessary for driving the panel are VH and VL that act as selection voltages for the Y electrodes, VM that acts as the non-selection voltage for the Y electrodes, and V×0 to V×4 that act as drive voltages for the X electrodes. VM is the center potential of voltages applied to the panel, and the other voltages are in the following relationships: (VH−VM) (VM−VL) and (V×0−V×1) =(V×1−V×2)=(V×2−V×3)=(V×3−V×4). The center potential V×2 on the X electrode side is at the same potential as VM. For a panel with a 1/240 duty ratio, for example, (VH−VL) is on the order of 25 V and (V×0−V×1) is on the order of 1.6 V.
Input power source that is input from the exterior of the liquid crystal display device comprises VCC for the logic portions of the driver ICs and VEE (=VH−VL) for creating the liquid crystal panel drive voltages, with respect to GND as a reference potential (0 V), and, as described above, VEE is a high voltage in comparison with VCC. It should be noted that VDDy and VSSy in
FIG. 49
are voltages for the logic portion of the Y driver, and VCC and GND are connected thereto directly. Similarly, VDDx and VSSx are voltages for the logic portion of the X driver, where VDDx−VSSx=VCC if GND is 0 V. The resisting voltage necessary for the X driver is (V×0−V×4), which is on the order of 7 V for a panel with a 1/240 duty ratio, for example. VEE and GND are used without modification as VH and VL, respectively. Voltages divided by resistors R
1
to R
6
between VEE and GND, with their impedances lowered by op-amps OP
1
to OP
6
, are used as V×0 to V×4 and VSSx. To ensure that the relationship (VDDx−VSSx)=VCC is satisfied, the resistances of R
7
to R
10
are set such that R
7
=R
8
and R
9
=R
10
. OP
1
to OP
6
operate on VEE and VCC has no direct effect on the formation of the panel drive voltages.
The description now turns to the power consumption that occurs when the power circuit of
FIG. 49
is used. The voltage applied
Lewis David L.
Oliff & Berridg,e PLC
Seiko Epson Corporation
Shalwala Bipin
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