Computer graphics processing and selective visual display system – Display driving control circuitry – Display power source
Patent
1996-09-11
1999-11-16
Luu, Matthew
Computer graphics processing and selective visual display system
Display driving control circuitry
Display power source
345 89, 345 95, G09G 336, G09G 500
Patent
active
059866497
DESCRIPTION:
BRIEF SUMMARY
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, V0 to V5 are in the relationship VD=(V0-V1)=(V1-V2)=(V3-V4) =(V4-V5), 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 V0 to V5, VEE is used without modification as V0 and GND as V5. V1+0V4 are obtained by division by resistances R1 to R5 between VEE and GND, the impedances of these outputs are lowered by operational amplifiers (op-amps) OP1 to OP4, and the resultant outputs are used as the remaining V1 to V4. OP1 to OP4 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 V1 or V4. If the non-selected Y electrodes are at V1, the voltage applied to the X electrodes is V0 or V2; if the non-selected Y electrodes are at V4, the voltage applied to the X electrodes is V3 or V5.
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 V0 to V2 when the voltage of the non-selected Y electrodes is V1. If the capacitance of the liquid crystal layer between the X and Y electrodes is assumed to be Cpn, a charge of Cpn.times.(V0-V1) flows from V0 and into V1 when the voltage at the X electrode changes from V0 to V1 (see D in FIG. 48). When the voltage at the X electrode then changes from V1 to V2, a charge of Cpn.times.(V1-V2) flows from V1 and into V2 (see E). Since V0-V1=V1-V2 in this example, the charge flowing into V1 and the charge flowing out of V1 are equal. Therefore, the balance of the charges flowing into and out of V1 is zero, so that a charge of Cpn.times.(V0-V2) effectively flows from V0 and into V2 (see F). This charge passes through the op-amp OP2 and eventually flows to GND (see G). However, this charge migrates within OP2 so that it does no effective work along the path to GND, so that thermal losses are generated and OP2 simply becomes hotter. If it is assumed that the panel charging/discharging current in this case is Ipn and GND is 0V, the power consumption due to this Ipn is: Ipn.times.VEE. As is clear from G in FIG. 48, the effective utilization factor of Ipn is: (V0-V2)/VEE. For a duty
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Lewis David L.
Luu Matthew
Seiko Epson Corporation
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