Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Reexamination Certificate
1999-10-22
2002-07-30
Wong, Dong (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C345S041000, C345S051000, C345S052000, C345S055000, C345S210000
Reexamination Certificate
active
06426594
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for driving an electro-optical device such as a liquid crystal display device, a driving circuit for driving an electro-optical device, an electro-optical device, and an electronic apparatus.
2. Description of Related Art
Related Art
A first background art associated with a method for driving a liquid crystal display device (based on a multi-line selection) is disclosed in International Application published as WO93/18501. In this method for driving a liquid crystal display device, a liquid crystal display panel includes scanning electrodes and signal electrodes arranged in a matrix such that the scanning electrodes and signal electrodes intersect each other, and pixels are formed in a matrix at intersections thereof. The scanning electrodes are organized into groups, each group consisting of a particular number of scanning electrodes which are selected at the same time, and the scanning electrodes are sequentially selected on a group-by-group basis.
FIG. 6
illustrates an example of a set of waveforms for the case where four lines of scanning electrodes (four scanning electrodes) are selected at a time according to this driving method. In
FIG. 6
, Y
1
to Y
8
denote the waveforms of scanning voltages applied to the scanning electrodes, and X
1
denotes the waveform of a signal voltage applied to a signal electrode. A selection voltage V
3
or −V
3
is applied to the scanning electrodes for a selection period (H) of each of four fields
1
f
-
4
f
of one frame (F).
In this driving method, when there are a relatively large number of scanning electrodes, a liquid crystal of type 2 indicated in root-means-square voltage luminance characteristic of liquid crystal shown in
FIG. 4
having a small value in terms of (saturation voltage)/(threshold voltage)=(Vs
2
/Vt
2
) is employed although a large driving voltage is required. In the case where there are a small number of scanning electrodes (for example when there are no more than about 32 scanning electrodes), a liquid crystal of type 1 having a low threshold voltage and having a large value in terms of (saturation voltage)/(threshold voltage)=(Vs
1
/Vt
1
) is employed so that the liquid crystal can be driven by a low voltage.
The operation of driving a liquid crystal of type 2 in accordance with the conventional method shown in
FIG. 6
is discussed below. Herein, the liquid crystal is assumed to be driven by voltages which give a maximum value in terms of the ratio of the root-means-square value of on-voltage to the root-means-square value of off-voltage. More specifically, if a liquid crystal of type 2 with a threshold voltage Vt
2
of 2.2 V is used and if the liquid crystal panel includes 64 lines of scanning electrodes, then V
3
is set to about 6.7 V, and V2 to about 3.35 V. In the case where there are 120 scanning lines to be driven, V
3
is set to about 8.9 V, and V2 to about 3.26 V. In any case, seven levels of driving voltages are required. Besides, the scanning electrode driving circuit is needed to output a high selection voltage. Thus, the difference between the selection voltage output from the scanning electrode driving circuit and the signal voltage output from the signal electrode driving circuit becomes great.
As a result, the conventional driving method requires a complicated power supply circuit and consumes a large amount of electric power. Furthermore, it is difficult to form both the scanning electrode driving circuit and the signal electrode driving circuit on a single IC chip. Referring to
FIG. 14
, a conventional power supply circuit is described below.
In this power supply circuit, a single input voltage Vcc relative to a ground voltage GND is input. A latch pulse LP is also input to the power supply circuit. Using Vcc and GND as power supply and in response to the latch pulse LP, a clock generator
21
generates a plurality of clock signals with different timing used by charge pump circuits. A negative sixfold boosting circuit
22
multiplies GND with respect to Vcc by 6 in a negative direction by means of charge pumping, thereby generating a voltage VEE. When Vcc=3.3 V, VEE becomes −16.5 V. In accordance with VEE, a contrast adjacent circuit
23
generates a selection voltage −V
3
which gives optimum contrast. This selection voltage −V
3
serves as a negative selection voltage applied to the scanning electrodes. A twofold boosting circuit
24
multiplies GND with respect to the selection voltage −V
3
by 2 by means of charge pumping thereby generating a positive selection voltage V
3
. A negative twofold boosting circuit
25
multiplies GND with respect to Vcc by 2 in the negative direction by means of charge pumping thereby generating a voltage −V
2
. ½ dropping circuits
26
and
27
generate V
1
by equally dividing between voltages Vcc and GND, and also generate −V
1
by equally dividing between voltages GND and (−V
2
), by a charge pumping operation. GND is directly employed as a center voltage VC. A voltage V
2
which is symmetric to −V
2
about GND is generated by directly employing Vcc. Thus, all voltages required to drive the liquid crystal panel are obtained. In this power supply circuit, output voltages V
3
, V
2
, V
1
, VC, −V
1
, −V
2
, −V
3
are symmetric about GND. A circuit
28
generates a voltage which is higher than −V
3
by Vcc and supplies the resultant voltage as a logic voltage −VDDy to the scanning electrode driving circuit.
In the conventional technique, seven levels of driving voltages used to drive the liquid crystal display device are generated in the above-described manner using the power supply circuit. However, as described above, the power supply circuit needs a very complicated circuit configuration.
The liquid crystal of type 1 shown in
FIG. 4
with a smaller threshold voltage is also used because this type of liquid crystal can be driven with a smaller voltage and thus consumes lower power. However, although liquid crystal display devices with such a liquid crystal having a low threshold voltage can be driven by a low voltage, the ratio of the root-means-square value of on-voltage to the root-means-square value of off-voltage applied to the liquid crystal is large, and thus, it is difficult to deal with a large number of scanning lines. If an attempt to drive a large number of scanning electrodes is made, degradation in contrast and irregularity results. Therefore, the upper practical limit of the number of scanning lines which can be driven is about 16 to 32.
In the conventional optimized amplitude selective addressing method, each scanning electrode is selected once during each frame period. In contrast, in the driving method in which a plurality of lines are selected at a time, selection periods are equally distributed in terms of time over each frame, while retaining normal orthogonality in the selection of scanning lines. Furthermore, in this method, scanning electrodes are selected in such a manner that a particular group (block) including a predetermined number of scanning electrodes is selected at a time, so that selected scanning electrodes are spatially distributed. Herein, the term “normal” means that all scanning voltages have an equal root-means-square value amplitude) during each frame period.
The term “orthogonal” means that when the amplitude of a voltage applied to a particular scanning electrode is multiplied by and added to the amplitude of a voltage applied to another arbitrary scanning electrode for respective selection periods over one frame period, the sum of the voltage amplitudes becomes 0. In simple matrix liquid crystal display devices, normal orthogonality is an essential prerequisite to the operation of turning each pixel on and off, independently of each other.
(Second Related Art)
A second background technique in the art of electro-optical devices such as a liquid crystal device is disposing a driving circuit in a single-chip f
Oliff & Berridg,e PLC
Seiko Epson Corporation
Tran Thuy Vinh
Wong Dong
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