Liquid crystal driving devices

Details Liquid crystal driving devices Liquid crystal driving devices

2002-06-28

2004-08-31

Lao, Lun-Yi (Department: 2673)

Computer graphics processing and selective visual display system

Plural physical display element control system

Display elements arranged in matrix

C345S089000, C345S100000, C345S690000, C345S204000, C345S545000, C345S536000

Reexamination Certificate

active

06784868

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to liquid crystal driving devices for driving matrix-type liquid crystal display panels having a tone display capability.
BACKGROUND OF THE INVENTION
Conventionally, there have been proposed various techniques for reducing power consumption of liquid crystal display devices. For example, Japanese Publication for Unexamined Patent Application No. 281933/1997 (Tokukaihei 9-281933) (published on Oct. 31, 1997) discloses a technique in which a display memory is installed in a driver LSI of the liquid crystal panel to limit external access to the display memory when displaying a still image in particular, so that the power consumption of the liquid crystal display device can be reduced.
Such a liquid crystal display device is described below with reference to FIG.
10
. It is assumed here that a liquid crystal panel
1003
is used in a matrix-type liquid crystal display device and is AC-driven according to a line-sequential scanning method and a voltage average method, both of which have been commonly used in this type of liquid crystal display.
The liquid crystal panel
1003
has such a structure that an STN liquid crystal element is placed between a pair of glass substrates, wherein, for example, a plurality of segment lines and a plurality of common lines are respectively disposed on the inner surfaces of the glass substrates orthogonal to each other, and each intersection of the segment lines and the common lines makes up a pixel.
Before explaining black-and-white display operations of the liquid crystal display device, an explanation will be given to a liquid crystal driver for use in liquid crystal display devices for portable phones and the like, in which a common driver, a segment driver, a display data memory, and a control circuit of the display data memory are packaged in one chip, for example.
A liquid crystal driver
1002
is provided therein a display data memory
1006
, which corresponds one to one to the pixels of the liquid crystal panel
1003
. For example, in order to carry out display in 248×68 dots, the display data memory
1006
requires a capacity of 248×68=16864 bits.
The liquid crystal driver
1002
outputs a segment signal that turns the display data memory
1006
to either 0 or 1 for ON/OFF of each pixel of the liquid crystal panel
1003
. A line address counter
1009
selects data of one line of the liquid crystal panel
1003
from the display data memory
1006
, and the segment signal is sent to the segment driver
1004
in the form of data so selected.
The segment driver
1004
, in response to the segment signal, outputs a voltage for driving the liquid crystal panel
1003
according to the display data and feeds it to the segment of the liquid crystal panel
1003
.
The display data is applied to the display data memory
1006
according to the data bus width of a CPU
1001
, by incrementing the X address and Y address of an X address counter
1007
and a Y address counter
1008
.
A common driver
1005
successively outputs line by line a scanning pulse that indicates an ON line of the liquid crystal panel
1003
and feeds it to the common of the liquid crystal panel
1003
. ON pixels and OFF pixels of a line are decided by the voltages applied to the common and segment of the liquid crystal panel
1003
. By successively driving the common lines, any characters or graphics can be displayed on the liquid crystal panel
1003
.
As described, display is carried out on the liquid crystal panel
1003
by the display data memory
1006
, the line address counter
1009
, the segment driver
1004
, and the common driver
1005
. Therefore, no transfer of display data is required between the CPU
1001
and the liquid crystal driver
1002
so long as the display remains the same. Further, a change of display is accompanied by data transfer that can be separately carried out from the transfer of display data that is used for the display of the liquid crystal panel
1003
. This allows for a slower data transfer speed and thereby reduces power consumption.
The display data memory
1006
corresponds to the pixels, one bit per pixel, so that two-level display of black and white is carried out.
Multi-tone display that displays intermediaries between black and white, rather than two levels of black and white, can be carried out by a frame rate control (FRC) method which realizes multi-tone display by varying the number of times each pixel of the liquid crystal panel
1003
is switched ON when displaying a single display data with a period of plural frames, or by a pulse modulation method in which a pulse width is varied in one frame to change ON time.
In order to carry out multi-tone display by these methods, the capacity of the display data memory
1006
needs to be increased to increase the number of bits per pixel of the memory, and the tone data of each pixel needs to be stored. For example, 3 bits are required for each pixel in display of 8 tones, and 6 bits for 64 tones and 8 bits for 256 tones.
Miniaturization of LSIs has advanced over the last years and it is now possible to increase the capacity of the display data memory
1006
to readily accommodate multi tones and multi colors.
However, increasing the number of display pixels and thus the number of common lines in the liquid crystal panel
1003
shortens the ON time of the liquid crystal pixels in one frame. Thus, when the liquid crystal panel
1003
is used to carry out large-screen or high-resolution display by the frame rate control method or pulse modulation method, flicker is caused in a display of tones with a short ON time and a poor display quality results.
In order to avoid such a problem, there has been proposed a driving method known as a dual-scan display mode, in which two segment drivers are used to simultaneously and respectively drive the upper and lower segments that have been prepared by dividing the display screen of the liquid crystal panel, i.e., the segment lines, into upper and lower parts.
Note that, a simple-matrix display mode in which the display screen of the liquid crystal panel is not divided into upper and lower parts will be called a single-scan display mode.
The dual-scan display mode, compared with the single-scan display mode, requires two driving circuits for the upper part and lower part of the liquid crystal panel, and therefore increases the scale of LSI mounting and the complexity of the driving circuits.
The advantage of the dual-scan display mode over the single-scan display mode, however, is that an ON time twice as long as that of the single-scan display mode (1/2 duty ratio) can be obtained. This prevents flicker on the display screen and improves display quality.
Meanwhile, in order to carry out multi-tone and multi-color display in the matrix-type STN liquid crystal display device while at the same time suppressing power consumption, a driver with a large display memory capacity needs to be used for the driving in the dual-scan display mode.
While such a liquid crystal display device for displaying a high-quality and multi-tone image is in demand, there has also been a demand for low-cost liquid crystal display devices which do not require many tones for display.
It is therefore advantageous, in terms of productivity and cost, to share the driver between these two different types of liquid crystal display devices.
However, an increase of display memory capacity is generally associated with an increase in unit cost of the driver, which increases production cost of the liquid crystal display device. Also, display panels with a small number of tones, because of their low unit sale price, cannot generally afford expensive drivers with a large memory capacity for displaying multi tones. It is therefore required to prepare and use a plurality of different types of drivers having different display memory capacities, according to different numbers of tones required for the display, depending on the use of the driver.
That is, two kinds of drivers are required: one for the liquid crystal display device

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