Circuit for generating a constant voltage from a plurality...

Details Circuit for generating a constant voltage from a plurality... Circuit for generating a constant voltage from a plurality...

1998-01-23

2001-01-16

Shankar, Vijay (Department: 2778)

Computer graphics processing and selective visual display system

Plural physical display element control system

Display elements arranged in matrix

C345S087000

Reexamination Certificate

active

06175349

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display apparatus and a voltage generation circuit for a liquid crystal display apparatus. More particularly, the present invention relates to a liquid crystal display apparatus for driving a pixel by applying a predetermined modulation voltage corresponding to an illuminated display/unilluminated display to a data electrode, and applying a predetermined programming voltage to a scanning electrode in line sequence, and a voltage generation circuit therefor.
2. Description of the Background Art
In these few years, a liquid crystal display is adapted in the products of various fields such as in the application of AV (Audio and Visual) and OA (Office Automation) owing to the advantages of lightweight, thin and small size, and low power consumption features.
Particularly for those driven by a battery such as a portable equipment, the requirement of reducing power consumption as much as possible is great. Development of a reflective liquid crystal display that does not use a back light of relatively great power consumption and research for reducing the power consumption of the liquid crystal display per se are in progress.
Referring to
FIG. 1
, a conventional liquid crystal display includes a display panel
1701
, a scanning electrode signal driver
1702
for applying a predetermined voltage to a scanning electrode line of display panel
1701
in line sequence, a data electrode signal driver
1703
for applying a predetermined voltage to a data electrode line according to the display information, a voltage generation unit
1706
for generating a voltage to be applied to the Ad liquid crystal display, and a control unit
1705
for providing a control signal to scanning electrode signal driver
1702
, data electrode signal driver
1703
and voltage generation unit
1706
for displaying the input information from an input signal line
1704
. Voltage generation unit
1706
includes a DC/DC converter
2101
that will be described afterwards.
Referring to
FIG. 2
, a display panel
1701
includes a plurality of pixels arranged in a matrix. Each pixel includes a liquid crystal display element
1801
connected between a corresponding scanning electrode line (Y
1
-Ym) and data electrode line (X
1
-Xn).
Referring to
FIG. 1
again, scanning electrode signal driver
1702
includes a shift transistor not shown, and an analog switch. Data electrode signal driver
1703
includes a shift register not shown, a latch circuit, and an analog switch. Scanning electrode signal driver
1702
applies a predetermined voltage to respective scanning electrode lines (Y
1
-Ym) according to a latch pulse LP and an alternating signal M.
Referring to
FIG. 3
, scanning electrode signal driver
1702
operates as set forth in the following. In response to latch pulse LP and alternating signal M from control unit
1705
, a voltage
1907
a
of a voltage value VH or a voltage
1907
d
of a voltage value VL from voltage generation unit
1706
is applied during selected periods
1903
and
1904
, and a voltage
1907
b
of a voltage value VM is applied during a nonselected period for a selected line.
As to an applied waveform
1908
of line Y
i
. a voltage
1907
a
of a voltage value VH is applied to line Y
i
during a selected period
1903
in an A frame
1901
in response to latch pulse LP and alternating signal M. In the next B frame
1902
, a voltage
1907
d
of a voltage value VL is applied during selected period
1904
in response to latch pulse LP and alternating signal M. In a nonselected state a voltage
1907
b
of a voltage value VM is applied to line Y
i
.
Application of a direct current component will cause degradation in the characteristics of the liquid crystal material in a liquid crystal display. It is therefore necessary to apply a symmetrical waveform with respect to voltage
1907
b
of voltage value VM. Therefore, voltage value VL must satisfy VH−VM=VM−VL.
As to an applied waveform
1909
of line Y
i+1
, voltage
1907
d
of voltage value VL is applied to line Y
i+1
during a selected period
1905
of A frame
1901
, and voltage
1907
a
of a voltage value VH is applied to line Y
i+1
at a selected period
1909
in B frame
1902
.
Referring to
FIG. 4
, data electrode signal driver
1703
operates as set forth in the following. In response to latch pulse LP and alternating signal M from control unit
1705
and a data signal D, a voltage
2008
a
of a voltage value V
1
sent from voltage generation unit
1706
, and a voltage
2008
b
of a voltage value VS are applied to the selected line. Here, voltage value V
1
=2×VM.
An applied waveform
2009
to the X
j
th data electrode line and an applied waveform
2001
to a position (X
j
, Y
i
) will be described. A waveform indicated by solid line
2009
a
is applied to the X
j
th data electrode line when the data corresponds to an illuminated display according to latch pulse LP, alternating signal M and data signal D. In the case of a nonilluminated display, a waveform indicated by broken line
2009
b
is applied to the X
j
th data electrode line.
For example, if the liquid crystal display element of position (X
j
, Y
i
) corresponds to an illuminated display, a voltage of voltage value VS is applied to the X
j
th data electrode line during a Y
i
line selected period
2003
in A frame
2001
, and a voltage of voltage value V
1
is applied to the X
j
th data electrode line in a Y
i
line selected period
2004
in B frame
2002
.
The applied waveform to line Y
i
is indicated by waveform
2010
. The applied waveform to a liquid crystal display element of (X
j
, Y
i
) is indicated by waveform
2011
. Solid line
2011
a
corresponds to a waveform of an illuminated display, and broken line
2011
b
corresponds to a waveform of an unilluminated display.
The value of the applied voltage to a liquid crystal display element of (X
j
, Y
i
) is |VH| and |V
1
−VL| in A frame and B frame, respectively, for an illuminated display. For an unilluminatd display, the applied voltage is |VH−V
1
| and |VL| in A frame and B frame, respectively. The applied voltage to a liquid crystal display element must be equal in A frame
2001
and B frame
2002
. Therefore,
|
VL|=|VH−V
1|
This gives us −VL=VH−V
1
. Considering the voltage of voltage value VM,
 
VH−VM=VM−VL
  (1)
is achieved by V
1
=2×VM.
Conventionally, a DC/DC converter is used in a voltage generation unit
1706
to generate a plurality of voltages having the above relationship as described in FIG.
1
.
When a DC/DC converter
2101
is used as shown in
FIG. 5
, the input of a voltage of voltage value VD results in the output of a plurality of voltage values VH, V
1
, VM, VS and VL having the above-described relationship.
The above-described DC/DC converter has a disadvantage that the voltage conversion efficiency at the low current area is extremely low as shown in FIG.
6
. This means that only a conversion efficiency of 15-25% can be achieved when the output current is 1-2 mA. Furthermore, a DC/DC converter occupies a definite size since it is a hybrid IC. There was a disadvantage that the size of the mounting substrate must be greater than the size of the DC/DC converter.
The present information society has seen significant development in portable information equipments. The demand for a lighter, thinner, and smaller display with lower power consumption seems insatiable in the field of portable information equipment. However, the usage of a DC/DC converter results in a great loss of power with constraints in the physical dimension, so that these requirements cannot be met. There is also a problem that the cost of the product cannot be reduced since a DC/DC converter is expensive.
SUMMARY OF THE INVENTION
In view of the foregoing, an object of the present invention is to provide a voltage generation c

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