Computer graphics processing and selective visual display system – Display driving control circuitry – Display power source
Reexamination Certificate
1997-12-05
2001-05-01
Luu, Matthew (Department: 2779)
Computer graphics processing and selective visual display system
Display driving control circuitry
Display power source
C345S212000, C345S204000, C327S544000
Reexamination Certificate
active
06225992
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to liquid crystal display (LCD) devices, and more particularly, to an apparatus and method for generating a set of bias voltages for an LCD driver to drive an LCD panel, which can provide a large current for a voltage divider to provide adequate bias voltages to the LCD driver at the instant when the LCD waveforms are being switched from one state to another, and a small current in other times for reduced power consumption to save energy.
2. Description of Related Art
Liquid crystal display (LCD) devices are digital display devices widely used on digital watches, palmtop game machines, and various other electronic instruments for display of data or graphics thereon.
FIG. 1
is a schematic diagram of a conventional LCD device, which includes an LCD driver
10
for driving an LCD panel
14
to display data thereon. The LCD driver
10
is coupled to a voltage divider
12
consisting of a number of serially connected 100 k&OHgr; resistors which can divide an external system voltage V
CC
into a number of apportioned voltages [V
1
, V
2
, V
3
, V
4
] which serve as bias voltages for the LCD driver
10
to output a plurality of analog LCD waveforms, including, for example, eight common signals COM
1
-COM
8
and a number of segment signals SEG
1
-SEG
40
, to the LCD panel
14
. These LCD waveforms represent the data or graphics that are to be displayed on the LCD panel
14
.
Typically, the system voltage V
CC
is supplied by a battery unit. However, one drawback to batteries is that the output voltage thereof will be constantly decreasing during use. A brand new battery unit having an output voltage of 1.7 V (volt) at the beginning, for example, will be decreased in the output voltage to 1.2 V after a period of use. Therefore, for a battery unit consisting of three serially connected batteries, the output voltage thereof will be gradually decreased from 5.1 V (1.7V×3=5.1V) to 3.6 V (1.2V×3=3.6V) during the period of use. In a game machine (for example the BRICK GAME machine) having a resolution of 320 dots, the contrast ratio of the LCD panel thereof will be optimal when the output voltage of the battery unit is within the range from 3.8 V to 4.6 V. The contrast ratio will be overly high when the output voltage of the battery units is over 4.6 V during the beginning of use, and then become inadequate when the output voltage of the battery units is below 3.8 V near the end of the life of use.
The system voltage V
CC
is divided by the voltage divider
12
shown in
FIG. 1
into a number of bias voltages [V
1
, V
2
, V
3
, V
4
] for the LCD driver
10
to drive the LCD panel
14
. By conventional method, a variable resistor V
R
is connected in series to the voltage divider
12
for adjusting the magnitude of a DC current I
d
(hereinafter referred to as bias current) flowing through the resistors in the voltage divider
12
. The resistance of V
R
is typically adjusted to a large value, but this will cause the bias voltage I
d
to be low. A conventional solution to this problem is to provide an array of capacitors [C
1
, C
2
, C
3
, C
4
] connected to the nodes where the bias voltages [V
1
, V
2
, V
3
, V
4
] are produced. These capacitors [C
1
, C
2
, C
3
, C
4
] can stabilize the bias voltages [V
1
, V
2
, V
3
, V
4
] and also allow for a large magnitude to the bias current I
d
at the instant when the LCD waveforms are being switched from one state to another.
The provision of the capacitors [C
1
, C
2
, C
3
, C
4
] shown in
FIG. 1
, however, needs an increased number of I/O pads on the IC chip of the LCD driver to connect, and represents an increase in component cost. For low-priced palmtop game machines, this means an increase in the manufacturing cost, which will cause the products less competitive on the market. Moreover, under the pad limit requirements, the increased number of I/O pads will force the manufacturer to use low-end fabrication processes (such as the 0.8 &mgr;m technology) instead of advanced ones (such as the 0.6 &mgr;m technology) to fabricate the IC chip of the LCD. In other words, when there is a limit to the number of I/O pads, the feature size of the IC chip cannot be further reduced even though the 0.6 &mgr;m fabrication process is used instead of the 0.8 &mgr;m technology.
It is, therefore, a primary research effort in the semiconductor industry to find a solution which allows the elimination of the above-mentioned capacitors (i.e., the capacitors [C
1
, C
2
, C
3
, C
4
] shown in
FIG. 1
) coupled to the LCD driver so as to reduce the component cost and also allow the use of the more advanced 0.6 &mgr;m technology to fabricate the LCD IC chip.
One solution is to lower the resistance of the resistors in the voltage divider so as to raise the magnitude of the bias current I
d
flowing through the voltage divider, thus allowing for an adequate level for the bias voltages supplied to the LCD driver. An inadequate level for the bias voltage would cause spikes to occur in the LCD waveforms.
One drawback to the foregoing solution, however, is that the bias current I
d
will be excessive that causes unnecessary power consumption and thus a waste of energy. For example, assume V
CC
=5V and 100 k&OHgr; resistors are used to constitute the voltage divider
12
, the bias current I
d
flowing through the voltage divider
12
shown in
FIG. 1
will be
I
d
=V
CC
/(100×5) k&OHgr;=5/500=10 &mgr;A (microampere).
However, when 15 k&OHgr; resistors are used in place of the 100 k&OHgr; resistors in the voltage divider
12
, the bias current I
d
will become
I
d
=V
CC
/(15×5) k&OHgr;=5/75=67 &mgr;A,
which is significantly much larger than the previous 10 &mgr;A current. This large amount of current is not useful for the operation of the LCD driver
10
but wasted instead. This causes unnecessary power consumption.
The provision of capacitors coupled to the LCD driver is also an impractical scheme since it is impossible to provide an adequate capacitance to capacitors in an IC chip which is very small in size. To do this, the size of the IC chip will become large, which is usually not desired.
One practical solution to the foregoing problem is to connect a variable resistor V
R
in series to the voltage divider
12
as illustrated in
FIG. 1
, so as to adjust for a suitable level for the bias voltages. For example, when the output voltage of the battery unit exceeds 4.6 V, the variable resistor V
R
can be adjusted until the level of V
LCD
is lowered to 4.2 V (which is the optimal level for the LCD driver
10
). On the other hand, when the output voltage of the battery unit is below 4.2 V, the variable resistor V
R
can be adjusted to zero resistance so as to provide the maximum possible level for V
LCD
.
The foregoing solution of using the variable resistor V
R
, however, is still not considered a satisfactory one to provide the best adjustment for the bias current. In view of this, an automatic brightness control apparatus for an LCD device is disclosed in ROC Publication No. 231,148. This patent has two preferred embodiments, respectively illustrated schematically in FIG.
2
and FIG.
3
. As shown, the patent of ROC Publication No. 231,148 includes a microprocessor
20
, a voltage divider
21
, a resistor circuit
22
, and an LCD panel
23
. The microprocessor
20
is a 4-bit unit having a pair of input ports P
8
.
0
, P
8
.
1
connected to the voltage divider
21
and a pair of output ports ALCD
1
, ALCD
2
connected to the resistor circuit
22
. Further, the microprocessor
20
has a brightness control port VLCD connected via an internal resistor to a voltage source V
DD
. The output of the VLCD port is controllable by adjusting the resistance of the resistors R
1
, R
2
in the resistor circuit
22
so as to allow the LCD panel
23
to display data with a desired brightness and contrast.
The foregoing patent, however, has several
Chen Wesley
Cheng Michael
Hsu Jerry
Hung Chris
Blackman Anthony J.
Luu Matthew
Rabin & Champagne, P.C.
United Microelectronics Corp.
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