Method and circuit for controlling contrast in liquid...

Details Method and circuit for controlling contrast in liquid... Method and circuit for controlling contrast in liquid...

2000-03-27

2003-07-29

Chow, Dennis-Doon (Department: 2675)

Computer graphics processing and selective visual display system

Plural physical display element control system

Display elements arranged in matrix

C345S095000, C345S210000

Reexamination Certificate

active

06600466

ABSTRACT:

FIELD OF THE INVENTION
This invention is in the field of electronic circuits and is more particularly related to biasing circuits for LCD drivers.
BACKGROUND OF THE INVENTION
Liquid crystal display (LCD) materials are well known by those skilled in the art of electronic design. LCD materials obey an optical response curve as shown in prior art FIG.
1
. On the X-axis is the RMS (root mean squared) voltage across a pixel of the LCD material. On the Y-axis is the reflectance of the LCD pixel. The lower the reflectance, the darker the pixel. A “1” on the reflectance axis represents 100% light reflected (the pixel is off). A “0” on the reflectance axis represents 100% light absorbed and the pixel is on. Practically, 100% reflectance or absorption is not achieved and designers operate about the points labelled V
OFF
and V
ON
. A designer must ensure that the RMS driving voltage driving each individual pixel falls within this critical transition region to achieve adequate LCD contrast. However, the location of the transition region of the optical repsonse curve is a strong function of the LCD material. Therefore as LCD materials vary, so to does the location of the curve's transition region. Bias circuits attempt to generate bias voltages that satisfy the appropriate threshold magnitudes (V
OFF
and V
ON
) across all LCD operating voltages and LCD material variations.
FIG. 2
is a prior art LCD bias circuit
10
, that generates a plurality of bias voltages, V
LCD1
, V
LCD2
, V
LCD3
, V
LCD4
and V
LCD5
. A resistor ladder consisting of matched resistors labelled R
1
and resistor R
2
establish the voltage ratios of the bias voltages. For example, if R
1
=100K and R
2
=270K the following ratios are established between the bias voltages:
V
LCD1
=0.85(
V
DD
−V
LCD5
)+
V
LCD5
,
V
LCD2
=0.70(
V
DD
−V
LCD5
)+
V
LCD5
,
V
LCD3
=0.30(
V
DD
=V
LCD5
)+
V
LCD5
,
V
LCD4
=0.15(
V
DD
=V
LCI5
)+
V
LCD5
.
Therefore the bias voltages in prior art circuit
10
are a function of the value of V
LCD5
. The bias voltages V
LCD1
−V
LCD4
are fixed by the establishment of V
LCD5
. Operational amplifiers
12
,
14
,
16
and
18
are unity gain buffers. LCD bias, which is defined by [(V
LCD3
−V
LCD5
)/2]/V
LCD
. Substituting V
LCD3
above into the equation for bias and simplifying, one obtains a constant (0.15). Bias is therefore fixed in the prior art solution.
The voltage value of V
LCD5
is controlled by a voltage doubler circuit
22
in conjunction with a contrast control circuit
20
. Contrast control circuit
20
is a 32 bit linear control circuit that varies the voltage at node V linearly between 0V and V
DD
.
This design solution is undesirable because variations in LCD voltage cause a shift in V
OFF
and therefore move the operating point outside the transition region. This design alters the contrast manually with a contrast knob or with keystrokes which effectuates the 32 bit control. Therefore contrast control must be manipulated manually. Further, the voltage output of the clock doubler circuit (V
LCD5
) is unregulated, causing it to vary as batteries wear and LCD loadings change. Regulation of voltages V
DD
and V
LCD5
is expensive because it requires further voltage regulation circuitry. Further still, Q
1
within contrast control circuit
20
draws substantial current resulting in inefficient power loss.
It, accordingly, is an object of this invention to provide a circuit and method of dynamically monitoring and controlling the LCD bias so that as LCD operating voltage varies, LCD bias may be dynamically adjusted to provide proper V
OFF
voltage, thereby overcoming the difficulties and limitations of the prior art. Other objects and advantages of the invention will be apparent to those of ordinary skill in the art having reference to the following specification and drawings.
SUMMARY OF THE INVENTION
A method of controlling contrast in LCDs using dynamic LCD biasing includes the step of identifying an expected bias function as a function of LCD material, LCD operating voltage, and LCD duty cycle. The expected bias function is then approximated to obtain a linear description of the expected bias function. A voltage is generated that follows the linear description of the expected bias function. The step of generating the voltage results in dynamic LCD biasing.


REFERENCES:
patent: 5250937 (1993-10-01), Kikuo et al.
patent: 5404150 (1995-04-01), Murata

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