RLCD transconductance sample and hold column buffer

Details RLCD transconductance sample and hold column buffer RLCD transconductance sample and hold column buffer

2002-09-11

2004-04-06

Shankar, Vijay (Department: 2673)

Computer graphics processing and selective visual display system

Plural physical display element control system

Display elements arranged in matrix

C345S087000

Reexamination Certificate

active

06717564

ABSTRACT:

TECHNICAL FIELD
This invention pertains to the field of electronic circuits for driving reflective liquid crystal displays (RLCD).
BACKGROUND AND SUMMARY OF THE INVENTION
In an RLCD having a matrix of m horizontal rows and n vertical columns, each m-n intersection forms a cell or picture element (pixel). By applying an electric potential difference, such as 7.5 volts (v), across a cell, a phase change occurs in the crystalline structure at the cell site causing the pixel to change the incident light polarization vector orientation, thereby blocking the light from emerging from the electro-optical system. Removing the voltage across the pixel causes the liquid crystal in the pixel structure to return to the initial “bright” state. Variations in the applied voltage level produce a plurality of different gray shades between the light and dark limits.
FIG. 1
illustrates an example block diagram of a conventional column driving arrangement for an RLCD device. A column driver
18
provides a ramp voltage to each of a plurality of column lines
20
, progressively applying a voltage corresponding to each gray-scale level. A counter
12
sequentially progresses through each gray-scale value, typically 0-256, although other levels of gray-scale resolution may be provided. A look-up-table LUT
14
maps each gray-scale value to a voltage that corresponds to this value; this mapping is a function of the particular RLCD, and is typically non-linear. The voltage value is converted to an analog voltage level by a digital-to-analog converter (DAC)
16
, and this analog voltage provides the input to the driver
18
. As discussed further below, the driver
18
is typically a high-current device.
The load that each column line
20
presents to the driver
18
is represented as a capacitance
28
, which represents the sum of the capacitances of the individual pixels in the column and the capacitance of the lines to these pixels. Each column line
20
includes a switch
26
that serves as a sample-and-hold gate, wherein the capacitance
28
serves as the “hold” storage element. Each column switch
26
is controlled by a comparator
24
that compares the current count of the counter
12
to the desired gray-scale level for the column, which is stored in a data memory
22
. When the count from the counter
12
reaches the desired gray-scale level for the column, the comparator
24
opens the switch
26
, placing the capacitance
28
in the hold-state, holding the current value of the ramp voltage from the driver
18
. Not illustrated, a row-controller subsequently applies the voltage on the capacitance
28
to the pixel at the intersection of the column and the selected row.
At the end of each row-cycle, all of the capacitances
28
are discharged and the above process is repeated. Because this discharge must occur quickly (typically within 30 nanoseconds), and must discharge all of the capacitances
28
(typically 5-10 nanofarads), the peak current of the discharge can be as high as a few amperes. In a conventional RLCD, the driver
18
is configured to provide this high-current capacity.
A number of drawbacks can be attributed to the conventional RLCD column driver arrangement of FIG.
1
. As noted above, the driver
18
must be configured to accommodate a high discharge current. Additionally, when each switch
26
is opened, a transient is fed back to the driver
18
from the gate of the switch
26
. This transient can be substantial, particularly when a large number of switches
26
open simultaneously, such as when a line segment of uniform gray-scale is being displayed. This transient modifies the voltage level from the driver
18
, causing it to differ from the voltage provided by the LUT
14
corresponding to the current gray-scale value in the counter
12
. Any columns that have not yet entered the hold-state will receive this erroneous voltage, and will display an improper gray-scale level. This transient effect is commonly termed “horizontal crosstalk”. Further, the common connection of multiple column lines
20
to the driver
28
provides a substantial “antenna”, and is susceptible to noise transients as well.
In this invention, a column driving arrangement for an RLCD device is provided that isolates the source of a ramp voltage corresponding to gray-scale levels from the sample-and-hold gates of the individual columns. Preferably, this isolation is provided by an operational transconductance amplifier (OTA) at each column that provides a controlled current for charging the column capacitance to the appropriate gray-scale voltage level. The capacitor effects an integration of the current, thereby providing a noise-filtering effect. Additionally, a each column capacitance is individually discharged, thereby obviating the need for a common high-current discharge device.


REFERENCES:
patent: 5459483 (1995-10-01), Edwards
patent: 6091390 (2000-07-01), Sim
patent: 6148048 (2000-11-01), Kerth et al.
patent: 6369853 (2002-04-01), Merrill et al.
patent: 6476864 (2002-11-01), Borg et al.
patent: 6489904 (2002-12-01), Hisano
patent: 6512544 (2003-01-01), Merrill et al.

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