Coded data generation or conversion – Analog to or from digital conversion – Digital to analog conversion
Reexamination Certificate
1999-12-16
2002-04-16
Jeanpierre, Peguy (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
Digital to analog conversion
C345S098000, C345S211000
Reexamination Certificate
active
06373419
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a DA converter incorporated in liquid crystal driving and other devices, and further relates to a liquid crystal driving device incorporating such a DA converter.
BACKGROUND OF THE INVENTION
DA (digital-to-analogue) converter externally receives a digital signal and convert it to an analogue signal. For example, in a liquid crystal driving device of an active matrix liquid crystal display device, a DA converter is used to convert input signals, which constitute display data, from digital to analogue for an output to a liquid crystal display section. Some DA converters of this kind include an OP-amplifier composed of MOS transistors.
The following description will explain an arrangement of a liquid crystal display device incorporating the aforementioned DA converter, an arrangement of a TFT liquid crystal panel in that liquid crystal display device, a liquid crystal drive waveform of the liquid crystal panel, and an arrangement of a source driver provided to the liquid crystal display device, in reference to FIG.
7
through
FIG. 11
which illustrate an arrangement in accordance with the present invention. In reference to FIG.
12
and
FIG. 13
, the following description will also explain a conventional arrangement of the aforementioned DA converter. Note in the following description that the present invention and the conventional technology share those common arrangements as presented in FIG.
7
through FIG.
11
. The foregoing conventional arrangement is described in
CMOS Device Handbook
, edited by the Editing Committee for CMOS Device Handbook and published by The Nikkan Kogyo Shimbun Ltd. on Sep. 29, 1987.
FIG. 7
constitutes a block diagram showing an arrangement of a TFT (Thin Film Transistor) liquid crystal display device as a typical active matrix liquid crystal display device.
This liquid crystal display device is divided into two parts: a liquid crystal display section and a liquid crystal drive circuit (liquid crystal driving device) for driving the liquid crystal display section. The liquid crystal display section includes a TFT liquid crystal panel
901
; the liquid crystal panel
901
includes therein liquid crystal display elements (not shown) and opposite electrodes (common electrodes)
906
(will be mentioned in detail later).
Meanwhile, the liquid crystal drive circuit includes source drivers
902
and gate drivers
902
, each driver being built as an IC (Integrated Circuit), a controller
904
, and a liquid crystal drive power supply
905
. The controller
904
provides display data D and control signals SI to the inputs of the source drivers
902
, and control signals S
2
to the inputs of the gate drivers
903
. Hence, the controller
904
provides vertical synchronized signals to the gate drivers
903
and horizontal synchronized signals to the source drivers
902
and the gate drivers
903
.
The externally provided display data is transmitted through the controller
904
to the source drivers
902
as the display data D with its digital form being retained. The source driver
902
time-sequentially latches the incoming display data, and thereafter, converts the display data from digital to analogue in synchronization with the horizontal synchronized signals received from the controller
904
. Then, the analogue voltages (half-tone display voltages) obtained from the DA conversion for half-tone display use are transmitted via liquid crystal drive voltage output terminals and source signal lines
1004
(will be mentioned in detail later) to the respective liquid crystal display elements (not shown) in the liquid crystal panel
901
.
FIG. 8
shows an arrangement of the liquid crystal panel
901
, including pixel electrodes
1001
, pixel capacitors
1002
, TFTs
1003
as elements for turning on/off voltage application to the pixels, source signal lines
1004
, gate signal lines
1005
, and opposite electrodes
1006
(equivalent to the opposite electrodes
906
in FIG.
7
). In
FIG. 8
, the encircled area ‘A’ represents a liquid crystal display element for one pixel. The source drivers
902
couple half-tone display voltages to the source signal lines
1004
according to brightness of the pixels used for a display. The gate drivers
903
couple scan signals to the gate signal lines
1005
so as to sequentially turn on the vertically lined TFTs
1003
. Through the TFT
1003
which is in an On state, the voltage in the source signal line
1004
is applied to the pixel electrode
1001
connected to the drain of that TFT
1003
, causing accumulation of charges in the pixel capacitor
1002
formed between the pixel electrode
1001
and the opposite electrode
1006
. The accumulation of charges alters the optical transmittance of the liquid crystal and realizes a display.
FIG.
9
and
FIG. 10
show liquid crystal drive waveforms as examples.
1101
and
1201
each denote a drive waveform of the source driver
902
.
1102
and
1202
each denote a drive waveform of the gate driver
903
.
1103
and
1203
each denote a potential of the opposite electrode.
1104
and
1204
each denote a voltage waveform of the pixel electrode. The voltage applied across the liquid crystal material is equivalent to the potential difference between the pixel electrode
1001
and the opposite electrode
1006
, which is shown as shaded areas in FIG.
9
and FIG.
10
. For example, in
FIG. 9
, the TFT
1003
is turned on when the drive waveform
1102
of the gate driver is in high level, causing the difference between the drive waveform
1101
of the source driver and the potential of the opposite electrode
1103
to be applied to the pixel electrode
1001
. Subsequently, the drive waveform
1102
of the gate driver changes to low level, causing the TFT
1003
to change to an Off state. Here, the aforementioned voltage is retained across the pixel due to the presence of the pixel capacitor
1002
. The same explanation holds true with the case in FIG.
10
. FIG.
9
and
FIG. 10
show different voltages being applied across the liquid crystal material: a higher voltage is applied in the case shown in
FIG. 9
than in the case shown in FIG.
10
. In this manner, a multiple half-tone display is achieved by applying variable analogue voltage across the liquid crystal and thus altering the optical transmittance of the liquid crystal in an analogue manner. The number of analogue voltages available for application across the liquid crystal determines the number of half-tones displayed.
FIG. 11
shows a block diagram of the source driver
902
as an example. Display data, provided externally as digital signals, consist of display data, DR, DG, and DB for R (red), G (green), and B (blue); the display data is temporarily latched by an input latch circuit
1301
, and thereafter stored time-sequentially in a sampling memory
1303
based on the operation of a shift register
1302
which receives a start pulse SP and shifts with a clock CK; the whole data is then simultaneously transferred to a hold memory
1304
in accordance with a horizontal synchronized signal (not shown). “S” represents cascade outputs. A standard voltage generating circuit
1309
generates standard voltages of differing levels according to a reference voltage VR. The hold memory
1304
transmits the data through a level shifter circuit
1305
to a DA converter circuit (digital to analogue converter circuit)
1306
where the data is converted to analogue voltages based on the standard voltages of differing levels provided by the standard voltage generating circuit
1309
. Then, an output circuit
1307
provides outputs as half-tone display voltages that are transmitted through liquid crystal drive voltage output terminals
1308
to liquid crystal display elements (see “A” in FIG.
8
).
In this manner, the standard voltage generating circuit
1309
, the DA converter circuit
1306
, and the output circuit
1307
constitute a DA converter. Further, in the liquid crystal display device, a liquid crystal drive circuit is arranged using the DA converter in the aforemention
Jeanpierre Peguy
Lauture Joseph
Sharp Kabushiki Kaisha
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