Coded data generation or conversion – Analog to or from digital conversion – Digital to analog conversion
Reexamination Certificate
2001-07-25
2002-08-27
JeanPierre, Peguy (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
Digital to analog conversion
C341S153000
Reexamination Certificate
active
06441763
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
A DA (digital-to-analog) converter converts externally inputted digital signals to analog signals. For example, in a liquid crystal driving device of an active matrix liquid crystal display device, a DA converter is used to convert externally inputted digital signals, which constitute display data, to analog signals, and to send the analog signals to a liquid crystal display section. Some DA converters of this kind include an OP-amplifier composed of MOS transistors.
FIGS. 10 and 11
show a detailed structure of a DA converter
100
incorporated in a liquid crystal driving device as mentioned above, which converts display data in the form of digital signals to analog voltages for outputs.
The DA converter
100
is made up of a selector circuit
106
[corresponding to a selector circuit
16
of a DA converter
10
of the present invention (FIG.
9
)], which is a DA conversion circuit; a voltage follower circuit
107
(corresponding to a voltage follower circuit
17
in FIG.
9
), which is an output circuit; and a reference voltage generating circuit
108
(corresponding to a reference voltage generating circuit
18
in FIG.
9
).
FIG. 10
shows an example of a structure of a DA converter incorporated in a 64-level gradation liquid crystal driving which outputs 64 analog voltages in accordance with 6-bit digital signals (Bit
5
to Bit
0
).
FIG. 11
is an enlarged view of the section from V
48
to V
64
of the reference voltage generating circuit
108
and the selector circuit
106
(the section B in FIG.
10
). The structure of the reference voltage generating circuit
108
and the selector circuit
106
shown in
FIG. 10
is arranged such that the structural pattern shown in
FIG. 11
are repeated.
The reference voltage generating circuit
108
generates a plurality of reference voltages (64 reference voltages in this example) in accordance with the display data provided in the form of digital signals. The selector circuit
106
, which is made up of MOS transistor switches, selects one of the reference voltages for output. The structure of the switches will be described later in detail.
The voltage follower circuit
107
outputs the voltage selected by the selector circuit
106
through a liquid crystal driving voltage output terminal (corresponding to a liquid crystal driving voltage output terminal
17
t
in
FIG. 9
) to a liquid crystal display element,; as a liquid crystal driving signal.
Generally, the reference voltage generating circuit
108
is commonly used for a plurality of liquid crystal driving voltage output terminals.
Meanwhile, one selector circuit
106
and one voltage follower circuit
107
are provided for each liquid crystal driving voltage output terminal. In the case of a color display, a liquid crystal driving voltage output terminal is used corresponding to each color, thus one selector circuit
106
and one voltage follower circuit
107
are provided for each color in a pixel. That is, supposing that the total number of pixels in a liquid crystal panel (corresponding to a liquid crystal panel
21
in
FIG. 5
) is N, and liquid crystal driving voltage output terminals for red, green, and blue are expressed as R, G, and B, respectively with a subscript n (n=1, 2, . . . , N), the liquid crystal panel has liquid crystal driving voltage output terminals R
1
, G
1
, B
1
, R
2
, G
2
, B
2
, . . . ) R
N
, G
N
, B
N
, therefore 3N selector circuits
106
and 3N voltage follower circuits
107
are required.
The following description will explain the structure and operation of the DA converter
100
incorporated ;in the liquid crystal driving device.
The reference voltage generating circuit
108
is structured such that 64 resistor elements are connected in series, and a largest liquid crystal driving voltage V
64
and a smallest liquid crystal driving voltage V
0
are inputted to the respective end terminals. Therefore, 64 voltages (V
0
to V
63
) are generated respectively from between the resistor elements, in proportion to the resistance of the resistor elements connected. These 64 different voltages generated from the reference voltage generating circuit
108
are inputted to the selector circuit
106
.
In the selector circuit
106
, the MOS transistor switches are arranged so as to select one of the 64 inputted voltages for output in accordance with the display data composed of a 6-bit digital signal. That is, the switches are turned on or off in response to each piece of display data composed of a 6-bit digital signal (Bit
0
to Bit
5
), thus one of the 64 inputted voltages is selected for output. The following description will explain voltage selection procedures in detail.
In a 6-bit digital signal, Bit
5
is MSB (Most Significant Bit), and Bit
0
is LSB (least Significant Bit). The switches are paired in two to form switch pairs. Therefore, Bit
0
is provided with 32 switch pairs (64 switches), and Bit
1
is provided with 16 switch pairs (32 switches). AS moving up from a certain bit to a next bit, the number of switch pairs decreases by half, down to a single switch pair (two switches) for Bit
5
. So the total number of the switch pairs in the selector circuit
106
amounts to 1+2+2
2
+2
3
+2
4
+2
5
=63 (126 switches).
The two switches of a switch pair operate such that, if the corresponding bit is “0”, the upper switch in the figure turns off, and the lower switch turns on. In contrast, if the corresponding bit is “1”, the upper switch turns on, and the lower switch turns off. For example, referring to the example in
FIG. 12
, Bit
5
, Bit
4
, . . . , Bit
0
is “111111”, so all the upper switches are on and all the lower switches are off, allowing the voltage V
63
to be outputted from an output terminal of the selector circuit
106
. Further, for example, if Bit
5
, Bit
4
, . . . , Bit
0
is “000001”, the voltage V
1
is outputted from an output terminal of the selector circuit
106
.
The voltage follower circuit
107
provides a voltage identical to the analog voltage outputted from the selector circuit
106
, for output via the liquid crystal driving voltage output terminal as a liquid crystal driving signal having a smaller internal resistance.
There is a trend in recent years for liquid crystal display devices to include more minute structure and display more gradation levels, resulting in increases in the size of circuits in liquid crystal driving devices. Meanwhile, as liquid crystal display devices find applications in more fields, there are increasingly higher demands for cheaper liquid crystal display devices and stronger needs to reduce manufacturing costs by manufacturing smaller liquid crystal driving devices. Further, in terms of portability, there are strong demands for smaller liquid crystal display devices including liquid crystal driving devices, which adds the importance of the reduction in the size of liquid crystal driving devices.
However, when the foregoing conventional DA converter
100
is used as a liquid crystal driving device of a liquid crystal display device, the number of circuit composing elements increases sharply in accordance with an increase in the number of the gradation levels to be displayed. For example, in the case of a liquid crystal driving device providing a 64-level gradation display using 6-bit digital signals, 64 resistor elements are required in the reference voltage generating circuit
108
, and
126
switches are required for every pixel to form the selector circuit
106
. In the same manner, in the case of a liquid crystal driving device providing a 256-level gradation display using 8-bit digital signals, 256 resistor elements are required in the reference voltage generating circuit
108
, and
510
switches are required for every pixel to form the selector circuit
106
, as it includes 1+
Jean-Pierre Peguy
Lauture Joseph J
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