Coded data generation or conversion – Analog to or from digital conversion – Digital to analog conversion
Reexamination Certificate
2003-04-22
2004-07-27
Young, Brian (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
Digital to analog conversion
C341S153000
Reexamination Certificate
active
06768439
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a source driver circuit of active matrix organic light emitting diode displays (AM-OLED), more specifically, to a digital to analog (D/A) converter for a current-driven type source driver circuit.
BACKGROUND OF THE INVENTION
With rapid advancement of the fabrication technology of thin film transistors, the OLEDs in new generation are widely applied in various portable electronic products for providing more fine images due to the fact it has advantages of high emitting efficiency, fast responding rate, power saving, non-restriction of visible angle, smaller size, less weight, high brightness and all-color images.
Please refer to
FIG. 1
, the circuit structure
10
of each unit pixel in current-driven type OLED device is shown. When the signal on the scan line is at low potential level, the PMOS transistors P
1
, P
2
and P
3
are sequential switched on. Because the transistor P
3
connected to the voltage source·Vdd can conduct with the current source
15
on the data line via the transistor P
2
, the drain current of transistor P
3
will be equal to the current I
data
of the current source
15
. For providing the required current I
data
of the transistor P
3
, the capacitor
20
connected to the gate of the transistor P
3
can be charged to adjust the gate potential of the transistor P
3
and to provide a desired Vgs between the gate and the source thereof. Then, when the signal on the scan line is at high potential level, the PMOS transistors P
1
& P
2
are switched off, and the NMOS transistor N
1
is switched on. At this time, the charged capacitor
20
can maintain the potential difference between the gate and the source of transistor P
3
at a specific value Vgs, so as to provide a drain current of I
data
and to drive the light emitting diode
25
to emit light via the conducting NMOS transistor N
1
.
In general, for providing more harmonizing and uniform colored images, the data current I
data
on the data line is adjusted to have multiple gray levels. Please refer to
FIG. 2
, the current source
30
for producing the current with 16 gray levels designed by J. Kanicki is illustrated. The current source
30
comprises four transistors M
1
, M
2
, M
3
& M
4
with the W/L (channel width to channel length) ratio of 1:2:4:8. And the sources of these transistors are all connected to a common voltage source VD
0
, as to the drains thereof are connected to another transistor DR, so as to have these drains at the potential level of the reference voltage VDR
0
when the transistor DR is switched on. The gates of transistors M
1
~M
4
are connected separately to control terminals of VD
1
, VD
2
, VD
3
and VD
4
, and switched on or off depending on the voltages of these control terminals. It is noted that because these transistors (M
1
~M
4
) have different W/L ratios individually, the drain currents thereof also have the ratio of 1:2:4:8. Thus, by turning on some transistors, the sum of currents I
data
can has the desired gray level. For instance, when only one transistor M
1
is turned on the data current I
data
is equal to I; and when the M
1
& M
2
both are turned on, the data current I
data
is equal to
3
I.
However, it is noted that the aforementioned current source has many disadvantages. First, for manufacturing the four transistors with the W/L ratio of 1:2:4:8, it is very important to control precisely the layout dimension of each transistor. Under this condition, any inaccuracy occurring in the process will cause the W/L ratio deviate and have the severe gray level shift. Especially, when more current gray levels are desired, it is necessary to manufacture the transistors with a large W/L ratio, that will cause the layout and design of transistors more difficult. For example, when 32 current gray levels are desired, it is required to fabricate two transistors with 16 times dimension difference, for producing the W/L ratio of 1:2:4:8:16. Second, because so far most LEDs are fabricated on the low temperature poly-silicon substrates, it is difficult to manufacture the thin film transistors that meet the required specifications and parameter due to the affections of the substrate properties.
Under such conditions, the distribution of current gray levels maybe have uneven shift due to the errors of the threshold voltage and mobility of the transistors M
1
~M
4
. Especially, as shown in
FIG. 2
, only four transistors are applied to produce
16
current gray levels, so any one transistor with shift parameters will affect the distribution of the current gray level and the uniform degree of the LED display. Further, as shown in
FIG. 1
, all pixel devices on the same data line have the currents supplied by one identical set of transistors (M
1
~M
4
), therefore these four transistors would degrade very fast so as to cause the threshold voltage and mobility of each transistor change severely and to decrease the uniform degree of the LED displays, and even to alter the gray level of images.
For overcoming the above issues effectively, in some process of fabricating LED displays, the approach of thermometer code current cell decoder is introduced. Please refer to
FIG. 3
, a set of thermometer code current cell decoder
35
with six bits is illustrated. This thermometer code current cell decoder
35
comprises a column decoder
351
, a row decoder
352
, and a plurality of current cells
36
arranged in an 8×8 square array form. Each current cell
36
has a local current source
361
and a local decoder
362
.
For the aforementioned current cell decoder
35
of six bits, the less signal bit (LSB) D
1
, D
2
, & D
3
are direct input to the column decoder
351
. Because each signal bit possess the binary 0 or 1, eight sets of different column selection signals (C) are produced after the decoding procedures by the column decoder
351
. As to the more signal bit (MSB) D
4
, D, & D
6
are input to the row decoder
352
, so as to produce eight sets of row selection signals (Ri) after decoding procedures. When these column selection signals Cj or row selection signals Ri are input to the current cell
36
, through the logical operating circuits of the local decoder
362
, the current source
361
can be decided to turned on or off for outputting current Io. For example, the local decoder
362
can be applied to perform the logical computing of ((Ri and Cj) or Ri+1), for deciding to output the current Io or not.
It is noted that, because each current cell
36
for producing output current Io has the identical current source, the multiple current gray levels can be produced according to the number of conducted current cells. For instance, when fifteen current cells are turned on, the summed data current I
data
is equal to 15 Io; or when twenty-six current cells are turned on, the data current I
data
is equal to 26 Io. Thus, the aforementioned issues about uniform degree of displaying images due to the variations of threshold voltage and mobility of the TFTs can be improved. Besides, because it is not applying the current source constitute of the four transistors (M
1
~M
4
) to supply the pixels on one identical data line, the operating times of each transistor are reduced considerably, so as to improve the degradation of transistors in the prior art.
However, it is necessary to reserve a considerable large area on the substrate for fabricating a large number of decoders and current cells when the approach of thermometer code current cell decoder is introduced. Especially with the increasing gray-scales of the LED displays (6 bits, 8 bits.), the whole area occupied by the current driven circuit also increase fast proportional to the square of bits. Therefore, the approach still cannot be applied to or satisfy with the displays with higher gray-scales.
Additional, such thermometer code current cell array are operated to conduct the specific number of the local current sources one by one according their arrangement and sequence for producing the desired current gray level. As shown in
FIG. 3
, when the current of
AU Optronics Corp.
Lauture Joseph
Troxell Law Office PLLC
Young Brian
LandOfFree
D/A converter for current-driven type source driver circuit... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with D/A converter for current-driven type source driver circuit..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and D/A converter for current-driven type source driver circuit... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3211174