ADD8708WCPZ-REEL7 AD [Analog Devices], ADD8708WCPZ-REEL7 Datasheet - Page 10

• Manufacturer Part Number: ADD8708WCPZ-REEL7
• Description: 18-Channel Gamma Buffer with Regulator
• Manufacturer: AD [Analog Devices]

ADD8708
APPLICATION NOTES
The ADD8708 is a mask-programmable gamma reference
generator that allows source drivers to be optimized for the
different combinations of liquid crystals, glass sizes, etc. in
large LCD panels. It generates 18 gamma reference outputs
that can be mask-programmed in 0.2% increments using the
500 matched internal resistors (see Figure 20), so that every
point on the curve can be targeted within 0.1% of the desired
value.
In a typical panel application, the selected source drivers have
an internal gamma curve that is not ideal for the specific panel
(see Figure 21). The ADD8708 allows the gamma curve in the
source drivers to be adjusted appropriately, and also ensures
that all the source drivers have the same gamma curve.
The matching and tracking accuracy of the internal resistors is
typically 0.1% with worst-case deviation from the desired curve
within 0.4% of the ideal gamma curve, over temperature.
The ADD8708 also includes a low-dropout linear regulator to
provide a stable reference level for the gamma curve for
optimum panel performance.
16
14
12
10
8
6
4
2
0
Figure 20. 500 Mask-Programmable Resistor String
CORRECTED BY ADD8708
Figure 21. Original and Corrected Gamma Curves
TAP POINT 4
TAP POINT 3
TAP POINT 2
TAP POINT 1
PANEL GAMMA CURVE
GAMMA REFERENCE INPUT POINTS
EACH R = 30Ω
TYPICALLY
ORIGINAL GAMMA CURVE
IN SOURCE DRIVERS
TAP POINT 500
TAP POINT 499
TAP POINT 498
TAP POINT 497
Rev. 0 | Page 10 of 16
1
TAP POINT SELECTION
The ADD8708 uses a single resistor string consisting of 500
individual elements. The tap points are mask programmable
and completely independent of each other. See the Tap Point
and Regulator Voltage Request Form in this data sheet.
Tap point voltages can be derived from the following equation:
where TP
Table 5. Typical Mask Implementation
V
_______________________________
ADD8708 release samples do not have these typical values. The values on the
samples are nonmonotonic and can be provided upon request.
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
DD
OUT
OUT
OUT
OUT
OUT
OUT
OUT
OUT
OUT
OUT
OUT
OUT
OUT
OUT
OUT
OUT
OUT
OUT
= 16 V, V
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
X
is the desired tap point for the X
Tap Point (X)
500
396
369
361
354
350
341
317
299
225
211
177
167
163
154
146
118
7
REG OUT
Figure 22. Gamma Buffers Tap Point Circuit
V
IN
X
V
= 14.4 V, 0 ≤ X ≤ 500
OUT
500–TP
X
TP
V
=
REG OUT
X
X
TP
500
X
Voltage
14.400
11.405
10.627
10.397
10.195
10.080
9.821
9.130
8.611
6.480
6.077
5.098
4.810
4.694
4.435
4.205
3.398
0.202
×
V
REG
1
OUT
V
OUT
th
channel.
X
Units
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V