ADD8702 Ver la hoja de datos (PDF) - Analog Devices
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ADD8702 Datasheet PDF : 8 Pages
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ADD8702
APPLICATIONS
Figure 1 is a block diagram of the configuration of an XGA-
compatible (1024 ϫ 768) TFT color panel with the ADD8702
VHIGH
A12
providing gamma correction reference voltages to the source
drivers and an integrated VCOM driver for LCD common node.
VIN11
A11
16V
BUFFER RNULL
VIN10
A10
GMA 12
GMA 11
GMA 10
CT
RL
VIN9
A9
GMA 9
Figure 2. Bandwidth Measurement Information
Panel size and resolution determine the number of gamma
VIN8
A8
reference voltages required. For a 256-grayscale level, 8-bit color
scheme, 6 ϫ 2 external reference nodes should be sufficient to
match the characteristics of the LCD driver to the characteristics
VIN7
A7
of the actual LCD panel for improved picture quality. External
reference gamma correction voltages are often generated using
a simple resistor ladder. Using the ADD8702, the resistor
VIN6
A6
ladder is incorporated in the IC for reduced cost and number
of components.
VIN5
A5
GMA 8
GMA 7
GMA 6
GMA 5
16V
0.1s
VIN4
A4
BUFFER 33⍀
1k⍀
10V
5V
VTH
VTH
5V
0V
VIN3
A3
GMA 4
GMA 3
Figure 3. Transient Load Regulation Test Circuit
The ADD8702 is designed to meet the rail-to-rail capability
needed by the application, yet offers the lowest cost per channel
solution. The ADD8702 gamma buffers offer 10 mA continuous
drive current capability. To be more competitive, the design
maximizes the die area by allowing specific channels to swing to
the positive rail and negative rail. So it is imperative that the
channels swinging close to the supply rail be used for the posi-
tive gamma references and the channels swinging close to GND
be used for the negative gamma references. The VCOM buffer can
handle up to 35 mA continuous output current and can drive up
to 1,000 pF pure capacitive load. Provision is available to adjust
the VCOM voltage to a desired level. Refer to Figure 4 for an
example of an application circuit for adjusting the output of the
VCOM buffer.
VIN2
A2
GMA 2
VLOW
VDD
A1
VCOM AMP
32k⍀
GND
VCOM ADJUST
2k⍀
ADD8702
GMA 1
LCD SOURCE DRIVER
LCD COMMON PLANE
Figure 4. Application Circuit
Table I. ADD8702 – 000 Mask Option, Resistor Tap Points
(0 ≤ ؋ ≤ 500) VDD = 12.5 V, VHIGH = 12.5 V, and VLOW = GND
Tap Point (؋) Voltage
Unit
VGMA1
8
VGMA2
57
VGMA3
84
VGMA4
115
VGMA5
139
VGMA6
194
VGMA7
218
VGMA8
298
VGMA9
371
VGMA10 418
VGMA11 442
VGMA12 488
VCOM
200
0.2
V
1.43
V
2.11
V
2.89
V
3.48
V
4.86
V
5.45
V
7.45
V
9.29
V
10.45
V
11.04
V
12.2
V
5
V
REV. 0
–7–
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