ADD8706ARUZ Analog Devices Inc, ADD8706ARUZ Datasheet - Page 9

• Manufacturer Part Number: ADD8706ARUZ
• Description: IC OPAMP 5+1CHAN 16V 16TSSOP
• Manufacturer: Analog Devices Inc

Specifications of ADD8706ARUZ

• Applications: TFT-LCD Panels: Gamma Buffer, VCOM Driver
• Number Of Circuits: 5
• -3db Bandwidth: 6MHz
• Slew Rate: 6 V/µs
• Current - Supply: 4.5mA
• Current - Output / Channel: 35mA
• Voltage - Supply, Single/dual (±): 4.5 V ~ 16 V
• Mounting Type: Surface Mount
• Package / Case: 16-TSSOP (0.173", 4.40mm Width)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

APPLICATION INFORMATION
THEORY
The ADD8706 is designed for use in LCD gamma correction
circuits. This is an ideal on-chip solution for low-end panels. It
provides five gamma voltages and a V
voltages provide the reference voltages for the column driver
RDACs. Due to the capacitive nature of LCD panels, it is
necessary for these drivers to provide high capacitive load drive.
The V
pixels. The V
This requires the V
to 35 mA.
INPUT/OUTPUT CHARACTERISTICS
The ADD8706 has five buffers specifically designed for the
needs of an LCD panel. Figure 20 shows a typical gamma
correction curve for a normally white twisted nematic LCD
panel. The symmetric curve comes from the need to reverse the
polarity on the LC pixels to avoid “burning” in the image.
Therefore, the application requires gamma voltages that come
close to both supply rails. To accommodate this transfer
function, the five ADD8706 buffers have been designed with
three different buffer designs in one package.
The nature of LCD panels introduces a large amount of
parasitic capacitance from the column drivers as well as the
capacitance associated with the liquid crystals via the common
plane. This makes capacitive drive capability an important
factor when designing the gamma correction circuit.
V
COM
V
V
V
V
V
V
V
V
V
V
V
G10
DD
G1
G2
G3
G4
G5
G6
G7
G8
G9
SS
0
output is the center voltage common to all the LCD
COM
Figure 20. LCD Gamma Correction Curve
circuit is common to all the pixels in the panel.
COM
16
driver to supply continuous currents up
GRAY SCALE BITS
32
COM
output. These gamma
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The outputs of the buffers and amplifier have been designed to
match the performance needs of the gamma correction and
V
capabilities differ. The difference in current drive and input
voltage range determine the buffer and amplifier use.
Buffer A has an NPN emitter-follower input stage, which
provides an input range that includes the top rail, but is limited
to 1.7 V away from the bottom rail. It is designed to source
15 mA of continuous current, making this buffer ideal for
providing the top voltage on the RDAC string.
Buffers B, C, and D use a single-supply PNP input stage with an
intermediate common-mode voltage range. The output was
designed to sink or source up to 15 mA of continuous current.
The limited input range and equivalent sink and source current
make these buffers suitable for the middle voltage ranges on the
RDAC string.
Buffer E also uses a single-supply PNP input stage, but the
output is designed to sink only up to 15 mA of continuous
current. This buffer is designed for the RDAC’s lower range.
Amplifier F is designed with an input range limited to midscale
applications. It is capable of delivering 35 mA of continuous
current. These qualities make Amplifier F suitable for V
applications.
IMPORTANT NOTE
Because of the asymmetric nature of Buffers A and E, care must
be taken to connect an input that forces the amplifiers to
operate in their most productive output states. Buffer A has very
limited sink capabilities, while Buffer E does not source well. Set
the Buffer A input to enable the amplifier output to source
current and set the Buffer E input to force a sinking output
current. This means making sure the input is above the
midpoint of the common-mode input range for Buffer A and
below the midpoint for Buffer E. Mathematically speaking,
make sure V
Figure 21 shows an application using the ADD8706 to generate
10 gamma outputs. Note that the five outputs are routed
through another resistor network to generate the extra five
output voltages, which feed into the column driver.
COM
circuits. All have rail-to-rail outputs, but the current drive
IN
> V
S
/2 for Buffer A and V
IN
< V
S
/2 for Buffer E.
ADD8706
COM