ADD8706 Просмотр технического описания (PDF) - Analog Devices

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ADD8706

16 V 5 + 1-Channel Operational Amplifier

Analog Devices 

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 VCOM output. These gamma

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 VCOM output is the center voltage common to all the LCD

pixels. The VCOM circuit is common to all the pixels in the panel.

This requires the VCOM driver to supply continuous currents up

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.

VDD

VG1

VG2

VG3

VG4

VG5

VG6

VG7

VG8

VG9

VG10

VSS

0

16

32

48

64

GRAY SCALE BITS

Figure 20. LCD Gamma Correction Curve

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.

ADD8706

The outputs of the buffers and amplifier have been designed to

match the performance needs of the gamma correction and

VCOM circuits. All have rail-to-rail outputs, but the current drive

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 VCOM

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 VIN > VS/2 for Buffer A and VIN < VS/2 for Buffer E.

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.

Rev. 0 | Page 9 of 12

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