ADD8704

APPLICATION INFORMATION

THEORY

The ADD8704 is designed for use in LCD gamma correction

circuits. Depending on the panel architecture, between 4 and 18

different gamma voltages may be needed. 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.

In addition to providing gamma reference voltages, these parts

is designed to supply continuous currents up to 35 mA.

INPUT

The ADD8704 has four amplifiers specifically designed for the

needs of an LCD panel. F

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. The

application therefore requires gamma voltages that come close

to both supply rails. To accommodate this transfer function, the

ADD8704 has been designed to have four different amplifiers in

one package.

igure 36

Figure 36. LCD Gamma Correction Curve

GRAY SCALE BITS

016

32

48

64

Amplifier A has a single-supply PNP input stage followed by a

folded cascode stage. This provides an input range that goes to

the bottom rail. This amplifier can therefore be used to provide

the bottom voltage on the RDAC string.

Amplifier B (PNP folded cascode) swings to the low rail as well,

but it provides 35 mA continuous output current versus 15 mA.

This buffer is suitable for lower RDAC range, middle RDAC

Amplifier C is a rail-to-rail input range that makes the

ADD8704 suitable for use anywhere on the RDAC as well as for

Amplifier D has an NPN follower input stage. This covers the

upper rail to GND plus 1.7 V. This amplifier is suitable for the

upper range of the RDAC.

OUTPUT

The outputs of the amplifiers have been designed to match the

performance needs of the gamma correction circuit. All four of

the amplifiers have rail-to-rail outputs, but the current drive

capabilities differ. Since amplifier A is suited for voltages close

it sources; it can sink 15 mA of continuous current. Likewise,

sources more current. Amplifier D can source 15 mA of

continuous current. Amplifiers B and C are designed for use as

and source equal amounts of current. Since they are used as

continuous current.

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.

IMPORTANT NOTE

Because of the asymmetric nature of amplifiers A and D, care

must be taken to connect an input that forces the amplifiers to

operate in their most productive output states. Amplifier D has

very limited sink capabilities, while amplifier A does not source

well. If more than one ADD8704 is used, set the amplifier D

input to enable the amplifier output to source current and set

the amplifier A input to force a sinking output current. This

means making sure the input is above the midpoint of the

common-mode input range for amplifier D and below the

midpoint for amplifier A. Mathematically speaking, make sure

Figure 37 shows an example using 4 ADD8704s to generate 10

gamma outputs. Note that the top three resistor tap-points are

connected to the amplifier D inputs, thus assuring these

channels will source current. Likewise, the bottom three resistor

tap-points are connected to the amplifier A inputs to provide

sinking output currents.

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