REV. 0

ADD8701

–7–

BUFFERS 10 TO 12

MARKER SET @ 10kHz

MARKER READING = 36.6nV/ Hz

FREQUENCY – Hz

05

10

15

20

25

–10

0

10

20

30

40

50

60

70

TPC 29. Voltage Noise Density

vs. Frequency

VCOM AND BUFFERS 1 TO 9

MARKER SET @ 10kHz

MARKER READING = 25.7nV/ Hz

FREQUENCY – Hz

05

10

15

20

25

–10

0

10

20

30

40

50

60

70

TPC 28. Voltage Noise Density

vs. Frequency

APPLICATIONS

LCD Gamma Reference Buffers

In high resolution TFT-LCD displays, gamma correction must

be performed to correct the nonlinearity in the LCD panel’s

transmission characteristics. A typical TFT-LCD panel consisting

of 256 grayscale levels takes an 8-bit digital word to select an

appropriate gamma reference voltage. An 8-bit source driver may

use 12 analog voltages that match the characteristic gamma curve

for optimum panel picture quality. The ADD8701 is specifically

designed to generate analog reference voltages to meet the gamma

characteristics of an LCD panel used by the source driver. The

gamma reference buffers offer 10 mA drive capability.

The ADD8701 is designed to meet the rail-to-rail capability

needed by the application and yet offers a low cost-per-channel

solution. The design maximizes the die area by offering channels

to swing to the positive and negative rails. It is imperative that

the channels swinging close to the supply rail be used for the

positive gamma references and that the channels swinging close

to GND be used for the negative gamma references. See Figure 2

for an example of the application circuit.

LCD VCOM Buffer

The output of the VCOM buffer is designed to control the voltage

on the back plate of the LCD display. The buffer must be capable

of sinking and sourcing capacitive pulse current. The amplifier

stability is designed for high load capacitance. A high quality

ceramic capacitor is recommended to supply short duration current

pulses at the output. The VCOM buffer of the ADD8701 can

handle up to 35 mA of continuous output current and can drive

up to 1,000 nF of pure capacitive load.

Unused Buffers

Inputs of any unused buffer should be tied to the ground plane.

GMA

A12

GMA

A11

GMA

A10

GMA

A9

GMA

A8

GMA

A7

GMA

A6

GMA

A5

GMA

A4

GMA

A3

GMA

A2

GMA

A1

ADD8701

LCD SOURCE DRIVER

Figure 2. Application Circuit