REV. 0

ADD8702

–7–

APPLICATIONS

Figure 1 is a block diagram of the configuration of an XGA-

compatible (1024

768) TFT color panel with the ADD8702

providing gamma correction reference voltages to the source

BUFFER

16V

Figure 2. Bandwidth Measurement Information

Panel size and resolution determine the number of gamma

reference voltages required. For a 256-grayscale level, 8-bit color

scheme, 6

2 external reference nodes should be sufficient to

match the characteristics of the LCD driver to the characteristics

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

ladder is incorporated in the IC for reduced cost and number

of components.

BUFFER

16V

5V

33

1k

10V

5V

0V

0.1 s

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

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

to 1,000 pF pure capacitive load. Provision is available to adjust

example of an application circuit for adjusting the output of the

GMA 12

GMA 11

GMA 10

GMA 9

GMA 8

GMA 7

GMA 6

GMA 5

GMA 4

GMA 3

GMA 2

GMA 1

ADD8702

LCD SOURCE DRIVER

A1

LCD COMMON PLANE

2k

32k

GND

A5

A4

A3

A2

A10

A8

A7

A6

A9

A12

A11

Figure 4. Application Circuit

Table I. ADD8702 – 000 Mask Option, Resistor Tap Points

(0

≤ ≤ 500) V

Tap Point ( )

Voltage

Unit

VGMA1

8

0.2

V

VGMA2

57

1.43

V

VGMA3

84

2.11

V

VGMA4

115

2.89

V

VGMA5

139

3.48

V

VGMA6

194

4.86

V

VGMA7

218

5.45

V

VGMA8

298

7.45

V

VGMA9

371

9.29

V

VGMA10

418

10.45

V

VGMA11

442

11.04

V

VGMA12

488

12.2

V

200

5

V