ADD8708WCPZ-REEL7 View Datasheet(PDF) - Analog Devices

Part Name

Description

Manufacturer

ADD8708WCPZ-REEL7

18-Channel Gamma Buffer with Regulator

Analog Devices 

ADD8708

APPLICATION NOTES

The ADD8708 is a mask-programmable gamma reference

generator that allows source drivers to be optimized for the

different combinations of liquid crystals, glass sizes, etc. in

large LCD panels. It generates 18 gamma reference outputs

that can be mask-programmed in 0.2% increments using the

500 matched internal resistors (see Figure 20), so that every

point on the curve can be targeted within 0.1% of the desired

value.

TAP POINT 4

TAP POINT 500

TAP POINT 3

TAP POINT 499

TAP POINT 2

TAP POINT 498

TAP POINT 1

TAP POINT 497

EACH R = 30Ω

TYPICALLY

Figure 20. 500 Mask-Programmable Resistor String

In a typical panel application, the selected source drivers have

an internal gamma curve that is not ideal for the specific panel

(see Figure 21). The ADD8708 allows the gamma curve in the

source drivers to be adjusted appropriately, and also ensures

that all the source drivers have the same gamma curve.

16

14

12

10

8

ORIGINAL GAMMA CURVE

IN SOURCE DRIVERS

6

PANEL GAMMA CURVE

CORRECTED BY ADD8708

4

2

0

GAMMA REFERENCE INPUT POINTS

Figure 21. Original and Corrected Gamma Curves

The matching and tracking accuracy of the internal resistors is

typically 0.1% with worst-case deviation from the desired curve

within 0.4% of the ideal gamma curve, over temperature.

The ADD8708 also includes a low-dropout linear regulator to

provide a stable reference level for the gamma curve for

optimum panel performance.

TAP POINT SELECTION

The ADD8708 uses a single resistor string consisting of 500

individual elements. The tap points are mask programmable

and completely independent of each other. See the Tap Point

and Regulator Voltage Request Form in this data sheet.

VREG OUT

VINX

500–TPX

TPX

VOUTX

Figure 22. Gamma Buffers Tap Point Circuit

Tap point voltages can be derived from the following equation:

VOUT X

=

TPX

500

× VREG

OUT

where TPX is the desired tap point for the Xth channel.

Table 5. Typical Mask Implementation1

VDD = 16 V, VREG OUT = 14.4 V, 0 ≤ X ≤ 500

Tap Point (X) Voltage

VOUT18

500

14.400

VOUT17

396

11.405

VOUT16

369

10.627

VOUT15

361

10.397

VOUT14

354

10.195

VOUT13

350

10.080

VOUT12

341

9.821

VOUT11

317

9.130

VOUT10

299

8.611

VOUT9

225

6.480

VOUT8

211

6.077

VOUT7

177

5.098

VOUT6

167

4.810

VOUT5

163

4.694

VOUT4

154

4.435

VOUT3

146

4.205

VOUT2

118

3.398

VOUT1

7

0.202

Units

V

V

V

V

V

V

V

V

V

V

V

V

V

V

V

V

V

V

_______________________________

1ADD8708 release samples do not have these typical values. The values on the

samples are nonmonotonic and can be provided upon request.

Rev. 0 | Page 10 of 16

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