ADD8709ASTZ-REEL7 View Datasheet(PDF) - Analog Devices

Part Name

Description

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ADD8709ASTZ-REEL7

18-Channel Gamma Buffer with Regulator

Analog Devices 

ADD8709

APPLICATION NOTES

The ADD8709 is a gamma reference generator that allows

source drivers to be optimized for the different combinations

of liquid crystals, glass sizes, etc. in large LCD panels.

In a typical panel application, the selected source drivers have

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

as shown in Figure 19. The ADD8709 allows the gamma curve

in the source drivers to be adjusted appropriately, and also

ensures that all of 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 ADD8709

4

2

0

GAMMA REFERENCE INPUT POINTS

Figure 19. Original and Corrected Gamma Curves

The ADD8709 also includes a low dropout linear regulator to

provide a stable reference level for the gamma curve for best

panel performance.

VOLTAGE REGULATOR

The on-board voltage regulator provides a regulated voltage to

the external resistors to set different gamma voltages.

The output of the regulator is set by the two resistors, R1 and R2,

and an internal reference voltage of 1.2V. In the ADD8709, R1

and R2 are external components. The output voltage can is

found by

VREG OUT = VREF × (R2/R1 + 1)

R2

55kΩ

R1

5kΩ

VREF +

1.2V –

VREG OUT

Figure 20. Voltage Regulator

MAXIMUM POWER DISSIPATION

The maximum safe power dissipation in the ADD8709 package

is limited by the associated rise in junction temperature (TJ) on

the die. At approximately 150°C, the glass transition temper-

ature, the properties of the plastic change. Even temporarily

exceeding this temperature limit may change the stresses that

the package exerts on the die, permanently shifting the para-

metric performance of the ADD8709. Exceeding a junction

temperature of 175°C for an extended period can result in

changes in the silicon devices, potentially causing failure.

OPERATING TEMPERATURE RANGE

The junction temperature is

TJ = TAMB + θJA × PDIS

where:

TAMB = ambient temperature.

θJA = junction-to-ambient thermal resistance, in °C/watt.

PDIS = power dissipated in the device, in watts.

For the ADD8709, PDIS can be calculated by

PDIS = VDD × IDQ + Σ(IOUT X(+) × (VDD − VOUT X)) +

Σ(−IOUT X(-) × VOUTX) + (VDD – VREG OUT) × ILOAD

where:

VDD × IDQ = nominal system power requirements.

IOUT X(+) × (VDD − VOUT X) = positive-current amplifier load power

dissipation (current comes from VDD).

−IOU XT(-) × VOUT X = negative-current amplifier load power

dissipation (current goes to GND).

(VDD – VREG OUT) × ILOAD = regulator load power dissipation.

Example 1

To calculate an estimated power consumption of the ADD8709

assume:

VDD × IDQ = 16 V × 15 mA = 0.240 W.

(VDD – VREG OUT) × ILOAD = (16 V – 14.4 V) × 5 mA = 0.008 W.

Rev. A | Page 10 of 16

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