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AD8382

VBIAS Generation—V1, V2 Input Pin

Functionality

In order to avoid image flicker, a symmetrical ac voltage is

required and a bias voltage of approximately 1 V minimum

must be maintained across the pixels of HTPS LCDs. The

AD8382 provides two methods of maintaining this bias voltage.

INTERNAL BIAS VOLTAGE GENERATION

Standard systems that internally generate the bias voltage

reserve the uppermost code range for the bias voltage and use

the remaining code range to encode the video for gamma

correction. A high degree of ac symmetry is guaranteed by the

AD8382 in these systems.

The V1 and V2 inputs in these systems are tied together and are

normally connected to VCOM, as shown in Figure 42.

VBIAS = 1V

VCOM

VFS = 5V

RESERVED

CODE

RANGE

VBIAS = 1V

VCOM

AD8382

4095

3280

Figure 42. V1, V2 Connection and Transfer Function in a Typical Standard

System

EXTERNAL BIAS VOLTAGE GENERATION

In systems that require improved brightness resolution and

higher accuracy, the V1 and V2 inputs, connected to external

voltage references, provide necessary bias voltage, VBIAS, while

allowing the full code range to be used for gamma correction.

To ensure a symmetrical ac voltage at the AD8382’s outputs,

VBIAS must remain constant for both states of INV. Thus, V1

and V2 are defined as:

V1 = VCOM − VBIAS

V2 = VCOM + VBIAS

APPLICATIONS CIRCUIT

The circuit in Figure 41 ensures VBIAS symmetry to within 1

mV with a minimum component count. Bypass capacitors are

omitted for clarity.

AVCC = 15.5V

VZ = 5.1V

DVCC = 3.3V

VCOM = 7V

R1 = 6k

Ω

R2 = 1k

Ω

V2 = 8V

V1 = 6V

AD8382

AD8132

V–

V+

VCOM

–IN

+IN

Figure 43. External VBIAS Generator with the AD8132

VBIAS = 1V

VCOM

VFS = 4V

VBIAS = 1V

4095

Figure 44. The AD8382 Transfer Function in a Typical High Accuracy System

5.7

6.2

6.7

7.2

7.7

8.2

8.7

9.2

9.7

10.2 10.7

–8.75

–6.25

–3.75

1.25

3.75

6.25

7.50

8.75

–1.25

–7.50

–5.00

0.00

2.50

5.00

–2.50

V+ – V– (V)

TA = 85°C

TA = 25°C

Figure 45. Typical Asymmetry at the Outputs of the AD8132 vs. Power Supply

for the Application Circuit

The AD8132 typically produces a symmetrical output at 85°C

when its supply, (V+) – (V–), is 7.2 V (Figure 45).

Rev. 0 | Page 17 of 24

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