AD8384ASVZ datasheet(18/24 Pages) AD

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AD8384

Rev. 0 | Page 18 of 24

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

AD8384 provides two methods of maintaining this bias voltage.

Internal Bias Voltage Generation

Standard systems that internally generate the bias voltage

reserve the upper-most code range for the bias voltage, and use

the remaining code range to encode the video for gamma

correction. In these systems, a high degree of ac symmetry is

guaranteed by the AD8384.

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

normally connected to VCOM, as shown in Figure 17.

VBIAS = 1V

VCOM

VFS = 5V

RESERVED

CODE

RANGE

VBIAS = 1V

VCOM

AD8384

1023

820

Figure 17. 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 the necessary bias voltage (VBIAS)

while allowing the full code range to be used for gamma

correction.

To ensure a symmetrical ac voltage at the outputs of the

AD8384, VBIAS must remain constant for both states of INV.

Therefore, V1 and V2 are defined as

V1 = VCOM – VBIAS

V2 = VCOM + VBIAS

APPLICATIONS CIRCUIT

The circuit in Figure 18 ensures VBIAS symmetry to within

1 mV with a minimum component count. Bypass capacitors are

not shown for clarity.

Note from the curve in Figure 20 that the AD8132 (Figure 18)

typically produces a symmetrical output at 85°C when its

supply, (V+) – (V–), is at 7.2 V.

AVCC = 15.5V

VZ = 5.1V

DVCC = 3.3V

VCOM = 7V

R1 = 6k

Ω

R2 = 1k

Ω

V2 = 8V

V1 = 6V

AD8384

AD8132

V–

V+

VCOM

–IN

+IN

Figure 18. External VBIAS Generator with the AD8132

VBIAS = 1V

VCOM

VFS = 4V

VBIAS = 1V

1023

Figure 19. AD8384 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 20. Typical Asymmetry at the Outputs of the AD8132 vs. Its Power

Supply for the Application Circuit

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