AD8385ASVZ datasheet(21/24 Pages) AD

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AD8385

Rev. 0 | Page 21 of 24

APPLICATIONS CIRCUIT

The following circuit ensures VBIAS symmetry to within 1 mV

with a minimum component count. Bypass capacitors are not

shown for clarity.

AVCC = 15.5V

VZ = 5.1V

DVCC = 3.3V

VCOM = 7V

R1 = 6k

Ω

R2 = 1k

Ω

V2 = 8V

V1 = 6V

AD8385

AD8132

V–

V+

VOCM

–IN

+IN

Figure 19. External VBIAS Generator with the AD8132

VBIAS = 1V

VCOM

VFS = 4V

VBIAS = 1V

1023

Figure 20. AD8385 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 21. Typical Asymmetry at the Outputs of the AD8132 vs. Its Power

Supply for the Application Circuit

Figure 21 shows that the AD8132 (Figure 19) typically produces

a symmetrical output at 85°C when its supply, (V+) – (V–), is

at 7.2 V.

PCB DESIGN FOR OPTIMIZED THERMAL

PERFORMANCE

The total maximum power dissipation of the AD8385 is partly

load-dependent. In a 12-channel 60 Hz XGA system running at

a 65 MHz pixel rate, the total maximum power dissipation is

2.3 W at an LCD channel input capacitance of 150 pF. At a

100 MHz pixel rate, the total maximum power dissipation can

exceed 3 W.

To limit the operating junction temperature at or below the

guaranteed maximum, the package, in conjunction with the

PCB, must effectively conduct heat away from the junction.

The AD8385 package is designed to provide enhanced thermal

characteristics through the exposed die paddle on the bottom

surface of the package. To take full advantage of this feature, the

exposed paddle must be in direct thermal contact with the PCB,

which then serves as a heat sink.

A thermally effective PCB must incorporate two thermal pads

and a thermal via structure. The thermal pad on the top PCB

layer provides a solderable contact surface on the top surface of

the PCB. The thermal pad on the bottom PCB layer provides a

surface in direct contact with the ambient. The thermal via

structure provides a thermal path to the inner and bottom

layers of the PCB to remove heat.

THERMAL PAD DESIGN

To minimize thermal performance degradation of production

PCBs, the contact area between the thermal pad and the PCB

should be maximized. Therefore, the size of the thermal pad on

the top PCB layer should match the exposed paddle size. The

second thermal pad of at least the same size should be placed on

the bottom side of the PCB. At least one thermal pad should be

in direct thermal contact with a plane such as AVCC or GND.

THERMAL VIA STRUCTURE DESIGN

Effective heat transfer from the top to the inner and bottom

layers of the PCB requires thermal vias incorporated into the

thermal pad design. Thermal performance increases logarith-

mically with the number of vias.

Near optimal thermal performance of production PCBs is

attained only when tightly spaced thermal vias are placed on

the full extent of the thermal pad.

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