AD8384ASVZ datasheet(19/24 Pages) AD

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AD8384

Rev. 0 | Page 19 of 24

PCB DESIGN FOR OPTIMIZED THERMAL

PERFORMANCE

The AD8384’s total maximum power dissipation is partly load

dependent. In a 6-channel 60Hz XGA system running at a

65 MHz clock rate, the total maximum power dissipation is

1.6 W at a 150 pF LCD input capacitance.

At a clock rate of 100 MHz, the total maximum power

dissipation can exceed 2 W, as shown in Table 14, for a black-to-

white video output voltage swing of 4 V and 5 V.

Table 14. Power Dissipation

VSWING = 5 V

VSWING = 4 V

CLOAD

(pF)

PQUIESCENT

(W)

PDYNAMIC

(W)

PTOTAL

(W)

PDYNAMIC

(W)

PTOTAL

(W)

150

1.12

0.82

1.94

0.71

1.83

200

1.12

1.01

2.13

0.86

1.98

250

1.12

1.21

2.33

1.01

2.13

300

1.12

1.41

2.53

1.17

2.29

Although the maximum safe operating junction temperature is

higher, the AD8384 is 100% tested at a junction temperature of

125°C. Consequently, the maximum guaranteed operating

junction temperature is 125°C. To limit the maximum junction

temperature at or below the guaranteed maximum, the package,

in conjunction with the PCB, must effectively conduct heat

away from the junction.

The AD8384 package is designed to provide enhanced thermal

characteristics through the exposed die paddle on the bottom

surface of the package. In order 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 surface

of the PCB 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. The second

thermal pad of 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 an external 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

logarithmically with the number of vias.

Near optimum 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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