ADSY8401 datasheet(13/16 Pages) AD

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ADSY8401

Rev. 0 | Page 13 of 16

PCB DESIGN FOR OPTIMIZED THERMAL PERFORMANCE

The total maximum power dissipation of the ADSY8401 is

partly load-dependent. In a typical 60 Hz XGA system, the total

maximum power dissipation is ≈ 1 W. The ADSY8401 package

is designed to provide superior thermal characteristics, partly

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 a thermal pad and

a thermal via structure. The thermal pad provides a solderable

contact surface on the top surface of the PCB. 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 logarith-

mically 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.

Table 6. Recommended Land Pattern Dimensions

Land Pattern

Dimensions

Top and Bottom Layers

Pad size

0.5 mm × 0.25 mm

Pad pitch

0.5 mm

Thermal pad size

5.25 mm × 5.25 mm

Thermal via structure

0.25 mm diameter vias on

0.5 mm grid

Thermal Pad and Thermal Via Connections

Thermal pads are connected to the AGND or AVCC plane. The

thermal pad on the solder side is connected to a plane. The use

of thermal spokes is not recommended when connecting the

thermal pads or via structure to the plane.

Solder Masking

To minimize the formation of solder voids due to solder flowing

into the via holes (solder wicking), the via diameter should be

small. Solder masking of the via holes on the top layer of the

PCB plugs the via holes, inhibiting solder flow into the holes. To

optimize the thermal pad coverage, the solder mask diameter

should be no more than 0.1 mm larger than the via diameter.

Table 7. Recommended Solder Mask Dimensions

Solder Mask

Dimensions

Top layer

Pads

Set by customer’s PCB design rules

Thermal vias

0.25 mm diameter circular mask centered

on the vias

Bottom layer

Set by customer’s PCB design rules

LAND PATTERN–BOTTOM LAYER

SOLDER MASK–TOP LAYER

LAND PATTERN–TOP LAYER

7mm

Figure 13. PCB Layers

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