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Rev E: 3/2/07

SP7655 Wide Input Voltage Range 8A, 300kHz, Buck Regulator

© 2007 Sipex Corporation

SP765X Thermal Resistance

The SP765X family has been tested with a

variety of footprint layouts along with different

copper area and thermal resistance has been

measured. The layouts were done on 4 layer

FR4 PCB with the top and bottom layers using

3 ounces of copper and the power and ground

layers using 1 ounce of copper.

FortheMinimumfootprint,onlyabout0.1square

inch (of 3 ounces of) copper was used on the top

or footprint layer, and this layer had no vias to

connect to the 3 other layers. For the Medium

footprint, about 0.7 square inches (of 3 ounces

of) copper was used on the top layer, but vias

were used to connect to the other 3 layers. For

the Maximum footprint, about 1.0 square inch

(of 3 ounces of) copper was used on the top layer

and many vias were used to connect to the 3

other layers.

The results show that only about 0.7 square

inches (of 3 ounces of) copper on the top layer

and vias connecting to the 3 other layers are

needed to get the best thermal resistance of

36°C/W. Adding area on the top beyond the 0.7

square inches did not reduce thermal resistance.

SP765X Thermal Resistance

4 Layer Board:

Top Layer 3ounces Copper

GND Layer 1ounce Copper

Power Layer 1ounce Copper

Bottom Layer 3ounces Copper

Minimum Footprint: 44°C/W

Top Layer: 0.1 square inch

No Vias to other 3 Layers

Medium Footprint: 36°C/W

Top Layer: 0.7 square inch

Vias to other 3 Layers

Maximum Footprint: 36°C/W

Top Layer: 1.0 square inch

Vias to other 3 Layers

APPLICATIONS INFORMATION

Using a minimum of 0.1 square inches of (3

ounces of) copper on the top layer with no vias

connecting to the 3 other layers produced a

thermal resistance of 44°C/W. This thermal

impedance is only 22% higher than the medium

and large footprint layouts, indicating that space

constrained designs can still benefit thermally

from the Powerblox family of ICs. This indi-

cates that a minimum footprint of 0.1 square

inch, if used on a 4 layer board, can produce

44°C/W thermal resistance. This approach is

still very worthwhile if used in a space con-

strained design.

The following page shows the footprint layouts

from an ORCAD file. The thermal data was

taken for still air, not with forced air. If forced

air is used, some improvement in thermal resis-

tance would be seen.