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Package Specifications (Continued)

To choose the proper heatsink, the system designer must

make sure that the calculated

CA falls above the curve

(shaded area). The curve itself defines the minimum tem-

perature rise above ambient that the heatsink can main-

tain.

See Figure 8-1 as an example of a particular heatsink

under consideration.

Figure 8-1. Heatsink Example

Example 1

Assume P (max) = 5W and TA (max) = 40°C.

Therefore:

In this case, the heatsink under consideration is more than

adequate since at 5W worst case, it can maintain a 40°C

case temperature rise above ambient (

CA = 9) when a

maximum of 45°C (

CA = 8) is required.

Example 2

Assume P (max) = 10W and TA (max) = 40°C.

Therefore:

In this case, the heatsink under consideration is NOT ade-

quate to maintain the 45°C case temperature rise above

ambient for a 9W processor.

For more information on thermal design considerations or

heatsink properties, refer to the Product Selection Guide of

any leading vendor of Thermal Engineering solutions.

CA = 45/9 = 5

Heat Dissipated - Watts

CA = 45/5 = 9

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