Package Specifications (Continued)
To choose the proper heatsink, the system designer must
make sure that the calculated θCA falls above the curve
(shaded area). The curve itself defines the minimum tem-
perature rise above ambient that the heatsink can main-
tain.
Example 1
Assume P (max) = 5W and TA (max) = 40°C.
Therefore:
T
– T
C
A
See Figure 8-1 as an example of a particular heatsink
under consideration.
θ
θ
= ---------------------
CA
P
(85 – 40)
= ----------------------
5
CA
θ
CA = 45/5 = 9
50
40
30
20
10
0
θ
= 9
CA
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 (θCA = 9) when a
maximum of 45°C (θCA = 8) is required.
θ
CA = 45/9 = 5
Example 2
2
4
6
8
10
Assume P (max) = 10W and TA (max) = 40°C.
Heat Dissipated - Watts
Therefore:
Figure 8-1. Heatsink Example
T
– T
C
A
θ
θ
= ---------------------
CA
P
(85 – 40)
= ----------------------
9
CA
θ
= 5
CA
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.
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