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Revision 1.1
G
Package Specifications
(Continued)
8.1.1
Table 8-2 shows the maximum allowed thermal resistance of a heatsink for particular operating environments. The calcu-
lated values, defined as
θ
CA
, represent the required ability of a particular heatsink to transfer heat generated by the SC1100
from its case into the air, thereby maintaining the case temperature at or below 85
°
C. Because
θ
CA
is a measure of thermal
resistivity, it is inversely proportional to the heatsinks ability to dissipate heat or its thermal conductivity.
Heatsink Considerations
Note:
A “perfect” heatsink would be able to maintain a case temperature equal to that of the ambient air inside the system
chassis.
Looking at Table 8-2, it can be seen that as ambient temperature (T
A
) increases,
θ
CA
decreases, and that as power con-
sumption of the processor (P) increases,
θ
CA
decreases. Thus, the ability of the heatsink to dissipate thermal energy must
increase as the processor power increases and as the temperature inside the enclosure increases.
While
θ
CA
is a useful parameter to calculate, heatsinks are not typically specified in terms of a single
θ
CA
.This is because
the thermal resistivity of a heatsink is not constant across power or temperature. In fact, heatsinks become slightly less effi-
cient as the amount of heat they are trying to dissipate increases. For this reason, heatsinks are typically specified by
graphs that plot heat dissipation (in watts) vs. mounting surface (case) temperature rise above ambient (in
°
C). This method
is necessary because ambient and case temperatures fluctuate constantly during normal operation of the system. The sys-
tem designer must be careful to choose the proper heatsink by matching the required
θ
CA
with the thermal dissipation curve
of the device under the entire range of operating conditions in order to make sure that the maximum case temperature
(from
Table 7-2 on page 281
) is never exceeded. To choose the proper heatsink, the system designer must make sure that
the calculated
θ
CA
falls above the curve (shaded area). The curve itself defines the minimum temperature rise above ambi-
ent that the heatsink can maintain.
Table 8-2. Case-to-Ambient Thermal Resistance Examples @ 85
°
C
Core Voltage
(V
CORE
)
(Nominal)
Core
Frequency
Maximum
Power (W)
θ
CA
for Different Ambient Temperatures (
°
C/W)
20
°
C
25
°
C
30
°
C
35
°
C
40
°
C
TBD
300 MHz
TBD
TBD
TBD
TBD
TBD
TBD
2.0V
266 MHz
2.98
21.84
20.16
18.48
16.8
15.12
1.8V
233 MHz
2.77
23.52
21.71
19.9
18.09
16.28