GX1 Datasheet, PDF (208/247 Pages) National Semiconductor (TI) – Processor Series Low Power Integrated x86 Solution

Package Specifications (Continued)
Core Voltage
(V_CC2)
2.0V
(Nominal)
1.8V
(Nominal)
1.6V
(Nominal)
Table 7-2. Case-to-Ambient Thermal Resistance Examples @ 85°C
Core
Frequency
Maximum
Power (W)
θ_CA for Different Ambient Temperatures (°C/W)
20°C
25°C
30°C
35°C
40°C
300 MHz
3.7
17
16
15
13
12
266 MHz
3.0
233 MHz
2.8
200 MHz
2.3
22
20
19
17
15
23
22
20
18
16
29
26
24
22
20
7.1.1 Heatsink Considerations
Table 7-2 shows the maximum allowed thermal resistance
of a heatsink for particular operating environments. The
calculated values, defined as θ_CA, represent the required
ability of a particular heatsink to transfer heat generated by
the processor 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 heatsink’s ability to dissipate heat or it’s thermal conduc-
tivity.
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 7-2, it can be seen that as ambient tem-
perature (T_A) increases, θ_CA decreases, and that as power
consumption of the processor (P) increases, θ_CA
decreases. Thus, the ability of the heatsink to dissipate ther-
mal 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 efficient as the amount of heat they are trying
to dissipate increases. For this reason, heatsinks are typi-
cally specified by graphs that plot heat dissipation (in watts)
vs. mounting surface (case) temperature rise above ambi-
ent (in °C). This method is necessary because ambient and
case temperatures fluctuate constantly during normal oper-
ation of the system. The system 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 Table
6-4 on page 190) is never exceeded. To choose the proper
heatsink, the system designer must make sure that the cal-
culated θ_CA falls above the curve (shaded area). The curve
itself defines the minimum temperature rise above ambient
that the heatsink can maintain.
See Figure 7-1 as an example of a particular heatsink
under consideration.
θ_{CA = 45/5 = 9}
50
40
30
θ_{CA = 45/9 = 5}
20
10
0
2
4
6
8
10
Heat Dissipated - Watts
Figure 7-1. Heatsink Example
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