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Legacy Fan Speed Control
Figure 7-41. Temperature Range = 5 °C
An alternate would be to consider a slightly larger value such as T
this case the design is trading off the acoustic margin for thermal margin.
There is increased granularity in the fan speeds.
Fan speed oscillation are significantly reduced
Maximum fan speed is lower
The rate of change of
the beginning of the fan acceleration than as the maximum speed is approached. By
having the fan start to accelerate at a lower T
keep up with rate of change in processor power. The rate of change in acoustics (dBA)
is more linear with RPM. When comparing these two metrics the choice of a larger
T
value becomes a more acceptable trade off. Figure 7-42 graphs the system at
RANGE
the same conditions as in Figure 7-41 but T
Thermal and Mechanical Design Guidelines
Fan RPM
3500
3000
2500
2000
1500
1000
500
0
vs. RPM is an exponential curve with a larger decrease at
CA
Tdiode
Tcontrol
Time (s)
reading the thermal solution can
SENSOR
= 10 °C.
RANGE
Tlow
80
75
70
65
60
55
50
45
40
= 10 °C. In
RANGE
113
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