Acoustic Fan Speed Control
R
Thermal/Mechanical Design Guide 57
Figure 19. Temperature Range = 5 °C
0
500
1000
1500
2000
2500
3000
3500
Time (s)
RPM
40
45
50
55
60
65
70
75
80
Tdiode (C)
Fan RPM Tdiode Tcontrol Tlow
An alternate would be to consider a slightly larger value such as T
RANGE
= 10 °C. In 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 Ψ
CA
vs. RPM is an exponential curve with a larger decrease at the beginning
of the fan acceleration than as the maximum speed is approached. By having the fan start to
accelerate at a lower T
DIODE
reading the thermal solution can 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
RANGE
value becomes a more acceptable
trade off.
Figure 20 graphs the system at the same conditions as in Figure 19, but T
RANGE
= 10°.
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