Legacy Fan Speed Control
112 Thermal and Mechanical Design Guidelines
These are the minimum parameters required to implement acoustic fan speed control.
See
Figure 7-40 for an example. There may be vendor specific options that offer
enhanced functionality. See the appropriate vendor datasheet on how to implement
those features.
Figure 7-40. Fan Speed Control
Diode Temperature
(°C)
T
CONTROL
Min Speed
Fan Speed
(RPM)
Full Speed
T
LOW
X %
Fan Speed
(% PWM Duty Cycle)
100 %
Diode Temperature
(°C)
T
CONTROL
Min Speed
Fan Speed
(RPM)
Full Speed
T
LOW
X %
Fan Speed
(% PWM Duty Cycle)
100 %
E.2.1.1 Temperature to begin fan acceleration
The first item to consider is the value for T
LOW
. The FSC device needs a minimum
temperature to set as the threshold to begin increasing PWM duty cycle to the fan.
The system designer might initially consider a small temperature range (T
CONTROL
–
T
LOW
= T
RANGE
), 5 °C to accelerate the fan. That would delay the fan accelerating for
the longest time after an increase in T
SENSOR
. There are a number of issues that should
be considered with this strategy
• There is little granularity in the fan speeds. For each 1 °C of increase in diode
temperature = 20% jump in PWM duty cycle %
• Fan speed oscillation as the thermal solution chases the on-die thermal sensor
temperature
• Having T
SENSOR
overshoot T
CONTROL
and the thermal profile causing the Thermal
Control Circuit to activate to reduce the temperature.
• In extreme cases THERMTRIP# activates and shuts down the processor
The first two cases can create a poor acoustic response for the user. The third case
the user could notice a drop in performance as the thermal control circuit reduces the
power.
Figure 7-41 is an example of this situation. The system begins at idle and the
Maxpower program is started at 65% workload.
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