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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
E.2.1.1
Temperature to begin fan acceleration
The first item to consider is the value for T
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
T
= T
LOW
the longest time after an increase in T
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
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.
112
Full Speed
Full Speed
Min Speed
Min Speed
), 5 °C to accelerate the fan. That would delay the fan accelerating for
RANGE
overshoot T
SENSOR
CONTROL
T
T
T
T
LOW
LOW
CONTROL
CONTROL
Diode Temperature
Diode Temperature
(°C)
(°C)
. The FSC device needs a minimum
LOW
. There are a number of issues that should
SENSOR
and the thermal profile causing the Thermal
Thermal and Mechanical Design Guidelines
Legacy Fan Speed Control
100 %
100 %
X %
X %
–
CONTROL
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