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© Andigilog, Inc. 2006
www.andigilog.com
October 2006 - 70A06010
aSC7621
Automatic Fan Speed Control using Maximum PWM Setting
The previously described and illustrated mode had no restriction on the maximum PWM setting. It is useful to limit the
maximum PWM command sent to the fan in order to minimize the acoustic impact. The Maximum PWM setting will clamp
the automatic fan PWM command at a user selected value. The Absolute Limit setting will still cause the PWM command to
be 100% and that will remain until the temperature falls below the Absolute Limit temperature by an amount equal to the
hysteresis setting. This will minimize the acoustic impact of having a temperature moving back and forth close to the
Absolute Limit.
The Absolute Limit may be set above or below the Fan Temp Limit plus Range. The PWM value will be overridden and will
follow the hysteresis curve in either case, but the acoustic impact will be different, running the fan to 100% PWM at a lower
temperature, but enhancing the cooling effect. Absolute Limit set on the low end is shown in Figure 8Figure 7. Setting it
above is shown in Figure 9.
It is important to consider the combination of Fan Temp Limit, Range, Maximum PWM and Absolute Limit and their impact
on cooling and acoustics. In addition, the capability to operate a fan from a combination of thermal zones allows a compound
linear slope to be achieved for further optimization.
Figure 8 Fan Control with Absolute Limit Set below Fan Temp Limit Plus Range
PWM set to Off
PWM 100%
Fan Temp Limit
less Hysteresis
Fan Temp
Limit
Hysteresis
(0
°C to 15 °C)
Range
(2
°C to 80 °C)
Fan Temp Limit
plus Range
Absolute
Limit
PW
M D
uty
Cyc
le l
ine
arly
Inc
rea
sin
g w
ith
Tem
p
PW
M D
uty
Cyc
le li
nea
rly
Dec
rea
sing
with
Tem
p
Fan Set to Off
Temperature
Fan
Speed
Linear Control Range
Fan Set to
Minimum
Hysteresis
(0
°C to 15 °C)
Maximum
PWM %
PWM set to Off
PWM 100%
Fan Temp Limit
less Hysteresis
Fan Temp
Limit
Hysteresis
(0
°C to 15 °C)
Range
(2
°C to 80 °C)
Fan Temp Limit
plus Range
Absolute
Limit
PW
M D
uty
Cyc
le l
ine
arly
Inc
rea
sin
g w
ith
Tem
p
PW
M D
uty
Cyc
le li
nea
rly
Dec
rea
sing
with
Tem
p
Fan Set to Off
Temperature
Fan
Speed
Linear Control Range
Fan Set to
Minimum
Hysteresis
(0
°C to 15 °C)
Maximum
PWM %
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