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© Andigilog, Inc. 2006
www.andigilog.com
October 2006 - 70A06010
aSC7621
Automatic Fan Control
Auto Fan Control Operating Mode
The aSC7621 includes the circuitry for automatic fan control. In Auto Fan Mode, the aSC7621 will automatically adjust the
PWM duty cycle of the PWM output. PWM outputs are assigned to a thermal zone based on the fan configuration registers.
At any time, the temperature of a zone exceeds its Absolute Limit, all PWM outputs will go to 100% duty cycle to provide
maximum cooling to the system.
Figure 7 Automatic Fan Speed Control Example
Example for PWM1 assigned to Zone 1:
•
Zone 1 Fan Temp Limit (Register 67h) is set to 50
°C (32h).
•
Zone 1 Range (Register 5Fh) is set to 8
°C (6xh).
•
Fan PWM Minimum (Register 64h) is set to 50% (80h).
In this case, the PWM duty cycle will be 50% at 50
°C.
Since (Zone 1 Fan Temp Limit) + (Zone 1 Range) = 50
°C + 8°C = 58°C, the fan will run at 100% duty cycle when the
temperature of the Zone 1 sensor reaches 58
°C.
Since the midpoint of the fan control range is 54
°C, and the median duty cycle is 75% (Halfway between the PWM Minimum
and 100%), PWM1 duty cycle would be 75% at 54
°C.
Above (Zone 1 Fan Temp Limit) + (Zone 1 Range), the duty cycle will be 100%.
5 °C
Hysteresis
Temperature °C
PWM set to Off or
Minimum Below
this Temperature
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
PWM
Duty
Cycl
e lin
early
Incre
asin
g wit
h Te
mp
PWM
Duty
Cyc
le lin
early
Dec
reas
ing w
ith T
emp
PWM Set to Off
Example
45
58
80
Min%
100%
PWM %
Minimum PWM set to 50%, fan speed increases linearly beyond 50 °C but will not return to
off until it has gone below Fan Temp Limit by the 5 °C Hysteresis setting to 45°C.
Temperature
PWM %
Linear Control Range
8 °C
Range
Off/
Min%
PWM Set to
Minimum
100%
0%
50
User Choice: Set to
Minimum or Off
5 °C
Hysteresis
Temperature °C
PWM set to Off or
Minimum Below
this Temperature
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
PWM
Duty
Cycl
e lin
early
Incre
asin
g wit
h Te
mp
PWM
Duty
Cyc
le lin
early
Dec
reas
ing w
ith T
emp
PWM Set to Off
Example
45
58
80
Min%
100%
PWM %
Minimum PWM set to 50%, fan speed increases linearly beyond 50 °C but will not return to
off until it has gone below Fan Temp Limit by the 5 °C Hysteresis setting to 45°C.
Temperature
PWM %
Linear Control Range
8 °C
Range
Off/
Min%
PWM Set to
Minimum
100%
0%
50
User Choice: Set to
Minimum or Off
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