Fan Control Device with Hardware Monitoring and Acoustic Noise Reduction Features
Datasheet
The speed of the fan is controlled by the duty cycle set for that device. The duty cycle for the minimum
fan speed must be programmed in Registers 64h-66h: PWMx Minimum Duty Cycle. This value
corresponds to the speed of the fan when the temperature reading is equal to the minimum
temperature LIMIT setting. As the actual temperature increases and is above the Zone LIMIT
temperature and below the Absolute Temperature Limit, the PWM will be determined by a linear
function based on the Auto Fan Speed Range bits in Registers 5Fh-61h.
Set the absolute temperature for each zone in Registers 6Ah-6Ch: Zone x Temp Absolute Limit (Auto
Fan Mode only). If the actual temperature is equal to or exceeds the absolute temperature in one or
more of the associated zones, all Fans operating in auto mode will be set to Full on, regardless of
which zone they are operating in (except those that are disabled or configured for Manual Mode).
Note: Fans can be disabled via the PWMx Configuration registers and the absolute temperature
safety feature can be disabled by writing 80h into the Zone x Temp Absolute Limit registers.
To set the mode to operate in auto mode, set Bits[7:5] Zone/Mode, located in Registers 5Ch-5Eh: PWM
Configuration Bits[7:5]=’000’ for PWM on Zone 1; Bits[7:5]=’001’ for PWM on Zone 2; Bits[7:5]=’010’
for PWM on Zone 3. If the “Hottest” option is chosen (101 or 110), then the PWM output is controlled
by the zone that results in the highest PWM duty cycle value.
Notes:
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Software can be alerted of an out-of-limit condition by the INT# pin if a status bit is set and enabled
and the interrupt function is enabled on either the PWM2 or TACH3 pins
Software can monitor the operation of the Fans through the Fan Tachometer Reading registers and
by the PWM x Current PWM duty registers. It can also monitor current temperature readings
through the Temperature Limit Registers if hardware monitoring is enabled.
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Fan control in auto mode is implemented without any input from external processor .
In auto “Zone” mode, the speed is adjusted automatically as shown in the figure below. Fans are
assigned to a zone(s). It is possible to have more than one fan assigned to a thermal zone or to have
multiple zones assigned to one fan.
Figure 6.2 on page 33 shows the control for the auto fan algorithm. The part allows a minimum
temperature to be set, below which the fan will not run or will run at minimum speed (depending on
the setting of the Min/OFF bits). A hysteresis value is included to prevent the fan continuously
switching on and off if the temperature is close to the minimum. A temperature range is specified over
which the part will automatically adjust the fan speed. If the fan is off and the current temperature is
below the minimum temperature, then when the temperature exceeds the minimum, the fan will “spin
up” by going on full for a programmable amount of time. Following this spin up time, the fan will go to
a duty cycle computed by the auto fan algorithm. As the temperature rises, the duty cycle will increase
until the fan is running at full-speed when the temperature reaches the minimum plus the range value.
The effect of this is a temperature feedback loop, which will cause the temperature to reach equilibrium
between the minimum temperature and the minimum temperature plus the range. Provided that the
fan has adequate cooling capacity for all environmental and power dissipation conditions, this system
will maintain the temperature within acceptable limits, while allowing the fan to run slower (and quieter)
when less cooling is required.
Revision 0.4 (04-05-05)
SMSC EMC6D102
DATA3S2HEET
SMSC [ SMSC CORPORATION ]
