Fan Control Device with Hardware Monitoring and Acoustic Noise Reduction Features
Datasheet
MIN/OFF bit = 0
(Fan off when temperature is below minimum)
MIN/OFF bit = 1
(Fan stays on when temperature is below minimum)
Temp
Temp
Tmax
=Tmin
+Trange
Tmax
=Tmin
+Trange
Hysteresis
Temperature
Tmin
Tmin
Time
Time
Time
PWM
Duty
Cycle
PWM
Duty
Cycle
Spin-Up
Time
Max
=FFh
Max
=FFh
Hysteresis
min
min
Time
Notes:
1. When exiting spin-up, the PWM is set to the current calculated PWM
2. The PWM is set to OFF when the current temperature is less than
(Tmin-Hysteresis Temp)
Figure 6.2 Automatic Fan Control
6.1.3.3
Spin Up
When a fan is being started from a stationary state (PWM duty cycle =00h), the part will cause the fan
to “spin up” by going to 100% duty cycle for a programmable amount of time to overcome the inertia
of the fan (i.e., to get the fan turning). Following this spin up time, the fan will go to the duty cycle
computed by the auto fan algorithm.
During spin-up, the PWM duty cycle is reported as 0%.
To limit the spin-up time and thereby reduce fan noise, the part uses feedback from the tachometers
to determine when each fan has started spinning properly. The following tachometer feedback is
included into the auto fan algorithm during spin-up.
Auto Fan operation during Spin Up:
The PWM goes to 100% duty cycle until the tachometer reading register is below the minimum limit
(see Figure 6.3), or the spin-up time expires, whichever comes first. This causes spin-up to continue
until the tachometer enters the valid count range, unless the spin up time expires. If the spin up expires
before the tachometer enters the valid range, an interrupt status bit will be set once spin-up expires.
Note that more than one tachometer may be associated with a PWM, in which case all tachometers
associated with a PWM must be in the valid range for spin-up to end.
SMSC EMC6D102
Revision 0.4 (04-05-05)
DATA3S3HEET
SMSC [ SMSC CORPORATION ]
