SMSC EMC2102 - Manual (Page 31)

Fan Speed Control with the EMC2102 Device
SMSC EMC2102
USER MANUAL
Revision 0.2 (09-17-07)
31
5.7.1.3 Case 3 - Valid RPM =< Target RPM << Spin-up Level RPM
In this case, the target speed is only a little greater than the valid speed, and both of them are much
smaller than the spin-up level speed (rpm60 or rpm75). The theoretical plot of this situation is shown
in
Figure 5.29, "Theoretical Plot of Case 3" .
Since the fan speed at the end of spin-up routine is higher than the minimum valid speed, the
EMC2102 should go to the normal operation after the spin-up routine. However, because the large
speed difference between the spin-up level speed (rpm60 or rpm75) and the target speed, the closed-
loop controller will try to make a big adjustment of its output to reach the target speed. With incorrect
settings (such as big output step or short update time), this adjustment could easily cause an
undershoot and make the fan speed below the valid rpm, and than make the spin-up routine restart.
Figure 5.28 Spin-up Case 2
Figure 5.29 Theoretical Plot of Case 3
Fan Drive Setting (51h)
RPM for
100% Drive
rpm60 or rpm75
Spin Up Time
¼ of Spin Up Time
Valid Speed
Check TACH
Fan Speed
(RPM)
Target Speed
Fan Speed