SMSC EMC2102 - Manual (Page 34)

Fan Speed Control with the EMC2102 Device
Revision 0.2 (09-17-07)
USER MANUAL
SMSC EMC2102
34
5.7.1.5 Case 5 - Correct RPM Relationships with a Slow Response DC Fan
In this case, the spin-up routine is too short to make the fan reach a speed higher than the valid speed.
Since the EMC2102 cannot detect a valid TACH at the end of spin-up routine, it will try to restart the
fan. Depending on the fan, it is possible that after several spin-up cycles, the fan can reach the valid
speed and operate normally.
Using the EVB-EMC2102 to test this situation with rpm60 (~4500 rpm), simply set the SPINUP_TIME
(53h[1:0]) = 250 ms, Valid TACH (56h) = 4400 rpm, and start the fan with TACH Target (57h) = 5500
rpm. Multiple spin-up routines will be observed (
Figure 5.34, "Spin-up Case 5" ).
To fix this problem, stop the fan and set the SPINUP_TIME = 2s, and restart the fan with 5500 rpm.
Increasing Spin-up time will make a longer 100% "kick" time and provide the fan with more power to
speed-up (
Figure 5.35, "Spin-up Case 5 Fix" ).
The problem also can be fixed by changing the spin up level from 60% to 75% if not done already, as
previously discussed.
Figure 5.34 Spin-up Case 5
Figure 5.35 Spin-up Case 5 Fix
Longer 100% kick provides more
driving power to speed-up the fan