Dual RPM-Based PWM Fan Controller with Hardware Thermal Shutdown
Datasheet
100%
(optional)
30% through 65%
New Target Count
Fan Step
Algorithm controlled drive
Prev Target
Count = FFh
¼ of Spin Up Time
Update Time
Spin Up Time
Target Count
Reached
Check TACH
Target Count
Changed
Figure 5.5 Spin Up Routine
5.8
Ramp Rate Control
The Fan Driver can be configured with automatic ramp rate control. Ramp rate control is accomplished
by adjusting the drive output settings based on the Maximum Fan Step Register settings and the
Update Time settings.
If the RPM based Fan Speed Control Algorithm is used, then this ramp rate control is automatically
used. The user programs a maximum step size for the fan drive setting and an update time. The
update time varies from 100ms to 1.6s while the fan drive maximum step can vary from 1 count to 31
counts.
When a new fan drive setting is entered, the delta from the next fan drive setting and the previous fan
drive setting is determined. If this delta is greater than the Max Step settings, then the fan drive setting
is incrementally adjusted every 100ms to 1.6s as determined by the Update Time until the target fan
drive setting is reached. See Figure 5.6.
SMSC EMC2104
Revision 1.74 (05-08-08)
DATA3S3HEET
SMSC [ SMSC CORPORATION ]
