Multiple RPM-Based PWM Fan Controller for Five Fans
Datasheet
100%
(optional)
30% through 65%
Fan Step
New Target Count
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 4.2 Spin Up Routine
4.6.1
Power Up Options
The EMC2305 allows for one of four fan speed options upon device power up depending on the status
of the pull-up resistor on the ADDR_SEL pin and the CLK pin. If the ADDR_SEL pin decode enables
the CLK pin (see Table 3.1), then the value of the pull-up resistor on the CLK pin is used to determine
the default fan drive setting (see Section 4.5.1).
If the Fan drive setting is set at a non-zero value (as determined by the CLK pin), then the drive setting
will be set to the desired setting. The Spin Up Routine will not be activated. This function does not
disable the Watchdog timer which will continue to function normally. See Section 4.8.
4.7
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 4.3.
SMSC EMC2305
Revision 1.1 (10-12-09)
DATA2S3HEET
SMSC [ SMSC CORPORATION ]
