Multiple RPM-Based PWM Fan Controller for Five Fans
Datasheet
4.5.3
Internal Clock
Alternately, the EMC2305 may be configured to use its internal clock as a clock output to drive other
fan driver devices. When configured to operate in this mode, the device uses its internal clock for
tachometer reading and drives the CLK pin using a push-pull driver.
4.6
Spin Up Routine
The EMC2305 also contains programmable circuitry to control the spin up behavior of the fan driver
to ensure proper fan operation.
The Spin Up Routine is initiated in Direct Setting mode when the setting value changes from 00h to
anything else.
When the Fan Speed Control Algorithm is enabled, the Spin Up Routine is initiated under the following
conditions:
1. The TACH Target Register value changes from a value of FFh to a value that is less than the Valid
TACH Count (see Section 5.15).
2. The RPM-based Fan Speed Control Algorithm’s measured TACH Reading Register value is greater
than the Valid TACH Count setting.
When the Spin Up Routine is operating, the fan driver is set to full scale (optional) for one quarter of
the total user defined spin up time. For the remaining spin up time, the fan driver output is set at a
user defined level (30% through 65% drive).
After the Spin Up Routine has finished, the EMC2305 measures the TACHx signal. If the measured
TACH Reading Register value is higher than the Valid TACH Count Register setting, the FAN_SPIN
status bit is set and the Spin Up Routine will automatically attempt to restart the fan.
Figure 4.2 shows an example of the Spin Up Routine in response to a programmed fan speed change
based on the first condition above.
Revision 1.1 (10-12-09)
SMSC EMC2305
DATA2S2HEET
SMSC [ SMSC CORPORATION ]
