Multiple RPM-Based PWM Fan Controller for Five Fans
Datasheet
For example, if a desired RPM rate for a 2-pole fan is 3000 RPMs, then the user would input the
hexidecimal equivalent of 1296 (51h in the TACH Target Register). This number represents the number
of 32.768KHz cycles that would occur during the time it takes the fan to complete a single revolution
when it is spinning at 3000RPMs.
The EMC2305’s RPM-based Fan Speed Control Algorithm has programmable configuration settings
for parameters such as ramp-rate control and spin up conditions. The fan driver automatically detects
and attempts to alleviate a stalled/stuck fan condition while also asserting the ALERT# pin. The
EMC2305 works with fans that operate up to 16,000 RPMs and provide a valid tachometer signal.
The fan controller will function either with an externally supplied 32.768kHz clock source or with it’s
own internal 32kHz oscillator depending on the required accuracy. The EMC2305 offers a clock output
that enables additional devices to be slaved to the same clock source.
4.3.1
Programming the RPM-based Fan Speed Control Algorithm
The RPM-based Fan Speed Control Algorithm is disabled upon device power up. The following
registers control the algorithm. The EMC2305 fan control registers are pre-loaded with defaults that
will work for a wide variety of fans so only the TACH Target Register is required to set a fan speed.
The other fan control registers can be used to fine-tune the algorithm behavior based on application
requirements.
Note that steps 1 - 6 are optional and need only be performed if the default settings do not provide
the desired fan response.
1. Set the Spin Up Configuration Register to the Spin Up Level and Spin Time desired.
2. Set the Fan Step Register to the desired step size.
3. Set the Fan Minimum Drive Register to the minimum drive value that will maintain fan operation.
4. Set the Update Time and Edges options in the Fan Configuration Register.
5. Set the Valid TACH Count Register to the highest tach count that indicates the fan is spinning.
Refer to AN17.4 RPM to TACH Counts Conversion for examples and tables for supported RPM
ranges (500, 1k, 2k, 4k).
6. Set the TACH Target Register to the desired tachometer count.
7. Enable the RPM-based Fan Speed Control Algorithm by setting the EN_ALGO bit.
4.4
Tachometer Measurement
The tachometer measurement circuitry is used in conjunction with the RPM-based Fan Speed Control
Algorithm to update the fan driver output. Additionally, it can be used in Direct Setting mode as a
diagnostic for host based fan control.
This method monitors the TACHx signal in real time. It constantly updates the tachometer
measurement by reporting the number of clocks between a user programmed number of edges on the
TACHx signal (see Table 5.12).
The tachometer measurement provides fast response times for the RPM-based Fan Speed Control
Algorithm and the data is presented as a count value that represents the fan RPM period.
APPLICATION NOTE: The tachometer measurement method works independently of the drive settings. If the
device is put into Direct Setting and the fan drive is set at a level that is lower than the fan
can operate (including zero drive), then the tachometer measurement may signal a Stalled
Fan condition and assert an interrupt.
4.4.1
Stalled Fan
A Stalled fan is detected if the tach counter exceeds the user-programmable Valid TACH Count setting.
If a stall is detected, the device will flag the fan as stalled and trigger an interrupt.
Revision 1.1 (10-12-09)
SMSC EMC2305
DATA2S0HEET
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