EMC2106_09 Datasheet, PDF (71/106 Pages) SMSC Corporation – Dual RPM-Based Linear Fan Controller with Hardware Thermal Shutdown

Dual RPM-Based Linear Fan Controller with Hardware Thermal Shutdown
Datasheet
Bits 6- 5 - RANGE[1:0] - Adjusts the range of reported and programmed tachometer reading values.
The RANGE bits determine the weighting of all TACH values (including the Valid TACH Count, TACH
Target, and TACH reading) as shown in Table 6.32.
Table 6.32 Range Decode
RANGE[1:0]
1
0
0
0
0
1
1
0
1
1
REPORTED MINIMUM
RPM
500
1000 (default)
2000
4000
TACH COUNT
MULTIPLIER
1
2
4
8
Bits 4-3 - EDGES[1:0] - determines the minimum number of edges that must be detected on the
TACHx signal to determine a single rotation. A typical fan measured 5 edges (for a 2-pole fan). For
more accurate tachometer measurement, the minimum number of edges measured may be increased.
Increasing the number of edges measured with respect to the number of poles of the fan will cause
the TACH Reading registers to indicate a fan speed that is higher or lower than the actual speed. In
order for the FSC Algorithm to operate correctly, the TACH Target must be updated by the user to
accommodate this shift. The Effective Tach Multiplier shown in Table 6.33 is used as a direct multiplier
term that is applied to the Actual RPM to achieve the Reported RPM. It should only be applied if the
number of edges measured does not match the number of edges expected based on the number of
poles of the fan (which is fixed for any given fan).
Contact SMSC for recommended settings when using fans with more or less than 2 poles.
EDGES[1:0]
1
0
0
1
1
0
0
1
0
1
Table 6.33 Minimum Edges for Fan Rotation
MINIMUM TACH
EDGES
3
5
7
9
NUMBER OF FAN POLES
EFFECTIVE TACH
MULTIPLIER (BASED ON 2
POLE FANS)
1 pole
0.5
2 poles (default)
1
3 poles
1.5
4 poles
2
Bit 2-0 - UPDATE - determines the base time between fan driver updates. The Update Time, along
with the Fan Step Register, is used to control the ramp rate of the drive response to provide a cleaner
transition of the actual fan operation as the desired fan speed changes. The Update Time is set as
shown in Table 6.34.
SMSC EMC2106
71
DATASHEET
Revision 1.78 (04-21-09)