RPM-Based PWM Fan Controller
Datasheet
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 5.12 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.
Table 5.12 Minimum Edges for Fan Rotation
EDGES[1:0]
EFFECTIVE TACH
MULTIPLIER (BASED ON 2
POLE FANS)
MINIMUM TACH
EDGES
1
0
NUMBER OF FAN POLES
0
0
1
1
0
1
0
1
3
5
7
9
1 pole
0.5
1
2 poles (default)
3 poles
1.5
2
4 poles
Bit 2-0 - UPDATE[2:0] - 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 5.13.
Table 5.13 Update Time
UPDATE[2:0]
2
1
0
UPDATE TIME
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
100ms
200ms
300ms
400ms (default)
500ms
800ms
1200ms
1600ms
5.10
Fan Configuration 2 Register
Table 5.14 Fan Configuration 2 Register
ADDR
R/W
REGISTER
B7
B6
B5
B4
B3
B2
B1
B0
DEFAULT
Fan
Configuration 2
EN_
RRC
GLITCH_
EN
33h
R/W
-
DER_OPT [1:0]
ERR_RNG[1:0]
-
28h
Revision 1.1 (10-12-09)
SMSC EMC2301
DATA3S0HEET
SMSC [ SMSC CORPORATION ]
