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
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]
1
0
0
1
1
0
1
0
1
0
MINIMUM TACH
EDGES
3
5
7
9
NUMBER OF FAN POLES
1 pole
2 poles (default)
3 poles
4 poles
EFFECTIVE TACH
MULTIPLIER (BASED ON 2
POLE FANS)
0.5
1
1.5
2
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 Update Time
UPDATE[2:0]
2
0
0
0
0
1
1
1
1
1
0
0
1
1
0
0
1
1
0
0
1
0
1
0
1
0
1
UPDATE TIME
100ms
200ms
300ms
400ms (default)
500ms
800ms
1200ms
1600ms
5.10
Fan Configuration 2 Register
Table 5.14 Fan Configuration 2 Register
ADDR
33h
R/W
R/W
REGISTER
Fan
Configuration 2
B7
-
B6
EN_
RRC
B5
GLITCH_
EN
B4
B3
B2
B1
B0
-
DEFAULT
28h
DER_OPT [1:0]
ERR_RNG[1:0]
Revision 1.1 (10-12-09)
30
SMSC EMC2301
DATASHEET
SMSC [ SMSC CORPORATION ]
