User guide

register to the desired fan frequency [read: speed]. The
8415Hz is divided down from the MAX6661 internal
clock, and has a ±25°C tolerance.
1) Determine the fan’s maximum tachometer frequency:
f
(TACH)
Hz = (rpm/60s / min) ✕ number of poles
Where poles = number of pulses per revolution.
Most fans are two poles; therefore, they have two
pulses per revolution.
2) Set the programmable FTD so that the frequency of
the fan tachometer divided by the prescaler value in
the FCD register falls in the 33Hz to 66Hz range.
3) Determine the value required for the fan FS register:
FS = 8415 / (f
TACH
✕ P)
Where P is the prescaler division ratio of the FCD
register.
Example: Fan A has a 2500rpm rating and 2 poles:
f
TACH
= 2500 / 60 ✕ 2 = 83.4Hz
MAX6661
Remote Temperature-Controlled Fan-Speed
Regulator with SPI-Compatible Interface
______________________________________________________________________________________ 17
N
TEMP DATA
FAN GAIN (FG)
8°C, 16°C, 32°C
RANGE
FAN-SPEED
CONTROL
1 TO 63
REF FREQ
8415Hz
FAN FULL
SCALE (FS)
127 TO 255
COUNTER
FAN
TACHOMETER
PERIOD (FTP)
FAN
TACHOMETER
PERIOD LIMIT (FTPL)
COMPARATOR
FAN TACHOMETER
DIVISOR (FTD)
1, 2, 4, 8
FAN OPEN/CLOSED
LOOP
UP/DOWN
DRIVER
DAC
FAN FAIL
V
FAN
TACH
FAN
TACH IN
Figure 7. MAX6661 Fan Loop Functional Diagram
REGISTER/ADDRESS FL (B1H = READ, B8H = WRITE)
COMMAND READ LIMIT/FAILURE REGISTER
Label
7
(MSB)
6
Data Bit
5
Data Bit
4
Data Bit
3
Data Bit
2
Data Bit
1
Data Bit
0
Data Bit
POR State 1 1 1 1 1 1 1 1
Table 11. Fan Tachometer Period Limit (RFTCL/WWFTCL)
Note: The fan tachometer period limit register is programmed with the maximum speed that is compared against the value in the FS
register to produce an error output to the status register.