MAX6641
SMBus-Compatible Temperature Monitor with
Automatic PWM Fan-Speed Controller
______________________________________________________________________________________ 11
Fan Instantaneous Duty Cycle (0Ah)
Read the fan instantaneous duty-cycle register to deter-
mine the duty cycle at PWMOUT at any time. The POR
state of the fan instantaneous duty-cycle register is 00h.
Remote- and Local-Diode
Fan-Start Temperature (0Bh, 0Ch)
These registers contain the temperature threshold val-
ues at which fan control begins in automatic mode. See
the Automatic PWM Duty-Cycle Control section for
details on setting the fan-start thresholds. The POR
state of the remote- and local-diode fan-start tempera-
ture registers is 00h.
Fan Configuration (0Dh)
The fan-configuration register controls the hysteresis
level, temperature step size, and whether the remote or
local diode controls the PWMOUT signal; see Table 1.
Set bit D7 of the fan-configuration register to zero to set
the hysteresis value to 5°C. Set bit D7 to 1 to set the
hysteresis value to 10°C. Set bit D6 to zero to set the
fan-control temperature step size to 1°C. Set bit D6 to 1
to set the fan-control temperature step size to 2°C. Set
bit D5 to 1 to control the fan with the remote-diode’s
temperature reading. Set bit D4 to 1 to control the fan
with the local-diode’s temperature reading. If both bits
D5 and D4 are high, the device uses the highest PWM
value. If both bits D5 and D4 are zero, the MAX6641
runs in manual fan-control mode where only the value
written to the fan-target duty-cycle register (09h) con-
trols the PWMOUT duty cycle. In manual fan-control
mode, the value written to the fan-target duty-cycle reg-
ister is not limited by the value in the maximum duty-
cycle register. It is, however, clipped to 240 if a value
above 240 is written. The POR state of the fan-configu-
ration register is 00h.
Duty-Cycle Rate of Change (0Eh)
Bits D7, D6, and D5 of the duty-cycle rate-of-change
register set the time between increments of the duty
cycle. Each increment is 2/240 of the duty cycle; see
Table 3. This allows the time from 33% to 100% duty
cycle to be adjusted from 5s to 320s. The rate-of-
change control is always active in manual mode. To
make instant changes, set bits D7, D6, D5 = 000. The
POR state of the duty-cycle rate-of-change register is
A0h (1s time between increments).
Duty-Cycle Step Size (0Fh)
Bits D7–D4 of the duty-cycle step-size register change
the size of the duty-cycle change for each temperature
step. The POR state of the duty-cycle step-size register
is 50h; see Table 4.
PWM Frequency Select (10h)
Set bits D7, D6, and D5 (select A, select B, and select
C) in the PWM frequency-select register to control the
PWMOUT frequency; see Table 5. The POR state of the
PWM frequency select register is 40h, 33Hz. The lower
frequencies are usually used when driving the fan’s
power-supply pin as in the Typical Application Circuit,
with 33Hz being the most common choice. The 35kHz
