Rainbow Electronics MAX6641 User Manual
Page 13
MAX6641
SMBus-Compatible Temperature Monitor with
Automatic PWM Fan-Speed Controller
______________________________________________________________________________________
13
The frequency-select register controls the frequency of
the PWM signal. When the PWM signal modulates the
power supply of the fan, a low PWM frequency (usually
33Hz) should be used to ensure the circuitry of the
brushless DC motor has enough time to operate. When
driving a fan with a PWM-to-DC circuit, as in
Figure
6,
the highest available frequency (35kHz) should be
used to minimize the size of the filter capacitors. When
using a fan with a PWM control input, the frequency
should normally be high as well, although some fans
have PWM inputs that accept low-frequency drive.
The duty cycle of the PWM can be controlled in two ways:
1) Manual PWM control by setting the duty cycle of
the fan directly through the fan-target duty-cycle
register (09h).
2) Automatic PWM control by setting the duty cycle
based on temperature.
Manual PWM Duty-Cycle Control
Setting bits D5 and D4 to zero in the fan-configuration
register (0Dh) enables manual PWMOUT control. In this
mode, the duty cycle written to the fan-target duty-
cycle register controls the PWMOUT duty cycle. The
value is clipped to a maximum of 240, which corre-
sponds to a 100% duty cycle. Any value above that is
limited to the maximum duty cycle. In manual control
mode, the value of the maximum duty-cycle register is
ignored and does not affect the duty cycle.
Automatic PWM Duty-Cycle Control
In the automatic control mode, the duty cycle is con-
trolled by the local or remote temperature, according to
the settings in the control registers. Below the value of
the fan-start temperature threshold (set by registers 03h
and 04h), the duty cycle is equal to the fan-start duty
cycle. Above the fan-start temperature, the duty cycle
increases by one duty-cycle step each time the tempera-
ture increases by one temperature step. Below the fan-
start temperature, the duty cycle is either 0% or it is
equal to the fan-start duty cycle, depending on the value
of bit D3 in the configuration byte register. See
Figure
8.
The target duty cycle is calculated based on the follow-
ing formula:
For temperature > fan-start temperature:
where:
DC = DutyCycle
FSDC = FanStartDutyCycle
T = Temperature
FST = FanStartTemperature
DCSS = DutyCycleStepSize
TS = TempStep
Duty cycle is recalculated after each temperature con-
version if temperature is increasing. If the temperature
begins to decrease, the duty cycle is not recalculated
until the temperature drops by 5
°C from the last peak
temperature. The duty cycle remains the same until the
temperature drops 5
°C from the last peak temperature
or the temperature rises above the last peak tempera-
ture. For example, if temperature goes up to +85
°C and
starts decreasing, duty cycle is not recalculated until
the temperature reaches +80
°C or the temperature
rises above +85
°C. If temperature decreases further,
the duty cycle is not updated until it reaches +75
°C.
For temperature < fan-start temperature and bit D3 of
the configuration byte register = 0:
DutyCycle = 0
For temperature < fan-start temperature and bit D3 of
the configuration byte register = 1:
Dutycycle = FanStartDutyCycle
Once the temperature crosses the fan-start tempera-
ture threshold, the temperature has to drop below the
fan-start temperature threshold minus the hysteresis
before the duty cycle returns to either 0% or fan-start
duty cycle. The value of the hysteresis is set by D7 of
the fan-configuration register.
DC
FSDC
T
FST
DCSS
TS
=
+
×
( )
-
FAN START
DUTY CYCLE
TEMPERATURE
DUTY CYCLE
REGISTER 02H,
BIT D3 = 1
DUTY CYCLE
STEP SIZE
FAN START
TEMPERATURE
TEMP
STEP
REGISTER 02H,
BIT D3 = 0
Figure 8. Automatic PWM Duty Control