brushless DC motor has enough time to operate. When
driving a fan with a PWM-to-DC circuit as shown in
Figure 5, 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 frequen-
cy normally should 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: setting the duty cycle of the fan
directly through the fan target duty-cycle registers
(0Bh and 0Ch).
2) Automatic PWM control: setting the duty cycle based
on temperature.
Manual PWM Duty-Cycle Control
Clearing the bits that select the temperature channels for
fan control (D5 and D4 for PWM1 and D3 and D2 for
PWM2) in the fan-configuration register (11h) enables
manual fan control. In this mode, the duty cycle written to
the fan target duty-cycle register directly controls the
corresponding fan. The value is clipped to a maximum of
240. Any value entered above that is changed to 240
automatically. In this control mode, the value in the maxi-
mum duty-cycle register is ignored and does not affect
the duty cycle used to control the fan.
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 fan-start
temperature, the duty cycle is either 0% or is equal to
the fan-start duty cycle, depending on the value of bit
D3 in the configuration byte register. Above the fan-
start temperature, the duty cycle increases by one
duty-cycle step each time the temperature increases by
one temperature step. The target duty cycle is calculat-
ed based on the following formula; for temperature >
FanStartTemperature:
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癈 from the last peak
temperature. The duty cycle remains the same until the
temperature drops 5癈 from the last peak temperature or
the temperature rises above the last peak temperature.
For example, if the temperature goes up to +85癈 and
starts decreasing, duty cycle is not recalculated until the
temperature reaches +80癈 or the temperature rises
above +85癈. If the temperature decreases further, the
duty cycle is not updated until it reaches +75癈.
For temperature < FanStartTemperature and D2 of
configuration register = 0:
DutyCycle = 0
For temperature < FanStartTemperature and D2 of
configuration register = 1:
DutyCycle = FanStartDutyCycle
Once the temperature crosses the fan-start temperature
threshold, the temperature has to drop below the fan-
start temperature threshold minus the hysteresis before
DC   FSDC    T   FST
DCSS
TS
=
+
?/DIV>
(  
)  
-
Dual-Channel Temperature Monitors and
Fan-Speed Controllers with Thermistor Inputs
_______________________________________________________________________________________   9
+3.3V
PWM
18k&
27k&
10k&
120k&
+3.3V
+12V
500k&
V
OUT
TO FAN
1礔
1礔
0.01礔
0.1礔
Figure 5. Driving a Fan with a PWM-to-DC Circuit
V
CC
PWM
4.7k&
5V
Figure 6. Controlling a PWM Input Fan with the MAX6615/
MAX6616s PWM Output (Typically, the 35kHz PWM
Frequency Is Used)