Rainbow Electronics MAX6616 User Manual
Page 9
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°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 temperature.
For example, if the 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 the temperature decreases further, the
duty cycle is not updated until it reaches +75°C.
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
=
+
×
( )
-
MAX6615/MAX6616
Dual-Channel Temperature Monitors and
Fan-Speed Controllers with Thermistor Inputs
_______________________________________________________________________________________
9
+3.3V
PWM
18k
Ω
27k
Ω
10k
Ω
120k
Ω
+3.3V
+12V
500k
Ω
V
OUT
TO FAN
1
µF
1
µF
0.01
µF
0.1
µF
Figure 5. Driving a Fan with a PWM-to-DC Circuit
V
CC
PWM
4.7k
Ω
5V
Figure 6. Controlling a PWM Input Fan with the MAX6615/
MAX6616s’ PWM Output (Typically, the 35kHz PWM
Frequency Is Used)