Rainbow Electronics MAX6678 - Manual (Page 8)

MAX6678
PWM Output
1) The PWMOUT_ signals are normally used in one of
three ways to control the fan’s speed: PWMOUT_ dri-
ves the gate of a MOSFET or the base of a bipolar
transistor in series with the fan’s power supply. The
Typical Application Circuit shows the PWMOUT_ dri-
ving an n-channel MOSFET. In this case, the PWM
invert bit (D4 in register 02h) is set to 1.
Figure 4
shows PWMOUT_ driving a p-channel MOSFET and
the PWM invert bit must be set to zero.
2) PWMOUT_ is converted (using an external circuit)
into a DC voltage that is proportional to duty cycle.
This duty-cycle-controlled voltage becomes the
power supply for the fan. This approach is less effi-
cient than 1), but can result in quieter fan operation.
Figure 5 shows an example of a circuit that converts
the PWM signal to a DC voltage. Because this circuit
produces a full-scale output voltage when PWMOUT
= 0V, bit D4 in register 02h should be set to zero.
3) PWMOUT_ directly drives the logic-level PWM
speed-control input on a fan that has this type of
input. This approach requires fewer external compo-
nents and combines the efficiency of 1) with the low
noise of 2). An example of PWMOUT_ driving a fan
with a speed-control input is shown in Figure 6. Bit
D4 in register 02h should be set to 1 when this con-
figuration is used.
Whenever the fan has to start turning from a motionless
state, PWMOUT_ is forced high for 2s. After this spin-up
period, the PWMOUT_ duty cycle settles to the prede-
termined value. Whenever spin-up is disabled (bit 2 in
the configuration byte = 1) and the fan is off, the duty
cycle changes immediately from zero to the nominal
value, ignoring the duty-cycle rate-of-change setting.
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 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 frequency
normally should be high as well, although some fans
have PWM inputs that accept low-frequency drive.
2-Channel Temperature Monitor with Dual Automatic
PWM Fan-Speed Controller and Five GPIOs
8 _______________________________________________________________________________________
V CC
PWMOUT
10kΩ
5V
P
Figure 4. Driving a P-Channel MOSFET for Top-Side PWM Fan
Drive
+3.3V
PWMOUT
18kΩ
27kΩ
10kΩ 120kΩ
+3.3V
+12V
500kΩ
V OUT
TO FAN
1µF
1µF
0.01µF
0.1µF
Figure 5. Driving a Fan with a PWM-to-DC Circuit
V
CC
PWMOUT
4.7kΩ
5V
Figure 6. Controlling a PWM Input Fan with the MAX6678’s
PWM Output (Typically, the 35kHz PWM Frequency Is Used)