Rainbow Electronics MAX6641 - Manual (Page 12)

frequency setting is used for controlling fans that have
logic-level PWM input pins for speed control. Duty-
cycle resolution is decreased from 2/240 to 4/240 at the
35kHz frequency setting.
PWM Output
The PWMOUT signal is normally used in one of three
ways to control the fan’s speed:
1)
PWMOUT drives 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 pin driving an n-channel MOSFET. In this
case, the PWM invert bit (D4 in register 02h) is set to
1.
Figure 5 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 6 shows an example of a circuit that con-
verts 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 com-
ponents 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 7.
Bit D4 in register 02h should be set to 1 when this
configuration 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 predeter-
mined value. If spin-up is disabled (bit 2 in the configu-
ration byte = 1), the duty cycle changes immediately
from zero to the nominal value, ignoring the duty-cycle
rate-of-change setting.
MAX6641
SMBus-Compatible Temperature Monitor with
Automatic PWM Fan-Speed Controller
12 ______________________________________________________________________________________
PWM
FREQUENCY
(Hz)
SELECT A SELECT B SELECT C
20 0 0 0
33 0 1 0
50 1 0 0
100 1 1 0
35k X X 1
Table 5. PWM Frequency Select (10h)
V
CC
PWMOUT
10k Ω
5V
P
Figure 5. Driving a P-Channel MOSFET for Top-Side PWM
Fan Drive
+3.3V
PWMOUT
18k Ω
27k Ω
10k Ω 120k Ω
P
+3.3V
+12V
500k Ω
V
OUT
TO FAN
1 µF
1 µF
0.01 µF
Figure 6. Driving a Fan with a PWM-to-DC Circuit
V CC
PWMOUT
4.7k Ω
5V
Figure 7. Controlling a PWM Input Fan with the MAX6641’s
PWM Output (Typically, the 35kHz PWM Frequency is Used)