Rainbow-electronics MAX6641 Bedienungsanleitung PDF herunterladen (Seite 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

100

35k

Table 5. PWM Frequency Select (10h)

PWMOUT

10kΩ

Figure 5. Driving a P-Channel MOSFET for Top-Side PWM

Fan Drive

+3.3V

PWMOUT

18kΩ

27kΩ

10kΩ 120kΩ

+3.3V

+12V

500kΩ

OUT

TO FAN

1µF

0.01µF

Figure 6. Driving a Fan with a PWM-to-DC Circuit

PWMOUT

4.7kΩ

Figure 7. Controlling a PWM Input Fan with the MAX6641’s

PWM Output (Typically, the 35kHz PWM Frequency is Used)

1 2 ... 7 8 9 10 11 12 13 14 15 16 17

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