MAX6678AEP92+【PDF 8/19 页】IC TEMP MONITOR 2CH PWM 20-QSOP

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PWM Output

1) The PWMOUT_ signals are normally used in one of

three ways to control the fans speed: PWMOUT_ dri-

ves the gate of a MOSFET or the base of a bipolar

transistor in series with the fans 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 _______________________________________________________________________________________

PWMOUT

10k&

Figure 4. Driving a P-Channel MOSFET for Top-Side PWM Fan

Drive

+3.3V

PWMOUT

18k&

27k&

10k&

120k&

+3.3V

+12V

500k&

OUT

TO FAN

1礔

0.01礔

0.1礔

Figure 5. Driving a Fan with a PWM-to-DC Circuit

PWMOUT

4.7k&

Figure 6. Controlling a PWM Input Fan with the MAX6678s

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

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