MAX6639 データシートの表示（PDF） - Maxim Integrated

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MAX6639

2-Channel Temperature Monitor with Dual, Automatic, PWM Fan-Speed Controller

Maxim Integrated 

MAX6639/MAX6639F

2-Channel Temperature Monitor with Dual,

Automatic, PWM Fan-Speed Controller

Table 10. Tachometer Pulses per

Revolution

REGISTERS 24h

OR 25h D[7:6]

TACHOMETER PULSES PER

REVOLUTION

00

1

01

2

10

3

11

4

Fan 1 and 2 Pulses and Min RPM (24h and 25h)

D[7:6]: This sets the number of tachometer pulses per

revolution for the fan. When set properly, a 2000RPM fan

with two pulses per revolution has the same tachometer

count as a 2000RPM fan with four pulses per revolution.

Table 10 lists tachometer pulses per revolution.

D[5:0]: This sets the minimum allowable fan tachometer

count (maximum speed). This limits the maximum

speed of the fan to reduce noise at high temperatures.

For reasonable speed resolution, the fan RPM range

should be set so this value is between approximately

30 and 60. If a maximum RPM limit is unnecessary, this

value can be set to the full-speed tachometer count.

Fan 1 and 2 Duty Cycle (26h and 27h)

These registers contain the present value of the PWM

duty cycle. In PWM fan-control mode, the desired (tar-

get) value of the PWM duty cycle can be written directly

into this register.

VCC

VFAN

(5V OR 12V)

4.7kΩ

PWM1

3V TO 5.5V

4.7kΩ

TACH1

TACH

OUTPUT

Figure 6. High-Side PWM Drive Circuit

Channel 1 and Channel 2 Fan-Start Temperature

(28h and 29h)

These registers contain the temperatures at which fan

control begins (in automatic RPM mode).

Applications Information

Fan-Drive Circuits

A variety of fan-drive circuit configurations can be used

with the MAX6639 to control the fan’s speed. Four of

the most common are shown in Figures 6 through 10.

PWM Power-Supply Drive (High Side or Low Side)

The simplest way to control the speed of a 3-wire (sup-

ply, ground, and tachometer output) fan is to modulate

its power supply with a PWM signal. The PWM frequen-

cy is typically in the 20Hz to 40Hz range, with 33Hz

being a common value. If the frequency is too high, the

fan’s internal control circuitry does not have sufficient

time to turn on during a power-supply pulse. If the fre-

quency is too low, the power-supply modulation

becomes more easily audible.

The PWM can take place on the high side (Figure 6) or

the low side (Figure 7) of the fan’s power supply. In

either case, if the tachometer is used, it is usually nec-

essary to periodically stretch a PWM pulse so there is

enough time to count the tachometer pulse edges for

speed measurement. The MAX6639 allows this pulse

stretching to be enabled or disabled to match the

needs of the application.

Pulse stretching can sometimes be audible if the fan

responds quickly to changes in the drive voltage. If the

acoustic effects of pulse stretching are too noticeable,

VCC

VFAN

(5V OR 12V)

3V TO 5.5V

4.7kΩ

TACH1

TACH

OUTPUT

3V TO 5.5V

4.7kΩ

PWM1

Figure 7. Low-Side Drive Circuit

16

Maxim Integrated

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