MAX6639(2005) Просмотр технического описания (PDF) - Maxim Integrated

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MAX6639
(Rev.:2005)

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

Maxim Integrated 

2-Channel Temperature Monitor with Dual,

Automatic, PWM Fan-Speed Controller

VCC

VFAN

(12V OR 5V)

VCC

3V TO 5.5V

4.7kΩ

PWM1

3V TO 5.5V

5V

4.7kΩ

TACH1

TACH

OUTPUT

Figure 8. High-Side PWM Drive with “Keep-Alive” Supply

VFAN

(5V OR 12V)

VCC

100kΩ

3.3V

4.7kΩ

PWM1

100kΩ

3V TO 5V

4.7kΩ

2N3904

2.2µF

33kΩ

91kΩ

10µF

TACH1

TACH OUTPUT

TACH

OUTPUT

Figure 9. High-Side Linear Drive Circuit

stretching while still allowing accurate tachometer feed-

back. The diode connects the fan to a low-voltage

power supply, which keeps the fan’s internal circuitry

powered even when the PWM drive is zero. Therefore,

the tachometer signal is always available and pulse

stretching can be turned off. Note that this approach

prevents the fan from turning completely off, so even

when the duty cycle is 0%, the fan may still spin.

4.7kΩ

PWM1

3V TO 5.5V

VFAN

(5V OR 12V)

4.7kΩ

TACH1

TACH

OUTPUT

Figure 10. 4-Wire Fan with PWM Speed-Control Input

Linear Fan Supply Drive

While many fans are compatible with PWM power-supply

drive, some are excessively noisy with this approach.

When this is the case, a good alternative is to control the

fan’s power-supply voltage with a variable DC power-

supply circuit. The circuit in Figure 10 accepts the PWM

signal as an input, filters the PWM, and converts it to a

DC voltage that then drives the fan. To minimize the size

of the filter capacitor, use the highest available PWM fre-

quency. Pulse stretching is not necessary when using a

linear fan supply. Note that this approach is not as effi-

cient as PWM drive, as the fan’s power-supply current

flows through the MOSFET, which can have an apprecia-

ble voltage across it. The total power is still less than

that of a fan running at full speed. Table 11 is a summa-

ry of fan-drive options.

4-Wire Fans

Some fans have an additional, fourth terminal that

accepts a logic-level PWM speed-control signal as

shown in Figure 10. These fans require no external

power circuitry and combine the low noise of linear

drive with the high efficiency of PWM power-supply

drive. Higher PWM frequencies are recommended

when using 4-wire fans.

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