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MAX6639 Datasheet(PDF) 17 Page - Maxim Integrated Products

Part No. MAX6639
Description  2-Channel Temperature Monitor with Dual, Automatic, PWM Fan-Speed Controller
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Manufacturer  MAXIM [Maxim Integrated Products]
Direct Link  https://www.maximintegrated.com/en.html
Logo MAXIM - Maxim Integrated Products

MAX6639 Datasheet(HTML) 17 Page - Maxim Integrated Products

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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.
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.
2-Channel Temperature Monitor with Dual,
Automatic, PWM Fan-Speed Controller
______________________________________________________________________________________
17
VCC
PWM1
4.7k
4.7k
TACH1
3V TO 5.5V
3V TO 5.5V
TACH
OUTPUT
VFAN
(5V OR 12V)
Figure 10. 4-Wire Fan with PWM Speed-Control Input
VCC
PWM1
4.7k
4.7k
TACH1
3V TO 5.5V
TACH
OUTPUT
VFAN
(12V OR 5V)
5V
Figure 8. High-Side PWM Drive with “Keep-Alive” Supply
VCC
PWM1
TACH1
4.7k
3V TO 5V
4.7k
100k
91k
33k
100k
3.3V
2N3904
2.2
µF
10
µF
TACH
OUTPUT
VFAN
(5V OR 12V)
TACH OUTPUT
Figure 9. High-Side Linear Drive Circuit


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