Datasheet

the circuit in Figure 8 can be used to eliminate pulse
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-sup-
ply circuit. The circuit in Figure 10 accepts the PWM sig-
nal 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
Maxim Integrated 17
MAX6639/MAX6639F
V
CC
PWM1
4.7kΩ
4.7kΩ
TACH1
3V TO 5.5V
3V TO 5.5V
TACH
OUTPUT
V
FAN
(5V OR 12V)
Figure 10. 4-Wire Fan with PWM Speed-Control Input
V
CC
PWM1
4.7kΩ
4.7kΩ
TACH1
3V TO 5.5V
TACH
OUTPUT
V
FAN
(12V OR 5V)
5V
Figure 8. High-Side PWM Drive with “Keep-Alive” Supply
V
CC
PWM1
TACH1
4.7kΩ
3V TO 5V
4.7kΩ
100kΩ
9.1kΩ
33kΩ
100kΩ
3.3V
2N3904
2.2μF
10μF
TACH
OUTPUT
V
FAN
(5V OR 12V)
TACH OUTPUT
Figure 9. High-Side Linear Drive Circuit