Microchip Technology DM164123 Data Sheet
Background
©
2007 Microchip Technology Inc.
DS41304B-page 17
2.4.4
Four-wire Fan Control
Four-wire fan control provides a PWM input, tachometer output, fan power and ground.
Instead of switching fan power, which must be done with two and three-wire fans, only
the PWM input line is switched (typically at 10 kHz–30 kHz). As duty cycle of the fan
increases, speed also increases non-linearly. The tachometer line is the same as the
three-wire fan, generating pulses based on the rotation of the permanent magnet of the
fan; however, the Hall sensor is always on. Therefore, PWM stretching is not needed
to ensure an accurate measurement.
Instead of switching fan power, which must be done with two and three-wire fans, only
the PWM input line is switched (typically at 10 kHz–30 kHz). As duty cycle of the fan
increases, speed also increases non-linearly. The tachometer line is the same as the
three-wire fan, generating pulses based on the rotation of the permanent magnet of the
fan; however, the Hall sensor is always on. Therefore, PWM stretching is not needed
to ensure an accurate measurement.