25-08-2017, 09:32 PM
4-Wire Pulse Width Modulation (PWM) Controlled Fans
4_Wire_PWM_Spec.pdf (Size: 114.9 KB / Downloads: 38)
Introduction
Overview
This specification defines the intended operation of a fan that implements the Pulse Width
Modulation (PWM) control signal on the 4-wire fan interface. The introduction of 4 wire PWM
controlled fans is a means to reduce the overall system acoustics. The expectation is a 4 wire
PWM controlled fan when properly implemented will be significantly quieter than a similar 3
wire fan.
Current
Steady state operation fan current draw (with 12.6 V applied, with fan operating in the free stream
condition) shall not exceed 1.5 A.
Fan current spike during start-up operation (with 12.6 V applied, with fan operating in the free
stream condition) shall be allowed to exceed 1.0 A, up to 2.2A maximum for a duration of no
greater than 1.0 sec.
Tachometer Output Signal
Fan shall provide tachometer output signal with the following characteristics:
• Two pulses per revolution
• Open-collector or open-drain type output
• Motherboard will have a pull up to 12V, maximum 12.6V
PWM Control Input Signal
The following requirements are measured at the PWM (control) pin of the fan cable connector see
Figure 7 and Table 1:
PWM Frequency: Target frequency 25 kHz, acceptable operational range 21 kHz to 28 kHz
Maximum voltage for logic low: VIL = 0.8 V
Absolute maximum current sourced: Imax = 5 mA (short circuit current)
Absolute maximum voltage level: VMax = 5.25 V (open circuit voltage)
This signal must be pulled up to a maximum of 5.25V within the fan.