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AMC6821 Configuration for Two, Three, and Four-wire Fans

Todd Grimes
Prodigy 10 points
Other Parts Discussed in Thread: AMC6821-Q1, AMC6821

I am interested in using AMC6821-Q1 (Intelligent Temperature Monitor and PWM Fan Controller) in one of our board designs, where a four-wire blower fan is going to be used.  The ~OVR, ~THERM, and ~FAN-FAULT signals will be used to shut down the board if an error condition is detected.

However, I would like to have the option of using either a two or three-wire blower as well, if for some reason the four-wire blower is not available.  For operation with the two-wire blower, it appears that disabling ~FAN-FAULT in Configuration Register 1 would de-assert the signal, thus keeping an error from being thrown. 

When using a three-wire blower the data sheet suggested that the power pin of the blower fan would need to be driven by the PWM output, which also enables the Tach of the Fan.    I assume this configuration would provide both speed control and tach features. 

However, the alternative three-wire fan that I am considering provides an open collector Tach output that can be pulled up by a resistor connected to a voltage.  For this case, I would not need the speed control, as the fan would be running at full speed, but would still want to read the Tach.  It seems that the PWM output could be disabled in the Configuration Registers after startup, allowing this configuration to work. 

Unless I have missed something in the data sheet, it appears that the AMC6821 would support either two (power only), three (power and tach), or four-wire (power, tach, and speed control) blower fans by simply programming the Configuration Registers appropriately.  I just would like some verification to make sure that I am correct with my assumptions.

Thanks.

  • 0
    TI__Expert 6925 points

    Hello Todd,

    It is certainly possible to use two, three, and four wire fans with the AMC6821 in the configurations you suggested simply by changing register values.

    For example:

    a) Two-wire (Only PWM control, no TACH pin monitoring)

    Possible operation modes:
    Software DCY Control Mode [FDRC1:FDRC0] = [00] , or
    Auto remote temperature fan control; [FDRC1:FDRC0] = [10]

    TACH pin setup:
    FAN-Fault-EN clear (‘0’)
    TACH-EN clear (‘0’)
    TACH-MODE set (‘1’)

    PWM Output:
    PWM-EN set (‘1’)

    b) Three-wire (Speed control)

    Possible operation mode:
    Software-RPM Control Mode [FDRC1:FDRC0] = [01]

    TACH pin setup:
    FAN-Fault-EN set (‘1’)
    TACH-EN set (‘1’)
    TACH-MODE clear (‘0’)

    PWM Output:
    PWM-EN set (‘1’)


    c) Three-wire (Speed monitoring only)

    Possible operation mode:
    Software DCY Control Mode [FDRC1:FDRC0] = [00]

    TACH pin setup:
    FAN-Fault-EN set (‘1’)
    TACH-EN set (‘1’)
    TACH-MODE set (‘1’)   ** Important to keep PWM-Output always in highZ

    PWM Output:
    PWM-EN clear (‘0’) makes PWM output highZ

    Regards,

  • 0 in reply to Rafael Ordonez
    TI__Intellectual 960 points

    Just one question: If use Two-wire fan, is there any possible to get TACH data? Does the "FAN FAILURE DETECTION" is still available? Thanks. 

  • 0 in reply to robinfeng
    TI__Genius 11750 points

    Howdy Robin,

    Unfortunately, two-wire fans are usually used for open-loop speed control and don't provide a TACH output pulse.  As a result, there is no easy way to determine the state of the fan.  The fans only have two leads, one of which connects to ground.  The other lead is used to control the fan speed by adjusting the input (dc) voltage, or through a PWM signal.

    Three or four-wire fans can be used for closed-loop speed control, as these fans provide a TACH output pulse that indicates fan speed, and would trigger the AMC6821 alarm if any fan faults are detected.

    Best Regards,

    Matt