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Original question:

AMC6821: PWM output shake in software RPM mode

AMC6821: PWM-OUT duty cycle behavior

Mike Yu
Intellectual 2290 points
Part Number: AMC6821

Hi Team,

While working with the customer, we found out that the PWM-OUT duty cycle will automatically changed in SW-DCY mode suddenly.

The reproduce steps are below.

  1. Set (0x01)Configuration Register1 to 0x3D
    > Set Default
  2. Set (0x22)DCY to 0x4C
    > Set Duty cycle 30%
  3. Set (0x00)Configuration Register0 to 0x9D
    > Set SW-DCY Mode, Invert mode, start bit

There are two question from the customer side.

  1. Why PWM goes to 100% high-level?
  2. Why PWM duty cycle goes to half from 30% to 15%?

Please reference the waveform and schematic as below. Thank you.

Waveform: Ch1 PWN-OUT, Ch2 TACH.

Schematic:

Best regards,

Mike

  • 0
    TI__Mastermind 43715 points

    Hi Mike,

    See the description for the TACH-MODE bit, which you cleared when writing default value to register 0x01. If your fan requires PWMINV=1, it should be set before writing TACH-MODE=0 for the measurement to work. 

    When the TACH-MODE bit is cleared ('0'), the PWM-Out pin is forced ON during RPM measurement, and internal correction circuitry is enabled to correct the error caused by this extra duty cycle.

    Can you double check the 15%? Setting the PWMINV should make 30% appear to be 70% and vice-versa. Read back the configuration registers to verify them. 15% might make sense if you were using the default FDRC=01.

    thanks,

    ren

  • 0 in reply to Ren Schackmann
    TI__Intellectual 2290 points

    Hi Ren,

    Thanks for the feedback, the customer are not see this situation when they set TACH-MODE=1. That is perfect.

    When they set TACH-MODE=0, 1

    The waveform shows the PWM actually 15% duty cycle after measurement. It will auto resume 30% after 101ms.

    Zoom in

    Set DCY=40%. B1-B2 is 70ms, shorter than 30% situation.

    We think this behavior is “and internal correction circuitry is enabled to correct the error caused by this extra duty cycle”.

    Is that correct? Thank you.

    Best regards,

    Mike

  • +1 in reply to Mike Yu
    TI__Mastermind 43715 points

    Mike,

    I'm confused. That quote 'internal correction circuitry is enabled to correct the error caused by this extra duty cycle' is associated with TACH-MODE 0. Does TACH MODE 1 resolve the customer issue?

    thanks,

    ren

  • 0 in reply to Ren Schackmann
    TI__Intellectual 2290 points

    Hi Ren,

    Thanks for the support on this, the should solve the customer question.

    Best regards,

    Mike