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Technical Query regarding Fan Controller IC - AMC6821

Rizwan Hirani
Intellectual 450 points
Other Parts Discussed in Thread: AM3505

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We have used AMC6821SQDBQRQ1 to control Fan speed using PWM out of this chip. Please note that we are using this chip in Software DCY mode, in which we have to set the duty cycle in DCY register We are not using auto temperature functionality or Tach mode this device. Please find attached snap of the schematic  showing connection of this IC.

So far we are able to perform proper read/write operation with fan controller IC using system host controller (Sitara AM3505) through I2C.Following are the main configuration we are using:

Software DCY Control Mode [FDRC1:FDRC0] = [00]

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

PWM Output:
PWM-EN set (‘1’)

PWM_INV set (‘1’) as we are using N-FET

But , it seems that we are not getting any PWM Pulse. It is always with 100% duty cycle. We have checked the Therm-En bit of status register 3 is not set which can make this device to run with 100 % duty cycle.

I would request  somebody to review the attached schematic and let us know if any issue with that , especially there are several input pins which are not connected and kept NC. Also let us know if we are missing any configuration setting.

 

Thanks,

Rizwan Hirnai

  • 0
    TI__Expert 6925 points

    Hello Rizwan,

    I have had the same issue before. It is caused by not having a pull-up resistor in the open drain pin 7 (THERM). Since it is an interrupt pin, if low, it triggers the duty cycle to 100% ignoring any value in the DCY register.

    I recommend having pull-up resistors in all open drain pins (THERM, OVR, FAN-FAULT, SMBALERT) even if they are not used.

    Regards,

  • 0 in reply to Rafael Ordonez
    Intellectual 450 points

    Hi Rafael,

     

    Thanks for the input. I doubted the same thing y'day, but when I read bit 7 in register 0x03 [Therm-IN]bit, it was showing '0', so I didn't patched pullup in my board. But I tried today and it worked perfectly.

    Thanks for the support.

    I would provide pull up to all pins in next revision of our design.

    Rizwan

  • 0 in reply to Rizwan Hirani
    TI__Expert 6925 points

    Hi Rizwan,

    I'm glad is working now. Since this is a very high priority interrupt, it might be activated during start up, which is why I think a pull-up is critical for this pin. 

    BTW, the screenshot you send is difficult to read, but it seems the PWM_MODE pin is has a pull-up and pull-down of 10kohms giving a Vdd/2 voltage to this pin. I believe this pin should have either a pull-up or a pull-down resistor not both, but I might be misreading the schematic.

    Regards,

  • 0 in reply to Rafael Ordonez
    Intellectual 450 points

    Hi Rafael,

     

    You are correct, in screen shot actually there is a pullup and pull down both are present, however we are using only pulldown and pullup is not populated.  The DNP note is not present schematic.

    Thanks,

    Rizwan