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TMS320F28075: CMPSS weird phenomenon

Young Hu
Expert 4185 points
Part Number: TMS320F28075


Hi champs,

My customer is designing digital power system using F28075. In their system. Recently they found a weird issue about CMPSS. The positive input comes from their current sensor whereas for the negative input they use the internal DAC. Take CMPSS1 for example as follows. 

However, during their debugging, they found if there is a input voltage close to 0V on CMPIN1N pin, the comparator output will be pulled high. In their code, DAC12 is chosen, and CMPIN1N pin is bypassed. So would you kindly help to review customer's initializing code and help to find the root cause?

Drv_CMPSS.cpp

Many thanks.

Regards,

Young

Thanks.

  • 0
    TI__Mastermind 28275 points

    Hi Young,

    When the inverting input source is switched to the COMPDAC, CMPINxN pin has no effect on the comparator so what you are seeing is not expected. I'm not seeing any issues with your configuration code. Going by your configuration code, can you provide some specifics?

    1. Which CMPSS are you seeing the issue on and is it COMPH or COMPL?

    2. What is the voltage on CMPINxN that causes this issue? Is it a range of voltages?

    3. What is the voltage on CMPINxP pin?

  • 0 in reply to Frank Ankapong
    TI__Expert 4185 points

    Hi Frank,

    Please find customer's reply as follows.

    1.      Which CMPSS are you seeing the issue on and is it COMPH or COMPL?

    It was CMPSS1,the issue happened on COMPH,COMPL is OK.And I am still testing other CMPSS to confirm whether they have the same issue.   

    2.      What is the voltage on CMPINxN that causes this issue? Is it a range of voltages?

    In our application, It’s -650(mV) at AD Input port. If it’s larger than -650(mV), there is no such issue.

    In the pictures below, purple wave is AD_A3(CMPSSIN1N)  input signal, green wave is outputXbar signal indicate COMPH output. These two pictures are refered as follows.

    3. What is the voltage on CMPINxP pin?

             It’s 1.5V.

    Purple wave is AD_A2 input(CMPSSIN1P),it does not reach the limit value

    Green wave is GPIO indicate COMPH output.

    Thanks.

    Regards,

    Young

  • 0 in reply to Young Hu
    TI__Guru 62975 points
    Young,

    The -650mV CMPSS input is violating the absolute maximum input voltage range. At that voltage, the ESD protection diodes are biased and conducting current; the internal VSSA plane near the pin is being pulled below 0V through the diode. I suspect that the strange behavior is caused by the injection of negative voltage into the internal VSSA supply.

    I would highly recommend that the system be modified to protect the device inputs from voltages that exceed the VSSA to VDDA range. At the very least, there should be current-limiting resistors in series with the CMPSS inputs to prevent damage and to limit the amount of supply disturbance when the input voltage exceeds the VSSA to VDDA range.

    -Tommy