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DRV8889-Q1: OL detection when OCP happen

Tosh+
Expert 7890 points
Part Number: DRV8889-Q1

Hello DRV8xx team,

I got a question from customer(Hitachi/Astemo :OEM is TOYOTA)

When OCP is active and turn off FET, does DRV8889 detect "Open Load"?
Based on the datasheet DRV8889 detects OL when IOL continue more than tOL.
Even though FET is off with OCP, does DRV still keep detecion of OL?

(I found similar information here)
e2e.ti.com/.../4721238

If you have condition of OCP to avoid this error please let us know.

Regards,
ti

  • +1
    TI__Mastermind 26235 points

    Hello Tosh-san,

    Whenever the OL detect is active and the winding current is zero while the driver is active for longer than tOL it will report an OL as described in the datasheet. Under normal circumstances this can happen at specific electrical angles mentioned in the datasheet.

    An OCP event will render the winding current zero and will trigger the OL because of this reason. This cannot be avoided unless OL is disabled which would not make sense.

    The e2e you referred in a different phenomenon of the OL detect where OL will not be reported even if there is an actual OL. This would happen for low step currents under normal operation of the stepper driver. 

    Regards, Murugavel

  • 0 in reply to Murugavel4637
    TI__Expert 7890 points

    Hello Murugavel,

    Thanks for your comment, let me ask you one more.

    In the datasheet description, "This situation does not arise in full-step mode, because the coil currents are never zero"
    Even though full-step mode, the winding current goes to zero when OCP happen (with turn off FET).
    Based on your comment, we understand that every condition with current-zero may flag OL fault. Is it correct?

    Regards,
    ti

  • +1 in reply to Tosh+
    TI__Mastermind 26235 points

    Hello Tosh-san,

    Yes, this is correct. 

    Regards, Murugavel

  • 0 in reply to Murugavel4637
    TI__Expert 7890 points

    Hello Murugavel,

    Please give me your help a littel bit more, because this trouble may involve OEM...
    We want to know the condition to detect open load.

    1) when DRVOFF=1(High), FET turn off. Is it still alive OL detection?
    2) when TSD, FET turn off as well. Is it still alive OL detection?

    I suppose, it's "NO" for both.

    Regards,
    ti

  • 0 in reply to Tosh+
    TI__Mastermind 26235 points

    Hello Tosh-san,

    1) When DRVOFF = 1, the complete H-bridge is disabled OL detection will not be detected. However the indexer and digital logic are still operating which means when STEP input is present indexer will keep advancing. It is possible the indexer may be stopped not in the 45 deg electrical angle position but in a position where one of the windings may be 0. If this condition happens when DRVOFF is changed to 0 and OL was enabled it may report a false OL. Just to keep mind.

    2) Please see below from "Fault Condition Summary" from the datasheet. During TSD the indexer and digital logic are still operating. I am verifying with our design team if OL will be masked during a TSD event. I'll know about this on Monday or Tuesday my time. Even if this is true, because the indexer and digital are operating the indexer may advance to a zero current position and when TSD condition is gone after cooling down this may likely cause an OL. So these conditions need to be kept in mind while designing.

     

    Is there a specific issue the customer is facing currently? If you could get the description of the exact issue we can analyze and provide a feedback specific to the customer issue condition. Thanks.

    Regards, Murugavel

  • 0 in reply to Murugavel4637
    TI__Expert 7890 points

    Hello Murugavel,

    Let me confirm OL detection mechanism.
    1) Below show timing diagram of OL and reflect to OL register. Please check if my understanding is correct.

    2) OL recoverly
    In the "Fault Condition Summary", OL report only and H-Bridge is operation.
    Whereas, 7.3.11.4 Open-Load Detection (OL) says the necessity CLR_FLT bit.
    In conclusion, even though OL happen and nFault driven Low, but H-Bridge still works normal. Is it correct?

    3) load current threshold (IOL) is 30mA typ. Condisering current direction IOL < +/-30mA, correct?


    Regards,
    ti

  • +1 in reply to Tosh+
    TI__Mastermind 26235 points

    Hello Tosh-san,

    1. Yes your understanding is correct.

    2. "In conclusion, even though OL happen and nFault driven Low, but H-Bridge still works normal. Is it correct?". Yes this is the behavior. Unless a CLR_FLT is done a new fault will not be able to produce an edge on the nFAULT.  Other than that the bridge will be operational as mentioned.

    3. Yes correct.

    Regards, Murugavel