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DRV11873: OCP function at start up

na na78
Guru 16770 points
Part Number: DRV11873

Hi

Could you please give me your expertise for the following question?

If Rcs is 3.3Kohm, OCP threshold would be set as 3A(typ) at start up.

According to the datasheet, 100% duty output would be applied to motor at startup.

So the resistor value between phases (U V W) would be VCC / Ilimit as maximum.

For example, 12V VCC and 1ohm U-W resistance are assumed, 12A is about to flow to motor, then OCP would be functional and motor spin would fail.

If phase resistance is 6ohm, the current can be 12/6 = 2A, so OCP would not be functional at start up.

Therefore, VCC depends on phase resistance of motor.  Is my understanding correct?

If correct, is it possible to increase phase resistance with external circuit to avoid OCP at startup?

BestRegards

  • 0
    TI__Mastermind 18900 points
    Hey na na78,

    Our expert has been notified and should reply soon.

    Best,
    -Cole
  • 0 in reply to Cole Macias
    TI__Intellectual 770 points

    na na78,

       "During overcurrent protection, DRV11873 only limits the current to the motor and it does not shut down the operation" as mentioned in datasheet. We will update datasheet for OCP function at start up.

       Please feel free to let me know if you have more concerns. Thank you.

    Thanks again,

    Janet 

  • 0 in reply to Janet Yi
    Guru 16770 points

    Hi Janet

    Thank you for your reply.

    I understand the device does not shut down the operation during overcurrent protection.
    That was already described in datasheet and I could refer it.

    And also, I can refer the following explanation from datasheet.

    "During motor start up, the overcurrent level is increased to 1.5 times the value set by RCS. If the overcurrent
    protection is triggered during the start up sequence, the motor will fail to start."

    Do you mean this is incorrect?
    Could you point out what part should be updated in datasheet?


    If OCP is assumed during startup, I believe the following situation would be possible.
    (As the premise, startup current should be limited by the value set by Ilimit x 1.5. )

    1) Motor starts up
    2) Overcurrect is detected during motor start up and goes into OCP
    3) Motor would stop starting up because top MOSFET would be turned off and bottom MOSFET is keeping on.
    4) The motor current would stop to flow and it goes below Ilimit
    5) OCP doesn't shut down the device so motor goes into start up sequence again

    I think situation "3)" is representing that "motor will fail to start".

    Could you please give us your view?

    BestRegards

  • 0 in reply to na na78
    TI__Intellectual 770 points

    na na78

      We would like to update datasheet regarding "If the overcurrent protection is triggered during the start up sequence, the motor will fail to start."

      As my understanding "motor would continue operation, just limit start up current to 1.5 * 6600/RCS" if need more than 1.5 * 6600/RCS start up current.

    Thank you,

    Janet

  • 0 in reply to Janet Yi
    Guru 16770 points

    Hi Janet

    Thank you for your reply.

    As the information from Pin Functions table, CS says following.

    "If MOSFETs' current is high, the part goes into overcurrent protection mode by turning off the top PWM MOSFET and keeping the bottom MOSFET on."

    I think this operation will cause to motor start failure.

    Is not this operation applied at motor start up? Or is it not applied anytime?

    BestRegards

  • 0 in reply to na na78
    TI__Intellectual 770 points

    na na78,

      Today I did a test "hit current limit" vs "no hit current limit" on a motor. Please see attachment.

      The behavior in pin table was observed, but didn't cause start up issue on hit current limit case.

    hope this help./cfs-file/__key/communityserver-discussions-components-files/38/11873_5F00_No_5F00_and_5F00_hit_5F00_current_5F00_limit.docx

    Thanks,

    Janet 

  • 0 in reply to Janet Yi
    Guru 16770 points
    Hi Janet

    Thank you for your reply and information.
    Let me make a reference.

    Thanks for your many support.

    BestRegards