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DRV8711: BEMF

Shota Mago
Expert 7710 points
Part Number: DRV8711

Hi team,

Let me confirm my understanding about sampling timing of BEMF voltage in the case of external stall detection.

  • When step input rises for zero current, BEMF pin starts sampling and keep it during SMPLTH.
  • When SMPLTH has expired, BEMF pin starts to hold the value.
  • SMPLTH must be longer than time of  recirculation in order to hold correct BEMF value.

I summarized my understanding in the scope capture as below.

If you notice something wrong, please let me know.

Best regards,

Shota Mago

  • 0
    TI__Guru** 114905 points

    Hi Mago-san,

    There is one small difference.

    1)When step input rises for zero current, BEMF pin starts sampling and keep it during SMPLTH.

    BEMF starts sampling after the current reaches zero at this step.

  • 0 in reply to Rick Duncan
    TI__Expert 7710 points

    Hi Rick-san

    Thanks for your quick reply despite being busy.

    I'm getting a little bit confused.

    If BEMF starts sampling after the current reaches zero, why voltage peaking on the BEMF pin is observed?

    Also,if there is anything wired phenomena on the scope capture, your comment is helpful.

    Best regards,

    Shota Mago

  • 0 in reply to Shota Mago
    TI__Expert 7710 points

    Hi Rick-san

    I captured BEMF waveform around zero current with slow decay mode.

    In the slow decay mode, BEMF pin seems to start sampling when motor current reaches zero.

    My previous scope capture was operated with mixed decay mode.

    Does sampling start timing depend on decay mode?

     

    Best regards,

    Shota Mago

  • 0 in reply to Shota Mago
    TI__Guru** 114905 points

    Hi Mago-san,

    Before the current reaches zero on a 0 current step, any voltage on the BEMF pin is the residual portion of the current decay.

    The complete BEMF circuit is active once the current reaches zero. When the current reaches zero, one side of the coil is connected to GND. The other side provides the BEMF sample.