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DRV2624 and Load Impedance

ASG
Prodigy 90 points
Other Parts Discussed in Thread: DRV2624, DRV2604
As I work with the DRV2624 chip in POC, questions about certain specs are coming up.  The ERM we'd like to use is 5.5 ohms +/-20%.
1. The datasheet calls out recommended operating conditions, suggesting RL load impedance of minimum 8 ohms.  In your experience, will this be a problem for the 5.5 ohm candidate ERM above, and why?
2. The datasheet calls out ZLoad "load impedance threshold for over-current detection" at 4 ohms typical.  Using the 5.5 ohm candidate ERM, should we expect any issues?
I notice other DRV chips e.g. DRV2604, carry the same specs.  Our ERM is typical of ERMs available, so I'd be surprised if DRV is not equipped to run it.  As you answer, please provide details because it may be acceptable, for instance, to run in open-loop mode when limitations apply only to closed-loop mode.
  • 0
    TI__Genius 10332 points

    Hello,

    We have to have tolerances in order to guarantee operation at low impedances. You should be ok running this ERM. I will look into this specification and get back to you with a more detailed answer. 

  • 0 in reply to Kelly Griffin
    Prodigy 90 points

    Kelly,

    Thanks for your reply.  When the diagnostics process is run on this same ERM (free-standing), it fails, i.e. the diagnostics bit (upper-most bit of status register) is set.

    I noticed here another thread on a similar topic--calibration, not diagnostics.  But, no resolution was offered even as the question was resolved.

    Can your team suggest: A) a set of detailed steps that is likely to help understand why diagnostics fails on this actuator, B) what resolved the calibration issue in the other thread, if related?

    Thanks, ASG

  • 0 in reply to ASG
    TI__Genius 10332 points
    ASG,

    Are you also using a similar ERM? We have found that the diagnostics fails because the ERM contains internal drive electronics that control the brushless DC motor. The internal electronics are to insure the coils are driven in the correct sequence. The internal drivers will block the back-EMF, so we recommend running in open loop mode.
  • 0 in reply to Kelly Griffin
    Prodigy 90 points
    Hi Kelly,

    Interesting about commutation electronics.

    The ERM that seems to fail diagnostics is Jinlong's Z7AL2B1690002, perhaps the same as Precision Microdrives' 307-103 (links below).
    www.vibration-motor.com/.../cylindrical-Z7AL2B1690002.html
    www.precisionmicrodrives.com/.../307-103-9mm-vibration-motor-25mm-type

    Some details:
    Upon completing diagnostics, DRV2624's status register (0x01) reports value 0x88, so "process done" and "diagnostics fail" are only bits set.
    Values for Z & K are as follows: Z: 16 K: 127 --> 2.549ohms, and fairly repeatable. (Upon removing the ERM, 0.0ohms is properly reported.)
    Perhaps the reported impedance is telling?

    Currently ERM is being controlled in open-loop mode, clamp voltages have been configured, but no auto-calibration step has been conducted. Please suggest any direction you can think of to assess why diagnostics might fail.

    Thanks, Alex
  • 0 in reply to ASG
    Prodigy 90 points

    Continued investigation shows that, although the DRV2624 seems to properly drive a load >8ohm (per data sheet suggested minimum), elevating the impedance above the 8 ohm mark does not prompt DRV to pass diagnostics.

    Any further thoughts from the community on why, after running diagnostics on DRV2624, the status register always has a "diagnostics fail" bit set?