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OL not detected on DRV8889A-Q1

Vlad Covataru
Prodigy 30 points

Hello experts,

Sorry for disturbing you.

I saw that there are some problems related to the open load detection on DRV8889/A-Q1.

Therefore I would request your help in solving an issue which I'm trying to understand since 3 months.

Below I will present the following situation.

We are using a stepper motor driver which is DRV8889/A-Q1 and we are struggling with the open load detection. I saw that there is a known issue related to the open load detection.

Required conditions in our application are the following:

Hold current 75 mA
Run current 500 mA
Working application conditions / configuration are the following

VREF - 1.7V
TRQ_DAC - 56.25%
SLEW_RATE - 105-V/us
DECAY - Smart tune ripple control
MICROSTEP_MODE - 1/16
OL_TIME - 200ms
EN_SR_BLANK - 0
With above conditions, we observed that on some chips the OL is not detected on the coil and on some chips the OL is detected instantly.

Can you please assist me in the configuration of the driver?

I would really appreciate your help.

It will be helpful to know if there is an issue with the OL.

Thanks a lot.

  • 0
    TI__Guru 55605 points

    Vlad,

    What you are most likely experiencing is noise in the comparator circuit that is resetting the OL detection loop and preventing the device from reporting an OL condiiton.

    Few things to try:

    1)  Reduce slew rate to the lowest setting.  Higher slew rates make the problem worse.

    2)  Reduce the micro-stepping down to 1/4 or 1/8.  This should also help the issue.  

    Regards,

    Ryan

  • 0 in reply to Ryan Kehr
    Prodigy 30 points

    Hello,

    Thanks for your answer.

    Reducing the micro-stepping is not a solution at least in my case.

    Slew rate with lowest setting is working and with TRQ_DAC.

    My answer is more in direction, I saw if IFS < 0.96A the detection will not work. IFS was around 0.5 A. Is this applying also in this situation ? 

    "Thank you for all the information. You mentioned current is 37.5% of the setting (to be confirmed). I'm assuming VREF = 3.3V which would translate to full scale current FSI of 1.5A x 0.375 = 0.5625A. There is a known issue with OL detection under the following conditions for this device. OL will not be detected when FSI < 0.96A and 1/64 or higher uSteps, FSI <= 1.5A and 1/128 & 1/256 uSteps. So for OL detect to work with the customers current setting we recommend to use 1/64 uSteps or lesser microsteps values 1/32, 1/16 etc. Also note the following conditions (below snippet from the datasheet) must be avoided when OL detection is enabled to avoid a false OL detection." - DRV8889-Q1: open load detection and tall detection - Motor drivers forum - Motor drivers - TI E2E support forums

  • 0 in reply to Vlad Covataru
    TI__Guru 55605 points

    Vlad,

    Yes, that post is applying to your situation as well.  Basically, with higher levels of micro-stepping, the currents will be very small near the zero crossing.  At currents approximately <15mA, the OL detection timer can be reset and cause it not to report a real OL condition.  Fast edges with higher slew rate makes the problem worse.

    Regards,

    Ryan

  • 0 in reply to Ryan Kehr
    Prodigy 30 points

    Thank you for your support in understanding better what is happening.

    Have a nice day!