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DRV8353M: Current sensing grief revisited

John Skeldon
Prodigy 30 points
Part Number: DRV8353M

Hi Guys , 

Its been a while - i have had some new boards made up of our current project and have come across a problem, i've searched the forum but cant find what i'm looking for 

the chip is DRV8353HMRTAT and for our 48v brushed motor control were up and running , thanks to help from you guys Slight smile

At the moment i have one of my boards running a motor (both ways - so h bridge is good ) but my 2 current sense outputs are stuck up at VREF , in my case 3.3v.

I've not seen this before and i'm unsure where else to look other than do i have a damaged chip ? the board did have a motor ground fault so i have my fault LED on ..

If you could give me a few pointers i'd be grateful .... the 3rd current sense is shorted out (and in this case to ground as well ). As ive said motor is spinning and i'm not in fault 

Confused ? 

  • 0
    TI__Mastermind 26046 points

    Hi John, 

    Can you share a schematic of your board? 

    Can you elaborate what you mean by motor ground fault?

    Thanks,
    Aaron

  • 0 in reply to Aaron Barrera
    Prodigy 30 points

    i have reverse battery protection on the 48v motor ground which uses a FET in the ground line ..the FET wasnt down properly (new staff building boards .. )

    This board works and has worked for a few months now so i can't see the problem i have a batch of 4 boards being built by new production staff (instead of me ) so i'm hoping its a build thing. I needed to do re-design part of the board but nothing to do with the motor drive side. 

    I'm asking the question as a just in case 

    Thanks

  • 0 in reply to John Skeldon
    TI__Mastermind 18916 points

    Hi John, 

    Thanks for the additional info - we are still looking into this and will try to give you a response by end of week 

    Best Regards, 
    Andrew 

  • 0 in reply to Andrew Liu
    TI__Mastermind 26046 points

    Hi John, 

    Current sense outputs (SOx) look okay on the schematic. Can you confirm that the GAIN pin is set to 47kohm to GND and not 0 ohm to 5V? If it's connected to 5V, then the GAIN setting may be too high and saturating the current sense output at VREF when there is positive current?

    Is it always stuck at VREF during motor operation, or only when you are sinking current through the shunt resistor when spinning the Brushed-DC motor? 

    Thanks,
    Aaron

  • 0 in reply to Aaron Barrera
    Prodigy 30 points

    in the yellow box there is a note about the resistors - as i was new to the chip if put "place holders " so i can change the settings. the Gain is set to its lowest setting i.e. 0 ohm to ground . the motor pulls less than 5Amps during normal operation about 1 Amp 

    all 3 current sense o/p's are stuck at vref - even with 0 duty going in to drive chip .

    NOTE : i've just had built a second board and this seams to be working ok , well current sense isnt stuck at vref ... i'm going to try fitting another drive chip and see what happens 

  • 0 in reply to John Skeldon
    TI__Mastermind 26046 points

    Hi John,

    Yes, I was confirming the settings based on the note in the yellow box. Thank you for confirming GAIN is the lowest setting tied to GND. 

    Can you check the pin impedance of each SOx pin to GND on that device and compare to a fresh device to see if there is any damage? (i.e. short or open compared to the normal impedance)

    It may be good as well to check that the SOx pins did not have a transient voltage, could you check to see if there is any ESD protection circuitry on the SOx lines or whether if the MCU is somehow pulling the SOx pins up to 3.3V?

    Thanks,
    Aaron

  • 0 in reply to Aaron Barrera
    Prodigy 30 points

    already fitted a replacement drive chip .., there is no ESD protection on the SOx lines ( it is best to have something even just a SOT-3 based device? )  the MCU wasn't pulling the lines up. The PCB now works as it should (i think , not fully tested it yet ).. Will keep you informed . 

    Thanks

    John

  • 0 in reply to John Skeldon
    TI__Mastermind 26046 points

    Hmmm...seems like the SOx output(s) has a short to VREF (can confirm with a DMM continuity check). 

    After testing, let me know if there is anything that isn't working correctly or if the issue occurs again. Could have been mishandled or had an ESD event appear on the first unit?

    Thanks,

    Aaron