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DRV8818EVM: Current waveforms overlap

Annie Liu
Genius 10295 points

Part Number: DRV8818EVM
Other Parts Discussed in Thread: DRV8818

Hi Team,

This is a board designed by the customer himself. DRV8818 sets 1/8 microstep, Vdecay=2.6V, Vcc=5V, Rx=27K, Cx=680pF, SRn=0, STEP=4KHz

But the motor vibrates and makes a lot of noise. The captured curve of the ISEN sampling resistor is as follows:

May I ask what is the reason for this? How to solve it?

Thanks,

Annie

  • 0
    TI__Mastermind 23801 points

    Hey Annie,

    Does the same thing happens if they drive the motor with the EVM set to all the same settings?  

    What is the peak current shown on this graph?  5V seems low to drive a motor, I wonder what would happen if driven by 12V or 18V instead (more power). 

    Please check 7.3.2 Current Regulation and the resistors on RCA and RCB pins, I wonder if TOFF or TBLANK are set too high for this step frequency.  

    Regards,

    Jacob Thompson

  • 0
    Prodigy 10 points

    Hi, Thompson,

    I don't have EVM in my hand, so I can't do this verification.

    The peak current corresponding to this figure is 1.6A, VCC (PIN10) is 5V, but VM (PIN28 & PIN15) is 12V.

    According to the parameters rx=27k, cx=680pf, toff=18.36us, tblank=0.952us. Is it set too high?

    In addition, I have tried to adjust the collocation of various parameters of Rx and CX, rx:12k~100kohm, cx:470pf~1500pf, but the current waveforms will overlap and hardly change.

  • 0 in reply to Zhihui Zhang
    TI__Mastermind 23801 points

    Hello,

    Do you have a current probe?  If so, please post an oscilloscope shot with a current probe on a motor phase, it will provide a clearer picture of what is happening.  During slow decay the voltage on Rsense will be shown as 0V, and during fast decay the value on Rsense will be inverted.  Because of this, the image above could be normal operating conditions for the motor and just looks odd because it is the voltage of Rsense.  You could also try using only fast decay or slow decay instead of mixed. 

    Is this Phase A or Phase B or both phases that are having issues?  We have had some issues with counterfeit parts having issues with 1 phase, would like to rule out the part being counterfeit. 

    Is this a new design?  If it is, please look into using a newer part such as the DRV8434.  It has integrated current sensing, smart tune/auto decay, and a few other modern features and should work better for you.    

    It would also be helpful if you could post a circuit schematic.  

    Regards,

    Jacob Thompson