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Stepper driver 8818 has minimum holding current?

tri truong
Prodigy 50 points
Other Parts Discussed in Thread: DRV8818

Hi all, please help!

This is the first time I work with a stepper motor also as stepper driver and I'm using the stepper driver 8818 now! The problem is I saw that the holding current always has a minimum value which I can decrease it anymore even with Vref = 0. I also have tried with different motors and I saw that this minimum value is also different with each different motor/ Does anyone have the solution or explanation for this? Please show me!

  • 0
    TI__Mastermind 26065 points

    Hi Tri

    What is the minimun level you get? Several mA or even smaller?

    There will be noise and offset of the internal comparator. It is possible to see a small current even at Vref 0.

    Best regards,

  • 0 in reply to Wilson Zuo
    Prodigy 50 points

    Hi Wilson,

    The minimum level get to hundreds mA. It's 190mA with SS242 Sanyo Denki step motor and 315mA with 5718 Power Step TM step motor. So there is an internal comparator inside the 8818 driver, I wonder which sources it compare? Can you tell me about that?

    Best regards,

    Tri

  • 0 in reply to tri truong
    TI__Mastermind 26065 points
    Hi Tri,

    If hundreds mA, there must be a different scenario to the reason of internal offset.

    Could you show the decay mode setting, the voltage and the RCx configuration?

    It is possible that you met an issue caused by too much injection in SLOW decay mode section. That is true be different with the motors. The current regulation is out of the range because the SLOW decay can not decay the current which injected during the Tblanking time below down to the setting level (Could still happen even you select mixed decay as the current rising section will still be SLOW decay).

    Using fast decay or change the RC value to reduce the Tblanking time may solve the problem if the reason is true as above.

    Best regards,
  • 0 in reply to Wilson Zuo
    Prodigy 50 points

    Thank you for quick reply, Wilson!

    The image below will describe how I config 8818 driver

    Best regard,

    Tri

  • 0 in reply to tri truong
    TI__Mastermind 26065 points
    Thank you! So by a simple checking, you can just set the decay pin voltage = logic LOW to set the decay mode as fast always. If you see the minimum current goes away, the root cause is identified. If you think fast decay is good enough for you, you can keep it. Or you can try to still using the mixed decay mode but set small C as Tblank = 1400*C.

    Tblank can be set < 1us. So you can try 470p. Just a rough suggestion. More carefully optimizing may need for your motor.

    Best regards,
  • 0 in reply to Wilson Zuo
    Prodigy 50 points
    Hi Wilson,

    After trying to change decay mode also as the C value. It still didn't work! Anyway, thank you very much for your supporting!

    Best regards,
    Tri
  • 0 in reply to tri truong
    TI__Mastermind 26065 points
    Hi Tri,

    That is strange, have you tried another board of device? When you set Vref = 0, have you just tested the points of the Vref pin and the GND pin close the device?

    Best regards,
  • 0 in reply to Wilson Zuo
    TI__Mastermind 26065 points
    By checking on DRV8818 EVM I can see the current can be regulated by the Vref voltage. But it is true we can see phase current at the rising section even when set Vref = 0 @SRn = 0. Could be more than 100mA seen from the phase. But if you look at the power supply current, it is still very small (<10mA normally) which means the average current is still controlled down to only like 10mA. This current we seen is related the slow decay on the rising cycle, also it has something to do with the SRn setting. If you put SRn to HIGH, you should see the current gone. But SRn=HIGH may give worse performance of the current regulation ripple and noise.

    I think there is not much side effect to the application since we are normally not using Vref to stop the current, but just to set as a half level control for holding the position.

    Best regards,
  • 0 in reply to Wilson Zuo
    Prodigy 50 points
    Dear Wilson

    So you are right! It's the phase current and if put SRn to HIGH, the current were gone and the stepper have ripple and noise, too!

    I'm now using the holding current as 1/3 as rated current and driving current as 7/10 as rated current for power purpose. The stepper ran good!

    Thank you very much again, Wilson!
    Tri