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Original question:

DRV8825 Decay mode problem

DRV8825: Loss of current control while in mixed decay mode

Ben Nelson
Prodigy 10 points
Part Number: DRV8825
Other Parts Discussed in Thread: , DRV8424

We have a prototype circuit board which uses the DRV8825. We have found that current control is lost when current setpoint is < 700mA. For example, a setting which we expect 200mA will actually show 700mA with a current probe. A few notes:

  • Current control is lost when using 1/2 and 1/4 step modes. Not lost when using Full step. Have not tested other modes
  • We are using mixed decay mode. Current control is maintained if we switch to fast decay. Current control is lost when using slow decay.
  • When using mixed decay, we expect to see fast decay until 75% of the PWM. When current is < 700mA we do not see fast mode. Only Tblank on time followed by slow decay
    • When current is > 700mA we do see fast decay as expected until 75% of the PWM, then slow decay.
  • Interestingly pulsing nReset low clears this issue. We see proper current control. However moving 1 step causes one of the phases to lose control. Have seen it happen on both phases.

Using our motor we have been able to reproduce this behavior on the eval board DRV8825EVM. Steps to reproduce on eval board:

  1. Remove jumper JP3 to use mixed decay
  2. Supply Vm with 24V
  3. Open DRV88xx EVM.exe
  4. Configure as follows. Expect half step mode with peak current output of 200mA
  6. Move 1 or more steps
  7. Observe one of the phases loses current control
  8. Connect JP3 to use fast decay
  9. Observe the out of control phase regains control

Current control is lost due to apparently using slow decay. Current trace shows Tblank rises current more than the off time decays, so it stutter steps upwards until reaching steady state around 700mA. We don't understand why mixed decay doesn't use fast decay in this situation. Is this a issue with the inductance of the motor? I don't have motor details immediately available. I have asked for them and will update when I get them.

  • 0
    Prodigy 10 points

    We are testing with this motor in a bipolar configuration. 5.6 mH and 2.8 Ohms
    catalog.orientalmotor.com/.../pk264a2a-sg3-6

  • 0 in reply to Ben Nelson
    TI__Guru 55655 points

    Ben,

    Is there an opportunity to use one of our newer motor drivers?  

    https://www.ti.com/product/DRV8428

    This device is much smaller in device size and solution size as it uses no external shunt resistors.  It is also right sized for your application in terms of current.

    Finally, it has Smart Tune technology that will choose the best decay mode for your motor by analyzing the current on a step-by-step basis.  

    I think your problem is the minimum Tblank time and current is running away on you.

    Regards,

    Ryan

  • 0 in reply to Ryan Kehr
    Prodigy 10 points

    Ryan,

    We are open to using a different driver. Currently we are using the DRV8434PWPR and DRV8424PWPR. However it is getting hard to find parts and we are looking into other options. Our application's full range is 0.2A to 2A so the 8428 will not work for us. Do you have any other suggestions? We decided to try the 8825 as there is significant stock of it currently.

    Agree Tblank time causes the current to run away when using slow decay mode. We don't understand why slow decay is being used instead of mixed below a certain current setpoint. Raising Vref so the current setpoint is above 700mA, we see mixed mode start to operate where it is in fast decay until 75% of the PWM. So we have decay mode configured correctly. 

    Ben

  • 0 in reply to Ben Nelson
    TI__Guru 55655 points

    Ben,

    Are you able to share scope captures so I can get a better idea of the issue?  What is the VREF voltage you are using for 0.2A and the sense resistors?

    If you are not opposed to a smaller QFN package, we have a lot of stock for the DRV8424/34RGE.  

    Regards,

    Ryan

  • 0 in reply to Ryan Kehr
    Prodigy 10 points

    Morning Ryan,

    Thank you for the suggestion to try the other package. I see about 2k stock combined. Unfortunately at our quantities that won't last us long. There is > 100k of the DRV8825 in stock between TI.com and Digikey.

    On the eval board, we used Vref = 0.28V and the default sense resistor of 0.2 Ohms. I'll share scope captures later today.

    Ben