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DRV8824: DRV8824

Frank Cardone43
Prodigy 130 points
Part Number: DRV8824
Other Parts Discussed in Thread: DRV8886AT, DRV8880, DRV8825

Greetings,

Can someone explain why as Vm goes above a certain value the voltage across Rs is not limited by Vrefx?

I am trying to understand the power dissipation in the DRV8824 and how the power is related to Vm motor voltage.

I have calculated the power dissipation using equation 2 from the data sheet. The power seems to be correct until I start raising the motor voltage. When I raise Vm above a certain level, the peak voltage on Rs (current sense) is not limited by the voltage threshold set with Vrefx. As Vm continues to move up and the voltage across Rs rises, the die temperature obviously rises.

Thank you,

Frank

  • 0
    TI__Guru** 114905 points
    Hi Frank,

    This may be loss of current regulation as described in the blog: e2e.ti.com/.../how-to-quickly-tune-your-stepper-motor

    Assuming you are using microstepping, the DRV8824 uses slow decay when the current is rising. This can lead to loss of regulation as shown in the slow/mixed 50% image in the blog.

    Newer devices like the DRV8880 and DRV8886AT with more decay settings control the increasing current to remove this loss of regulation.
  • 0 in reply to Rick Duncan
    Prodigy 130 points

    Hi Rick,

    Thank you for your response.

    I should have mentioned that the overheating is while the stepper motor is stopped and energized.

    I believe the problem has something to do with the "Blanking Time". I sized the Rs according to the data sheet BUT during the Blanking Time the ISEN is ignored. I'm trying to workout the value of Rs so that during the BT the current does not exceed Ifsa (eq4).

    I could use some insight/perspective into if this is a problem and the proper equation to use to limit current during the Blanking Time.

    Thank you again,

    Frank

  • 0 in reply to Frank Cardone43
    TI__Guru** 114905 points
    Hi Frank,

    What microstepping and decay modes are you using?

    For the explanation below, I am assuming you are using some level of microstepping and mixed or slow decay.

    Mixed decay uses slow decay when the current is increasing. When the motor is stopped and energized one phase will be in slow decay.

    As the VM voltage increases, the blanking time (typically 3.75us) can inject more current into the phase than the slow decay time (~16.25us) can remove. This can cause the current to rise until either an overtemperature or overcurrent event occurs.

    Changing the sense resistor does not help. Adding more resistance in the outputs

    There are a few solutions:

    1) Have you tried fast decay? This will help with loss of current regulation, but increase the current ripple at each step. You could also switch to fast decay only when holding.
    2) The DRV8825 (pin2pin compatible with the DRV8824) has a slower PWM frequency which provides more time for the current to decay (~36.25us) in slow decay mode.
    3) The newer devices mentioned earlier have more decay options.