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DRV8871: DRV8871

brittany cohen
Prodigy 30 points
Part Number: DRV8871

Hi there,

I currently designed a motor controller with the MC33926 controller for a dc brushed motor. after implementing this controller we noticed current spikes from the initial movements for the motor (as expected) and then implemented a PWM input to the controller to more steadily run our motor without creating such large current spikes.

After this implementation, I then began looking more into other motor controllers that offer current decay behaviors as appose to just using the freewheeling diodes in the MC33926.

My question is understanding the difference between different current decay modes vs PWM the input into a motor controller to prevent current spikes. Don't both decay modes and PWM the input deal with suppressing current spikes? What is really the main difference between these 2? 

Sincerely,

Bob 

  • 0
    TI__Guru 56650 points

    Bob,

    I am not familiar about  MC33926. DRV8871 does the peak current control. All the decay modes can regulate and control the peak current and prevent current spikes. For brushed DC motor, you may see a high current during the start up because less back EMF to against the input voltage when the motor speed is low.

    Regards,

    Wang

  • 0 in reply to Wang5577
    Prodigy 30 points

    Hi Wang,

    I have another question. The DRV8871 is shown to be used with an additional controller. I understand that if the current is above a threshold set, the motor's signal will be some PWM to cut the motor from drawing above the set current. My question regarding the additional controller is this: 

    If I have a device that I want always to be running at x amount of speed and the motor has a load applied to it. I would need an additional controller with a control loop to make sure that the PWM inputs going into the DRV8871 are making sure I am continually providing this x speed, right? The DRV8871 will not handle this internally? 

  • 0 in reply to brittany cohen
    TI__Guru 56650 points

    Hello, Bob,

    DRV8871 doesn't sense the motor speed. The host controller needs to sense the motor speed and adjust DRV8871 current regulation point to control the motor speed. 

    If you don't want to do the close loop speed control, you can pick a stepper motor and control the "STEP" frequency to do open loop speed control.

    Regards,

    Wang Li

  • 0 in reply to Wang5577
    Prodigy 30 points

    So instead of setting the DRV8871  to the current chopping threshold, we would adjust the ILIM pin accordingly to adjust the speed?  

  • 0 in reply to brittany cohen
    TI__Guru 55605 points

    Bob,

    For speed control, you would sense speed with external sensor and adjust input PWM accordingly.  Increase PWM duty cycle for faster speed...decrease for slower speed.  

    Regards,

    Ryan