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DRV8837 Inrush Current and Softstart

Erhan Ergun
Intellectual 270 points
Other Parts Discussed in Thread: DRV8837

Hello,

I have an application where i will drive 16  DC Motors by 1 MCU. DC Motors are 6V and 40ma.

Is there a way to start them softly. In some cases i need to start them within 1ms so i can not apply delays start of them and when i start all of them they consume exessive current (approximately 100ma for each dc motor).

From internet i got some circuits but they are for directional motors. One of them below.


I am planing to add inductors in series to motor pins in, i mean at the motor output of drv8837 in order to decrease inrush current but i am not sure of it.

Can you suggest me a soft start circuit for bidirectional motor drive. Thank you.

Note= I can not use PWM, there is 16 motors and 1 MCU and MCU not have that much PWM.

  • 0
    TI__Expert 8885 points

    Erhan,

    I could not able to see the figure you attached. Anyways, there are followinng ways to limit initial inrush:

    1. applying duty-cycle ramp i.e. gradually increase the dutycycle from 0 to 100%. This is most efficient way to get the inrush control.

    2.  You can also add series impdenace in motor, either resistance and inductor. This will come with expense of steady-state voltage drop during normal running. Inductor would be better than resistance in this regard.

    Best Regards

    Milan-Motor Application Team

     

  • 0 in reply to Milan Rajne
    Intellectual 270 points

    This is the circuit. Thank you for your reply.

  • 0 in reply to Erhan Ergun
    Intellectual 270 points

    I think i am going to apply 0805 package ferrite beads but not sure the value of it. Can you suggest a value or a specific inductor.

    Thank you

  • 0 in reply to Erhan Ergun
    TI__Expert 8885 points

    Erhan,

    Exact value of inductor calculation would require simulation, because being an electromechanical system, current rise time in DC motor's armature winding will depend upon, applied voltage, Armature impedance (R and L) and back-emf. Back-emf is proportional to speed which is determined by mechanical inertia and applied load to motor shaft.

    To avoid complex simulation, if we assume that back-emf is and armature resistance is zero for current rise time calculation, than is simple equation and L can be calculated:

    L =Vm* Tr/Ipk, for example, you want to limit the peak current to 40mA in 100usec, with applied Vm=12V

    L =30mH, that’s too big value because in actual case, back-emf and resistance will limit the current to much lower value. But at least now we have one ball-park figure to start with. You can choose any value 1/3 to 1/8 of calculated value and fine tune with actual experiment.

    Best Regards

    Milan-Motor Application Team