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Original question:

DRV8837: Capacitance between OUTA-OUTB for reducing Back EMF

DRV8837: DRV8837

SUBEEN KIM1
Prodigy 40 points
Part Number: DRV8837

Hello

I use drv8837 on our board.

Our board operates the Motor Linear Actuator. (Motor Model : xydh12)

Our board performs break operation after forward or reverse operation.

When breaking, the motor generates twice as much (about 24V) back EMF.

So LDO is impacted, causing problems with 3.3V generation.

This problem was solved by attaching a diode to the LDO output.

But I don't know how to protect DRV8837.

Isn't it a problem to use it as it is because it has its own protective circuit inside?

Thanks.

  • 0
    Prodigy 40 points

    Above picture is circuit about Motor Driver.

  • 0
    TI__Guru 56650 points

    Subeen,

    "When breaking, the motor generates twice as much (about 24V) back EMF."

    Hoe do you break the motor? If two low side FETs are on (OUT1 and OUT2 are "L") to break the motor, we should not see 24V.

  • 0 in reply to Wang5577
    Prodigy 40 points

    Hi WANG.

    Thanks for your reply.

    The below picture shows the waveform when braking after forward motion.

    You can see the voltage increase momentarily when braking.

    A burst occurs in the VM after a burst in the Foward.

    I don't know how to solve this phenomenon.

  • 0 in reply to SUBEEN KIM1
    TI__Guru 56650 points

    Subeen,

     If two low side FETs are on (OUT1 and OUT2 are "L") to break the motor, we should not see 24V.

    1. From the waveform, blue trace only keeps low for about 3ms after break. Can we extend that 3ms (keep OUT1 and OUT2 are "L") to 300ms?

    2. How much input capacitance do you put on the VM pin? Please increase it.

  • 0 in reply to Wang5577
    Prodigy 40 points

    HI Wang,

    I'm sorry, But My problem has not been solved.

    [ If two low side FETs are on (OUT1 and OUT2 are "L") to break the motor, we should not see 24V. ]

    The 24V is caused by the back electromotive force of the motor.

    Answer

    1. From the waveform, blue trace only keeps low for about 3ms after break. Can we extend that 3ms (keep OUT1 and OUT2 are "L") to 300ms?

    => We continue to keep low after 3 ms. However, 24V is generated by back electromotive force.

    2. How much input capacitance do you put on the VM pin? Please increase it.

    => 47uF 35V capacitor was used.
         Can putting a CAP on the VM pin protect the motor drive? I want to protect motor driver from the Back EMF.

    Sequence of problem occurrence

    (1) Back electromotive force(24V) is generated from the motor.

    (2) 24V voltage also appears on the VM (11V) line through OUT1 or OUT2.

    (3) The LDO Input(=VM) contains a voltage of 24V. So the voltage of the LDO OUT temporarily dropped to 2.8 to 2.9 volts.

    (4) MCU Reset (The MCU we use needs 3.3V.)


    Current progress

    (1) Diode was attached to LDO Input. (To prevent the MCU reset)

    (2) MCU reset resolved


    Questions

    (1) Some Hbridge motor drives are already applied to internal circuits.

         Is it possible to protect Back EMF in the DRV8837 internal circuit?

    (2) If the answer to the first question is no, How can I protect my motor drive from 24V?

    [ I tried this ]

    1) e2e.ti.com/.../887650 => Cdiff1, Cdiff2, Ccommon = 0.1uF [Be unresolved]

    2) I'm considering using a free wheeling diode. but I don't know how to apply it to H bridge.

    Thanks.

  • 0 in reply to SUBEEN KIM1
    TI__Guru 56650 points

    Subeen,

    (1) Is it possible to protect Back EMF in the DRV8837 internal circuit?

    DRV8837 doesn't have an internal circuit to clamp or protect from back EMF.

    (2) If the answer to the first question is no, How can I protect my motor drive from 24V?

    a. when brake the motor, keep OUT1 and OUT2 are "L" from more than 300ms, instead of 3ms.

    b. How much input capacitance do you put on the VM pin? Please increase it.