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DRV8848: selecting input idle state and protection circuit

Martin Mensch
Prodigy 80 points
Part Number: DRV8848

Hello ,

I have two questions about DRV8848:

- I see two possible idle states at xIN1 and xIN2: both low or both high, while the active state is a PWM on one of them. I tried both and it seems that my motor is running smoother and starts running at lower duty cycles with both high, avoiding a cuurent into the supply. It seems a bit unusual to me so I want to ask if it is safe for the IC.

- Is ist ok to simply connect the motor between OUT1 and OUT2 or should I better add some kind of protection circuit like a snubber RC or a TVS diode? I don't know my motors inductance but it has a short circuit current of about 5A. How to properly calculate a snubber RC circuit and how to select a TVS diode? Will a TVS withstand permanent switching?

Thanks for any help

Martin

  • 0
    TI__Guru** 114905 points
    Hi Martin,

    It is safe to operate the IC switching between both inputs high and one high/one low. In fact , this is the first part of current regulation (slow decay).

    It is also OK to connect the motor directly between OUT1 and OUT2. If the stall current is 5A, you may want to consider setting a current regulation , IFS, of 4A or less. In many cases, there is no need to use the stall current to start the motor.

    For protection in a system, a TVS or some other method of protecting against in circuit ESD is recommended. An example of this type of event would be someone touch the motor while the system is operation, and creating an EDS discharge.
  • 0 in reply to Rick Duncan
    Prodigy 80 points

    Hi Rick,

    thanks for your answer.

    Concerning the protection question I think I could have explained better. I am worrying about back-EMF voltage peaks getting to high and damaging the DRV. Are the reverse diodes of the MOSFETs fast enough to ensure motor current to flow continously?

    We had a case here where a DRV8848 crashed and it really had a small hole burnt into its case. Before it was running ok for hours. Now we wonder what is wrong with our circuit. I attach you an image of the DRV8848.

    Thanks for any hint.

    Martin

  • 0 in reply to Martin Mensch
    TI__Guru** 114905 points
    Hi Martin,

    The reverse diodes of the MOSFETs are fast enough. If you experienced damage, it could be due to voltage spikes.

    Please see the blog: e2e.ti.com/.../motor-drive-forum-top-faqs-3-methods-to-prevent-electrical-overstress for examples to prevent this.
  • 0 in reply to Rick Duncan
    Prodigy 80 points
    Hi Rick,
    thanks for this link. The blog gave me a number of further links so meanwhile I read some interesting papers.
    We decided now to use a TVS diode to clip VM pumping. Now we think about two options. One is placing it close to the DRV between VM and Gnd pins, the other is placing it close to the motor clamp between the OUT1 and OUT2 pins. We would value your opinion about these two options. What do you think?

    Thanks

    Martin
  • 0 in reply to Martin Mensch
    TI__Guru** 114905 points
    Hi Martin,

    Sorry for the delay. I would recommend close to the DRV between VM and GND. Placing the TVS here protects in both directions.