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DRV8323R: SEN_OCP vs. VDS_OCP function

biter terbi
Prodigy 160 points
Part Number: DRV8323R

Hello TI experts.

I'm looking at the documentation of the overcurrent protection and I can see there are two protections. One that senses overcurrent in the fets and another in the shunts.

The VDS_OCP says:

"all external MOSFETs are disabled and the nFAULT pin is driven low.
When the external MOSFETs are disabled in this way, the driver automatically uses a lower setting for the gate
drive current instead of the programmed IDRIVE setting. This setting lets any large current that may be present
to be switched off slowly to minimize any inductive kickback caused by parasitic capacitance in the system"

While the SEN_OCP says:

"all the external MOSFETs are disabled and the nFAULT pin is driven low"

So my question is if the SEN_OCP also lowers the IDRIVE setting to help slowly switch off the fets in the system.

Why would you use one or another?

Thank you very much!


  • 0
    TI__Mastermind 23426 points

    Hey Biter,

    Thank you for your question. I will aim to provide response before the end of the week.

    Best,
    Akshay

  • 0 in reply to Akshay Rajeev Menon
    TI__Mastermind 23426 points

    Hey Biter,

    The lowering Idrive is not mentioned on SenOCP so it would only be expected of VDSOCP and not SENOCP.

    The VDS OCP is mainly used as a protection against a hard short  and serves as a protection against device damage in shoot through/short conditions.

    The RDS on of the FET is used in VDS OCP whereas Sen OCP is based on shunt resistor. RDS on changes more based on temperature than shunt resistor so it would be more stable. However, VDS OCP has more granularity than SEN OCP. Ultimately you can use these features  to best suit your system needs.

    Best,
    Akshay

  • 0 in reply to Akshay Rajeev Menon
    Prodigy 160 points

    Hello Akshay. Thanks for your response.

    I don't understand the part where you say that the VDS_OCP is more stable than the SEN_OCP. Shunt value wouldn't change with temperature and rds will, so I understand the SEN_OCP would be more stable, right?

    Also the shunt is 1% tolerance, which means all the powertrains will have almost the same current protection.

    I'll use both protections like you mentioned, but I'd really like to know if SEN_OCP also lowers the IDRIVE setting. I've found a few issues in the documentation so I don't 100% trust it.

    Best regards

  • 0 in reply to biter terbi
    TI__Mastermind 23426 points

    Hey Biter, 

    Sorry for the confusion, you are correct, I meant SEN OCP is more stable. 

     I can consult with my team on SEN OCP and Idrive and let you know once I  get a confirmation.

    Best,

    Akshay

  • 0 in reply to Akshay Rajeev Menon
    TI__Mastermind 23426 points

    Hey Biter,

    SENOCP setting Idrive to the lowest state is not defined/tested for this device. So most likely then that is not the case based on the datasheet.

    If you would like to confirm the exact behavior for your application, you can do a SPI read on a SEN OCP shutdown event, to check of the IDRIVE setting has been modified on bench to validate.

    Best,

    Akshay

  • 0 in reply to Akshay Rajeev Menon
    Prodigy 160 points

    Akshay.

    I'll try to measure the voltage across VDS when the fault is triggered to check if the turn-off time is different than usual.

    If so, we can confidently say that protection is working.

    What do you think? If the measurement is done can you guys update the documentation?

    Thanks!

  • 0 in reply to biter terbi
    TI__Mastermind 23426 points

    Hey Biter,

    The VDS slew rate time would be good indicator and also reading the SPI value to see if the register for IDRIVE is getting updated. 

    I can definitely bring this up to my team for future testing/documentation.

    Best,

    Akshay

  • 0 in reply to Akshay Rajeev Menon
    TI__Mastermind 23426 points

    Hey Biter,

    Marking the thread as closed for now. If your question was addressed please mark the thread as resolved.

    Please let us know if you have more questions or if you have new questions please post them as a new thread.

    Best,
    Akshay

  • 0 in reply to Akshay Rajeev Menon
    Prodigy 160 points

    Ok, I have the answer for this question.

    VDS_OCP is meant to protect against really high currents, like a short circuit in the windings or a broken mosfet, this is why it just has 4 levels.

    SEN_OCP is meant to be used as your "normal" protection, given shunt resistance thermal drift is not as high as the rds drift in a mosfet.

    The Idrive lowering in case of an OCP is only used in the VDS_OCP, given the short circuit current must be extremely high. SEN_OCP does not have this feature.

    I've got this answer from a TI dev.

    I hope this helps, and if you could update the documentation with this it would be great.

    Thank you very much.