• State Resolved
  • Locked Locked
  • Replies 7 replies
  • Answers 2 answers
  • Subscribers 63 subscribers
  • Views 440 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LM5177: SCP & Constant Output Current Limit

Sam Ting
Expert 3590 points
Part Number: LM5177

Tool/software:

Hi team,

My customer needs both features at the same time. Could you share any ideas on how to achieve this? Thanks!

Regards,
Sam Ting

  • 0
    TI__Guru 58641 points

    Hi Sam,

    the LM5177 provides this already:

    - SCP is done via the sense resistor on CSA/CSB

    - Constant Output current limit can be done with the sense resistor at ISNSx

    See datasheet for more details.

    Best regards,

     Stefan

  • 0 in reply to Stefan Schauer
    TI__Expert 3590 points

    Hi Stefan,

    To achieve the constant output current limit, we set the trigger level of ISNSx lower than CSA/CSB setting, but this cause the hiccup mode/SCP never entry. So we want to know if there is a simple way (eq: add some logic circuit externally) to have both features at the same time.

    Regards,
    Sam Ting

  • 0 in reply to Sam Ting
    TI__Guru 58641 points

    Hi Sam,

    If the constant current limit is enabled both limiter are on at the same time.

    The current limit at CSA/CSB is for protecting the circuit against current damaging the components and therefore should be the last one triggering.

    The ISNS is used a constant current limiter and therefore the limit needs to be below the limit of the over current.

    You have both limiter active and working and depending on the configuration this could be seen in such an example:

    - Application with constant current limit on the output

    - running in Boost mode

    - input voltage increasing so that the inductor current get high enough to trigger the over current but the constant output current is still not limiting.

    Note: this is somehow constructed and requires some more conditions

    - the UVLO level needs to be low enough to not disable the device before (which should be the case if properly designed)

    Best regards,

     Stefan

  • 0 in reply to Stefan Schauer
    TI__Expert 3590 points

    Hi Stefan,

    Thanks for the info!
    We have a case with ISNSx < the current limit CSA/CSB, but found a problem in the output short condition. The output voltage was dropping and the output current was limited at the expected current level, but no hiccup mode in this situation. Engineers are therefore concerned about overstressing the power stages during long-term operation and need to find a workaround. 

    Regards,
    Sam Ting

  • 0 in reply to Sam Ting
    TI__Guru 58641 points

    Hi Sam,

    i am not sure if this is really an issue. If you have designed the circuit for an continuous average current and this average current is still maintained in short circuit conditions due to decreasing the output voltage the whole circuit should still be able to handle this without issues.

    Best regards,

     Stefan

  • 0 in reply to Stefan Schauer
    TI__Expert 3590 points

    Hi Stefan,

    Sorry for the confusion, I don't think this is an issue because all the above operations work as expected. In short, engineers need an indicator (instead of PG) to inform system that the device is working in this mode. 

    Regards,
    Sam Ting

  • +1 in reply to Sam Ting
    TI__Guru 58641 points

    Hi Sam,

    there is no dedicated signal available for that. You might observe the IMONOUT signal as this will go down when limiting the current in forward direction.

    Depends on which level you need and what exact detection requirements are needed.

    Best regards,

     Stefan