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LM5155-Q1: CSS needed?

Part Number: LM5155-Q1
Other Parts Discussed in Thread: LM5155

Hi team,

if soft start feature is not needed Css (for soft start) can be DNP(Do Not Populate)?

Regards,

Hirata

  • Hello Hirata-san, 

    Thanks for reaching out. We do not recommend operating the LM5155 without a soft start cap. The soft-start feature helps the converter gradually reach the steady state operating point, thus reducing start-up stresses and surges, furthermore the device regulates the FB pin to the SS pin voltage or the internal reference, whichever is lower. Please let me know if further support is needed. 

    Kind regards,
    EM

  • Hello EM-san,

    Thanks for your feedback.

    I am sorry i got new information.

    My customer is using Flyback topology like Fig10-10, and and using below SS configuration like Fi9-8.

    In the below Fig10-10, there is a description that "Optional Primary-Side Soft-Start".

    In this topology, Soft Start is still needed? or optional? 

    Regards,

    Hirata

  • Hello Hirata-san, 

    Thank you for your message. In an isolated flyback you need two soft start mechanisms: one is always the cap on the SS pin, while for the second one you can choose between the bipolar transistor between COMP and SS or the soft start at the secondary side, like in figure 9.9 in the datasheet. Please let me know if further help is needed. 

    Kind regards,
    EM 

  • Hello EM-san,

    Do you mean Soft Start is highly recommended for the isolated flyback as well?

    The reason I am asking is there is description that "optional Primary-Side Soft Start" in below figure(Fig10-10 in D/S)

    And for example do you recommend to use soft start feature even if they set the current limit threshold below the rating of the FET? 

    Thanks.

    Regards,

    Hirata

  • Hello Hirata-san, 

    Thank you for your message. I think in this case the image title is a bit misleading, so we might think about correcting the datasheet.  The soft start mechanism needs to be present in the isolated flyback and you can choose one option between: 

    • capacitor on SS pin + bipolar 
      or 
    • capacitor on SS pin + circuit on the secondary side (like fig. 9.9 )



      The soft start is of crucial importance and has to be present, this does not depend on the current limit setting as the SS mechanism kicks in during start up. In addition, the current limit should always be set lower than the MOSFET rating to avoid damaging it. Let me know if further help is needed. 

    Kind regards,

    EM

  • Hi EM-san,

    Thanks for your clarification. I understood.

    I appreciate your support.

    Regards,

    Hirata