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LM34966-Q1: soft start function

Chuqiang Wen
Expert 5845 points
Part Number: LM34966-Q1

Hi expert,

My customer is using LM34966-Q1 as flyback controller. Input: 12V battery, output: two windings(18V 4.4W and 18V 14W). Their schematic design is refer to below screenshot from datasheet. 

My question is if we remove the PNP transistor between COMP and SS pin, does soft start function still work? 

Thanks!

Ethan Wen

  • 0
    TI__Guru 65370 points

    Hello Ethan,

    The softstart internal to LM34966 influences the reference voltage, not the COMP voltage. If you remove the PNP, there is no softstart function anymore as COMP is allowed to get to the full voltage.

    Best regards,
    Brigitte

  • +1
    TI__Guru 58641 points

    Hi Ethan,

    No, it would not work because the pull up from VCC to COMP does "overwrite" the softstart functionality.

    In a boost application the softstart would reduce the output of the comp signal during the softstart phase but for an Flyback this does not work this way.

    So therefore you need the transistor to keep the COMP signal low (lower) during the softstart phase.

    Best regards,

     Stefan