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Original question:

LM74930-Q1: Charge pump behaviour in case of undervoltage fault condition

LM74930-Q1: Active Rectification, calculating derated gate charge

Oliver Filsch
Intellectual 390 points
Part Number: LM74930-Q1

Tool/software:

Hello,

I need active rectification up to 200kHz using LM74930-Q1.

TI's document SLVAET2 deals with this topic for similar ideal diode controllers. According TI's document SLVAET2, loading of the ideal diode controller's charge pump is:

My question: why is Qgm derated at Vgs=6V? Where do the 6V come from?

Thanks, Oliver.

  • +1
    TI__Mastermind 33855 points

    Hi Oliver,

    6V is an approximate value as after 6V the FET is considered fully ON (almost) with very minimal change in RDS. Also, If Vgs < 6V, this means VCAP-VS is also < 6V and FET may turn-off due to collapse of charge pump. 

    To conclude, the FET should be selected such that during rectification, VGS doesn't fall under 6V and Gate charge corresponding to 6V VGS can be supported by charge pump.

    Regards,

    Shiven Dhir