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how to select MOSFET of active snubber circuit for LTC3421

Dear experts,

Now we're designing a circuit using LTC3421, and in our application output voltage of LTC3421 is about 4.5V,

so we're adding schottky diode and active snubber network according to the demonstration circuit 565.

Could you please give me some advice how to select a MOSFET (Q1 on the demonstration circuit)?

Which specification of a MOSFET do I need to take care most?

What I’m concerned is that a ZXM61P02 which is selected for a demonstration circuit 565 has very small

Gate Threshold Voltage( Vgs(th) min:-0.7V, max:-1.5V).


The value of Vgs(th) could be affect on circuit behavior?

Thank you in advance for your help.

Best regards,

HATTORI, Tsutomu

  • There is no concern for the VGS(th) for this application. To choose a FET for a snubber, you’ll need to make sure:
    Vgs (max positive) > Vf (diode),  Vgs(max negative) < -VOUT. Vdss < -VOUT
    The FET thermal dissipation is able to handle the power needed to clamp the SW node.

    The FET in the demo circuit is already chosen to account for these aspects, and has been tested to work properly. That is the recommended FET. If you cannot use that FET, try finding a FET with very similar max specifications and thermal performance.