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LM3478 based SEPIC, application note AN-1484

Eneko Lopez de Subijana
Prodigy 10 points
Other Parts Discussed in Thread: LM3478

Hello community,

I am designing a SEPIC converter based on the LM3478 controller for the following requirements:

Vin 5 - 30 V
Vout 12 V
Iout max 0.83 A

In addition, I am using 100kHz as the switching frequency of the power MOSFET.

Right now, I am calculating the value of the components following the guidelines from the AN-1484 application note, but I have a problem understanding where "the gate drive current IG" value comes from [see page 7, expression no. 26].

Does anybody know where this value comes from? Or why is it equal to 0.3A?

Thanks in advance for your attention.

Eneko

  • 0
    TI__Genius 16025 points

    Hi Eneko,

    The application note references 5.7Vin as max input voltage. If for your application you consider 5Vinput you should expect the gate drive voltage to be very close to 5V (refer line 9, page 3 of the datasheet http://www.ti.com/lit/ds/symlink/lm3478.pdf). From line  7, of the same datasheet, the Rdson of the FET is 16ohms. This gives a gate drive current Ig of .3A.

    Hope this answers your question! Feel free to ask any more you may have.

    Thanks,

    Anston