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TPS5420 - Icout(RMS) equation question

John Czajkowski
Prodigy 20 points
Other Parts Discussed in Thread: TPS5420, TPS5430

I am hoping someone can clarify equation 11 on page 13 of the datasheet of the TPS5420.  The design example calculates a ripple current of 143 mA but i am unable to calculate the same value.  As far i can tell the equation is:

Icout(RMS) = 1/sqrt(12)*[ ( Vout * (Vin(max) - Vout) ) / (Vin(max)*Lout - Fsw*0.8*Nc)]

where

Vout = 5 V

Vin(max)=36 V

Lout = 33 uH

Fsw - 500 Khz

Nc = 1

Plugging in the aforementioned equation in the above equations produces a Icout(rms) of -.112 mA NOT 114 mA.

So my question is: Is the equation i am using correct, if yes why do the values not line up, and if no where am i going wrong?

Thanks

  • 0
    TI__Mastermind 35115 points

    Hi John,

    There appears to have been an issue when the pdf was generated. The subtraction between Lout and Fsw should be multiplication. The correct formula is then: Icout(RMS) = 1/sqrt(12)*[ ( Vout * (Vin(max) - Vout) ) / (Vin(max)*Lout*Fsw*0.8*Nc)]

    Also the 0.8 term in the denominator is there to add some tolerance. Lastly the 143mA in the calculation appears to be a copying error from the TPS5430 datasheet. I calculate 94mA.

    Best Regards,
    Anthony

  • 0 in reply to Anthony F
    Prodigy 20 points

    Anthony,

    Thanks you very much for correction and the speedy reply.

    Regards,

    John