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LM5122: Output voltage ripple calculations

Part Number: LM5122

Hi,

I need help clarifying a few equations from the datasheet. 

1) Is the Vripple_max(Cout) from Equation 33 of the datasheet peak to peak ripple voltage or just peak voltage?

2) Why is the LM5122 ripple equation different than eq. 12 & eq 13 of the "Basic Calculation of a Boost Converter's Power Stage" app note?  I tried both and got different answers.  The app note gave me a higher estimate.

Here are the eq. 12 & eq. 13 from the app note:

  • Hi Kelvin,

    Equation 12 and 13 are correct in calculating capacitance needed to achieve an output ripple requirement. 

    1) Equation 33 looks to be maximum peak-peak ripple. 

    2) Difference between equation 33 and equation 13 is that equation 13 assumes ripple is purely from ESR. Equation 33 looks like it includes ripple is from capacitor as well. You can derive equation 33 using equation 25+26 in the basic calculation of a boost converter sheet.

    If you assume inductor current ripple is small, voltage ripple due to ESR is equal to ESR*(Iout/1-D).

    Capacitive ripple is equal to (Iout*D)/(fs*Cout).

    If you add those two, and take out Iout/(1-D) from the combined equation, you end up Iout/(1-D)* [ESR + D*(1-D)*1/fs*1/Cout]. 

    D*(1-D) hit its maximum value at D = 0.5. Therefore, D*(1-D) = 0.25, or you can rewrite the worst case scenario as Iout/(1-D)*[ESR + 1/(4*fs*Cout)].

    Thanks,

    Richard