# How to choose switched regulator and calculate the filtering (LC) corresponding

I'm a new user of switched regulator and I want to take one with ultra low noise and ripple (I'll also add a filtering, a ferrite, an armouring). But I don't know how to choose them.
I've understood so far, I have to choose one with high frequency switched regulator in order to get less harmonic and avoid big LC.
I've to choose a low noise regulator and check by LTspice the ripple and get them as less as possible.
But should I take some others parameters in consideration?

I'm also wondering how to calculate the inductance and the capacitance of a switched regulator.
I found these formula in a website, but it doesn't work well:
L=Vout*(1-D)dI f
with D=Vout/Vin and dI=peak to peak
C=2*dI/(8*f*dV)

Thank you for help

• You must also read the data sheets carefully and see what they say. Jul 7, 2017 at 7:31

The datasheets contain the math that is correct for the specific part in question.

I would note that topology matters for noise, switched cap is worse then flyback is worse then boost is worse then buck is worse then quasi resonant in general.

Also, pushing extreme efficiency does not help with noise, you don't want the edge rates and dI/dt to be too stupidly high.

LT have some parts specifically designed to limit EMI, but do follow the layout suggestions, layout is everything in this game.

One other tip, sometimes you can make switcher noise substantially irrelevant by synchronising the switcher with something like an ADC clock (Makes the switcher spurs alias to DC), it is a useful trick when it works.

Do be careful with LC filters on the output, it is tempting to make L large, but that hurts both because of the slowed transient response and because the self resonance rapidly becomes painfully low, also watch the Q, you don't want much or an LC network can become horribly badly behaved at some frequencies.

Do not forget to filter the input side, especially in buck converters, but really any variety can cause problems with radiation from the input side power traces.