Output Capacitor Selection for Buck Converter

[white_devil_123]

Hey, I am trying to design a 24V/8V 8A Switching Power Supply at a 1MHz Switching Frequency with LM61480 Switching Regulator.

Here is the Datasheet to the regulator: https://www.ti.com/lit/ds/symlink/lm61480-q1.pdf

I have a doubt about the output capacitor selection. Can I use the usual formula, which is, "Cout = (peak-to-peak output current ripple)*(switching period)/(maximum allowable voltage output ripple)" for this switching regulator, too?

I did go through the datasheet but still confused.

[tszaboo]

TI has an online calculator called Webench, where you can design your SMPS.
You can create a design on the product page:
https://www.ti.com/product/LM61480
Try it!

[mtwieg]

Output ripple voltage will depend more on the capacitor ESR and ESL than capacitance. Also the capacitors must be rated for that ripple current.

Electrolytic capacitors will not be effective at filtering 1MHz ripple, you will need ceramic capacitors (electrolytic capacitors may also be used, but won't do much for 1MHz ripple).

[Faringdon]

I once did a 1MHz vicor module PSU for a co....they wanted the Cout  with a couple of 4.7uF MLCC's...i changed it for a few 1uF MLCCs...it was something like a quarter of the capacitance...and the output voltage ripple went  DOWN!.
(Because ESL was bigger in the higher capactiance MLCCs).
When  i reported it to them..they said i must have done it wrong , and fired me ...but i was explaining the situation as we walked along the corridor to the exit door...and they decided to look into it.....they took me back on.

[uer166]

, and fired me ....

Are you 10000% sure it was because of that issue, and that issue only?

[David Hess]

Output ripple voltage will depend more on the capacitor ESR and ESL than capacitance. Also the capacitors must be rated for that ripple current.

I agree; the impedance will be dominated by the ESR and ESL which depend more on factors other than capacitance, including the package.  Even with electrolytic capacitors at high frequencies, it is about impedance rather than capacitance.

[Faringdon]

Yes.
Please find the attached LTspice sim.....make out that for your capacitor bank.

simsurfing.com (by Murata?) for MLCCs is a free program which shows these charts