USING AN ELECTROLYTIC WITH TPS54332

Oscar,

I think your question applies not only to this particular device, but to using parallel capacitors for converters in general.

If you have a 1uF ceramic with 1mohm ESR in parallel with a 47uF electrolytic with 130mohm ESR, the two together will NOT give the same ripple as a 48uF capacitor with 1mohm ESR. It's all about impedance. A closer estimation of what the effective output cap/ESR is to convert each capacitor/ESR to a parallel equivalent, combine the parallel terms, and convert back to series. This would be a true reflection of the total impedance at the frequency at which you calculate the complex reactance for the conversion.

A good way to illustrate this is through a simulation. I attach a TINA-TI simulation below with a few buck converters with different output capacitor configurations. The first uses two separate capacitors as I mentioned above, the second uses series-to-parallel conversions to get an effective output cap, and the third uses the assumption that capacitances and ESR can be directly combined. The steady state simulation results show that the third approach is not a good approximation of the actual case, while the first and second circuits have nearly the same output voltage ripple.

So to answer your question, it depends. For what you suggest to work as you intend, the electrolytic cap will need to have fairly low ESR (high Q) on its own or there will be little improvement in the steady state output voltage ripple by adding a ceramic cap with low ESR in parallel.

FYI - for circuit 2 where I used series-to-parallel conversions (and back), you can refer to this article for an explanation on how to do the conversions:

Convert parallel impedances to series impedances - http://www.eetimes.com/document.asp?doc_id=1278134