Need help with an output capacitor

[illusive]

Hi all,
I'm trying to optimize the output filter for a buck converter. The IC is the LM2676. It has a switching friquency of 260KHz. I'm having hard time choosing the output filter capacitor after the inductor (47uH), mainly with what ESR should it be. I have to choices: The first one is two 330uF 60mohms ESR capacitors in parallel (for a total of 660uF and 30mohms ESR). The second one is a two 470uF 30mohms ESR in parallel (940uF 15mohms ESR total). The datasheets for these buck converters are saying that the lower ESR the better, but also that if the ESR is too low there may be instability, but they don't go into details. And now i cannot choose should i go with lower capacitance and higher ESR or the other way round. I'm trying to optimize the filter for the lowest ripple possible.

[uncle_bob]

Hi

Another thing to consider is the ripple current rating on the capacitors. Often that is a bigger deal than the ESR.

Have you tried the canned design software that TI has for that IC? The main reason to use those old parts is the software. A lot has come along in the 15 years since they came out. If the software isn't helping you ... I'd look at some newer parts.

Bob

[cat87]

The two 330uF in parallel sound OK to me. It's not always a good idea to go with the lowest ESR because the lower the ESR, the larger the current spikes will be when the load changes suddenly.
If either old fashioned pen-and-pencil calculations or design software from TI do not help, then you can always use trial and error and see which  option gives you the best result over a wide range of loads.
This are my two cents