# Is there a relevant difference between these capacitors?

I am planning to try designing a board that incorporates a switch-mode DC-DC converter, using this Texas Instruments reference design for their TPS54386PWPR regulator part:

The bill of materials for the Texas Instruments circuit specifies C1 as a 100 µF 25 V 20% aluminum capacitor (Panasonic part # EEE-FC1E101P) that is 10.2 mm tall. I would like to substitute a part that is no more than 8 mm tall for board clearance reasons.

I was able to find several possible alternatives on Digi-Key, including other Panasonic parts, but I do not understand which specifications might be relevant to choosing a suitable replacement and which are not.

For example, all of these match the basic parameters I (mostly) understand: capacitance, voltage and tolerance. But what else do I need to take into account?

• I guess if you post a schematic it isn't considered a shopping question. Good to know. LOL. – mikeY Jul 10 '13 at 18:56

Being a switching supply, the effective resistance (ESR) is important. The FC series is a low ESR type, so you'll want something with similar or lower ESR. (although with some circuits, if you go too low on the ESR it can become unstable, too).

Normal (high ESR) cheap capacitors will lose efficiency, may not work at all, and have shorter life. It's more important on the output capacitors, but is important at the input as well.

You will notice some of the more modern, high frequency switchers use more ceramic capacitors, as they have a very low ESR. Sometimes they will parallel both types, to try and get the best of both worlds.

Tolerance isn't really important here. Voltage rating should be higher than the input you plan to use (so if you intend to use, say, 24V input, you'd want 35V cap for safety margin). Life at temperature is important as well, if it will be in a higher temperature environment.

If you want to have something similar from panasonic, try the FK series. It is miniaturized, and somewhat reduced ESR relative of FC.

EEEFK1E101P is 8mm dia and 6.2mm height, otherwise similar capacitor.

For DC-DC converters input bulk capacitor depends mostly on ESR and Ripple current rating of the capacior.