Where to find Low ESR cap at sensible price

Hi All

I’m building an Arduino clone based device to run in a vehicle (motorcycle), so I wanted a regulator/converter that is desgned for vehicular applications (or will deal will the necessary quirks). This seemed to fit the bill perfectly: http://www.ti.com/lit/ds/symlink/lm2940-n.pdf @ $1.65 (needs to be under $5-6)

However it says it needs a low ESR 22uF cap (unbouded upwards). My application will result in an output current of about 30mA. The problem with that is that for the device to operate in the stable range at such low current, it needs a cap with an ESR of 0.1. to 1 (page 14 in datasheet) - also attached. I tried searching on digikey and everything I found (that wasn’t 10’s of $) required 150uF at 6v to get an ESR of 0.9 and dropping to 0.6 at 330uF. Presumably this it at room temp. (I couldn’t find anything in specs about change in EFR according to temp). I don’t mind going bigger than 22uF, but don’t want anything too physically big. I realize I probably need tantalum for more stability as temp changes, but couldn’t find anything at all

Can anyone point me in direction of a suitable component?

One other thing how impotant is the voltage rating? If I’m using Vout of 5v can i use any capacitor rated above 5v? Let’s say 50v for example. Is there a downside?



Best I can find ($1.59@1 for 68uF): http://www.newark.com/avx/tap686k016scs/capacitor-tant-68uf-16v-1-ohm-0/dp/54J5611 Newark allows searching by ESR so you can give it a whirl yourself.

Usually aim for a voltage rating about double the requirement. As the voltage rating of the cap increases the cap becomes physically larger and more expensive, but there's no harm in using too high a voltage rating.

The datasheet mentions that you can parallel tantalum and aluminium electrolytics, the tantalum could be around 4.7uF and the Al cap 15uF, meaning a saving in cost for the tantalum (which guarantees the low ESR).

Thanks Guys

Chagrin, am I mis-reading the data sheet? On P97, it says the 68uF has ESR of 1.8 (85-degrees and 6.3v)

Regards Mark

You can also parallel 2 cheap caps to halve the ESR...

MarkT: You can also parallel 2 cheap caps to halve the ESR...

I fired one across the room with under 1min of operation on the input of a switching supply recently. Perhaps many in parallel for that particular component rather than two would have been the go :)

Found the can about 4metres away. My ears rang for ages. My very own practical introduction to the importance of ESR! Geoff