At 09:06 AM 6/22/2003 -0700, Tal wrote:

>Hello,
>
>I am trying to design a DC/DC (8V down to 3.5 to 6VDC, 500ma) using
>National LM2671 (http://cache.national.com/ds/LM/LM2671.pdf).
>
>The section that discuss the input and output capacitor consideration
>specify the capacitors by their max RMS current. However, some
>capacitors data sheets do not specify this value and instead specify max
>ESR.
>
>My questions to the list are:
>
>1. Is there a relation between capacitor max RMS current and max ESR and
>if so, what is the formula  (does it assume max power dissipation by the
>capacitor?).

Jim Williams of Linear technology gives a good formula for this.
I think they have it in an app note, otherwise it's in one of his design books.

>2. Is it really critical to have low ESR or high current capacitors for
>the input and/or output of the DC/DC ? What happens if I will use
>'simple' capacitors, is it just a matter of efficiency ?

In a thermal printer design I did, I tried several "low esr" and "high frequency" caps, balancing not only the requirements of delivering 19A pulses of 32V, 300uS wide every mS, but also cost.   I ended up with three "cheap" caps, instead of the same $ worth of the expensive units.
It's been 8 years now, and we still aren't seeing any cap failures from the field. :)

>3. What kind of capacitors (electrolytes, tantalum, etc) are preferred
>for this application (volume manufacturing), assuming that component
>cost is very important ?

Tants are not good at high ripple currents.

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