Here is the relation between a capacitor Irms (max RMS current) and ESR:

   ESR = Pd / (Irms * Irms)

Where Pd is the max power dissipation of the capacitor.

Since only ESR, the resistive component of the capacitor dissipates
heat, this formula seem to make sense.

BTW, it is taken from page 18 (top of right column) of Nationa's LM3477
data sheet (http://cache.national.com/ds/LM/LM3477.pdf).

Tal

> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU] On Behalf Of Olin Lathrop
> Sent: Sunday, June 22, 2003 12:32 PM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: Capacitor spec for DC/DC application
>
>
> > 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?).
>
> Low ESR capacitors can generally tolerate higher ripple
> currents, but the two are separate specs.  The ESR rating is
> more relevant to whether the capacitor will function
> sufficiently like an ideal capacitor to allow the circuit to
> work correctly.  Max ripple current is for protecting the
> capacitor from damage.
>
> > 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 ?
>
> ESR can be critical depending on how tolerant the circuit is
> to a resistor effectively in series with the capacitor, which
> can also effect efficiency.  The ripple current spec is only
> critical if you'd like the capacacitor not to degrade early
> (like blowing up and covering everything near it with a thin
> layer of corrosive goo).
>
> > 3. What kind of capacitors (electrolytes, tantalum, etc)
> are preferred
> > for this application (volume manufacturing), assuming that
> component
> > cost is very important ?
>
> That depends on the circuit needs.  DC-DC converters can
> often be designed with a tradeoff between inductor and
> capacitor requirements.  Inductors are expensive and bulky
> too, so sometimes the cheapest answer is a more expensive
> cap.  Electrolytics are usually the worst, although some are
> much better than others.  Check out the Panasonis FC series,
> for example. Not surprisingly, they are more expensive than
> other electrolytics. Tantalums are the next step up, but are
> more bulky and expensive for the same capacitance and
> voltage.  The best for ESR and ripple current are ceramics,
> but of course come only in low capactitance values.  There is
> a lot of hot development on ceramic 1208 surface mount
> capacitors that have 10s of uF in the 5-20V range.  Several
> companies have these, and new products come out regularly and
> are significantly better than the ones just 6 months old.
>
>
> *****************************************************************
> Embed Inc, embedded system specialists in Littleton Massachusetts
> (978) 742-9014, http://www.embedinc.com
>
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