Hi Nicholas,

I'm looking at the Panasonic FC line in my handy Digikey catalog and
find Impedance (100kHz) listed.  This is a good surrogate for ESR in
switching power supply (SMPS) applications.  Another important spec for
SMPS applications is the ripple current rating.

ESR will effect the ripple voltage seen across the capacitor as a
consequence of the ripple current passing through the capacitor (lower
is better).  The ripple current rating (higher is better) of the
capacitor (or capacitor bank) relative to the actual ripple current
through the capacitor will give an indication of service life.

What constitutes "low" depends entirely upon the application.  Just
remember that ESR * (ripple current) = ripple voltage, no matter how
large the capacitor.  Usually, capacitors with low ESR will have higher
ripple current ratings.  This is largely a consequence of the need to
dissipate heat.  The trade off, other things being equal, is that low
ESR caps cost more than their higher ESR cousins at least in terms of
their initial price (when you have to replace the caps in your Picstart
wallwart remember, they saved a few cents).

Regards,
Dave

Nicholas Irias wrote:
>
> I have seen a number of IC datasheets that call for low ESR capacitors
> as external components.  But when you look up capacitors in a Digikey or
> other catalog, ESR is never quoted in terms of x ohms at 250kHz or any
> other frequency for that matter.  Even caps advertised as low ESR do not
> cite actual ESR values.
>
> And if you check out datasheets at manufacturer's web sites, the ESR
> values cited for "low ESR" caps vary substantially - anywhere from
> milliOhms to 1000+ ohms.
>
> How should I interpret an IC datasheet that recommends a low ESR cap?
> Should I just use a polyester cap and figure that ESR should be low?  Or
> is there some target resistance, at the appropriate frequency for the
> application, that I should be looking for on a capacitor datasheet?
>
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