Re: Any problems with using X5R/X7R caps as buck converter output capacitors?

On Tue, 29 Jan 2008 23:09:35 +0000, John Devereux
<jdREMOVE@xxxxxxxxxxxxxxxxxx> wrote:

"Joel Koltner" <zapwireDASHgroups@xxxxxxxxx> writes:

Before I start dropping some decent fraction of a dollar per capacitor (sorry,
Joerg, the PCB layout guy claims he doesn't have room for electrolytics), I
was wondering if anyone here has had any problems (or successes) using regular
ceramic chip caps with X5R or X7R dielectrics as their main output capacitors
in a buck converter? I need ESR under 250milliohms, which -- while ceramic
caps don't normally spec this -- seems much higher than any guesstimate at ESR
I can make by looking at impedance graphs from the likes of AVX (my
guesstimates range from about 10-100milliohms depending on the particular cap
I look at). I'm looking for some tens of uF here... probably a 22uF cap would
work well.

One thing to watch out for is the the ESR being too *low*. Some SMPS
chips are unstable without enough ESR on the output!

A number of linear regs and some of the Simple Switchers are unstable
with very low esr. Some datasheets say so, and some you have to inferr
from app notes. We use one Micrel buck switcher that's very pickey
about esr, and the only way to find out is by researching the output
cap that's used on the eval board.

Of course, all our boards have scads of 0.33 uF ceramic bypass caps
scattered around. They may not be on the power supply *** of the
schematic, but they're sure present electrically. Seems like an
electrolytic with some modest esr (even a polymer) damps the ceramics
enough to keep most things happy: pole, zero, pole.

Seems like roughly half the average board is power supplies these
days.



The chip ceramics I have tried appeared to have very low ESR - in the
region of 10mOhm or less.

But yes, I have used them successfully and much prefer them to
electrolytics now, where it is possible to use them. (I test stability
by connecting a 50 ohm output function generator to the input
/ output, and sweeping DC - few hundred kHz. This seems to highlight
any problems). Also check switch-on transient behaviour and current
limit recovery.

We bang the output with a load step square wave, and observe transient
recovery. That gives about the same info. If it rings much, it's not
stable.

John



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