Selecting > 1uF ceramic caps

[Tim Shoppa]

I'm migrating some designs from through-hole and SMT electrolytics
and am beginning to learn what's possible with large ( 1 - 10uF )
ceramic caps.

The electrolytics are typically subassembly bypass capacitors and
caps used in switched-capacitor converters (i.e. MAX232) although there
are occasional uses in things like RC one-shots and AF-frequency voltage
shifting.

Right now I'm trying to get my mind into the mindset where ceramics
are reasonable alternatives and I'm still not completely converted.

Panasonic has a few app notes extolling all the virtues of these parts,
but my question is: what are the gotchas?

For bypass purposes the +80/-20 percent parts seem to be price competitive
with electrolytics at the very low uF range. The 10 percent ceramic
parts aren't so competitive, nor are the +80/-20's above 10uF or so.

Tim.

 

[Tim Wescott]

The only problem that I know of personally is that the high-value ceramics
are microphonic. I don't know if it's piezoelectricity or if it's just
capacitance changes that cause a signal to appear when the cap is biased,
but this can cause you grief if you're using them in the signal processing
chain.

I can remember one famous episode at a former company where we ended up
using back-to-back tantalums in a video PLL circuit -- had we designed it
for tantalums from the start we would have created a nice stable negative or
positive voltage reference for the cap, but since it was after the fact...

"Tim Shoppa" <shoppa@trailing-edge.com> wrote in message
news:bec993c8.0402090914.a9d4106@posting.google.com...

I'm migrating some designs from through-hole and SMT electrolytics
and am beginning to learn what's possible with large ( 1 - 10uF )
ceramic caps.

The electrolytics are typically subassembly bypass capacitors and
caps used in switched-capacitor converters (i.e. MAX232) although there
are occasional uses in things like RC one-shots and AF-frequency voltage
shifting.

Right now I'm trying to get my mind into the mindset where ceramics
are reasonable alternatives and I'm still not completely converted.

Panasonic has a few app notes extolling all the virtues of these parts,
but my question is: what are the gotchas?

For bypass purposes the +80/-20 percent parts seem to be price competitive
with electrolytics at the very low uF range. The 10 percent ceramic
parts aren't so competitive, nor are the +80/-20's above 10uF or so.

Tim.

 

[Jeff]

Another would be the very low ESR causing instabilities. Mostly a problem
with regulators.

"Tim Wescott" <tim@wescottnospamdesign.com> wrote in message
news:102fhr7polqj992@corp.supernews.com...

The only problem that I know of personally is that the high-value ceramics
are microphonic. I don't know if it's piezoelectricity or if it's just
capacitance changes that cause a signal to appear when the cap is biased,
but this can cause you grief if you're using them in the signal processing
chain.

I can remember one famous episode at a former company where we ended up
using back-to-back tantalums in a video PLL circuit -- had we designed it
for tantalums from the start we would have created a nice stable negative
or
positive voltage reference for the cap, but since it was after the fact...

"Tim Shoppa" <shoppa@trailing-edge.com> wrote in message
news:bec993c8.0402090914.a9d4106@posting.google.com...
I'm migrating some designs from through-hole and SMT electrolytics
and am beginning to learn what's possible with large ( 1 - 10uF )
ceramic caps.

The electrolytics are typically subassembly bypass capacitors and
caps used in switched-capacitor converters (i.e. MAX232) although there
are occasional uses in things like RC one-shots and AF-frequency voltage
shifting.

Right now I'm trying to get my mind into the mindset where ceramics
are reasonable alternatives and I'm still not completely converted.

Panasonic has a few app notes extolling all the virtues of these parts,
but my question is: what are the gotchas?

For bypass purposes the +80/-20 percent parts seem to be price
competitive
with electrolytics at the very low uF range. The 10 percent ceramic
parts aren't so competitive, nor are the +80/-20's above 10uF or so.

Tim.

 

[R.Legg]

shoppa@trailing-edge.com (Tim Shoppa) wrote in message

Panasonic has a few app notes extolling all the virtues of these parts,
but my question is: what are the gotchas?

For bypass purposes the +80/-20 percent parts seem to be price competitive
with electrolytics at the very low uF range. The 10 percent ceramic
parts aren't so competitive, nor are the +80/-20's above 10uF or so.

The larger Y5V dielectric parts tend to to be composed using materials
that do not have a thermal profile permiting their use on the
wave-solder side of an assy. If your process expects this, they are
unsuitable.

X5R dielectrics don't usually suffer from this limitation - though you
seem to be shying away from these parts, due to price. High value
ceramics are unlikely to ever be price-competitive when compared to Al
electrolytics, regardless of the dielectric. Their use is usually
determined by other factors entirely ie form-factor, product density /
real estate / process control. Don't confuse the issue.

RL

 

[John Woodgate]

I read in sci.electronics.design that R.Legg <legg@magma.ca> wrote (in
<e715b5cc.0402091636.1feb174e@posting.google.com&gt about 'Selecting >
1uF ceramic caps', on Mon, 9 Feb 2004:

Don't confuse the issue.

Beg pardon but that's de facto newsgroup policy. Especially telling
newbies to 'use a PIC' or 'do a Fourier'. (;-)
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

 

[Jamie]

Ah,, Fourier Transforms.
FFT's
DFT's
IDFT's
i do them in programming Windows base sound card crap and
some High end ADC converters, i would hate to see one done in
a Pic chip.! or though i do have some code lying around here some
where that does a FFT and IFFT in 8051 i do think..


John Woodgate wrote:

I read in sci.electronics.design that R.Legg <legg@magma.ca> wrote (in
e715b5cc.0402091636.1feb174e@posting.google.com&gt about 'Selecting
1uF ceramic caps', on Mon, 9 Feb 2004:


Don't confuse the issue.


Beg pardon but that's de facto newsgroup policy. Especially telling
newbies to 'use a PIC' or 'do a Fourier'. (;-)

 

[Tim Shoppa]

legg@magma.ca (R.Legg) wrote in message news:<e715b5cc.0402091636.1feb174e@posting.google.com>...

X5R dielectrics don't usually suffer from this limitation - though you
seem to be shying away from these parts, due to price. High value
ceramics are unlikely to ever be price-competitive when compared to Al
electrolytics, regardless of the dielectric. Their use is usually
determined by other factors entirely ie form-factor, product density /
real estate / process control. Don't confuse the issue.

Real estate and product density - especially removing high-profile
components like lytics so the boards can be stacked more closely or
so that we can use lower-profile LED/LCD indicators and connectors - are
indeed driving this particular migration. It's very much a tradeoff,
as replacing a tall skinny radial electrolytic sometimes ends up requiring
more board real estate. ("Tall" means anything over 6mm at present!)

One rather radical redesign involves moving components so they are under a
large LCD glass display, in fact. I've seen this done in a couple of
mass-produced designs but never thought I'd be doing that sort of
density myself.

I'm just wondering out-loud how ceramic electrical characteristics might affect
existing designs. Certainly all the LDO regulator makers warn about using
bypass caps with too-low ESR; knowing that this is a possibility may
require putting down a few resistors in series, requiring a little
more real estate, but with only a few LDO regulators spread out over
many square feet of board this isn't a major real-estate issue. I've
gotten some ESR numbers from Panasonic and Murata but for the most
part they don't publish or guarantee these numbers for their ceramic parts.

Tim.

 

In some power apps, the lower ESR can combine with trace
induction to produce fatal powerup spikes.


On 9 Feb 2004 09:14:28 -0800, shoppa@trailing-edge.com (Tim Shoppa)
wrote:

I'm migrating some designs from through-hole and SMT electrolytics
and am beginning to learn what's possible with large ( 1 - 10uF )
ceramic caps.

The electrolytics are typically subassembly bypass capacitors and
caps used in switched-capacitor converters (i.e. MAX232) although there
are occasional uses in things like RC one-shots and AF-frequency voltage
shifting.

Right now I'm trying to get my mind into the mindset where ceramics
are reasonable alternatives and I'm still not completely converted.

Panasonic has a few app notes extolling all the virtues of these parts,
but my question is: what are the gotchas?

For bypass purposes the +80/-20 percent parts seem to be price competitive
with electrolytics at the very low uF range. The 10 percent ceramic
parts aren't so competitive, nor are the +80/-20's above 10uF or so.

Tim.