[SI-LIST] Re: Chip caps vs. Tantalum
- From: pwelling@xxxxxxxxxxxxxx
- To: Michael_Poimboeuf@xxxxxxxxxxxxxx, martin@xxxxxxxxxxxxxx
- Date: Mon, 19 Aug 2002 16:35:41 -0600
HI,
Sometimes a Tantalum dielectric is preferred.
One place where you would not want to consider ceramic capacitors is in low
frequency applications that require a low ESR.
You may not want ceramic capacitors in applications such as decoupling a VCO
or PLL charge pump (in addition to high frequency ceramic), for improved
jitter and phase noise performance. The ESR and frequency response are not
the best choice for a high performance design.
You may not want ceramic capacitors in audio and low level/low frequency
analog applications like amplifiers, filters, etc.
Some applications are sensitive to microphonics (flexing of the
multi-stacked dielectric and deposited electrodes creating a piezoelectric
effect that follows the vibration source). COG dielectrics help in this
situation, but have small values and require paralleling to get higher
values. Here you will need low ESR down to DC and if in a mechanically noisy
environment, the microphonics may introduce problems.
A definite drawback to Tantalum capacitors is they are polarized. Being a
polarized component, sometimes you must deal with biasing an AC signal,
reverse surge currents then become an issue to work around.
Ross
-----Original Message-----
From: Michael_Poimboeuf@xxxxxxxxxxxxxx
[mailto:Michael_Poimboeuf@xxxxxxxxxxxxxx]
Sent: Monday, August 19, 2002 3:01 PM
To: martin@xxxxxxxxxxxxxx
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Chip caps vs. Tantalum
There are lots of factors favoring ceramic over tantalum - as well as some
the
other way round.
The foremost factor in my experience is reliability and safety.
Tantalum caps are more prone to failure due to surge current from fast
turn-on.
If you search
the web for surge current failure modes in tantalum capacitors I think you
may
be able to
track down the papers regarding circuit designs to limit inrush current for
tantalum applications.
For instance:
https://www.onsemi.com/pub/Collateral/AN1542-D.PDF
I was involved in an unpublished study regarding computer system failure,
explosions, fire and
so forth due to inrush current failure... In my study we found that the
tantalum
capacitors failed
because they were overheated by the contract manufacturer during assembly
and
rework and
that inrush current caused them to fail (spectacularly) in the field
afterwards.
If chip caps give you sufficient Xc over the frequency and temperature range
you're designing
for, and if you can tolerate microphonic noise and other issues with
ceramics,
then ceramics
are better.
--
Michael Poimboeuf
mkp@xxxxxxxxxxxxxx
"Martin Euredjian" <martin@xxxxxxxxxxxxxx> on 08/19/2002 10:37:46 AM
Please respond to martin@xxxxxxxxxxxxxx
To: si-list@xxxxxxxxxxxxx
cc: (bcc: Michael K Poimboeuf/AM/Avid)
Subject: [SI-LIST] Chip caps vs. Tantalum
I've been trying to figure out what the differences might be between
large-value (1 to 100uF) chip caps and similarly valued Tantalums. This, of
course, as it pertains to PDS or switch-mode DC-DC converter design.
When/Why might one choose a chip cap over a Tantalum? What are the
advantages/disadvantages in comparison?
I've seen pictures of modern DC-DC modules that seem to use several
large-value chip caps. If that's the case, why did the designer go this
route?
Thank you,
===============================
Martin Euredjian
eCinema Systems, Inc.
voice: 661-305-9320
fax: 661-775-4876
martin@xxxxxxxxxxxxxx
www.ecinemasys.com
===============================
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