[SI-LIST] Re: Ceramic Vs Tantalum for bulk bypassing
- From: steve.barton@xxxxxxxxxxxxxxx
- To: joel@xxxxxxxxxx
- Date: Sat, 22 Apr 2006 16:22:47 -0700
Hi, Joel,
The ceramic versus tantalum capacitor choice can depend on the particular
parameters of the situation - the amount of current the bulk capacitance has
to supply, for how long and the range of frequencies to be bypassed. All
versus the physical size and cost of the capacitor.
Let me offer some comments:
* If the bulk capacitance is also the output capacitance of a Voltage
Regulator, and the capacitance has an ESR that is too low or too high, the
Regulator circuit may become unstable. Depends on the design of the
Regulator. Even if the bulk capacitance is separate from the output
capacitor, the added bulk capacitance appears in parallel with the output
capacitor and may affect the stability of the Regulator.
* The larger value ceramic capacitors will tend to have the poorer
performance Y5V and Z5U dielectrics. So, if your circuit has to work over a
wide temperature range, you must consider the change in capacitance with
temperature.
* My own rule of thumb is that I find ceramics to be (physical) size and
cost effective versus tantalums up to about 10 uF, possibly 22 uF tops. For
higher bulk bypass capacitance values, I tend to go with tantalum.
* But, of course, along with the tantalums, I put in lower value 1 nF, 10
nF, 100 nF and/or 1 uF ceramics to take care of the high frequency
bypassing.
For example, a 22 uF ceramic isn't much good for bypass at 900 MHz because
that frequency is way above the series resonance of the 22 uF ceramic.
* I remember a time about five years ago when there was a worldwide shortage
of tantalums - not enough mfg capacity for the demand. Remember the dot-com
boom! So, we in design/development were asked to look at new and existing
designs to see where we could replace tantalums with ceramics. This is a
situation where you can't do a blanket replacement without looking at the
individual circuits. For example, if a product had a 4.7 uF tantalum used
as the output capacitor for a linear voltage regulator, we couldn't simply
write an ECO to change it to a 4.7 uF ceramic. We had to check that the
regulator would remain stable with the lower ESR of the ceramic part.
* Cost is a big issue for large values of bulk capacitance - that's why we
still see groups of aluminum electrolytics in vertical cans on PC
motherboards.
* Of course, ceramics do have an advantage that they can be installed
backwards without blowing up when the power is turned on. Joke for mfg
people!
Hope the above info helps. Tend to agree with your comment where you say,
"Tantalum when used with lower value ceramics (.1uf) might be just as
effective as all ceramic"
Cheers,
Steve Barton
FLEXTRONICS Components Group
San Jose, California, USA
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Istvan Novak - Board Design Technology
Sent: Friday, April 21, 2006 10:32 AM
To: joel@xxxxxxxxxx
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Ceramic Vs Tantalum for bulk bypassing
Joel,
There is a hidden contradiction in the opening statement: the lower ESR of
the ceramic does not necessarily reduce noise; most of the time it increases
worst-case peak-to-peak noise by driving a minimum in the impedance profile.
The noise is reduced ONLY if the BW of excitation noise does not extend
beyond the SRF of the low-ESR part.
Another consideration is the minimum amount of bulk capacitance you need.
When de design calls for many thousand microfarads of bulk capacitance, it
is more effective to provide it with tantalum-like parts.
Regards,
Istvan
Joel Brown wrote:
>Which is better for bulk bypassing, ceramic or tantalum?
>Ceramic can be used up to rated voltage, tantalum needs 50% voltage
>derating.
>Ceramic has lower ESR which reduces noise due to surge currents, but
>the higher ESR of tantalum capacitors could damp ringing which reduces
>noise. Tantalum capacitors are less likely to crack under stress.
>Tantalum when used with lower value ceramics (.1uf) might be just as
>effective as all ceramic.
>Any other thoughts on this?
>
>Thanks - Joel
>
>
>
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