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I'm using a 22u 6.3V X5R 0805 ceramic capacitor to filter out a 3.3V supply (for a buck converter.) I've heard that applied voltage has an effect on the capacitance of certain ceramic caps; a higher voltage causes a reduction in capacitance. How true is this? Would it decrease too much as to not work as a suitable filter cap? I think 10-15u is probably the minimum for it to remain stable, although the ripple will probably increase at lower capacitance. Any ideas?

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This page has the following table:
alt text

So, if the 22uF is a 20% tolerance part such that is may actually be 17.6uF (-20%), and it is really 15uF (-15%) due to heating, it may be reduced to ~10.5uF (-30%) near its tolerance or ~14uF (-5%) near its operating point. To reduce the effect, use a higher breakdown value capacitor.

Although this webpage tests much higher breakdown voltage ceramics, the results are illustrative: alt text

This part, the 08056D226MAT2A by AVX, states that it meets its tolerance tested at 0.5 VRMS, not close to its breakdown:

For Cap > 10 μF, 0.5Vrms @ 120Hz

It also mentions that capacitance may change up to 12.5% over its load life, 7.5% due to soldering thermal shock, and 12% due to flexure fissures (cracks).

Info tidbit: The term X5R means it is composed of a class 2 dielectric (ie: ceramic) that will maintain its capacitance to within 15% (18.7uF - 25.3uF) over a temperature range -55oC to 85oC.
Something that may interest you, though it has more to do with the final application than the question -- the great 'pedia also mentions:

Due to its piezoelectric properties, they are subject to microphonics.

... And the previously linked page:

High-K ceramic capacitors can show significant piezoelectric effects; if you tap them they will produce a voltage spike. This is caused by the barium titanate, the main material in high K ceramics. The higher the K, the stronger this affect.

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  • \$\begingroup\$ Wow. That's a lot of capacitance loss. The exact part is GRM21BR60J226ME39L - uk.farnell.com/murata/grm21br60j226me39l/…. \$\endgroup\$ – Thomas O Jan 7 '11 at 19:44
  • \$\begingroup\$ I have footprint space for a 22u 10V 1206 cap - would it be advisable to place this instead of a 22u 6.3V 0805? I'm not currently placing it because of slightly higher cost of the 1206 cap vs. 0805 cap. \$\endgroup\$ – Thomas O Jan 7 '11 at 19:46
  • \$\begingroup\$ @ThomasO, Here's an 0805 10V (21c/3000; $1.05/1). Not sure about pros & cons of 0805 vs 1206, but 10V is definitely better than 6.3V. Are you going to be able to test the microphonics? Sounds interesting... \$\endgroup\$ – tyblu Jan 11 '11 at 6:30
  • \$\begingroup\$ @tyblu, It's nice, but quite expensive (twice the price) and it seems I need to order min. 3000. Also, it would be bad to depend on a single capacitor manufacturer. There are several suppliers for 22u/6.3V caps. \$\endgroup\$ – Thomas O Jan 11 '11 at 13:22
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    \$\begingroup\$ @ThomasO, Are you designing for the military or Apple? Replacing this part would be trivial if the original manufacturer went kaput. \$\endgroup\$ – tyblu Jan 11 '11 at 13:46

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