0
\$\begingroup\$

I'm reading a user document of a dev board. On page 12, there is an explanation of the power management subsystem. The board VBUS and VBAT lines have a decoupling cap rated at 6.3V, whereas the output voltage rail has a 10V rated capacitor. What affects the decision of putting a higher voltage rating cap on the the rails. Would it affect the circuit if all caps were rated 10V?

\$\endgroup\$
  • \$\begingroup\$ There is no reason for a well-designed internal regulator to demand this specific voltage value, and even not specifying its ESR. So I think this requirement is BS, both 6.3V and 10V should work just fine on 3.3V rails, just 6.3V is less stable in value, but cheaper. \$\endgroup\$ – Ale..chenski Mar 1 '19 at 2:46
1
\$\begingroup\$
  • greater Vmax voltage is better for reduced supply sensitivity to capacitance drop and often lower ESR but at the expense of size limitations for a 0402 preferred package size.

The short answer is these choices offered the design the best bang for the buck, meaning the lowest impedance choosing the biggest uF in the smallest size to limit ESL [nH], ESR [mΩ] and thus raise self-resonant frequency, SRF. It does not mean you will be able to find these easily and going to a bigger voltage may demand a bigger package that may reduce ESR but may lower SRF below the switcher spectrum of x MHz for example, which may make the cap look like an inductor.

offering limitations

You must spend enough time to understand the many non-ideal characteristics of capacitors to remember that Vmax - uF - ESR - $cost - footprint size ALL have tradeoffsenter image description here

Then spend much more time to be able to have an in-house number with qualified parts that match certain expectations for all the above. Start by reading more datasheets.

C0G won't work here because C0G or NP0 is low-density stable temp coeff. with a low but stable dielectric constant. NP0 means -/+0 ppm/'C {+/-30'ppm} so you cannot get big uF in NP0=C0G material. ( see 1st table above for C0G)

See page 32/34 to see the BOM, you will find the descriptions of 0402 6.3V caps @ 10uF

Other standard material codes are Y5Z,X5R,X7R but ther are zero or . Here Dialog has chosen the biggest value caps in the smallest package size (402 or 603) to provide the lowest impedance at the frequency of current where \$I_C=C*dV/dt\$ and \$f_{-3dB}=0.35/t_R\$ for rise time \$t_R=dt\$ and ripple voltage \$dV=ΔVpp\$ so $$C=\dfrac{I_C*0.35}{ΔV*f_{-3dB}}$$

enter image description here

\$\endgroup\$
  • \$\begingroup\$ Wow, thanks for the detailed response! I have a lot to read up on \$\endgroup\$ – rookie Mar 1 '19 at 1:38
  • \$\begingroup\$ I had to do the same for many days/weeks when I start reading Mil-Stds for caps, crystals etc for aerospace when I graduated. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Mar 1 '19 at 1:40
  • \$\begingroup\$ Keeping track of packaging effects on everything else is the worst. \$\endgroup\$ – DKNguyen Mar 1 '19 at 2:15
  • \$\begingroup\$ Yes and out of 0.5 million ceramic caps listed on D-K none are 10uF 0402 like item 25 not even 4V yet Mouser lists them, with 0 stock and 24 wk lead time \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Mar 1 '19 at 2:33
2
\$\begingroup\$

Real capacitors are far from ideal and different things affect their capacitance. For example, there are multiple types of ceramic capacitors that use different dielectrics and they all behave differently under temperature and DC-bias.

Some ceramic capacitors like Y5V can vary as much as -82% to +22% over their temperature rating! I'm not sure where these are used. I assume garbage consumer electronics. They come in very large capacitances for their physical size though...and they better with that -82% fluctuation.

X5R and X7R capacitors are better in that they vary about +/-15% over their temperature rating. These are typically used for decoupling for anything that matters.

But every type of ceramic except for C0G suffers from DC-bias effects. So if you apply a DC voltage to it (as decoupling capacitors are used) their capacitance decreases. Sometimes by 50% as you get near their voltage limit.

Don't use C0G for decoupling though. They are expensive and physically large for the capacitance they provide. They are also very low ESR (resistance) which is normally good but in decoupling applications can produce ringing. C0G is voltage bias and temperature stable and suffers from no piezo effects (other ceramics will produce a voltage under vibration which results in noise) so they are used in timing and signal path applications like filtering.

You can compensate for this by picking a capacitor rated at the size you want but pick a higher voltage than what you are running it at. Or you can pick one near the voltage you're running it at but choose one with a higher rated capacitance.

There are also tantalum capacitors (not ceramic capacitors) that are very sensitive to over-voltage and can also suffer reduced lifetime so you use them at half their voltage rating so bad things don't happen.

For electrolytics de-rating their voltage can extend their lifetime (also true of other capacitor types).

\$\endgroup\$
  • \$\begingroup\$ Thanks! I'm not familiar with the term C0G - can you elaborate? \$\endgroup\$ – rookie Mar 1 '19 at 0:28
  • \$\begingroup\$ It's just the code for a type of dielectric used in C0G/NP0. Two codes, same thing. Just like Y5V, Y5R, X5R, X7R, X8R, X8S, each digit tells you the temperature range and stability of the capacitor. Scroll down the wiki for the code tables. en.wikipedia.org/wiki/… \$\endgroup\$ – DKNguyen Mar 1 '19 at 0:29

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.