3
\$\begingroup\$

The STM32F family requires decoupling capacitors, but all of the documentation gives only limited (and vague) guidance on what types of capacitors to use. For example, in the datasheet for the STM32F103R6, page 73, they say that a particular 10nF capacitor between VDDA and VSSA should be "ceramic (good quality).” Is good quality a low-ESR ceramic like X5R, or a low voltage-dependence ceramic like C0G?

Also, here in AN4325 - Getting started with STM32F030xx and STM32F070xx series hardware development:

chemical capacitor

What is a "chemical capacitor"? Is it just a wet aluminum electrolytic?

My basic question is this: what kind of capacitors do people typically use for decoupling on STM32 MCUs? Would X5Rs 16V for the ceramics and aluminum polymer (not wet aluminum) also 16V for the bulks be acceptable choices? If not, what should I pick?

\$\endgroup\$
4
  • 1
    \$\begingroup\$ There's no obvious reason why you couldn't just use a 4.7uF ceramic. \$\endgroup\$
    – Lundin
    Commented Jan 9, 2023 at 7:43
  • 1
    \$\begingroup\$ Well, the common ceramics like X5R and X7R have capacitance that is voltage dependent. If the point of the 4.7uF cap is to absorb large voltage swings coming at low frequencies, then I can understand why they would not want a ceramic capacitor whose capacitance is going to change depending on how big of a voltage spike hits it. That said, I don't think 4.7uF is a magic value -- it could probably be 2uF or 10uF and still work, so overall I suppose you're right. But I think this is probably the reason they went out of their way to suggest a "chemical" (i.e., Al electrolytic) capacitor. \$\endgroup\$
    – jemalloc
    Commented Jan 9, 2023 at 9:53
  • \$\begingroup\$ Also, the answer below makes a good point that an aluminum electrolytic will have higher ESR than a ceramic capacitor and thus will dampen resonance. That could be another reason they suggest using an electrolytic for the 4.7uF cap. \$\endgroup\$
    – jemalloc
    Commented Jan 9, 2023 at 9:58
  • \$\begingroup\$ Aah wait I just realized that this might be one of the parts with a built-in regulator. For running straight from a single li-ion cell or similar. If so then it makes much more sense. Otherwise when supplying the MCU from a regulated power source then the chemistry (or ESR) shouldn't matter much. In which case you'd stick a large aluminium electrolyte bulk cap on the supply input. \$\endgroup\$
    – Lundin
    Commented Jan 9, 2023 at 10:24

1 Answer 1

Reset to default
3
\$\begingroup\$

Maybe translation quirks?

X5R or better will be fine; and I think they do mean electrolytic.

The bulk cap brings ESR which dampens resonances between connected low-ESR caps. Consider the component ESL and trace inductance between them; a model can be built on inspection of the layout.

Aluminum polymer may be too low ESR to serve that function, but a range of ESRs are available; select accordingly.

\$\endgroup\$
3
  • \$\begingroup\$ Yes, it could be translation, or maybe they don't want to be too prescriptive about this because they consider it somewhat application specific. \$\endgroup\$
    – jemalloc
    Commented Jan 9, 2023 at 5:34
  • \$\begingroup\$ Why is it called "bulk" cap? I mean what does "bulk" mean here? This not like 1 Mega Farad capacitor, it is just small micro Farad capacitor. \$\endgroup\$
    – gyuunyuu
    Commented Jun 29, 2023 at 14:20
  • 1
    \$\begingroup\$ @gyuunyuu Bulky in relation to the high-frequency capacitors around it; indeed, it need not be all that big in absolute terms. \$\endgroup\$
    – Tim Williams
    Commented Jun 29, 2023 at 17:50

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

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