I can fit both electrolytic 100uF and cer. 100nF next to IC, should I add both? - Page 1
Forum section: electronics / beginners

author: dusan
author_rank: Regular Contributor
timestamp: November 19, 2022, 03:13:23 pm
content: I can fit both electrolytic 100uF and cer. 100nF decoupling capacitors very close to IC, room is not a problem, should I add both? From capacitance perspective increasing 100uF to 100.1uF has minimal effect, but does a ceramic capacitor have some special qualities that electrolytic cap is lacking? Is it beneficial to add 100nF if I aready have 100uF right next to the chip?To be more specific, 100uF is about what I need. Does adding 100nF ceramic helps something?

author: Fraser
author_rank: Super Contributor
timestamp: November 19, 2022, 03:19:56 pm
content: The ceramic capacitor will deal with higher frequency noise that the electrolytic will not.

author: rsjsouza
author_rank: Super Contributor
timestamp: November 19, 2022, 03:24:12 pm
content: As Fraser put it well, the material of the capacitor's dielectric will have different physical (and consequently electrical) properties that allow it to behave differently depending on frequency, temperature, vibration, etc.In this particular case, the ceramic (or even polyester, mica, etc.) will have a better performance in higher frequencies, which is common in digital circuits.

author: mariush
author_rank: Super Contributor
timestamp: November 19, 2022, 03:44:57 pm
content: The 100nF capacitors are decoupling capacitors.Here's a few videos showing why they're important:

author: dusan
author_rank: Regular Contributor
timestamp: November 19, 2022, 04:05:32 pm
content: Thanks, I think these videos answered all my questions.

author: TimFox
author_rank: Super Contributor
timestamp: November 19, 2022, 04:18:58 pm
content: When both capacitors are used, the ceramic needs to be as close as practicable to the chip, but the electrolytic location is not so critical.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 19, 2022, 05:09:49 pm
content: Remember to NEVER EVER (unless you really know what you are doing and can actually measure for performance and EMC) do what is shown in the above video thumbnail of EEVBlog #859 (don't have time to watch the video now and I don't remember if Dave used that as an alarming example, or encouraged to do that bullshit).Then to the actual question. First, nowadays there rarely is need to use 100nF for bypassing. With bypass capacitors, the purpose is to giveoverall minimal absolute impedance. (Unit for impedance is ohm and only that should interest you - relative impedance graphs where Y axis autoranges between different parts confuse people!)How is this achieved?Forhigh frequencies, by minimizing ESL, that simply means:minimizing package size and distance to pins- distance also includes vias and such; using two vias in parallel is better than one, and so on. Forlow frequencies, by maximizing C. Use largest capacitance value you can get.So simply: use smallest package size you can handle. For me, it's 0402 unless I have strong reasons to go smaller, usually no. Now limiting yourself to 100nF with 0402 is completely arbitrary. You can easily get 1uF parts in 0402.They perform the same at high frequencies, but the 1uF part works better at lower frequencies. For IC power bypassing, excess capacitance does not hurt. For voltage regulators, references etc., see their stability criteria, however. Usually a lot of capacitance is only better.The graph shown in the above youtube video thumbnail hashidden assumptionthat larger C means larger package and larger ESL! Thisused to be truebetween 100nF and 1uF parts 30 years ago. Today, 100nF and 1uF is available in same 0402 package, and same ESL.TLDR: for bypassing, if in doubt, use 1uF instead of 100nF. If you know 100nF is enough, then sure, go for it.Then regarding the electrolytic capacitor. Electrolytic caps have one significant advantage: they have a lot of capacitance combined with significant ESR. ESR is a stabilizing element: quick voltage change means power loss in ESR, which is better than having the same voltage drop over the ESL. Why? Because the ESL stores the energy, and supplies it back, causing oscillations, which cause emissions (possibly at hundreds of MHz, failing EMC), or even worse, voltage can overshoot to levels that damage parts.Large values of MLCCs can cause significant voltage peaking and oscillation with long input wires (inductance). Further, different values of MLCCs put in parallel (see aforementioned video thumbnail) can oscillate with each other. Latter can be avoided by not doing those different values. Former - add the electrolytic at the power input, job pretty much done!Now if unsure about if your power distribution network might still oscillate, it does no harm to add those high-ESR capacitors here and there, if you have room and budget that is. Typical choices are aluminum electrolytics, or tantalums. MLCC with explicit series resistor is fine, too. Just remember this damping capacitor needs to have several times (2-3x bare minimum) the capacitance of the to-be-damped low-ESR (MLCC) bank. For example, ten 1uF MLCCs in parallel could be damped using a 47uF electrolytic, but 100uF or 220uF is fine, too. So called "low-ESR" types are usually fine because they are not actuallythatlow; but general purpose type is the best bet unless you actually have large ripple currents (e.g. switch mode converters, especially the input of buck).

author: Infraviolet
author_rank: Super Contributor
timestamp: November 21, 2022, 07:07:05 pm
content: I'd usually add two ceramics, 100nF and 10uF. The 10uF ceramic MLCC (0603 or 0805 format) are pricier than normal for caps, but not expensive really. The 100nF deals with short glitches, the 10uF helps protect against longer brownouts which might occur if the power rail powering the IC also turns on something with higher current demands. An electrolytic cap usually has too high an ESR and inductance to respond fast enough to help with decoupling. You can get 100uF ceramics though, but they are priced around £1 each, and only rated for low voltages, still smaller than equivalent electrolytic.