voltage regulator additional caps and which value needed? - Page 1
Forum section: electronics / beginners

author: mclien
author_rank: Contributor
timestamp: January 19, 2025, 04:19:30 pm
content: As I'm trying to get may PCB very small, I cannot use my c&p method from other MCU boards.I searched for a small footprint voltage regulator and found the MIC5528 (datasheet attached). Datasheet states to use 2.2uF caps on in and out voltage to ground.Another common voltage regulator seems to be the AP2112 (sort of a standard of the adafruit boards), which uses 10uF caps for that purpose. I often see thoses used with the SAMT21 type of MCU using an additional 1uF cap o the output voltage side.So I'm a bit lost if I need some additional caps (and what value) in my scenario (I attached a screenshot of that part of my circuit, too (the PMEG2020 is just 2 shottky diodes with common cathode in on package to allow USB and battery as input).So is C2 even necessary (If so, I guess the 1uF is most likely the wrong value)

author: RoGeorge
author_rank: Super Contributor
timestamp: January 19, 2025, 04:49:22 pm
content: The prefix 'm' means mili, use 'u' for micro.There is no a unique value good everywhere. Search the datasheet from the exact regulator you want to use. Not a generic datasheet, but the recent datasheet from the manufacturer you will use, and look there for the recommended values, and for the recommended type of capacitor.Note that the value is not enough, thetypeof capacitor matters. For example, if the datasheet says 10uF tantalum, and you use 10uF MLCC instead of tantalum, that won't be good even they both have 10uF.

author: mclien
author_rank: Contributor
timestamp: January 19, 2025, 05:20:00 pm
content: Quote from: RoGeorge on January 19, 2025, 04:49:22 pmoh, mea culpa. correctedQuoteI was of the impression the attached datasheet is the on from the manufactorer.QuoteSo according to the datasheet (p. 13) that should be 2.2uF. LowESR ceramic (type X5R/X7R).As the board can be extended with some RF features/circuits it might be good to follow this advice from the datasheet for the input side:"Additional high frequency capacitors, such as small-valued NPO dielectric-type capacitors, help filter out high frequency noise and are good practice in any RF-based circuit"I guess the value f that one is depended of the frequencies to expect? (the usual suspects would likely be 2.4/5/6 Ghz for WiFi and Bluetooth)

author: Siwastaja
author_rank: Super Contributor
timestamp: January 20, 2025, 01:03:29 pm
content: Definitely do not parallel 2.2uF and 1uF ceramics, it makes no sense at all. Such mistakes easily happen when you "copy-paste" designs together, e.g. load-side IC datasheet specifying 1uF and regulator specifying 2.2uF. In this case, single 2.2uF would do the job, if the regulator is close to the IC. Or, if there is distance between them, use the same 2.2uF part at both sides.One thing to be aware of is that some regulator ICs need certain amount of ESR to operate correctly, and therefore suggest usage of tantalum output capacitor; this recommendation should not be ignored. But if the regulator is MLCC-stable (i.e., stable down to some 10mOhm of ESR or so; and I recommend choosing such modern-day regulator), then just use enough output capacitance to meet the recommendation. They rarely tell you what is theactualcapacitance needed, instead suggesting a nominal MLCC value. This can be problematic because capacitor A might lose just 40% of capacitance at the DC bias, while capacitor B loses whopping 90%. What did the manufacturer mean with their recommendation? The practical answer is to avoid the worst offenders, namely those MLCCs that just offer too much capacitance for the package size; so if 2.2uF is recommended, you don't need to use 4.7uF because you found out your chosen capacitor loses 50% of capacitance at the DC bias, because every other capacitor does it, too, and manufacturer took that into account. But if you choose a 2.2uF part which actually delivers only 0.22uF under DC bias, you are probably in trouble; maybe the regulator truly needs 1uF?Usual internet advice of using the temperature tolerance 3-letter markings (like Y5V, X7R) to make conclusions related to other parameters than temperature tolerances, so to avoid certain stuff (like Y5V) and assuming that others (like X7R) have better DC bias characteristics, is wrong; X7R can colossally suck too. Similarly, the advice of picking higher rated voltage parts to then "derate" the voltage is similarly poor advice; a 10V and 25V part can be even the exact same part, both hitting 50% of rated capacitance at the same absolute voltage (e.g. 5V), not relative to their rating. Really, claimed volumetric energy density (claimed capacitance vs. package size) is the best indirect indicator of DC bias effect - in other words, if it seems too good to be true, it isn't. But nothing beats checking actual data if it's available.

author: mclien
author_rank: Contributor
timestamp: January 20, 2025, 03:32:16 pm
content: Thanks fro the detailed information. And the c&p "guessing" was exactly why I asked. I had an idea what the 1/10uF combination in the original circuit should do, which made my adjustment look fishy, which you confirmed.So I think I'll go with the 2.2uF on both sides (as recommended in the datasheet) and skip any additional cap there. both caps are placed directly beside the regulator (courtyards touching).Re-reading the datasheet, I think it was meant "use caps that are LowESR and X7R".So, if I send a BOM to the manufacturer, do I just list a lowESR cap or is it usual to name a max. ESR for the caps?

author: Siwastaja
author_rank: Super Contributor
timestamp: January 20, 2025, 07:23:44 pm
content: "Low ESR" is a dangerous name because it can mean pretty much anything without proper context.I mean, in some context "low ESR" meanshigh ESR! A certain regulator might be unstable with super low ESR of an MLCC (and MLCC israrelycalled a "low-ESR capacitor", because this low ESR goesby default), and requireshigherESR - that of an electrolytic capacitor - but maybe lowerESR than onsome otherelectrolytic capacitor. This is the case for "low ESR electrolytic capacitor", which is still high ESR compared to ceramic, but maybe 50% less than a "standard ESR" electrolytic of similar size.For example: put a 22uF MLCC on a PCB, measure power input at PCB using oscilloscope; take a meter or two of wire to connect it to the lab supply. Instead of ramping the supply up, hot-plug it to enabled output. See how the input spikes and rings due to the LC circuit between the wires and the capacitors. Now replace with 22uF electrolytic. Definitely no ringing; so much ESR that it's totally damped. Replace with low-ESR electrolytic. Still no ringing at all; ESR still dampens it.Bill of materials: you're the designer; choosing parts is one of your most fundamental tasks, so provide part numbers. Only in very trivial cases you should let the CM choose parts for you; typical examples would be some pull-up resistors etc. with non-critical characteristics. If you need, for example, an electrolytic capacitor (low ESR or standard ESR), I suggest doing your own homework choosing the part, and specify part number; that way you get the manufacturer you trust, temperature spec, lifetime spec, ripple current spec etc. ESR value isn't even part of usual capacitor datasheet so manufacturers can't do it for you.Plus, it's always a good idea to specify at least one alternative part number to speed up the process of substitutions in case of unavailability.

author: mclien
author_rank: Contributor
timestamp: January 22, 2025, 03:30:48 pm
content: So the datasheet says "low-ESR ceramic chip capacitors, X5R or X7R".From reading your post, I have the impression, that isn't a precise enough "specification" to work with?I might have understood this wrong, but that seems a common enough part to not specify a specific part number.For background: The manufacturer I intend to use also offers sourcing parts to best possible conditions.

author: Siwastaja
author_rank: Super Contributor
timestamp: January 22, 2025, 04:06:42 pm
content: Quote from: mclien on January 22, 2025, 03:30:48 pmAll ceramic capacitors are very low ESR, that part is irrelevant; the datasheet does not use "low-ESR" as an attribute to the noun "ceramic capacitor", they actually mean "low-ESR capacitor, such as ceramic capacitor".X5R and X7R are temperature ratings only and you should know at what temperatures your design will be exposed to. Many datasheet writers still believe in myths and magical thinking that X5R or X7R mean something else than temperature ratings and somehow "work better".The biggest problem is capacitance under DC bias. There is easily 5x variation here between randomly chosen parts! Some 2.2uF is really 0.2uF, another might be 1uF. How much does the regulator actually need? No idea. But by specifying a part number yourself, you can at least prevent the manufacturer from using absolute worst offenders.QuoteWorks for parts with little to go wrong, and not many parameters to specify. If you try to specify "capacitance under DC bias of 5V at least 0.75uF" then the manufacturer is either giving up with your BOM, or outright ignoring your specification.You can try to control this by secondary means, i.e., specifying X7R and large enough package (i.e. 0805 instead of 0603 for 2.2uF size), but that's just risk management, increasing the odds of it working; the "design-by-luck" paradigm. It's not uncommon at all, though, and we all do it to some extent.

author: mclien
author_rank: Contributor
timestamp: January 22, 2025, 07:10:31 pm
content: Thanks for all the explaining.I was aware of the "temperature only" specification. (And the idea, that with the higher temp stability it might be a better quality in general).As for my specific problem, the datasheet mentions that the output cap can be increased, but won't increase performance and is optimized for 2.2uF, so I can at least use higher capacitance there to have more margin for error.Still leaves the question how to find the reliable producer for the right capacitance on the input side.EDIT:There. Find a supplier, who offers the DC bias Voltage char.(Still leaves the question, if the writer of the voltage reg. took taht in account already. It is menat for upt to 6V DC after all)

author: mclien
author_rank: Contributor
timestamp: January 24, 2025, 09:43:06 am
content: Based on your really useful explanations, I did quite some more searching.What still don't comprehend is how few of the manufacturers offer "DC bias voltage - capacity" charts in their datasheets. I did only find one. But they did it to an amazing level (at least as I understand it):https://ds.yuden.co.jp/TYCOMPAS/ut/specificationSearcher?SR2=LM%2CMP&cid=C&u=MIf you pick a cap in that list, you actually get an interactive chart.As for my project I have to rework my layout, which is fine and will use caps based on the what I did find in the datasheets.Something like this:for the 2.2uF, which should be exact : 0805 sized cap, which still has 95% at 5V (a 16/25V rated one)for the V out, which excepts higher capacity: 0603 size, nominal 4.7uF, which still hold 2.3uF at 5V0402 1uF for the core voltage of the MCU (which is 1.2V, which is low enough to have nearly full capacity)The other caps are 100nF, which still have enough capacity at 5V.