Resistance in series with LDO output ceramic capacitor - Page 1
Forum section: electronics / beginners

author: pilmeni
author_rank: Newbie
timestamp: December 18, 2021, 08:39:55 pm
content: Hello,I am adding an LDO to my design with a requirement of > 4.7 uF output cap with an ESR of 33 mOhm - 2.2 Ohm. I would like to use a 10 uF cap, which has an ESR as low as 1.5 mOhm. Are there any reasons why I cannot add a series resistor to the output cap? Are there any special considerations which should be given to the resistor selection?Thank you!

author: wraper
author_rank: Unknown
timestamp: December 18, 2021, 08:49:38 pm
content: You can use a series resistor. However the best way would be to simply use a modern MLCC tolerant LDO. Instead of working around the deficiencies of the old LDO.

author: tszaboo
author_rank: Super Contributor
timestamp: December 19, 2021, 11:05:37 am
content: This is done on 1117 type LDO regulators occasionally. They are designed for Tantalum caps, but they are not the goto parts in many new designs.For the resistor, just make sure it can handle the current, maybe use a bigger package than 0402...

author: Siwastaja
author_rank: Super Contributor
timestamp: December 19, 2021, 11:11:37 am
content: Explicit series resistor is fine. Note, this resistor goes in series with the cap, not in series with the whole output (cap + load), but I think you already understand this. Very little power is actually dissipated in the resistor so a 0805 SMD part for example is fine.Another typical option is a tantalum cap. While I wouldn't recommend a tantalum on the input side, it's fine fine in current-limited output of the regulator, and provides stable ESR specified in the datasheet. Satisfying zero-ESR-instable LDOs (when they are otherwise appealing) is nearly the only application where I use tantalum caps.But I agree with wraper that really the best option is to pick a modern regulator which is stable with MLCC output cap (i.e., down to 10mOhm or even below). Just look at the parametric search of your favorite distributor with the requirements you need, and then look at a few different parts. Won't take long to find one specified MLCC-stable.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 19, 2021, 12:46:22 pm
content: The resistor normally sees very little actual loss, so 0201 size would be OK, though larger is fine. One should still keep the inductance down. So using trace resistance for some 50 mOhms is tricky.For the capacitor keep in mind that a 10 µF X7R cap may not actually have >4.7 µF of capacitance with a DC voltage applied. It depends on the type and voltage.

author: Simon
author_rank: Unknown
timestamp: December 19, 2021, 02:35:38 pm
content: You usually want to use a ceramic capacitor that is rated for 5+ times the voltage it is used at unless you have the manufacturers data that gives a capacitance to voltage graph. Few do. So all my generic bypass 0.1µF capacitors are 50V rated for 5V chips. At higher capacitances this is more of a problem so it pays to get decent caps that give you the data. Another option is polymer electrolytic's. Often I use some ceramic with a polymer to get the best of both worlds.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 19, 2021, 02:38:33 pm
content: Note also that polymer caps are available in a range of ESR, as are tantalum. That range is generally lower than tant, making tant the more traditional one-component solution for this need -- but others are available, just go searching for the ESR you need, you'll find it.The R+C is also good for damping supply resonances, typically at the start and/or end of a chain of loads, with their bypass caps along the way. A filter network is formed, between the stray trace inductance L between caps, and the caps themselves, C; when this is terminated into ESR = sqrt(L/C), it will be well-damped and not show an impedance peak at the resonant frequency. This is less relevant in a 4-layer (GND+VCC planes) design, where the inductance between loads is extremely small (the planes act as nearly ideal connections), and so all the caps more effectively act in parallel.Note that small caps, with low ESR, can be used in parallel with the big lossy cap. All the regulator needs is the ESR to be dominant at its cutoff frequency (which isn't usually given, but you may be able to determine from characteristic curves, e.g. following the diagonal slope of PSRR down to whatever frequency corresponds to its 0dB intercept). A few 0.1u's in parallel is fine, which is why you can get away with powering, well, most everything, eh?Tim

author: Siwastaja
author_rank: Super Contributor
timestamp: December 19, 2021, 07:20:06 pm
content: Quote from: Simon on December 19, 2021, 02:35:38 pmThis is not true at all. It's the opposite, ceramic caps are fine without voltage derating, and although not recommendable, often survive significant overvoltages, even long term. I regularly design in a 50V MLCC for regulated 48V bus, no problem here (although I do try to choose a 63V rated part, of course.)Some try to mitigate risk of cracking by derating voltage. This is very iffy way of dealing with it; cracking is a mechanical problem, not caused by the voltage. Higher voltage rating only means slightly lower risk of short because the electrodes are further away. In reality, the risk is practically the same and really only solved by fixing the mechanical root cause.Some derate voltage trying to compensate for the capacity loss at DC bias, because actual capacitance suffers more and more under larger and larger DC voltage. But to combat this, using some fixed rule-of-thumb voltage derating is some of the worst possible ways. Really the only proper way is to look up the actual C vs. V graph and choose accordingly. Failing that, guesstimates based onpackage size(including height, so volume really!) workwaybetter than guesttimates based on voltage ratings.Really, a manufacturer can basically sell the same, or very similar part with 25V and 50V rating; the giveaway is the same package volume. In such case, the 25V part would require only half of the derating; for example, both such parts could offer enough capacitance for the application at 15V.

author: Simon
author_rank: Unknown
timestamp: December 20, 2021, 08:39:57 am
content: Well everything I have been told is the opposite and many don't give the graphs. On the one hand you say that ceramics do not loose capacitance then you say they do. If it's about "bias" well a capacitor sitting on a 5V rail is "biased" at 5V. The only people I have found to give data on individual caps is Wurth Elektronik and I use their ceramics specifically in EMC filter design because unlike others they give all the data about each part so that I can make an informed choice and not guess.A single component is already a filter all by itself - know thought parasitic's, or thy choices are null and void!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 20, 2021, 11:02:09 am
content: There are two kinds of voltage limits. Just as for inductors, there are two current limits.The voltage rating is merely the voltage which is guaranteed safe to apply. (Or for inductors, gives the rated temp rise.)It saysnothingabout capacitance.As it happens, saturation occurs, typically taking an arbitrary drop say -30%, and noting the voltage (or current for inductors) at which this occurs. For ceramic caps, this can be a minuscule fraction of the rated voltage. Or it can be above the rating. The rating is meaningless here.Several give characteristics, though they're rarely linked from Digikey, you have to go search on their websites. Samsung, the DK links are often broken, if present at all; Murata, look it up on SimSurfing; KEMET, K-SIM; etc. Several don't have any data at all.Conversely, ferrite beads never give a saturation figure, but may give bias current curves in the datasheet or characteristics; inductors always(?) give a saturation figure, though you have to look at the datasheet to see what threshold they picked.Note that C0G and similar type 1 ceramics have no dependency on voltage, so are good from any source.Tantalum voltage ratings work the same way, and have stable capacitance so C(V) isn't an issue. Instead, they have problems with microcracking of the dielectric, and since their breakdown is often catastrophic (thermite reaction), a 2-3x overrating may be judicious.Whereas electrolytics fail more gracefully, and may blow up (gas and/or vapor buildup --> bulging --> venting) under excessive voltage, but also if done slowly, may tolerate surprisingly high voltages for a long time (but, C, ESR and lifetime will likely be out of spec).Tim

author: Kleinstein
author_rank: Super Contributor
timestamp: December 20, 2021, 01:06:57 pm
content: Quote from: T3sl4co1l on December 20, 2021, 11:02:09 amThe catastrophic tatalum cap failure happens with too powerful a supply. This is why one should never (also not with with higher voltage rating) use Ta cap where there is no good current limit - unless one is prepared for fireworks.Overrating the voltage is more like avoiding failure in frist place, even if not catastrophic. There can still be failures, though less likely.

author: Simon
author_rank: Unknown
timestamp: December 20, 2021, 01:11:14 pm
content: Any component specification is almost arbitrary. It's a complomise. I'm sure the same electrlytics are made for the same value and rated at a higher voltage with less working hours. That is why we reduce the working voltage so that 1'000 hrs becomes 100'000 hrs.

author: wraper
author_rank: Unknown
timestamp: December 20, 2021, 01:20:53 pm
content: Quote from: Simon on December 20, 2021, 01:11:14 pmSorry Simon but that's BS. For tantallum capacitors voltage derating does not mean they will work longer but that a chance of failure due to micro defects appearing during soldering process vastly decreases. For electrolytic capacitors running at lower voltage does not really improve their lifetime too, ripple current is what actually matters.

author: Simon
author_rank: Unknown
timestamp: December 20, 2021, 02:41:08 pm
content: there are various factors is what I meant, they are specified to work at those limits for those hours, we de-rate them on design because even a 5000 hour electrolytic will not last long enough. If you think about it you start to wonder about how the parts are designed and how they get their ratings in the first place. No part will break at the exact voltage it is limited to. but using it there is rather silly. Or for short term use you will be fine and can abuse the part.