Regulator stability and ESR of ceramic caps - Page 1
Forum section: electronics / beginners

author: Starlord
author_rank: Frequent Contributor
timestamp: August 15, 2015, 10:48:55 pm
content: I have a question about a couple regulators.The ESR requirements for the two regulators are listed on the following pages:Page 15:http://www.ti.com/lit/ds/symlink/lp2980-n.pdfPage 10:http://www.ti.com/lit/ds/symlink/lp2985-18.pdfAs you can see, the 2985 can accept an ESR slightly lower than .01 ohms with a 5V output but exactly how low isn't clear from the graph, nor is it clear how the ESR requirements would change with the capacitance, though based on the lower voltage output graphs I would surmise the ESR can be slightly lower with increased capacitance.As for the 2980, at 10uF it almost reaches 0.01 ohms, and more capacitance clearly leads to lower acceptable ESR ratings.But then I have this 10uF capacitor I'd like to use because it's a house part at my PCB manufacturer and therefore cheaper:https://s3.amazonaws.com/devpublic1/datasheets/C3216X5R1C106M160AA-265193.pdfFor the first 100KHz it seems like it meets the specs, but around 100khz-1Mhz it may dip below the allowed ESR for the 2985. I'm not sure though because the 2985's graphs don't show a 10uF cap and it's not clear exactly how low it goes with the 2.2uF at >2.5V, or the 4.7uF at <2.5V and I don't know if the 4.7uF graph even applies at the higher voltage.I guess I could select a different cap, but I'd just like to know if I am worrying over nothing and that cap is close enough, or the higher frequencies don't matter as much for stability.Actually I don't even know what unstable actually means as far as the regulator goes. Will the voltage go wild? Will there be more noise?

author: rfbroadband
author_rank: Unknown
timestamp: August 16, 2015, 02:13:33 am
content: I would strongly recommend to stay within the manufacturers recommendations. The ESR of the caps will affect stability. If you want to use a different cap, built the circuit using your cap, verify that stability is guaranteed and you should be good.

author: unitedatoms
author_rank: Frequent Contributor
timestamp: August 16, 2015, 02:27:11 am
content: There is 2002 paper from same manufacturer explaining why cap ESR should be no less than some set value (for LDO regulators).TLDR: F=1/2*Pi*R*C is the frequency where the loop gain is sufficient to maintain regulation. The lower is ESR the higher is frequency, the lesser is gain, the more transitions through overshoot (more than 4 oscillations is bad) is needed to regulate. Eventually when ESR is too low, the oscillation will not stop.http://www.ti.com/lit/an/slva115/slva115.pdf

author: singapol
author_rank: Frequent Contributor
timestamp: August 16, 2015, 03:11:51 am
content: lp2980 =50mA.lp2985 = 150mA. low noiseI don't know what your application is but from a practical aspect if you want reliabilty regardingcapacitors after said regulators you would be penny wise pound foolish if you just use a "housepart"cap.If you measure low esr caps with a hand held esr meter chances are it will not read 0.01ohm or 10mOhm unless it's a really "good" cap.We can argue that " oh the measurement is not done with lab grade equipment and environment " but the fact is you get what you pay for.Every mothers' son know that since ancient times ( I'm exagerating) tantalum caps was THE cap to use and still is but there are those exotic polymer caps like Sanyo Oscon that are also good.Although you may wing it with low esr 105 degreeC switchmode power cap which is cheap too.Every component has it's quirks and weaknesses, it's the balancing or compromise you are willing to tolerate.Stability in terms of electronics is staying away from oscillation thresholds as when transistors switch at " high speeds" heat is generated which is the number one enemy of semiconductors which then lead to failure.http://www.petervis.com/Raspberry_PI/dead-raspberry-pi/LP2980-2V5-RG3-pinout.htmlEdit: BTW the caps discussed are not ceramic as they only come in small values so a 10uf is never a ceramic.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 16, 2015, 06:56:26 am
content: It's not clear at what frequency, exactly, the ESR should be measured at. Perhaps near the cutoff frequency (i.e., Xc ~= R, i.e., around the resonance valley in the ceramic cap's Z(F) curve).It's at least borderline, and probably too low at the worst. For sure, it's a decade away from the ideal value (~0.1 ohm is in the middle of the stable region), so whatever you do, you'll be a damn sight safer with that kind of ESR added in.Play it safe and add the resistor.There's an off chance you can find an aluminum polymer that size with high enough ESR, or a tantalum low enough. If you don't mind these types, of course (more expensive, and "house parts" may not fit the spec, or exist).Tim

author: Kleinstein
author_rank: Super Contributor
timestamp: August 16, 2015, 12:52:59 pm
content: Adding a resistor is often a option, however this should be a low inductance type. Other MLCC will not have significant higher ESR though - so you may need a different capacitor technology, the resistor or a different regulator. There are newer regulators specified for MLCC caps at the output.For just 10 mOhms just a cm of thin line (keep low inductance in mind) on the PCB might do the job.The critical frequencies are likely in the 100 kHz to 5 MHz range.

author: Starlord
author_rank: Frequent Contributor
timestamp: August 16, 2015, 02:34:39 pm
content: How can I find a low inductance resistor?I selected all 0.1 ohm chip resistors that are of a suitable size and power capability:http://www.digikey.com/short/tjmvpbBut there's no inductance listed, nor do I know what "low" inductance constitutes, and I looked through a couple datasheets for panasonic and yageo (yageo was the only capacitor manufacturer that actually graphed the ESR in their datasheets that I could find) but there's no inductance listed.

author: Starlord
author_rank: Frequent Contributor
timestamp: August 16, 2015, 02:49:04 pm
content: Also regarding "newer" regulators allowing ceramic caps... I've been looking for such a regulator and I haven't been able to find one.Oh I've found plenty which state they work with ceramic caps, but actually finding a ceramic cap that fits their parameters is like searching for a needle in a haystack. Only Yageo actually lists the ESR over a range of frequencies and even then they only do so for like three sample capacitors each of which are different.Here's all the TI regulators that might be suitable:http://www.digikey.com/short/tjmvm3I've gone through them all and some of the tps ones might work but their PSRR isn't fantastic in the audio region.Here's one for example:http://www.ti.com/lit/ds/symlink/tps709.pdfUnfortunately though it can accept down to 0 ohms ESR, it only accepts up to 0.2 ohms ESR and if you look at the graph on that house part 10uF cap I listed in my first post, towards the low end of the spectrum the ESR creeps just a bit above that.Here's all the potentially suitable ones b other manufacturers:http://www.digikey.com/short/tjmvznThis one for example is particularly cheap and has a particularly high PSRR all across the board:http://www.digikey.com/product-detail/en/TAR5S50UTE85LF/TAR5S50UTE85LFCT-ND/4304172But while they say ceramic caps are okay the stability graph cuts off at 0.02 ohms and it is unclear if below that is an unstable region, and all the capacitors I have ESR graphs for dip below that.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 16, 2015, 05:44:07 pm
content: Chip resistors are ~1nH/mm in length.Get a 0.1 ohm resistor in your favorite SMT size, it'll be fine.Fine traces are actually quite terrible as resistors; for frequencies above 10kHz or so, copper conductors have more reactance than resistance.Don't worry about the ESR at low frequencies. It's most important around whatever the regulator's transition frequency is (somewhere in the 10kHz-1MHz range, most often).If you need more PSRR than regulation, consider an unregulated filter over an LDO. Or an HDO (classic 7805, etc.) if you can spare the voltage headroom. An excellent filter is the "capacitance multiplier", which costs about 0.7V (for the first stage, less on subsequent stages), and performs orders of magnitude above and beyond what any LDO will claim.Tim

author: singapol
author_rank: Frequent Contributor
timestamp: August 16, 2015, 05:53:07 pm
content: Quote from: Starlord on August 16, 2015, 02:49:04 pmThe light bulb finally light up for me regarding your questions.QuoteSo your application is for audio. Your mlcc is a smt device am I right, why do you want to use a ceramic for audio? Does it sound good? I think you may have misinterpreted the regulator data.The stability data/graphs showa band of esr valuesthat the regulator is stable. See pg.10 for greater than 2.5V. So for LP2985 it's just under 1 ohm down just under 10 mili ohm.You might want to find another regulator if you are not happy about it's power rejection ratio. A common regulator for audio is TL431 fixed or adjustable.Read this article if you still insist on a ceramic cap if you want low noise performance of your regulator. I doubt the input and output phase difference in the audio band will adversely affect regulator stability.http://www.kyocera.co.jp/prdct/electro/pdf/technical/highcv.pdf

author: Starlord
author_rank: Frequent Contributor
timestamp: August 16, 2015, 07:42:27 pm
content: I want to use ceramic caps because they're cheap, they're small, they don't wear out, and they don't catch fire.I have a couple large electrolytic capacitors on the board for bulk capacitance, but I don't have a lot of room on the board to add this regulator, let alone more electrolytic caps, and I refuse to use tantalum because I don't want my incompetence to be responsible for burning someone's house down. Plus they're just too costly.And aside from the hiss at low volume levels, yeah the audio sounds great for a 40W amplifier on a board that's smaller than a credit card and requires no heat sink.Here's a board using the same amplifier. He's got way more caps though because he put all the output filters and things on there whereas I went for a bare bones approach because I've got a microcontroller on the same board supplying the audio to the amp so I needed to conserve space:https://www.eevblog.com/forum/buysellwanted/some-really-tiny-50w50w-amps-for-portable-speakers/QuoteYeah, but 10 milliohm is more than the lowest ESR on the 10uF cap graph and the few graphs I could find for some Yageo caps. Capacitor manufacturers don't seem to put that data in their datasheets often.But as someone else suggested I could stick a resistor in series to get the ESR into the right range. So I guess I'll have to do that if I can't find a suitable capacitor and regulator combo that I'm satisfied will work together.

author: Zero999
author_rank: Super Contributor
timestamp: August 16, 2015, 08:01:18 pm
content: Quote from: Starlord on August 16, 2015, 07:42:27 pmCeramic capacitors do wear out. Their capacitance declines with age. Ceramic capacitors also fail short circuit which can cause a fire, even though the capacitors themselves are non-flammable.QuoteTantalum capacitors are unlikely to catch fire if used properly. They do not age like ceramics do, are not piezoelectric and do not reduce their capacitance as the voltage is increased.http://www.kemet.com/Lists/TechnicalArticles/Attachments/93/2008-11%20Update%20-%20Ceramic%20versus%20Tantalum.pdf

author: Starlord
author_rank: Frequent Contributor
timestamp: August 16, 2015, 08:29:16 pm
content: QuoteWell, they don't dry out in two years and fail like the electrolytics on every motherboard I've ever had.

author: free_electron
author_rank: Super Contributor
timestamp: August 16, 2015, 10:36:35 pm
content: Quote from: Hero999 on August 16, 2015, 08:01:18 pmsure.. and pigs fly.i've seen many tantalums go kaboom. it is enough that, during assembly of the board, it was improperly washed. or it was depanelised wrongly so there was stress on the cap causing microcrcks. these develop over time, moisture gets in and kaboom.tantalums are a disaster waiting to happen.more , it is now a conflict mineral... prices fluctuate wildly.get rid of that crap. MLCC was specifically developed to kick tantalum to the museum.

author: singapol
author_rank: Frequent Contributor
timestamp: August 17, 2015, 05:19:35 am
content: Quote from: Starlord on August 16, 2015, 07:42:27 pmSo you want to be an entreprenur and engineer, you have my respect.Designing safe electronics istop priority,yes don't burn down your customers house.It will ruin your business with expensive lawsuits.So you want to make and sell switching amp in credit card format.You already have bulk capacitance and you want to filter the high frequencies.As you may have read one mlcc can notfilter efficently so the commonly accepted way is to parallel different size caps.If they are smt devices I don't see a problem.If you are marketing this amp to non audiophiles it's not a problem as they don't know betterbut for audiophiles is a non starter.If you have enough ground plane to heatsink 2X40W goodfor you.Your obsession with esr and output impedance is tripping you up. As long as you have done due diligence and testing preferably with spectrum analyser to check stability it should not be a problem.Engineering is the art of compromise...sure never compromise on safety!

author: Starlord
author_rank: Frequent Contributor
timestamp: August 17, 2015, 02:09:28 pm
content: Quote from: T3sl4co1l on August 16, 2015, 05:44:07 pmOkay, that sounds good, I'll do that and use the 10uF house part with the LP2980. That will give me an ESR of .1 to .2 ohms which is in the region of stability for that cap size on that regulator across the whole frequency spectrum and since .1 ohm is enough to put the cap in the region of stability and it's unlikely any 10uF cap will have an ESR of over 10 ohms, if the manufacturer ever changes that house part or I choose my own 10uF cap, it shouldn't impact the regulator's stability.With this combination of components and low current draw, the regulator should have a PSRR of 40 to 70db across all frequencies.Thanks for all the help guys!

author: Starlord
author_rank: Frequent Contributor
timestamp: August 17, 2015, 02:16:32 pm
content: Actually one last question:I'm going to be placing this regulator right next to the DAC and it will be supplying only the DAC. The DAC datasheet suggests 10uF and .1uF caps on the input. If I have that 10uF cap on my regulator output that should suffice for that, but what do I do about the .1uF? Do I need to worry about it's ESR as well, or is it too small to affect anything? And with my supply so close to the IC, do I even need it?

author: singapol
author_rank: Frequent Contributor
timestamp: August 17, 2015, 05:18:12 pm
content: Quote from: Starlord on August 17, 2015, 02:16:32 pmA 0.1uf measured 17.8 ohms on my handheld esr meter at 100khz. What esr does LP2890 like?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 17, 2015, 06:04:22 pm
content: Small caps don't matter, as long as they total less than a third of the bulk cap. In fact, you get the best of both worlds, because you have low impedance from the regulator at low frequencies, and low impedance from the bypass capacitor at high frequencies (with a small rise inbetween, but not much more than the ESR of the bulk cap).Tim