Linear Vregs and PSRR. - Page 1
Forum section: electronics / projects

author: kvresto
author_rank: Regular Contributor
timestamp: May 09, 2015, 01:09:34 am
content: Hi Everyone, I have another question on the subject of PSRR, for those who have some experience in this area.I have two similar regulators both with sort of similar PSRR. The TLV1117, and the TPS79633.One is listed as suitable for "Post-Regulation for Switching Supplies", and the other is not listed as such.Are they both equally suitable for "Post-Regulation for Switching Supplies"? I guess further to this, how much PSRR is required to make a Vreg suitable?Thanks kv.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: May 09, 2015, 01:26:14 am
content: Depends on you, not the regulator. How much noise at what frequency can you live with?

author: kvresto
author_rank: Regular Contributor
timestamp: May 09, 2015, 01:39:09 am
content: Fair enough. Say I want to power a couple of opamps and a 14bit ADC, so I don't know a noise value at the moment, but its always in nice to think you can get 1LSB out of the ADC. I know what you are going to say, in this case your opamps could be noisier perhaps, and what is the SNR of your ADC, so lets say I want the output of the Vreg good enough to power a precision analog circuit.I'm just researching and gathering info at the moment. Later I will make a choice of components etc.Thanks.PS: Great info in your last post on my other thread.

author: kjs
author_rank: Regular Contributor
timestamp: May 09, 2015, 07:29:49 am
content: the PSRR is usually just a number at a certain frequency. What you have to look at is the curve which shows PSRR over frequency. Then select the one which has the better PSRR at the switching frequency of your pre-regulator.However, your board layout plays a critical role too. If not done right a lot of noise will just go around the post regulator.......

author: kvresto
author_rank: Regular Contributor
timestamp: May 09, 2015, 08:45:00 am
content: QuoteI guess the best approach would be to follow the manufacturers guidelines, and also keep all loops as small as possible.cheers

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 09, 2015, 09:00:11 am
content: Run through the numbers -- set up a basic skeleton system, and track the attenuation ratios to each node. You'll be interested in input coupling (due to protection diodes, bias resistors, etc. as applicable), buffer/gain PSRR, reference PSRR, and any filtering along the way (you can model the power distribution network of a typical 2-layer board as a series of small inductances and bypass capacitors, in the topology of the routing itself; or call it close enough to "ideal ground" for a 4-layer solid ground plane type).LDOs need not even be at all important -- the voltage reference won't care if it's supplied with regulated power, being a regulator itself, and as long as it's well filtered and in spec, the op-amps don't care at all. (Well, better stabilized DC might give slightly better ref stability, but that's fine whether from a switching or linear source. It would be more of a concern from a linear-only supply, but we wouldn't be worrying about switching noise then, would we?) A passive filter and/or capacitance multiplier will be plenty to ensure low AC noise -- and don't forget that regulators themselves carry not just transmitted noise (due to PSRR), but generate quite a lot of (bandgap) noise too.So if you're finding inductors expensive, the C-mult. is good, or you can just use an RC filter if you're feeling simpler still.And don't forget, while you're at it -- the power supply network is a series of L and C components, like any other filter; it's intended more to have a low impedance, than to have a controlled impedance (and transmission/cutoff characteristics), but the same rules apply, and it has to be suitably dampened, and yes this means sometimes including higher impedances in order to achieve better worst-case results (e.g., adding ESR to capacitors to numb down the resonant peaks against stray inductances).Tim

author: kvresto
author_rank: Regular Contributor
timestamp: May 09, 2015, 09:44:26 am
content: Thanks Tim. A bit to digest. Just a comment, I have found in my testing that high frequency spikes, at regular intervals (switching frequency) to be the biggest issue. They almost disappear after an LC filter, but I have a layout for an LM2593 (one of 2 I've tried) that has these spikes persisting into the output after the filter, although hugely, hugely attenuated, and another layout version where they are for all intents and purposes gone (same filter). I wonder if the last paragraph in your response has application in what I've seen.kv

author: SArepairman
author_rank: Frequent Contributor
timestamp: May 10, 2015, 01:55:05 am
content: adding artificial ESR to capacitors is best done with composite resistors if you want to try it out. to my understanding anyway

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 10, 2015, 02:21:45 am
content: Most likely common mode: identify switching current paths between input and output grounds, and avoid them where the bypass caps come together.This applies even for three terminal modules. If the internal filtering for one pin occurs at a point where ground loop voltage puts noise on it relative to the other pair, you have a problem.There's really nothing mysterious about ground loop or common mode; you just have to clear your mental block of seeing "ground" as a monolithic entity, and realize that everything has finite impedance.Cables with no galvanic connection to "ground", running through free space, might have a common mode transmission line impedance in the vicinity of 100-1000 ohms, depending on distance to nearby grounds and other cables. If the cables are truly open (not shielded within an enclosure), much of that impedance will be the direct radiation impedance as well.Even short bits of wire or track have impedances in the ohms at RF frequencies; large ground planes can support internal 2D standing waves.The worst topology for a switcher is a linear "input on one end, output on the opposite end" layout. This puts the switching current right in the middle of the ground path. The best topology is something like a cul-de-sac (a dead-end road with a thermometer-bulb shaped turn-around at the end): the input and output can be filtered, together, at a common ground point, distant from the switching current. This way, even if the layout of the switcher section is bad (it might have common mode ground noise in its area), the nasties can be filtered out.If the input and output are naturally extremely noisy (or the end use is extremely sensitive), there isn't much you can do about it without taking up a lot of space and expense: one or two stages of LC filtering won't do much more than 80dB, even with the best layouts, due to ESL of the components. A filter with a cutoff in the 10kHz range might have squigglies (i.e., additional poles and zeroes) up in the 1-100MHz range. To do better, a multi-stage, tapered approach would be necessary -- chaining filters of increasing cutoff frequencies, including ferrite beads (additional loss to absorb rather than reflect the undesired frequencies), smaller caps, shielding and so on.My radio project here,http://seventransistorlabs.com/Radio_20m/has two DC-DC converters on the front end -- a Meanwell 12V supply, followed by a 6.3V and 100V dual converter. The latter is fully shielded and well filtered (in and out); using the radio itself, I cannot detect any noise coming from either source, except using an inductive probe very close to them (the converter is open on one side, so I can pick up quite a few bars of noise within).It might be worth noting that the converter is a discrete design, so probably has less switching noise than an integrated one, especially one made with modern components (with switching frequencies into the MHz, and switching edges into the single ns!). The stuff over 20MHz is the noise manufacturers are the first to ignore (ripple measurements are done with 20MHz BW, standard -- yes, it's BS, but that's what they do), yet is the most troublesome (it seeps into bipolar op-amps as variable input offset, radios and PLLs as spurs, and ADCs as noise) and hardest to deal with (ferrite beads help absorb it, but it re-radiates easily, so often requires additional shielding to fully treat in sensitive situations!).Of course, my radio has tons of copper clad and bypass caps, so I have that luxury; a smaller, cost-driven product wouldn't!Tim

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: May 10, 2015, 02:42:46 am
content: What do ferrite beads "do" with the "noise"? Turn the energy into heat? How efficient can they be at radiating? And by radiating, I mean a 100MHz sine wave's energy is being sent into space, isn't the radiation resistance too high for such a small blip as a ferrite bead at 100MHz? I don't get it.I mean I've happily sprinkled them on boards for the last 20 or so years, looked at datasheets, tossed numbers around, even measured results, but what does it *mean*?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 10, 2015, 03:28:10 am
content: An FB is just an impedance -- check the datasheet (if it goes into any detail, that is). The impedance is largely real at higher frequencies, meaning it's resistive, lossy, self heating.Note that FBs saturate at very modest currents (even large "6A" parts die off in the vicinity of 100mA), so the impedance you're inserting need not be as useful as the headline claims.If you throw them in, willy nilly, it's very likely you're using them wrong. They're not usually enough inductance to matter in terms of low frequency response, and with the low saturation, that inductance is only manifest at small signals anyway. So a power inrush surge, for example, might saturate it to the point where it looks like a piece of wire of equivalent length (~10s nH?), and the power supply network comes up as if it were made of wire (little overshoot, well dampened by large bypass caps).So, they might be applied "wrong", but that's usually harmless (which I suppose is good business for both FB manufacturers and appnote writers).As for absorption, it can be beneficial. Suppose you light a room with a white bulb in the middle. Suppose you don't want any of that light to get out. If you construct the room entirely from polished aluminum panels, it will be very well lit indeed inside that room, but little light will escape. If you use matte black panels instead, it will be dimmer inside, and still, little light will escape. But if you have the possibility of radiation from that room -- the panels have holes, or don't quite overlap perfectly -- which are you better with, the shiny or the black version?The exact same goes for filters, except one-dimensionally within wires (hopefully..) rather than in a 3D box. The random holes or gaps represent possible radiation-susceptibility paths in your circuit. If you're reflecting all that energy (with a high-Q filter), it stays bottled up on the SMPS side, which is a good thing. But that also potentially makes it "brighter", too -- more current bouncing around the switching components and filter capacitors. A purely dissipative filter would be very lossy indeed (you don't want to absorb all the reactive energy from the switcher, because it needs that to operate at high efficiency!), but some compromise is clearly desirable. This is very broadly where ferrite beads come in -- they're cheap as hell, handy enough, and do a reasonable job of breaking high frequency resonances.Because ferrite beads saturate so easily, they're best confined to small signal traces (analog and digital inputs and outputs), and common mode ("current compensating") roles (beads around cables, common mode chokes).Often, the resistance is used explicitly: suppose you have a main board attached to a display board with a ribbon cable. That cable has some equivalent inductance, and the boards have some equivalent capacitance; together, they resonate as a dipole structure, at some characteristic frequency (maybe in the lower 100s MHz). Over most of the band, the structure has a low impedance, and nothing couples into your ribbon cable's signals (say you're passing around some 5V CMOS logic signals, so you have a good 2 or 3V of headroom before logic thresholds start going completely nuts). But at that one characteristic frequency, give or take a few percent (it might have a Q factor of 30ish?), the impedance spikes, and the voltage between boards, and the current through the cable, becomes huge. Suddenly, you've got, I don't know, 30V between boards, from a modest 3V/m external stimulus, and some fraction of that 30V will not be shared evenly between all the wires in the cable. And it only takes 10% of that to blow your logic levels and get gibberish, which will easily be provided (the coupling factor between lines in a ribbon cable isn't terrific, even if you grounded every other signal -- which you should do for signal quality purposes anyway).Now, add a ferrite bead to that cable, maybe a modest 100 ohms near that resonant frequency. Now there's a modest impedance over a wide range of frequencies, which means it will act like a shitty antenna over a wide range. The induced voltage will absolutely be larger at other frequencies -- but, still well within the logic threshold. (The induced *difference* won't actually be worse at all, because the ferrite bead acts in common to all signal and ground traces.) And at the peak, it will be severely dampened, so your worst case scenario goes from "oh shit" to "meh"!So, FBs are best around cables, and not so much for filtering purposes, but for dampening purposes, which improves on that worst case scenario. FBs aren't great for filtering -- if you wanted a good filter, would you prefer an L || R over a high Q inductor of the same value? Hell no! But the attenuation can have knock-on value, like dampening parasitic modes and radiation (even within filters of much lower cutoff frequencies).Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 10, 2015, 03:33:36 am
content: Quote from: evb149 on May 10, 2015, 03:13:31 amThis example is probably not as bad as it sounds -- there's no such thing as a 24 bit SAR ADC, so you don't get aliasing. You get integrating (multi-slope or S-D) ADCs, which average over those noises.You will still have to keep RF out of the op-amps and stuff; and at the 24 bit level, that's probably just as true of CMOS and FET amps as it is of BJT types. (CMOS amps are noisy as hell, but if the sample rate is fairly low, you can probably beat that significantly, by using a chopper or autozero type.)I would dare say a radio or spec is the most sensitive, for obvious reasons. But really, anything with high dynamic range will do, which still includes integrating ADCs, SDR and so on.Tim

author: kvresto
author_rank: Regular Contributor
timestamp: May 10, 2015, 09:22:54 am
content: .