How to remove display noise on 5V rail - Page 1
Forum section: electronics / beginners

author: Saimoun
author_rank: Frequent Contributor
timestamp: February 20, 2022, 02:21:49 pm
content: HiI have a project that runs of USB 5V, which is used to power:a LED display (7-segments, 3 digits)a digital 3.3V circuitan analog (audio) circuitThe digital part of the circuit runs perfectly fine after a (relatively) high-PSRR 3.3V LDO.The analog part of the circuit needs to run at 5V since 3.3V is a bit too low audio volumes for the project.Powering the analog circuit directly from the USB Vbus is clearly a no-go because of the noise from the LED display. Attached is the 5V rail when the display is showing 187.So the question is:how do I get a clean 5V from this?What are your suggestions?Thank you!SimonPS: The whole audio circuit is made for a single rail of max 5.5V so I know one solution is to make proper +/-15V rails but that would be much more complicated and would require changing all components on the audio circuit.

author: radiolistener
author_rank: Super Contributor
timestamp: February 20, 2022, 07:44:06 pm
content: Quote from: simonlasnier on February 20, 2022, 02:21:49 pmUse filter: ferrite beads and ceramic capacitors 10 nF + 100 nF + 1000 nF + 47 uF

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 20, 2022, 10:52:53 pm
content: Stacking values of ceramics has essentially no effect...Also, ferrite beads won't have much if any filtering value at the frequencies shown in the waveform.OP, can you afford a few hundred mV drop for a tight LDO, say 4.5V?Also, have you verified that this is, in fact, causing enough noise to be a problem? The waveform by itself doesn't mean much; op-amps have reasonable PSRR. Maybe some filtering in the 100s kHz to take the edges down (where PSRR is usually falling off pretty hard).Otherwise, if you need dynamic range you're probably better off using just way more voltage than you have available period, i.e., use a boost converter and filter it carefully to avoid noise. Doesn't need to be whole-ass +/-15V but more than 5V in any case, and bipolar isn't hard to have once you're doing a converter at all, so you can if you want to.As for non-solution solutions -- I take it there's no avoiding the output level, i.e. you can't just turn up the whatever it's feeding?Tim

author: Vovk_Z
author_rank: Super Contributor
timestamp: February 21, 2022, 12:46:38 pm
content: Quote from: simonlasnier on February 20, 2022, 02:21:49 pmI would add L- or Pi-filters (RC, CRC, LC, CLC) with small R, L and any C (1uF ... 100uF) between power input and all of those circuits. So you'll decrease an influence one circuit into another.Will you have "clean 5V" or not depends on what you mean "clean", but adequate filters definitely must help a lot.

author: Saimoun
author_rank: Frequent Contributor
timestamp: February 21, 2022, 01:26:12 pm
content: Thank you all for your answers!)I have to mention I asked about this before - sorry for creating a new thread - actually Tim you wrote quite a few things on my last thread.But in the last thread I asked specifically how to design a boost converter (thinking this was the solution) so I thought asking again giving this time the overall problem (i.e. getting a clean 5V). Also the thread is 5 months old I do not know if bumping it up is the best idea. Just for reference:https://www.eevblog.com/forum/beginners/boost-converter-input-usb-5v-output-about-6v/Just FYI I do not have the best oscilloscope, so the way I test is I listen directly to the audio output (I have good ears, and also that's at the end the direct impact on the users).To answer your questions:* Yes this is a problem - it sounds like very loud buzz* The audio circuit has a DAC, an opamp and an analog switch, and yes of course having the DAC outputting maximum level helps. Problem is I saved money by making audio levels digital, so the opamp is actually set at a fixed +8dB. That works very well and is very quiet when running with a clean 3.3V supply (with the High-PSRR 3.3V LDO).* I already tried basic filtering using capacitors and LC filters - that did not help at all with the current circuit.I really like the 4.5V LDO idea, but for the PSRR to work I probably need a bit more headroom right? And also USB Vbus can theoretically fall to 4.75VWhat I hear from your answers are two solutions:1) The Boost converter to raise the voltage to like 6V + filtering (with a 5V LDO f.x.) - this will work for sure, I've been avoiding it because of my total lack of experience about DC-DC converters2) Changing the audio circuit to have a proper analog volume control

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 21, 2022, 04:02:53 pm
content: What's your analog circuit look like? Sounds like it has terrible PSRR -- something to look into.Tim

author: Saimoun
author_rank: Frequent Contributor
timestamp: February 21, 2022, 08:34:49 pm
content: Ha maybe you're right!It's pretty simple though - a DAC then an opamp in a Multiple Feedback Low-Pass Filter (like that:http://sim.okawa-denshi.jp/en/OPtazyuLowkeisan.htm), and then a coupling cap to bring the center to 0V.I attached the schematics. It's not shown but all ICs are decoupled with 4.7uF and 0.1uF caps.The opamp has a analog gain control using an analog switch. This way the gain be -22dB or +8dB. Then from the DAC I shift bits (i.e. moving by 6dB), so I have a digital volume from 0dB to -6-6-6-6=-24dB (4 bits shifts). So with both the analog and the digital gain I get from -48dB to +8dB volumes in 6dB steps (10 volumes in total).This is the part that could potentially be improved?Or maybe it's simply the components which have low PSRR:DAC:https://datasheet.lcsc.com/lcsc/1811022110_TM-Shenzhen-Titan-Micro-Elec-TM8211_C92003.pdfOpamp:https://datasheet.lcsc.com/lcsc/1809211219_Gainsil-GS8632-SR_C157703.pdfAnalog switch:https://datasheet.lcsc.com/lcsc/1811021622_BL-Shanghai-Belling-BL1551_C82528.pdfThese are all chinese brands so I know the datasheets are not the bestOn the other side, apart from this noise issue they all work flawlessly! And like I said with the High-PSRR 3.3V I cannot hear the display noise, even at +8dB gain.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 21, 2022, 09:06:43 pm
content: Replace the bottom half of the HalfVCCA divider with a TL431.Tim

author: Saimoun
author_rank: Frequent Contributor
timestamp: February 21, 2022, 11:16:19 pm
content: Ha ha ha - gosh I feel like an idiot for not seeing it, but yea the HalfVCCA has not much protection/rejection of power supply ripple!!So you think this is the weak point of the circuit, then?I did not know the TL431, very interesting, thank you for the tip!I guess you meant smth like that?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 22, 2022, 12:02:07 am
content: Yup, also you won't need (or want!) the capacitors (because stability, see datasheet), and there are variants of '431 and others with different voltages and lower bias current if you need to save some.The ref will still be somewhat noisy (~70uV, I think it is?) but will have better PSRR. There are quieter ones out there. And, better to DC couple it if you can, using the DAC's ref for bias (or rather, half of it, sitting at mid scale at idle if it's an unsigned DAC -- which it has to do anyway since it's single-supply, though the digital side might not be represented that way exactly), so the DAC side coupling cap and bias network aren't needed at all.Tim

author: Saimoun
author_rank: Frequent Contributor
timestamp: February 22, 2022, 09:11:56 am
content: Ok interesting, you mean DC couple it so instead of being 2.5V it is exactly set at the mid-voltage from the DAC? Then you're right I will not need C14But how do I DC couple the TL431?Gosh I actually just realized maybe I do not need anything of this... I am not using the Right channel from the DAC, so it is always set to zero, i.e. the mid-voltage. Could I then feed this directly into the opamp? I'll try

author: Saimoun
author_rank: Frequent Contributor
timestamp: February 22, 2022, 10:47:32 am
content: Soooo - I just tried going directlyUSB Vbus -> audio VCCA (with a few decoupling caps)This time with the Half VCCA powered from the right channel of the DAC, through a 4.7k resistor and with a decoupling 100n cap.Conclusion: it's really, really bad, even at -22dB gain I get 120mVp-p in 250 ohm headphones - that is LOUD, like listening for too long to the buzz makes you dizzyAlso the USB Vbus is very unstable, just moving the cable a bit makes the voltage move by 100-200mV, in the headphones that sounds like rumbling and scratching.So not sure if using the right channel of the DAC was a bad idea, or if the bad-PSRR of the audio circuit is somewhere else?

author: Saimoun
author_rank: Frequent Contributor
timestamp: February 22, 2022, 12:15:40 pm
content: Alright - doing more investigation, leaving the DAC output floating, checking the difference between the VCCA noise and the DAC output noise:* VCCA noise: 100mV RMS* DAC Right output noise: 50mV RMSSo that's a 3dB PSRR at those frequencies, I think that says it all?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 22, 2022, 11:20:44 pm
content: Is the DAC VCC referenced? That would seem to be a bigger issue...Tim

author: Saimoun
author_rank: Frequent Contributor
timestamp: February 24, 2022, 07:24:56 am
content: I am not sure what that means, and how do I know it?The DAC is a "dumb" 16bits R-2R circuit as I understood.

author: srb1954
author_rank: Super Contributor
timestamp: February 24, 2022, 07:55:23 am
content: Quote from: simonlasnier on February 24, 2022, 07:24:56 amFrom the data sheet it looks like the DAC is Vcc referenced. It is just made up of a bunch of analog switches that alternately switch Vcc or Gnd to an R-2R network. When a switch connects to Vcc some of noise on Vcc will appear on the DAC output. While the MSB is switched to Vcc you will probably get a PSRR of only 6dB.The application circuit shows additional filtering on the Vcc supply to the DAC but, given the magnitude of noise you have on your Vcc, you will need much more effective filtering than shown in the datasheet. It may be that running the DAC off its own LDO is a better solution but if that is not possible you will need to change the DAC to one that has a separate Vref pin that can be supplied from a clean reference voltage.

author: Saimoun
author_rank: Frequent Contributor
timestamp: February 24, 2022, 09:20:03 am
content: Thank you srb1954! I'm learning every day hereSo I guess the DAC is indeed the weak link of the circuit. I'll see if I can find another one, or at least put it behind a high-PSRR LDO while the rest of the audio circuit can run off the 5V

author: Saimoun
author_rank: Frequent Contributor
timestamp: February 24, 2022, 07:09:29 pm
content: So - mindblowing (for me at least) discovery of the day - when you remove the DAC of the equation, the audio circuit itself has a very good PSRR!!I cannot believe I've had that problem for months and never realized that it was possible for an audio circuit to do the supply rejection by itself (I always assumed for audio you need a clean power supply).The circuit I just tried was:* DAC powered with a separate High-PSRR 3.3V LDO* Mid-voltage (HalfVCCA) from the right channel of the DAC - that works beautifully because I can get the DAC right channel to just output 2.5V* rest of the audio circuit powered directly from USB Vbus (no filtering other than a few caps)At -22dB gain, I literally hear nothing it's dead silent. At +8dB I can hear the buzz noise of the display, but still very faint.So now we're back to the original problem: how do I filter the noise on the 5V rail, to reduce it enough so it does not appear even with +8dB of gain on the opamp? I guess I could try an LC filter?

author: Vovk_Z
author_rank: Super Contributor
timestamp: February 25, 2022, 05:11:40 pm
content: Yes, start with simple LC filter. 1-10 uF and 47-100-470-1000 uF.

author: Saimoun
author_rank: Frequent Contributor
timestamp: February 26, 2022, 01:04:07 pm
content: Quote from: Vovk_Z on February 25, 2022, 05:11:40 pmDo you mean 1-10uH?Also, just wondering: I am making the filter on USB Vbus so technically no more than 10uF allowed because of in-rush currents. But putting say 33uF is fine if it's after the inductor since the inductor will limit the in-rush current of the capacitor, correct?

author: Vovk_Z
author_rank: Super Contributor
timestamp: February 26, 2022, 10:17:43 pm
content: Yes, I mean uH.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 26, 2022, 11:17:06 pm
content: Inrush through the filter is determined by its impedance. Zo = sqrt(10uH / 47uF) is 0.46Ω and 5V divided by that is 10A peak inrush, well above the 500mA limit (or 2 or 3 if it's set that way). You'd need very much more inductance to afford higher capacitances, and then you have the problem that your filter output is equally squishy (i.e. about 10 ohms, for frequencies near cutoff), which is probably not ideal.The LDO will be fine. Get an especially high PSRR type, or use two cascaded even. (If they're not adjustable, you can stack one on top of a diode in the ground return path to get, say, 4V from a 3.3V reg. Mind that, ground pin currents may be quite small, or variable, in which case consider adding a bias resistor from that reg's output to the diode. If Ignd varies say 10uA to 1mA, reducing the spread to like 10:1 or less would be fine, i.e. 90uA bias minimum, and a fraction of a mA would do better. So like a 1k there.)Also mind ground-return paths. You may be at the point where ground loops become relevant. Does your PCB have a solid GND plane? What's component placement like? Is your output shared with anything (sound card loopback, other equipment?), or is it completely floating (e.g. headphones)?Tim

author: Saimoun
author_rank: Frequent Contributor
timestamp: February 27, 2022, 01:19:12 pm
content: Quote from: T3sl4co1l on February 26, 2022, 11:17:06 pmThank you both for the replies.@Tim: I am confused about the calculation. If 10uF is the usual allowed, then with 10uH and 10uF that gives 1impedance, i.e. 5A peak inrush, still way above the 500mA? I guess my question is where does the "10uF standard" come from?The LDO for the DAC is not the problem - that works already. What I am looking into now is how to get a slightly cleaner 5V rail to power the opamp with.I've been very careful about ground and yes the PCB has a solid GND plane. It's a 2 layer though so it is cut a bit, but I always try to place the components and traces to avoid cutting the plane completely, so it's still very solid. And the output is either balanced line (so no ground noise on the audio) or headphones (floating).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 28, 2022, 01:14:53 am
content: Yeah, presumably they permit some inrush -- this is more apparent considering typical USB power switches (that the host might be using). They're usually something like a protected MOSFET, that goes into current limiting somewhere above 500mA (might be loosely calibrated, say anywhere from 600mA to 2A), and then is thermally protected so that, while it's dissipating those, whatever, up to 10W say, but it's a teeny little die in an SMT package -- it heats up quickly, and might stay on for only a few hundred µs. Which is enough to charge 10µF (in 83 to 25µs) plus reasonable overhead, but will consistently fail for higher loads and indeed might never start up into say 47uF+, especially if load current is being drawn while it's still coming up.The ones with polyfuses, might not give a lick, but also the inrush current momentarily shorts out its own supply. If that's like a PC, the supply probably has enough brunt to deal with it. Think I have that on a USB hub, which basically has everything all wired together, and the polyfuses hardly matter because the 5V 2A adaptor will easily brown out under such conditions. So plugging in a heavy load might reset (or corrupt) everything on the hub.As for the op-amp, how much current does it need (total, quiescent plus peak signal output)? Maybe the high impedance filter is indeed adequate. Maybe you only need to "take the edge off" -- note that PSRR improves at low frequencies, so it should be okay at DC, and maybe at 100s of Hz, even low kHz, but some extra filtering in the 10s or 100s kHz might be warranted, for example.Do you have an output noise waveform, by the way? You showed the supply rail going up and down like a skyline; amp PSRR will more or less differentiate that, giving peaks on every transition rather than whole levels. So you should only need to filter the peaks.Note also, you can fake an inductor with a gyrator circuit -- at some expense to the voltage drop and/or bias current required by the circuit -- or use more nonlinear means of supplying and filtering. Or use a "capacitor multiplier", if you can afford the ~0.7V drop.By "nonlinear", I'd think something like: wire up a current source into a relatively large capacitor; the current is set somewhere between op-amp peak consumption, and USB max output (so, maybe somewhere in the 20 to 400mA range). Because the current is in excess of the load, normally it sits saturated, so the amp gets full supply. But if the input jumps up in voltage, that jump isn't transmitted directly to the output, instead the current limit causes the capacitor to charge slowly up to the new level. And when the input steps down, hmm, ideally you'd want a diode sort of function so that the load cap is allowed to simply discharge down (by load current alone) to the new level, but most circuits I think are going to have a resistive or wrong-way-diode characteristic, so some refinement would be necessary over the stock CCS circuit. Let's see... I think a Baker clamp is enough to do it? Yeah, that's not too bad then.And, such a circuit is nonlinear, so when it's saturated, there's low resistance between input and output, and a certain baseline level of noise can be transmitted; this is still filtered by the capacitor, but its ESR passes some, still. It has the effect of a blowoff valve, so that noise above a certain amplitude is forced to "delay", getting smeared out into a more gradual slope by the capacitance. Which means for your purposes, maybe you still need the filter for better small-signal response, but also, you are specifically shielded from the large-signal regime, where transitions, noise amplitude, whatever is large enough to hit the CCS region, is blocked from passing.Finally, once you've got the CCS in there -- that solves the inrush problem, and you can use whatever beefy capacitance / filter stack you need. Indeed, the attenuation of the CCS might not even matter much (say the noise amplitude is always in the ~0.2V range, so it never really gets a chance to hit the CCS range from noise alone), but it's still valuable for this reason alone.Tim

author: Saimoun
author_rank: Frequent Contributor
timestamp: March 02, 2022, 02:57:30 pm
content: Thank you Tim again that is a really big help! Very appreciatedQuote from: T3sl4co1l on February 28, 2022, 01:14:53 amI really need to buy a decent oscilloscope one day, but until now this one has been fine - my ears is the precise oscilloscope ha haI attached a picture - yellow is the 5V supply, blue is the audio output (not playing anything) with +11.7dB gain. That's with no filtering other than a 4.7uF and 0.1uF cap.Quote from: T3sl4co1l on February 28, 2022, 01:14:53 amI think the current requirements are really low:* Quiescent current is max 0.65mA at 25 degrees* Max output I might need some help here** The "loudest" the application sends is a square wave of 25ms every 150ms (i.e. a fifth of the time)** Measuring it with the oscilloscope gives me the max Vrms into 32 ohm, which is 0.5Vrms** So that gives 15.6mA, divided by 5 (because only a fifth of the time), and times 2 (because there are 2 channels)==> That gives me a grand total of about 7mA - I would say that is quite overestimated thinking the USB usage of the whole thing is about 40mA? (and the LED display is probably using most of that)Anyways, for now I was actually thinking of making literally an RLC filter - just to be on the safe side to avoid any oscillations, and also because it will filter even better.Can I save one resistor and use the DC resistance of the coil? For example if I use say a 100uH which has a 7.5 ohm DCR with a 4.7uF cap. Does that make an RLC with R=7.5, L=100uH and C=4.7uF?That would cut at 7.3kHz and might be enough (I'll try).If that does not work I'll look into the gyrator or capacitor multiplier circuits

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 02, 2022, 03:29:01 pm
content: Can you zoom in on those spikes? Also does that not have a "hi res" mode or anything? Very few bits in the acquisition there...Only need a cutoff corresponding to somewhat longer than the time constant of that. Even an RC filter might do, but CLC will do even better.Okay, so 32 ohm load, and peak is what matters, and you have a square wave worst case so that's that. No duty cycle, take peak not average.Another thing to think about is short-circuit current, partly in case of actual shorts, and partly because loads can have weird impedances. Opamps are usually rated on this, not very accurately, but say 30-60mA is a fine enough range for design purposes.Then say delta less than 0.5V for say 20mA step load, and that'll be 0.5/0.02 = 25 ohms or less, so RLC around that; for 4.7uF that'd be under 3mH, which I don't think you're in any danger of hitting.And then, enough ESR to keep it stable and that's fine.Yeah, 7.5 ohms DCR is more than enough for 100uH + 4.7uF (Zo = 4.6 ohm), and at that load current, it's not much of a problem in terms of voltage drop. And even without those spikes being well resolved, some kHz is probably about right.Tim

author: Saimoun
author_rank: Frequent Contributor
timestamp: March 03, 2022, 01:02:02 pm
content: Quote from: T3sl4co1l on March 02, 2022, 03:29:01 pmI knowUnfortunately not - It does have an oversampling which can help a tiny bit but still all you see is noise.So I tried a few things:RCLC filter (if that's a thing), with 10 ohm, 4.7uF, 100uH (+1.5 ohm DCR), 4.7uF. That must have cut a bit, at least on the oscilloscope on the 5V supply the corners a tiny little bit more round. But listening to the audio output I cannot really hear much differencecapacitor multiplier with 4.7k ohm and 4.7uF: that really works - the 5V supply actually looks clean on the oscilloscope, also the noise from the display is entirely gone, even at +11dB gain. The problem is the voltage drop which is quite high (drops to about 4.2V it seems). I mean the whole point is the make the audio louder, if I go through all this trouble to raise from 3.3 to 4.2... Maybe not worth itI attached my circuit for the capacitor multiplier.But now that the supply is clean I realized there is quite a lot of white noise (again it is not that bad - but the whole point of this is to reduce the noise so...). I found out it comes mainly from the fact I used the Right channel of the DAC as the reference voltage HalfVCCA. The left channel is going through a 2nd order high pass filter (through the opamp). I tried using the TL431 - great IC! - and that solved that problem.So new question is: is there a way to reduce that voltage drop in the capacitor multiplier? I guess the choice of transistor would matter? For now I used an NPN 2N2222A for testing.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 03, 2022, 02:11:48 pm
content: Sure, use germanium.Or boost up the base voltage slightly. Not trivial, as you already know regarding boost supplies. Little current is needed at least, but still something. And if you use any kind of converter or charge pump, you're injecting noise into that supply and it needs even more filtering.I guess it would be fun, amusing, to use a photodiode, or a BJT in breakdown*, to try and source that extra couple 100 mV.*This goes something like: E-B breakdown generates free charge carriers, and some light which is quickly absorbed nearby also creating free charge carriers. The fraction of these which find their way into the B-C junction, basically act like a photodiode, thus generating an opposite voltage at very feeble current. You'd need to do this with an RF transistor (Veb < 5V), and I'm not sure how much you actually get out of those types (since with the higher doping, diffusion distances are shorter; but the junction is thinner too, so maybe that doesn't matter?).Is the DAC really noisier? TL431 isn't especially quiet by itself... DAC shouldn't be any worse than its ref.Tim

author: Saimoun
author_rank: Frequent Contributor
timestamp: March 03, 2022, 02:45:59 pm
content: Regarding the DAC noise, I guess I could put a 100n (or even bigger) capacitor - do DAC's like capacitive loads?I would say since this one is a R-2R type it's just a bunch of resistors so it should be ok, correct?Regarding the photodiode/BJT breakdown, that sounds a bit out of my abilitiesAnd I guess changing the resistor or cap in the capacitor multiplier will have no impact on the voltage drop, right? (since it comes from the transistor)

author: Saimoun
author_rank: Frequent Contributor
timestamp: March 03, 2022, 02:57:19 pm
content: I do feel like I'm going around in circles, always going back to: well I gotta make a boost converterSay I make a boost converter and then filter the output with a capacitor multiplier - can I count on the drop voltage to always be the same or should I add an LDO after?

author: mawyatt
author_rank: Super Contributor
timestamp: March 03, 2022, 03:03:31 pm
content: If current draw isn't high then look at a LDO with really good HF PSRR rejection. The LT3045 was mentioned on another thread by NiHaoMike, this has superb HF PSRR rejection.https://www.analog.com/media/en/technical-documentation/data-sheets/30451fa.pdfBest

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 03, 2022, 03:05:32 pm
content: Cmult voltage drop varies with load because it's a resistor into a diode drop, more or less; with extra voltage overhead besides, this will be even more irrelevant. You still need stable VREF for the DAC of course.Tim

author: Saimoun
author_rank: Frequent Contributor
timestamp: March 03, 2022, 04:29:01 pm
content: @mawyatt: thanks for the reply - LDO would work great but I need (ideally) 5V from a USB VbusQuote from: T3sl4co1l on March 03, 2022, 03:05:32 pmI'm confused - so it does vary but not much for me to worry about it? My idea would be to make a boost converter to like 5.8V and then put a capacitor multiplier to end on about 5V. Then use this directly as opamp VCC.I think for the DAC I will just use a 3.3V LDO, then I know its power is clean (since clearly the DAC has no PSRR whatsoever).What about adding capacitive load on the DAC's output?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 03, 2022, 04:59:32 pm
content: It varies in two ways, well three:1. PSRR is only good at high frequencies. That makes sense, it's a filter. So, it's still one-to-one at DC.2. The output impedance (change in supply voltage for change in load current) is a diode drop in series, which dominates high frequency impedance, and contributes to DC resistance.3. Drop across the resistor, roughly proportional to load (it acts as R / hFE at the output).PSRR at AC is your primary problem, as I understand it, so these are okay. The op-amp won't care about varying supply voltage, as long as it's not banging its head into the rails on peaks. The DAC needs a constant REF, else the output gets AM modulated by the supply.If the DAC is buffered, it likely wants a series resistance (47 ohms or more?) before a big capacitive load. If unbuffered, go nuts.Tim

author: Vovk_Z
author_rank: Super Contributor
timestamp: March 03, 2022, 10:40:47 pm
content: 7.5 R may be too much, I would begin with a smaller inductance with a smaller internal DC resistance.

author: Saimoun
author_rank: Frequent Contributor
timestamp: March 04, 2022, 08:00:25 am
content: Quote from: T3sl4co1l on March 03, 2022, 04:59:32 pmI see, makes sense@Vovk_Z: why would you think that?I'm gonna try and make a boost converter anyways.I did some measurements with a multimeter, and it seems peak current usage is about 33mA. That was at the highest volume, constant sound and with 0ohm load on both left&right channels. I tried with 32ohm headphones and the max was about 22mA.EDIT: did one more measurement, this time by putting a 3.3 ohm resistor in series with the 5V supply and measuring the voltage drop with the oscilloscope.The peaks go to 280mV, so actually about85mA peak! The 33mA from the multimeter makes sense since those peaks are only about a third of the time.So I guess I should design the boost converter to handle about 100mA peak, and about 10mA RMS (i.e. 33/5 since in normal operation sound is only played at most a fifth of the time)?

author: Saimoun
author_rank: Frequent Contributor
timestamp: March 11, 2022, 08:27:23 am
content: HelloJust giving an update. So I found this IC which is a synchronous converter already set to 5V, then I'll put an RLC filter after that.It's very efficient because it's synchronous and I only need an inductor - so less things to choose wrong (avoiding smoking issues, likehttps://www.eevblog.com/forum/beginners/my-boost-converter-is-smoking-d).Also it's designed for 300mA output current so much closer to my usage.The design I decided is:Code:(original 5V from USB Vbus)5V -> LED Display-> 3.3V LDO -> MCU & other digital circuitry-> synchronous boost converter to 5V -> RLC filter -> audio opamp-> 3.3V LDO -> DACMaybe that sounds a bit crazy to go up and down like that, but this way the noise from the display is definitely isolated, and since the converter is after an LDO I am not limited in capacitance like I would be on the USB VBus.Maybe the last 3.3V LDO is unnecessary, I'll make the design so I can easily test powering the DAC directly from the 5V after the RLC filter.I attached the (google translated) datasheet of the synchronous converter, called ME2188.Regarding the design:Tim - I'll take your advice and use the 22uH suggested in the datasheetBut again I'm struggling knowing what current the inductor should be able to endure.* I know it's probably the fault of the datasheet giving very little info.* I also tried putting the values inhttp://schmidt-walter-schaltnetzteile.de/smps_e/aww_smps_e.html(I know it's for asynchronous, so taking this with a pinch of salt), using 3.3V input, 5V output, 22uH, 0.1A output current, 120kHz (since the IC does PFM and I guess it's 320kHz at 0.3A), that gave me an indcutor ripple current of 0.43A. So a switching current of 0.38A.* So I found a 1210 inductor, 620mA, 22uH ±10%, with DCR= 270mΩ - that should be ok I think?For the caps I'll use 4.7uF MLCC at the input of the converter, and an electrolytic 47uF at the output.For the RLC filter I'll put a 1 ohm 0603 to avoid oscillations and then just reuse the same parts as the converter, 22uH and 47uF.That's a lot of info - hope that's ok I'm thinking it might help someone that passes by this thread in the futureMy specific questions:The IC recommends to use tantalum capacitor for the output - is it a bad idea to use electrolytic?It also recommends a 0.1uF ceramic on the output (before the big one I guess) - never saw that on a converter IC before... ?Am I in the ballpark with my 0.62A inductor or completely off?Thank you!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 11, 2022, 04:44:25 pm
content: Cute, a 3-terminal sync reg!So inductor ratings, low enough resistance that it's not annoying of course (within thermal ratings certainly, and any efficiency gain you want on top of that), and saturation current >= peak current used. So, max DC (again, of the regulator!) plus peak ripple (or half of peak-to-peak or delta I).In that size, multilayer ferrite is probably fine, otherwise most anything wound ferrite or composite powder should be fine. Small chip inductors come in all sorts of styles, take your pick. Raw-ass ferrite beads wouldn't be suitable (not that you'd find one with so much inductance, nor sold as such; multilayer FBs are made exactly the same way though, so, just that you won't know by eye in that case, don't let them get mixed), and powdered iron toroids can be an issue as far as losses (not that they'll be a problem for immediate purposes either -- you'd be pondering those for 100s W capacity converters). Shielded is usually better than unshielded; it kinda doesn't matter for tiny stuff like this, but is nice for low noise, and power stuff where not only does it reduce emissions but an open-core inductor with a lot of voltage on it might start cooking (induction heating) neighboring components.Unfortunately the datasheet doesn't really tell you what or what things need to be what they are. No internal compensation is listed. PFM could be as simple as hysteretic control again, or it could be an independent oscillator skewed by operating conditions. Also not obvious if it's synchronous rectified, or just free running (always driving LX high/low); efficiency seems reasonable to lowish currents so maybe it's the former. It could also be a burst mode control, with consequent higher ripple. It does say very low current at no load, suggesting some kind of burst mode. But no way to really tell from here. Such is the downside of shopping in China...That they suggest tantalum, implies they want ESR. Electrolytic can be used as well, just shop around for something comparable (0.1 to 1 ohm ESR?). Ceramic can also be used with external ESR. If it works fine with ceramic alone (do a step transient test -- again, a scope is extremely useful here), well there you go.Also, you probably want one a little step up from that, with only the 6V rating? Since you were looking for 5.9 or so. I suppose it doesn't matter as it's fixed 5.0V anyway, but more might be nice for the overhead you wanted.Tim

author: Saimoun
author_rank: Frequent Contributor
timestamp: March 16, 2022, 10:52:48 am
content: Thanks again Tim and apologies for the late replyQuote from: T3sl4co1l on March 11, 2022, 04:44:25 pmI'm still confused - the max DC current of the regulator, is that 300mA (output current) or 1A (LX max current)?Or is it not possible to know because the datasheet is wonky (I mean it does say absolute max LXpin current 1A and then later on "limiting current" max 1.2A...)?Ok about the capacitor - thanksAnd as for the 5V output - because here the regulator is after the 3.3V LDO I do not need to remove the Display noise anymore, but only the switching regulator's noise, so the RLC filter should be enough.Say I was to change that with a non-chinese part, i.e. a regulator with:* 5V fixed output* synchronous* max output current about 150mA* small and cheapwhich part would you recommend?