Switcher post regulator - Page 1
Forum section: electronics / projects

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: May 03, 2015, 03:27:38 pm
content: I want to get a switcher's output to be more like a linear's noise-wise, you know, within a certain range of "that looks about right". In other words, I have no rigid requirements or hard numbers, just want something close. Oh and there's no space for an LC filter.Here are some interesting LDOshttp://www.micrel.com/index.php/products/power-management-ics/ldos/linear-power-filters.html?zoom_highlight=%22ripple+blocker%22This "ripple blocker" seems to work on the general idea of adding the inverted noise waveform to the output, I guess.Thing is, I want 5V output , the Ripple Blockers aren't specced for >3.3V.So I'm wondering, how is the the PSRR rating of a regular boring old LDO like the Microchip MCP1826 achieved? And what does it mean when it's rated in negative dB? It's a ratio, so which part is negative? I don't get it.If I use a switcher that runs at 1MHz, will the MCP part attenuate the 1MHz tone by 40dB?Is the fact that this graph is shown at 10mA a warning that at higher currents it won't attenuate as much?I googled and read a lot of app notes, it seems it's divided into ferrite bead approaches which keep the load's noise out of the rest of the circuit (at >>1MHz), or LC based filtering.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 03, 2015, 05:21:48 pm
content: Common to follow it with a linear circuit.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 03, 2015, 06:57:25 pm
content: I think they're saying gain, not rejection. Yes, calling it "PSRR" (rejectionratio..) and negative is dumb...How much dropout do you have to work with? LDOs are generally terrible. The valley at 500kHz in that graph is almost certainly exclusively due to the 22uF ceramic on the output; performance at higher frequencies will be dominated by layout (in other words, you lied when you said you don't have room for LC filters...because at some point, you have no choice!).At least you can usually design switchers to have low noise below 100k, which will help with a regulator like that.For better wideband rejection, you might consider a capacitor follower. The output won't be as well regulated, but if clean power is more important than accurate power, that's a win. (Anyway, accurate power almost always involves bandgap noise, so over some bandwidth, it will never be "quiet". If you get into the really high-falootin', low noise references, this does change a bit.)Tim

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: May 03, 2015, 08:51:36 pm
content: Yeah, I was sure I was going to have to play Sherlock Holmes to suss out the reality....Headroom is whatever I want, 1V seems very generous and even if it dissipates 1W it's still far better than the fully linear regulator I intend to replace." you lied when you said you don't have room for LC filters"Well, unless I go full 1960s on the design...http://upload.wikimedia.org/wikipedia/commons/thumb/c/c3/Cordwoodcircuit.agr.jpg/500px-Cordwoodcircuit.agr.jpgMy little TO-3 wafer is pretty full. I was thinking of putting a to-220 style LDO bent over, that might do the trick to free up some space.Here are the metrics, if I can put a scope on the output of a 7805 and my module and the trace is about the same thickness. That's what I want...I know there are parts that would probably make the design better, but I'm trying to balance cost and ease of assembly, not everyone will be able to assemble QFN at home, or 0402s.This project is just for giggles. I want to replace the linear regulators inside the Commodore 1541 disk drive. I'm worried 1MHz noise being injected into the Commodore 64 and making awful lines on the video.The layout is just preliminary.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 03, 2015, 08:56:10 pm
content: Bah, there's at least enough space for another L1.And don't forget, you can always go up! SMT parts are usually rather low profile, so you might have to make other compromises (THT parts with bent over, SMT'd leads?).

author: Frant
author_rank: Regular Contributor
timestamp: May 03, 2015, 11:48:25 pm
content: The TPS7A4700 (http://www.ti.com/lit/gpn/tps7a47) has the highest PSRR and noise performance I have ever seen. Some other voltage regulators may exhibit higher PSRR values at frequencies up to the order of 100 kHz, but this one goes up to 10 MHz.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: May 04, 2015, 12:02:01 am
content: Quote from: Frant on May 03, 2015, 11:48:25 pmWhoah, that's a nice part!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 04, 2015, 12:29:49 am
content: Hmm...Huge red flag: bipolar part, NO MENTION of ground current under dropout conditions. None whatsoever that I could see.You wouldn't be using it for battery operated applications anyway (quiescent current is fairly high), so it's probably not a problem, but something to keep in mind.The PSRR is quite good.Tim

author: Gyro
author_rank: Super Contributor
timestamp: May 04, 2015, 09:35:03 am
content: Not sure if this approach might help - in concept at least....Finesse Voltage Regulator Noisehttp://www.wenzel.com/documents/finesse.html

author: flyt100
author_rank: Newbie
timestamp: May 06, 2015, 01:17:19 am
content: Here is a new LDO that could be a fit. GREAT PSRR over frequency. It does specify GND current in dropout...http://www.linear.com/product/LT3042

author: Marco
author_rank: Super Contributor
timestamp: May 06, 2015, 01:52:17 am
content: Quote from: Alex Eisenhut on May 03, 2015, 03:27:38 pmI think it's simply the extra headroom from the charge pump for the base driver of the NPN pass transistor which is the key.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: May 06, 2015, 11:25:50 pm
content: Quote from: flyt100 on May 06, 2015, 01:17:19 amAwesome. Tricky package for beginners though. Maybe for later.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: May 06, 2015, 11:27:38 pm
content: Quote from: Marco on May 06, 2015, 01:52:17 amThere is no NPN pass transistor.http://www.micrel.com/rippleblocker/index_files/APEC_Ripple_BlockerFINALWhitePaper.pdf"an active filter circuit to provide a clean output voltage"

author: kvresto
author_rank: Regular Contributor
timestamp: May 06, 2015, 11:56:34 pm
content: Hi Everyone.Sorry to push in on the conversation. but a comment from T3sl4co1l has got my interest,Quote.T3sl4co1l , can you elaborate on this, its the first time I've heard the term "Capacitor follower"?PS: I've just assembled a LM2593 150Khz switcher, and a LC post filter, the results are good as far as output noise and ripple, but large realestate, and some unwanted low amplitude high frequency spikes I have not solved yet.Also I was looking at the TC1262 by Microchip, will this be better? Have a look at page5 and "Output Noise vs. Frequency" graph. Admittedly I haven't looked if there is a 5V version in this family.cheerskvresto.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: May 07, 2015, 12:31:30 am
content: Quote from: kvresto on May 06, 2015, 11:56:34 pmI thought he meant this trickhttp://www.radio-electronics.com/info/circuits/transistor/capacitance-multiplier-circuit.php

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 07, 2015, 02:00:24 am
content: Ah yea, multiplier not follower.For even higher attenuation, you can chain RCRC's along the base, dropping maybe 200mV at a time; by chaining BJT followers, only the first one incurs full Vbe loss, the rest run at low Vce (the 200mV or whatever).Obviously, the drop varies with load and temperature (Vbe as log(Iout), RC drop as R*Iload/hFE), so if you need precise DC, this isn't a good idea, but if you're pressed for low noise, precision probably isn't a high priority, and it's a good win.Phil Hobbs (http://electrooptical.net/and frequent poster on SED) is fond of this circuit.Tim

author: kvresto
author_rank: Regular Contributor
timestamp: May 07, 2015, 02:18:59 am
content: Ok, I've learned about a new cct in my case.Would I use this directly after a switcher or after the LC filter following a switcher? OR...?kvresto

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: May 07, 2015, 11:43:08 pm
content: Quote from: T3sl4co1l on May 03, 2015, 06:57:25 pmI guess you caught me. I dumped the post regulator and just put a BMFL and BMFC on the ouput. I can tolerate losing 50mV at 500mA if it brings like 40-50 dB crushing power on the tones. This is all guesswork at this point, nothing built yet.

author: Marco
author_rank: Super Contributor
timestamp: May 08, 2015, 12:39:47 am
content: Quote from: Alex Eisenhut on May 06, 2015, 11:27:38 pmThat's kind of a nitpick, N-channel MOSFET then. The reason for the inherent superiority of source followers is the same as for emitter followers.QuoteThe MIC94300 is basically a capacitor multiplier with a NMOS pass transistor, with a charge pump to be able to drive the NMOS, the capacitor multiplier is the active filter they are talking about.With the MIC94310 LDO there is no more capacitor multiplier ... only the NMOS output."Standard P-Channel LDOs have high PSRR at DC but will start losing ripple rejectionabove 10kHz, as shown in Figure 9. This roll-off is caused by the gain-bandwidth limitof the error amplifier. The MIC94310 does not have this limitation because it uses anNMOS pass device. This NMOS is configured as a source follower and has intrinsicallygood PSRR due to its low output impedance. This means that a smaller loop gain isneeded to achieve the same PSRR when using a PMOS as a pass device. Therefore, athigher frequencies where the loop gain drops, an NMOS pass device can still maintain agood PSRR. The MIC94310 surpasses the performance of a standard LDO at frequenciesfrom 60kHz to 10MHz. This makes the MIC94310 a perfect choice to use as a regulator"

author: kvresto
author_rank: Regular Contributor
timestamp: May 08, 2015, 07:51:03 am
content: BMFL and BMFC? what? even google has no idea. Do I need to do some maths BMFL/BMFC = L/C, are we talking about an LC filter?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 08, 2015, 10:55:06 am
content: BMF = about 10-100 times larger value than needed, I would say.Even without BMFs, it's hard to argue with at least a few ferrite beads to take the edge off. None of those high-PSRR regulators claim anything above a few megs, where using a regulator is pointless anyway; you have little choice but to use passives out there.Tim

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: May 08, 2015, 11:47:26 am
content: I'm still tossing parts around on a schematic. I guess I have a lot more to read about LDOs in general, later. (Good thing I tossed it out.)There is no Spice model for the TPS54232, I'll try to find a similar part this w/e. I tossed my school notes on switching regulators. I apparently had a clue 20 years ago, now I need to re-learn all the stuff.I added a PTC fuse on the input just to protect myself when I do build hardware.Oh, and BMF = BIG MF, MF is a favorite expression of Samuel L Jackson. O0

author: kvresto
author_rank: Regular Contributor
timestamp: May 08, 2015, 01:11:21 pm
content: ok, I'm laughing, kinda embarrassingly. At least my maths was good:)

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: May 09, 2015, 01:15:02 am
content: So it makes for a nice packing exercise. I might try some fancy 3D stacking if I make another round PCB, .75 inches in diameter and stacked .5 inch high to really get that TO-3 look and feel. I might put the switcher in there upside down for best shielding, and put input and output conditioning on the TO-3 part.Hmm!I'm the kind of guy who starts with the PCB then I simulate. Maybe I'll put the schematic up for review and harsh remarks.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 09, 2015, 08:45:24 am
content: Those caps are awfully small in relation to the inductors. Are you sure you'll have enough value there -- at rated voltage and temperature? This module likely isn't going to stay cool, even with an efficient chip!Inductor size, I guess, seems reasonable given the gross ratings I'd expect from the form factor, or you can shrink both by choosing an even higher frequency chip (should be able to get some in the >MHz range?).Also, there's no input filtering (beyond the bypass caps). Is there other analog stuff this might back-feed into -- is that a concern?Remember, the input side of a buckis the noisier side!Tim

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: May 09, 2015, 02:03:44 pm
content: Quote from: T3sl4co1l on May 09, 2015, 08:45:24 amHey, I said be harsh to my *schematic*! I expect nothing but praise for my layout.The big cap on the input, top left, is a tantalum that is removed now and I'll put more ceramic caps, with the resistance of the PTC fuse that should damp out a bit the oscillations. Output current is more on the order of half an amp for the application, not the ~3 amps from the form factor.As for the feeding back, I was wondering about that, the linear transformer feeding the circuit has two secondaries and about 4 inches of loose wiring from the transformer to the bridge and main input caps. Inter-winding capacitance is not known (to me) but for sure it'll be the right value to let that 1MHz signal jump all over the place.But yeah, coil and capacitor selection continues. I'm also considering the .75 inch "top hat PCB" to put the switcher on there and filtering on the base.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: May 09, 2015, 02:41:42 pm
content: For example, L3 is far too much of a BMFL and there are ceramic inductors that are far smaller in size, which allows space for BMFC C9. Etc..I'm starting from the TI datasheet and porting over the equations one by one. Their stupid software is too clumsy for me.https://docs.google.com/spreadsheets/d/1v11noxAnF4kvpm88LzwxX3Mkwh_AWj5LsXuw3eF2t6A/edit#gid=0The datasheet kinda sucks though.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 10, 2015, 02:52:37 am
content: Don't know where those numbers are coming from...I always go back to basics. V = L * dI/dt. This does it all for a few inputs,http://schmidt-walter-schaltnetzteile.de/smps_e/abw_smps_e.htmle.g.Vin min = 5Vin max = 20Vin calc = 10Vout = 5Iout = 0.5f = 1000L = 5E-6The "proposal" is automatically generated (solves L for dI = 20% or something like that), which is rather conservative, more suitable for old powdered iron chokes than compact ferrites or modern powder composites. So don't be afraid to overwrite that figure and put in something a bit more aggressive, like I've done here.Note it says "Tip: The lowest value of L is achieved if ?IL=2·Iin for Vin_min." You usually want to be around BCM (the boundary between CCM and DCM, where inductor current is just barely returning to zero each cycle) to maximize density. Assuming that you can, since such operation does increase inductor losses. (Nothing a modern SMT can't handle.)5uH (say, 4.7 to 10uH real value in circuit?) seems pretty reasonable to me. That'll be much smaller, which will allow you to put much bigger capacitors around. Remember those have to be >>5V parts (X7R or better) to maintain value. Probably 16V at the output, 25V or more at the input. A single 1206 part should be more than enough (for each cap), plus additional filtering as needed/desired.The layout itself -- don't get me wrong, it looks pretty good. I never do anything that dense, though I always have the area to expand into, as well... I probably wouldn't do much better myself. And it's a neat constraint to work against, as long as you have the time to optimize it of course.Tim