Quiet & low cost +/-V rails  - Page 1
Forum section: electronics / projects

author: fcb
author_rank: Super Contributor
timestamp: August 06, 2014, 07:01:18 pm
content: I'm working on a project at present that is powered from 4xAAA cells (i.e. 3.7 to 5.7V after diode drop), the analog sections need +/-6V at around 15-25mA (less on the negative rail).My first iteration used 3x ICL7660's and an LDO in the following config:2x ICL7660's as voltage doublers in parallel (giving a +6.8v to +10.8v rail)1x LP2951 as regulator fed from new rail giving 6.0V1x ICL7660 as voltage inverter on the output of the LP2951 (giving -6V rail)Most of the electronics in the unit is audio, so I'm keen to keep the noise down. It just seems like an expensive way of doing things.I did contemplate building a pair of anti-phase charge pumps out of 74HC04's (driven off 24KHz, derived from a 48KHz clock)? Anyone got a smart (low cost and small board space!) idea...?

author: Precipice
author_rank: Frequent Contributor
timestamp: August 06, 2014, 07:19:31 pm
content: I've just done a design using an LT1930ES5 to generate similar rails. P8 of the datasheet has the +-generator I ripped. I dropped the diodes down to BAT54S, since I don't need the full current and they're already on my BoM.No idea yet how noisy it is - but switching frequency is out at 1.2MHz (or you can get the 2.2MHz 'A' part), so it should be safely out of the way of audio.$2 at 100-off, so not super-cheap (but unusually good for an LTC part!). Certainly compares well with a pile of ICL7660s unless they've suddenly got cheap. It's tiny, too (SOT23) - and the support components are small, which was a driver for me, but you don't say.Edit: you do say small. In that case, I reckon this is worth a look. If you're doing audio stuff, I really don't think you want to be deriving your PSU from 48KHz. That way lies much annoyance.

author: void_error
author_rank: Frequent Contributor
timestamp: August 06, 2014, 09:11:21 pm
content: Quote from: fcb on August 06, 2014, 07:01:18 pmYou could indeed use a 74HC04 (or MC14584, same thing but works up to 15V) and boost its output current using a PNP-NPN pair or low power MOSFETs.Add the voltage doubler / voltage inverter bits of circuitry to that and you only have to use one chip. 48kHz is a waste of space because you'll need higher value capacitors, go for around 300kHz.

author: cellularmitosis
author_rank: Unknown
timestamp: August 06, 2014, 10:25:08 pm
content: I recently worked trying to filter out the noise produced by an ICL7660:https://www.eevblog.com/forum/projects/an-evening-with-the-icl7660/

author: fcb
author_rank: Super Contributor
timestamp: August 07, 2014, 08:39:34 am
content: Precipice: That's a nice part, I think it's a little bit too expensive though (I can't find in stock and for $2). I have to say, the idea of the small footprint does appeal.void_error: I'm probably going to try a 74HC04 on the bench today, I choose 48KHz as I have that clock (with a nice 50% duty-cycle) available nearby, certainly I could take the BCK (3.072Mhz) and feed it into a /8 and get 384KHz which might push the noise up the band - I worry about diode losses increasing.. Perhaps i'll try that as well.cellularmitosis: Nice other thread link, interesting stuff. I like the faraday pan...One other idea is to use a 555 as the driver, nice powerful output stage. Will report back my chosen solution when i've spent some time on the bench.

author: Precipice
author_rank: Frequent Contributor
timestamp: August 07, 2014, 08:55:55 am
content: Quote from: fcb on August 07, 2014, 08:39:34 amAh - I've got a reel of 2000 here, if you want a smallish number to prototype with.And, from bitter experience - running your switchers well outside the audio range (and dodging AM radio, to be kind to others), makes filtering so much easier (smaller, cheaper).Also - bear in mind there are better versions of the 7660, which can actually deliver useful current. Nat Semi's LM2662MX f'rinstance.

author: fcb
author_rank: Super Contributor
timestamp: August 07, 2014, 09:17:42 am
content: LM2662MX runs upto 5.5v, I have to cater for at least 5.7v (probably safer to assume someone inserts freshly charged NiMh's in the unit which can sit higher than 1.5v). Nice part though.Thanks for the offer of a few parts to prototype with, I should be OK - it'll require some serious deadbugging or a PCB to prototype that SOT23-5 properly. Any other 'goto' HF SOT23 boost regulators out there?Also, is there a 'goto' LDO -V adjustable regulator (negative version of the LM1117)?

author: Precipice
author_rank: Frequent Contributor
timestamp: August 07, 2014, 09:40:15 am
content: Quote from: fcb on August 07, 2014, 09:17:42 amAh, yeah, that would be brave...>Thanks for the offer of a few parts to prototype with, I should be OK - it'll require some serious deadbugging or a PCB to prototype that SOT23-5 properly. Any other 'goto' HF SOT23 boost regulators out there?Not that I've used - but stick your requirements into LTC's and TI's selection / design forms, and see what spits out. No sane person can keep track of all their offerings!>Also, is there a 'goto' LDO -V adjustable regulator (negative version of the LM1117)?Yeah, there's a '3' version, same as there is for most of the rest of the sizes. They're far from low noise, though! (and low noise candidates don't fit on the same footprints). If you can't face buying or building proper low noise regulators, you can clean them up a lot, for not much effort - search for noise eater. Here's a sample.http://www.techlib.com/electronics/finesse.htmlIt's cheaper and smaller than trying to tame the noise with ever bigger and better capacitors...TI have a load of low noise LDOs, if you just want to buy something and stick it down.

author: David Hess
author_rank: Super Contributor
timestamp: August 07, 2014, 02:51:48 pm
content: If you are up to winding a small transformer for operation at your 48 kHz, then low noise regulated bipolar outputs can be produced by regulating the input voltage before chopping.

author: fcb
author_rank: Super Contributor
timestamp: August 07, 2014, 03:50:43 pm
content: Precipice: I'm familiar with the LM337, but I can't seem to find a low-drop out version (in a SOIC package). Interesting link you posted, a "noise eater", it looks like a recipe for an oscillator - perhaps I'm reading it wrong though - does it work by creating a shunt regulator that's controlled by the noise on the line??I suspect I will have to build a discrete regulator for the -ve side.David_Hess: I've done a few transformer based supplies based on 48/96KHz switching (generating multiple isolated supplies in audio gear), I don't fancy winding a transformer small enough for my application though.

author: Precipice
author_rank: Frequent Contributor
timestamp: August 07, 2014, 04:15:31 pm
content: Quote from: fcb on August 07, 2014, 03:50:43 pmLM337LM/NOPB -http://uk.rs-online.com/web/p/linear-voltage-regulators/5361322/I don't know how low dropout you need, but it's not dreadful. TI have LM337L, with some shockingly sparse data!Quote from: fcb on August 07, 2014, 03:50:43 pmYeah, with a series resistor before it. I've never used one - but they seem well regarded by low-noise obsessives.>I suspect I will have to build a discrete regulator for the -ve side.It's not the end of the world...However, TI have a decent slew of -VE LDOs, including some claiming ultra low noise...http://www.ti.com/lsds/ti/power-management/linear-regulators-ldo-products.page#p32=1&p182=Negative OutputEdit:URL fixed, and no, none of them are low noise.

author: David Hess
author_rank: Super Contributor
timestamp: August 07, 2014, 04:25:32 pm
content: Quote from: fcb on August 07, 2014, 03:50:43 pmThey are not as common and I have never found a low dropout direct replacement for the LM337 either. This is close:http://www.linear.com/product/LT1175QuoteThat is exactly how it works although I have seen transformer coupled series pass versions as well.QuoteI am sure Coilcraft makes one intended for low power isolated supplies which would be suitable. They are used for galvanically isolating various interfaces like Ethernet, RS-232, and USB. They are also used for isolated gate drivers in larger power supplies.There are all kinds of dual output switching regulator designs which could be used but the filtering needed for low noise raises the cost.

author: RiverTown
author_rank: Regular Contributor
timestamp: August 07, 2014, 04:43:55 pm
content: Best way to search for an ICs, in this case negative LDO, is to use yours supplier parametric search.There are dozen different - LDOs, but that don't mean that your supplier have all them in stock.I must warn you negative LDOs aren't very cheap, I think that it is hard to find one that is under 1- 1.5 bucks in one of quantities.

author: fcb
author_rank: Super Contributor
timestamp: August 07, 2014, 05:26:40 pm
content: Quote from: RiverTown on August 07, 2014, 04:43:55 pmYes. I'm familiar with my favorite suppliers parametric search....Yes. There are a few -ve regulators about, just not small LDO variants..No. The bulk of the negative regulators are cheap (not quite as cheap as +ve), but they mainly seem to be LM337 or 79x variants.

author: Precipice
author_rank: Frequent Contributor
timestamp: August 07, 2014, 05:29:01 pm
content: Out of interest, how low a dropout do you need?(and why are we still discussing all this linear stuff? Fast switchers make for easy filters!)

author: fcb
author_rank: Super Contributor
timestamp: August 07, 2014, 05:36:49 pm
content: Dropout voltage:It will depend on the final PSU topology somewhat, but my *guess* is(3.7v * 2) - 2 * Vfd = 6.8v (vfd is around 0.3v, also assume cells at 1.0v each)So at best, the worst case scenario is -6.8v, although the output Z of ICL7660 will reduce this.I'm after -6v, so probably 0.5v dropout as a *guess*.

author: Precipice
author_rank: Frequent Contributor
timestamp: August 07, 2014, 05:44:36 pm
content: Oh crumbs, proper-low.Go on, ditch the 7660s...Don't forget that, if your current requirements are low, diode/cap inverters/doublers/triplers are cheap and small, if not terribly efficient). You'd only need one high power driver (C555?) and a bunch of passives (and then you could maybe afford some headrooom for your regulators)

author: RiverTown
author_rank: Regular Contributor
timestamp: August 07, 2014, 05:49:08 pm
content: Quote from: fcb on August 07, 2014, 05:26:40 pmWho is your supplier, it is weird that they don't have any negative LDOs in stock?Take a look at Farnell website, yes they are expensive supplier, but they have few of them.Quote from: fcb on August 07, 2014, 05:26:40 pmNor LM337 nor /9xx series are the LDO regulators. Linear negative regulators- yes, LDO- not a chance.I were using TPS72325 in my last project. You can't say that it's cheap in small quantities.http://export.farnell.com/texas-instruments/tps72325dbvt/regulator-ldo-fixed-2-5v-sot-23/dp/2323571

author: f5r5e5d
author_rank: Frequent Contributor
timestamp: August 07, 2014, 06:24:11 pm
content: for the V multiply, inverting there should be some single chip inductive boost solutions with off the shelf coilsI used a single SOT-23-5 boost converter chip with dual winding smt toroid likehttp://www.bourns.com/data/global/pdfs/pm3600_series.pdffor split supplies in a similar I,V rangeyou can buy dual switcher in a chip too now LT1944 is one, or yet another approach ishttp://cds.linear.com/docs/en/design-note/dn223f.pdfin my application the switcher was on board, within an inch or two of 12 bit ADC, Av 30 input amp/active filters for the signal with no problems (of course with added linear Vreg of the supplies)

author: David Hess
author_rank: Super Contributor
timestamp: August 07, 2014, 06:39:58 pm
content: At 10s of milliamps and below, there are a number of operational amplifiers which can be used as low dropout negative regulators directly without a pass element. Even with an external pass transistor, short circuit protection would not be required if the current is limited at the input because of supply impedance or current limiting.

author: fcb
author_rank: Super Contributor
timestamp: August 07, 2014, 09:30:50 pm
content: RiverTown: The TPS72301 (adjustable version of the part you suggested), looks ideal - it's a bit pricey though.Precipice: The advantage of the 7660 on the negative rail is the lack of diode losses.f5r5e5d: Nice bourns part, a but big though - I've ordered a few for my samples draw.David Hess: A possibility, bound to be cheaper than a TPS72301, could probably use a pass element to help in the selection, I wonder if the LM358 could be used.

author: Precipice
author_rank: Frequent Contributor
timestamp: August 07, 2014, 09:39:56 pm
content: Quote from: fcb on August 07, 2014, 09:30:50 pmYeah, but they're huge (SO-8) and slow, and need big capacitors for the flying caps and the output cap, followed by filtering out all the audio frequency noise, and even then, you've got precious little headroom for your LDO, especially given its dreadful output impedance. A tripler would give you enough headroom for a cheaper Vreg, even after the diode losses. Less of a fight that way, but you're the one doing the project

author: RiverTown
author_rank: Regular Contributor
timestamp: August 07, 2014, 09:45:06 pm
content: I don't know what you want to power with this supply. Maybe you can just use boost switching (high frequency) converter, positive LDO and virtual ground.This would drastically decreased costs (no need for voltage inverter and negative LDO).

author: Mechatrommer
author_rank: Super Contributor
timestamp: August 07, 2014, 11:16:33 pm
content: just a quick thought... family of mornsun A_S-2W & B_S-2W Series. one example A0505S-2W

author: David Hess
author_rank: Super Contributor
timestamp: August 08, 2014, 12:34:01 am
content: Quote from: fcb on August 07, 2014, 09:30:50 pmAn LM358 has a Vbe drop when sinking current and is not all that powerful so its voltage drop would be about 2 volts at 20 milliamps according to its specifications.An LM7321/LM7322 or LM7331/LM7332 (single/dual) look to be ideal because they are fast for high PSRR and have high output current and unlimited capacitive load drive. Linear Technology has some suitable models as well. The dual parts could be used to regulate both rails with a suitable reference. I might try to design a true bipolar tracking supply this way if it was advantageous.RiverTown mentioned using a single regulator and virtual ground which these could also do well although there may be better parts for that.I am still more inclined to using a low power isolation transformer with active regulation of the sine or square drive on the input side for inherently low noise but would have to consider the above despite having designed low noise switchers and charge pumps in the past.

author: f5r5e5d
author_rank: Frequent Contributor
timestamp: August 08, 2014, 05:09:41 am
content: switched C V converters seem attractive for simplicity but fall short pretty much everywhere in comparison with today's switching magnetics based optionswith magnetics you can have continuous conduction current, lower ripple currents, higher operating frequency that allows smaller size, easier filtering

author: fcb
author_rank: Super Contributor
timestamp: August 08, 2014, 11:09:33 am
content: I've reworked my analog electronics to remove the -ve requirement (no virtual earth rubbish here). Going to boost the 4xAAA's up to 7V and then linear regulators back down to 6V/5V as needed. Microcontroller/DAC is driven by an LDO straight from the batteries.And I'm going to use a little SOT23-5 boost converter, here's a copy of my notes and an explanation below:LM2623AMM (VSSOP/8) - pulse skippingMIC2288YD5 (SOT23-5) - 1.35MHz, fixed freq PWMLM2622MM-ADJ (VSSOP/8) - special mention as nice triple output app.TPS61085 - (TSSOP/8) - quasi constant freq., 6V max onlyLM27313XMF (SOT23-5) - 1.6MHz, fixed freq PWMAPE1911MP (HSOIC/8) - 0.5MHz, fixed freqLT1930 - 1.2MHz, fixed freq PWM, expensiveLT1930 / LM27313 / MIC2288 have the same pinout.Diode: MBR0520 or B0520Inductor: 4.7uH or 10uH (Wurth 744032100 or 74404024100) - 1210 package gives lots of optionsCaps: 1206 packageLayout: Follow LM27313 page 8 exampleI'm probably going to use the MIC2288 as it's a good mix of cost/availability, interestingly all the SOT23-5 regulators I came across seemed to have the same pin out and functions. I'm going with a fixed frequency converter and avoiding pulse skipping types - quite possible as my load is fairly constant (doesn't vary by more than 3:1 under worst case scenarios).

author: Precipice
author_rank: Frequent Contributor
timestamp: August 08, 2014, 11:30:37 am
content: Quote from: fcb on August 08, 2014, 11:09:33 amI saw, somewhere, a little dev board that took both SOT23-5 switchers and LDOs, since the pinouts are substantially similar. Can't remember where, possibly CERN?It looked like the sort of thing a chap should have a stash of, for prototyping these useful little chips without the hassle of spinning up a PCB or dead-bugging.In fact, having had a quick look, I'll do a proper design for this over the weekend, and get a panel of boards made7805 In-Gnd-Out pinout for plugging into a breadboard, and takes as many SOT23-5 switchers and linears (with associated Ls, Cs and setting resistors)It's about time I tried out panelising and V-scoring PCBs through Elecrow...

author: tszaboo
author_rank: Super Contributor
timestamp: August 08, 2014, 11:42:24 am
content: If you are still interested, I've succesfully used boost-invert converters which are made for displays to power my circuits. I dont know how much noise they would make, but it was powering my mixed signal part with less than -100 dB visible noise on the samples at moderate frequencies. I'm talking about ICs like LT1945, I've used a different on though. It didn't even needed linear regulator after, possibly because it works at several hundred kilohertz.

author: f5r5e5d
author_rank: Frequent Contributor
timestamp: August 08, 2014, 01:01:32 pm
content: in the supply I mentioned the cycle skipping did cause audible noise at low load - literally audible - either inductor core magnetostriction or piezo hi K caps - it was very low level high pitched sound but noticeable working on the open board on the bench after hours especially since it came and went with fooling with the circuit

author: fcb
author_rank: Super Contributor
timestamp: August 08, 2014, 01:10:17 pm
content: Yeah - I avoid PFM/cycle skipping supplies like the plague.In-fact I try and avoid PWM if possible in audio, it will be interesting to see how these little SOT23-5's work in my product.

author: Precipice
author_rank: Frequent Contributor
timestamp: August 10, 2014, 12:41:37 pm
content: Quote from: Precipice on August 08, 2014, 11:30:37 amWell, that was only mildly annoying. It takes a fair number of SOT23-5 LDOs and buck regulators, and fits where a TO220 would on a breadboard or bodge board.Topside is compulsory stuff (including capacitors in the feedback divider, to make some TI devices TI stable), and space for a tant, to give unstable LDOs something to drive, rather than a ceramic. Caps are 0805, inductor is 1210, which I think should work for switchers out to 600mA.Bottom side is optional stuff - a trimpot for voltage setting, an anti-cockup TVS and a resistor/zener to allow driving of enable pins that can't be driven by the supply voltage.Since I seem to have a load of boost and buck-boost chips hanging around, I think I'll knock out similar boards for them - something that can generate 3V3 or 5V from a lithium cell would be nice. Might have to rely on the battery for undervolt lockout though.Right, back to the work I've been trying to avoid!

author: fcb
author_rank: Super Contributor
timestamp: August 10, 2014, 06:04:55 pm
content: Nice little board, I wouldn't bother with the parts on the back though - mission creep, and requires a dbl sided load.I would be tempted to write that it is a "BOOST" somewhere on the unit (back side), and the specs for the boost, maybe make a little range of them for those of us too lazy to bother:a) BOOST 1-2cells to 3.3Vb) BUCK anything to 3.3Vc) BOOST 1-2cells to 5.0Vd) BOOST anything to 12V