Switching step-down, low quiescent current? - Page 1
Forum section: electronics / beginners

author: tld
author_rank: Regular Contributor
timestamp: October 16, 2012, 09:42:27 am
content: Hi,I'm working on a battery-powered (12V SLA, no car) design, and I'm having some trouble finding the right part(s) for a power-supply. This is intended to run for a long time without battery-replacement. If possible, 6-9 months would be nice. The battery should be charged every 6 months anyway, to keep it happy, and 3 months margin wouldn't hurt. Nominal voltage for the project is 3.3V, though some sections are flexible.Aside from the power-supply, there's two sections of the design. One section will always be on, and is based around an attiny13a, for it's low power requirements. The attiny13a itself has a power-draw in the uA-range if done right, but realistically I wouldn't be surprised if I end up with 2-3mA for this section.The other section is featuring an atmega328p and an xbee series 2 pro as the most power-hungy parts. This section should be on for about 1% of the time, drawing it's maximum (about 350mA I think) for <0.01% of the time. Biggest power-draw is xbee when it's transmitting, but that's a fairly fast operation, and only happens a handful of times a minute. Wakeup for the section is twice a minute.My problem isn't that I can't find a solution to this, my problem is that I find so many ways that I could solve it, and I'm too fresh at this for my gut-feeling to be well-developed.Easiest would be to conclude that I'm not drawing that much current anyway, just drop in a linear regulator, and be done with it, but where's the edutainment in that?Other ideas I've come up with would be to use a switchmode regulator, have the attiny-section turn it on before I need the atmega/xbee-section, let a cap charge along with that, power down the switchmode, and let the attiny-section simply be powered from that cap. The attiny-section can live fine off of 2V, so this should be possible.Since the voltage drop over a linear would be from 12V to 3.3V, and at 350mA, I think switchmode-losses and losses from cap-selfdischarge would probably be less, making this an improvement over a pure linear reg.I could run a dual-regulator setup, with a low-quiescent low-current regulator powering the attiny-section, only waking up a switchmode regulator for the hungrier section.I could use a transformer, possibly followed by an LDO, to perhaps gain some efficiency over a pure linear reg.I could use a tps62122, which *almost* fits perfectly, offering 75mA and only 10uA in low-power mode. Had the power-hungrier section been satisfied with 75mA, my search would probably have stopped here. I could perhaps use it to charge a cap though, giving a large enough buffer to allow the power-hungry section to do it's thing.Another option would be a max5033, allowing for up to 500mA, but with quiescent current up to 350uA.Right now, I think the best bet might be one of these frou:* tps62122, with cap-buffer for power-hungry section* max5033 only* max5033, running only part-time, with attiny-section running off of a cap the remainder of the time* max5033 for power-hungry section (only on when needed), ultra low quiescent linear regulator for attinyPerhaps the best would be to build and evaluate all four.I'd like to check with you guys though, if anyone happens to know of a part similar to tps62122, but allowing for higher currents, or have any other advice or input.I have a vague suspicion I might not be the first guy ever faced with a problem like this.Oh, and my skills are limited (but developing). I can do soic-parts and similar, but would like to get by without qfn, thermally enhanced msop, and the like.Hoping someone can poke or prod me in the right general direction.- tld

author: Rerouter
author_rank: Super Contributor
timestamp: October 16, 2012, 10:14:15 am
content: i would reccomend your always on segment trigger the switchers enable pin method, as for the actual supply for the attiny, you would want to watch out on the minimum load, as most methods dont like under 1mA,you can use a low voltage op amp driving a npn which pulls down a signal pnp if you want a linear supply with stupidly low leakage, just put a 1M or greater resistance across the pnp emmitor and base,(the goal is that the leakage current [generaly a nA or so] does not generate more than 0.3V across it) which means you have only the pnp leakage as you minimum load, and the npn's leakage going down the drain to ground, (some can be down in pA) this means its only those leakages and the op amps quiesient current being wastedequally a deep cycle lead acid or better would be required for that kind of treatment, what your chasing is self discharge per month, considering you want the rig to survive 9 months, you need less than 11%/month minimum, ideally closer to 5.5%, (yes your rig can run at far lower voltages but the battery will be in a bad way)hope it helps

author: ejeffrey
author_rank: Super Contributor
timestamp: October 16, 2012, 10:33:33 am
content: Is the battery fixed? A bank of low-discharge NiMH batteries might also be a good solution. They will tolerate 50%+ discharge better than the SLA and have lower self discharge rates for more usable capacity. Then you could just choose the number you need in series to get the voltage you want instead of the 12 V operating voltage. 3 cells in series gives you 3.6 volts. I think the attiny will run off that directly, but if not you can use a micropower LDO. Power the high power section directly off the unregulated battery voltage or with a separate high power regulator, but use a MOSFET controlled by the attiny to switch it on and off. Depending on how it works, you can do low side switching with just a FET (high power section floats at battery voltage when off), high side switching with a PFET, or high side switching with an NFET and a charge pump to drive the gate voltage.

author: peter.mitchell
author_rank: Super Contributor
timestamp: October 16, 2012, 10:43:59 am
content: I don't know what capacity battery you'd be using but even in the really small capacities, lets say 7ah 12v (pretty standard), in perfect conditions (no self discharge or anything) thats like 30Mah@12v a day for 7.5 months.. That's a fair amount of juice.What kind of application is this in?

author: Rerouter
author_rank: Super Contributor
timestamp: October 16, 2012, 11:09:41 am
content: from the sound of it its a remotedata logger, the kind you leave out in the middle of no where, but as it only has a range of a few hundred or so meters so i would think its a drive by one, waking up every 2 minutes to see if a reciever is near, and transmitting if it sees one,as for battery capacity, going off his assumptions, 9 months is roughly 6500 hours,his average current draw because of such small percentages still comes within 2.5mA max,so 0.0025A * 6500H comes to 16.25AH capacity, before self discharge (sure you got your math right?)with a self discharge of lets say 5% per month, he needs to add 16.25*9*0.05 = 7.31AH of capacity, or about 23.6AH for a 100% dicharge over 9 months, or a 66% discharge over 6,still not quite ideal, lets aim at 50% again for 6 months, it would be (23.6/5*6.6) about 31.5AH capacity, ]as for the actual switcher, i say one with an enable pin only because most of them have a very low current sleep state when not enabled,

author: peter.mitchell
author_rank: Super Contributor
timestamp: October 16, 2012, 02:53:09 pm
content: Yeah, i probably goofed up the math somewhere. On a side note, if it is a remote datalogger, could you attach some kind of solar to it? I mean, if you're going to have it accessed remotely You'd have the antenna mounted on something already...31.5Ah... that's about the size of a small car battery... you could probably put some kinda solar for the price difference between a single 7Ah + solar and a much larger battery and no solar.

author: tld
author_rank: Regular Contributor
timestamp: October 16, 2012, 03:29:21 pm
content: Quote from: Rerouter on October 16, 2012, 11:09:41 amI'll go over and give a more complete answer in a bit (need to make dinner for kid), but just wanted to get a quick reply in about the math.First of all, SLA-ratings are done at higher current-draws. At these low loads, it's not impossible that I'd get close to those 16.25Ah out of a 7Ah-rated battery, and still not reach dangerously low voltage. I should certainly get an actual 7Ah out of it, staying within safe levels (assuming low-selfdischarge deep cycle SLA at least).Let's (for the sake of argument) say that I end up with an average current-draw (both constant on and high-draw sides) of 3mA @ 3.3V.And let's take the swichmode regulator for high-draw side, cap for constant-on side design.If I'd get 83% efficiency, after accounting for both regulator-losses, and cap self-discharge, that 3mA @ 3.3V should give an average draw of 3/(12.5/3.3V)*0.83 = about 1mA average draw from the SLA. Using the example above, 0.001A * 6500H ~= 7Ah, fairly typical size for a cheap SLA.Okay, so I might come out with a slightly higher average power-draw, or slightly lower efficiency.Worstcase; if all else fails, double up... If I put two 7Ah-rated batteries in parallel, I should be able to get about 20Ah from them, given the low load.Seems do-able at least.(and yes, this is all ever so slightly simplified... I know SLAs are usually not happy spending too much time at any significant discharge levels and so on, but I'm digressing from the original point/question, which was more how I could combine regulator(s) and possibly other things (like transformers and/or caps), to reduce draw on them).Thanks for all the good input guys.