Low current (~100ma) buck voltage converter simple, low cost options (48V to 12V - Page 1
Forum section: electronics / projects

author: dominicM
author_rank: Regular Contributor
timestamp: July 18, 2014, 07:51:26 pm
content: I have 48V AC-DC power supply that I use to drive high power LED drivers. I also want to use the same 48V input to power Arduino Pro Mini.I considered linear voltage regulators but with LM317 the efficiency would be 2.77%!I also considered voltage divider but wasn't able to get it to work. With high resistor values the voltage dropped drastically when load was applied. With lower resistor values they would dissipate around 3W with one configuration for example.Lastly switching power regulators aren't ideal because the cost is quite high for input voltages over 40V such as over 2 euros for a single texas instruments LM25019. Then there's also additional components like inductors that are out of my scope of knowledge... The total cost for all components would be about 7 euros which is massive considering cheap ebay AC-DC power supplies are only 2-3euros, surely DC-DC conversion should be simpler than AC-DC?Are there any other alternatives? What would you do in this case?P.S. Sorry in advance if this is should be posted in beginner section, it didn't seem entirely beginner level.

author: mazurov
author_rank: Frequent Contributor
timestamp: July 18, 2014, 08:37:54 pm
content: If input voltage variation is small (as it should be while driving LEDs) you don't need the whole DC-DC converter. Build a buck power stage, make it synchronous if you want efficiency, then add a square wave generator with settable duty cycle. I made one recently to drive 12V LED heatsink fans from 48V, works very well. It is not going to be the cheapest solution though, 2x60V MOSFETS and buck MOSFET driver will set you back ~$US8-10. Even auto grade monolithic controllers capable of delivering your current are still cheaper.

author: dominicM
author_rank: Regular Contributor
timestamp: July 18, 2014, 08:51:28 pm
content: Yes, the input variation should be minimal, though it does not matter as the 48V input is fed to constant current buck driver which in turn drives the led.The cost is the same as the switching supply so it's not a great option. It seems odd that is it so expensive for such a low current solution. Is there anything else that could work? Maybe with lower required power output (~50mA)? Efficiency is important up to a point. 20% efficiency would be just fine but 2.77% is just horrible...

author: mij59
author_rank: Frequent Contributor
timestamp: July 19, 2014, 05:41:02 am
content: Hi,First you need to calculate the total required power for the 5V supply, at this point your just guessing.Maybe you need other voltages to complete the project.You could consider a separate power supply.The LM317 is a adjustable voltage regulator, a voltage regulator with a fixed output would also do.The standard version of the LM317 has a max. voltage rating of 37V, so you would need the LM317HV version.Efficiency is not so important in this case, a 5V linear regulator with an output current of 50mA would dissipate 2,15W.

author: Whales
author_rank: Super Contributor
timestamp: July 19, 2014, 10:19:05 am
content: For the LEDs: you could try assuming the 'load' would be stable and go for:- 555 timer providing an adjustable duty-cycle square wave (using a potentiometer)- control a MOSFET switching the 48V using this signal- smooth output via a capacitorExtra complexities:- a 12V or similar linear regulator will be needed to power the timer/driving of the MOSFET (less than a few mA will be drawn)- LED electrical characteristics change as they heat up, so you will probably want to adjust your duty cycle to suit the LEDs when they are 'hot'.For the Arduino: a linear regulator might still be fine. Calculate the max power draw & work out how many watts will be lost across the regulator.

author: mariush
author_rank: Super Contributor
timestamp: July 19, 2014, 11:45:50 am
content: Ok, so look at it another way.You need 5v @ 100mA ... that's 5*0.1 = 0.5 watts. For safety, let's say you need 0.15A and therefore the power required to produce 5v@0.15A is 0.75w. A good switching regulator will typically have an efficiency above 80%, especially with so high input voltage / output voltage difference but even with 75% efficiency, you'd need about 1 watt of power from the input.Switching regulators that are capable of working directly from 48v are few and expensive, so a good idea would be to reduce this voltage so that it will be within a range that's supported by most switching regulators. That would be 3v --- 38-40v.Let's go with 35v, just to be on the safe side (in case your 48v DC power supply is not quite 48v).With a 35v input voltage, your switching regulator will pull about 1w / 35v = 0.02857 A or about 28.57 mA. Let's round it up to 30mA.So you need to find something that drops the 48v to 32v and let's 30mA go through. Well, V = I*R so (48-35) = 0.03* R therefore R = 433.33 ohm. Smaller value means less voltage drop but more power wasted in resistor. Larger value means higher voltage drop and higher current limit.433 ohm is not a standard value, so let's go either:* 1 x 390 ohm = 390 ohm* 330 ohm + 100 ohm in series = 430 ohm* 270 ohm + 150 ohm in series = 420 ohm* 2 x 820 ohm in parallel = 410 ohmFor 390 ohm, you'd have (48-35) / 390 = 13/390 = 0.033 A and the power dissipated in the resistor will be P = I*I*R = 0.43w so in theory a 0.5w resistor will be enough but in practice a 1w rated resistor will be safer.With higher value resistors, the power dissipated will be lower, but not by much, it will still be safer to use a 1w rated resistor.So now you can choose any switching regulator that works with around 35v at input... even a cheap MC34063 switchin regulator works with up to 40v, so it would be fine:http://www.digikey.com/product-detail/en/MC34063AP/296-17766-5-ND/717432(note that some only work with up to 36v or 38v, depends on manufacturer)You can use online calculators (http://www.nomad.ee/micros/mc34063a/orhttp://dics.voicecontrol.ro/tutorials/mc34063/for example) or the one on Sourceforge (http://sourceforge.net/projects/mc34063uc/) to figure out the inductors and diodes and their specs. Everything should cost you less than 2$.Now keep in mind that the switching regulator will not always use 30mA. If you don't use 5v @ 0.1A, the switching regulator will not use 30mA. The switching regulator doesn't act like a LED so you can't leave it at that.48v + ----- 390 ohm --------- regulator --- 48v -At 30mA , the regulator appears as a 35v / 0.03 = 1166 ohm resistance. You have 30.85mA going through the 390 ohm resistor, 12v drop and 371 mW dissipated. The 1166 ohm "resistance" will see 35.97v on it and waste about 1.11w.But at 10mA, the regulator appears like a resistance of about 4700 ohm .. 390 ohm resistor will see 9.45mA flowing through it and drop only 3.68v, and the regulator will see 44.32v which is more than the 40v maximum so that's not good.(going to continue this post, editing it now)

author: cosmos
author_rank: Regular Contributor
timestamp: July 19, 2014, 12:09:45 pm
content: Quote from: mariush on July 19, 2014, 11:45:50 amI like your approach, but just want to caution that using resistors to bring the voltage under 40V means the solution depends on the load being there, no load means 48V again and risk that poof goes the switching regulator.Might want top look at using Zener(s) instead?With a ~10V Zener you should be able to go closer to the max rating of the switcher, hence less losses in the preregulation too.

author: mariush
author_rank: Super Contributor
timestamp: July 19, 2014, 12:30:16 pm
content: cosmos, I agree, and I was editing the post as you wrote the message. I'll edit this message and add more info and a possible solution.Now keep in mind that the switching regulator will not always use 30mA, it's not a LED to suck as much power as the 390ohm resistor will allow it to suck.If you don't use 5v @ 0.1A, the switching regulator will not use 30mA.48v + ----- 390 ohm --------- regulator --- 48v -If your device uses 100mA at 5v (5v*0.1A = 0.5w), the regulator will use about 0.66w to produce this if it has that 75% efficiency.So your regulator will behave like a resistor of about 2600 ohm in series with that 390 ohm regulator:390ohm : 16.05mA flow, 6.26v drop , 100mW wastedregulator: 16.05mA flow,41.74vdrop , 670mW wastedWell that's not good, we wanted 35v.At 10mA on 5v (5v * 0.01A = 0.05w), the regulator will use about 0.066w to produce this with 75% efficiency, so your regulator will behave like a 34 kohm resistor:390ohm : 1.4mA flow, 0.54v drop, 0.76mW wastedregulator: 1.4mA flow, 47.46v drop , 66mW wastedMuch worse. Now your regulator is close to 48v.So what can you do... Well, if you can make sure the regulator will always have a minimum load, for example by connecting a power on led to it to waste about 10mA, you can then add another resistor in parallel with the regulator to make the overall "virtual resistance" change.Let's go with a 1800 ohm resistor in parallel with the regulator.Now at the minimum load of 10mA, your regulator uses 66.6mW the 1.8k resistor takes some of the load:390 ohm : 32.34mA flow, 9.1v drop , 212mW wasted1800 ohm : 21.61mA flow, 38.9v drop, 840mW wastedregulator : 1.73mA flow, 38.9v drop , 67mW wastedSo the regulator is happy with 39v on the input and outputs 5mA at 75% efficiency and overall you're wasting 1,119mW to do it, or an overall efficiency of 4.45% - hey, it's bad, but still better than a linear regulator.With 50mA on 5v, things change a bit. With the same 75% efficiency, the regulator will need 5v * 0.05 * 100/75 = 0.333W or 333mW to produce that, so we're looking at:390 ohm : 29.4 mA flow, 11.47v drop, 337.63 mW wasted1800 ohm : 20.29mA flow, 36.53v drop , 741.16 mW wastedregulator : 9.13mA flow, 36.53v drop, 333.52 mW wastedYou now have a reasonable 36.5v on the regulator and you used 1411mW to output 250mW so about 17.17% efficiencyWith 100mA on 5v, it's even better. Now you have 666mW wasted in the regulator and about 33v on regulator, but you're only using about 1850mW to do it (in the picture below, 675mW in regulator, 1862mW overall, so 675*100/1862 = 36.25% efficiency).So both resistors will have to be 1w rated. Of course, you can play with the values in that circuit simulator and get some compromise. But just two resistors and a minimum load (a led using 10mA for example) can be enough to allow you to use any cheap 20 cents switching regulator (excluding inductors and diodes which add to the cost)

author: SeanB
author_rank: Super Contributor
timestamp: July 19, 2014, 12:35:44 pm
content: There are a lot of telecoms regulators designed to run a MCU off phone line voltage, either isolated or not, and they are pretty rugged and often have a cheap development board available for free inside corded phones with a digital display.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: July 19, 2014, 07:28:50 pm
content: Quote from: dominicM on July 18, 2014, 07:51:26 pmFind your budget and shop for buck modules on eBay.Or look on eBay first and set your budget...http://www.ebay.com/itm/DC-DC-LM2596HV-Buck-Converter-5V-60V-to-1-25V-26V-Power-Module-48V-to-3V-5V-12V-/301216970372?pt=LH_DefaultDomain_0&hash=item4621ee3684http://www.ebay.com/itm/DC-TO-DC-Buck-Converter-Step-Down-Module-18-48v-to-5V-1-5A-12V-1-5A-dual-out-/261422288256?pt=LH_DefaultDomain_0&hash=item3cddfb8d80Naturally I'd try them out on a dummy load first...And make sure you never exceed the maximum input voltage under any and all conditions.

author: dominicM
author_rank: Regular Contributor
timestamp: July 19, 2014, 10:01:12 pm
content: Quote from: mij59 on July 19, 2014, 05:41:02 amFirst I need to clarify that it does not need to be 5V. The assumption you made is that since it's arduino it's 5V but Arduino Pro Mini has built in regulator that can take anywhere from 3.5V to 12V. Since one sensor needs at least 5V my new range is 5V to 12V. The arduino itself if 3.3V version and this is my primary voltage for most sensors/devices.The absolute maximum current draw by arduino is 150mA, this is limited by the internal voltage regulator among other things. Nominal (measured) amperage is ~63mA with minimal change except possibly at start-up/shutdown or another unknown (to me) event.LM317 maximum voltage is not 37V, in fact it has no specified max voltage. It has maximum voltage differential of 40V which means it would work with 48V input and 8V (or higher) output: 48-8=40VThe problem with low efficiency is that I would like to keep my device on 24/7. 2.15W wasted heat + power used by device would add up quite quickly over a month 24/7 especially considering I plan on having one of these in every room.

author: dominicM
author_rank: Regular Contributor
timestamp: July 19, 2014, 10:10:17 pm
content: Quote from: Alex Eisenhut on July 19, 2014, 07:28:50 pmAs my options look increasingly limited, I might have to resort to ebay cheaposWanted to avoid this as I would need to have 2 individual PCB's with cables running from 1 to the other and back, very messy...

author: dominicM
author_rank: Regular Contributor
timestamp: July 19, 2014, 10:13:53 pm
content: Quote from: Whales on July 19, 2014, 10:19:05 amWell, I am using LED drivers that take 48V as input and output constant current to the LED directly so I don't need anything else to drive the LED's. Potentiometer would be no good as well as everything will be controlled over wireless comms. I am not really sure what you mean by assuming the load?

author: mariush
author_rank: Super Contributor
timestamp: July 19, 2014, 10:29:12 pm
content: Quote1kwh = $0.10 - $0.30, even cheaper depending where you live.2w * 24 * 31 = 1488w = 1.488 kWh or less than half a dollar a month.Electricity costs is not a problem, heat is. With 48v in and 12v out and 100mA, you're going to dissipate 36v * 0.10 = 3.6w - without heatsink, the regulator will heat up about 55-60c/w so you'll need a heatsink that's going to be large.A switching regulator will not waste as much power as heat, but like I explained switching regulators that accept 40v+ are more expensive. It's cheaper to use a couple of 1w resistors to dissipate a couple of watts (in worst case) safely. At 60mA, if you check my post above, you'll have less than 1w wasted in the two resistors.And by the way, the Arduino Pro Mini uses a MIC5205 as a voltage regulator:http://www.micrel.com/_PDF/mic5205.pdfIf you check the datasheet, at page 3 you'll see that the regulator works at up to 16v - they probably say 12v so that stupid kids won't damage the chip with 16v or maybe because the copper pad on which the regulator sits may not be large enough to keep the chip cool.

author: dominicM
author_rank: Regular Contributor
timestamp: July 19, 2014, 11:46:32 pm
content: Quote from: mariush on July 19, 2014, 10:29:12 pmI am not in the US and the power cost could be as high as 0.35USD. Like you mention the heat dissipated is 3.6W + 100ma load. with 4 devices in the home (could be more) thats 3.7W * 24 * 31 = 2.752W * 0.35USD = ~3.4USD * 4 devices = 13.5USD. This is insanely high for something like this. It should be no more than 1USD a month (with some power saving tricks). The device is meant to reduce power consumption so it's not something I can live with long term.Will have to look into voltage dividers more then as it's even simpler than LM317 and more efficient like you say.It's a clone so it's probably a good idea to stay under the limits just in case. And I actually did blow one up with a badly regulated dc adapterCapacitor went, maybe it was just defective or it couldn't handle ~15.6 voltage spike at start-up...

author: dominicM
author_rank: Regular Contributor
timestamp: July 20, 2014, 01:10:24 am
content: @mariushThat took me a while to read and understandI think I will order MC34063. Not sure where you saw 20 cent regulators though, the cheapest I saw was 40 cents(euro)It's still far from ideal but I don't think there's a much better solution. Will use your resistor suggestion and will try to source 36V PSU (few table-top are available in 36V) in the future or even 24V with lower Wattage led might work. Will have to try it out and see how if it works.Also would using 12V instead of 5V make a significant difference or is it fairly close?Thanks a lot for such a detailed answer!

author: mariush
author_rank: Super Contributor
timestamp: July 20, 2014, 01:30:45 am
content: I'm in Romania, so I added the 24% VAT to the price listed on the sites and converted to dollars using google:21 cents each if you buy 25pcs :http://www.tme.eu/en/details/mc33063ad/voltage-regulators-dc-dc-circuits/texas-instruments/#(33063 is just a (better) version of 34063, both are made by lots of manufacturers)DIP version is a bit more 'expensive' at $0.38 if you buy 10 pcs :http://www.tme.eu/en/details/ka34063/voltage-regulators-dc-dc-circuits/fairchild-semiconductor/#RS-Components have them for 0.24 euro if you buy 40, a bit more if you buy smaller quantity:http://ro.rsdelivers.com/product/on-semiconductor/mc33063avdg/swregulator-15a-step-up-down-inv-soic8/7862401.aspxFarnell has them for about 40-50 us cents each if you buy 10pcs or more. You can get clones and no-name chinese versions of 34063 on ebay, probably 5-10 for $0.99. You can get car usb chargers that have 34063 in them for about 2-3$... they're cheap and simple to use, but have almost no protections so you can kill the chips easily.

author: dominicM
author_rank: Regular Contributor
timestamp: July 20, 2014, 01:37:17 am
content: Ah, yes that's quite cheap, oddly enough though ebay has them for 2 euro+ for one plus shipping...

author: dannyf
author_rank: Super Contributor
timestamp: July 20, 2014, 03:17:27 am
content: A resistor divider will most surely not work.you can have a 48v regulator. You can have two serial regulators , or a pre-regulator plus a regulator, ...

author: dominicM
author_rank: Regular Contributor
timestamp: July 20, 2014, 02:30:11 pm
content: Can you be more specific? Why wouldn't resistor divider not work?How would "serial regulator" work? Can you give an example of what pre-regulator I could use?

author: dominicM
author_rank: Regular Contributor
timestamp: July 20, 2014, 02:40:54 pm
content: @mariushI remembered that I attempted to use this regulator:http://www.mouser.com/catalog/specsheets/MPS_MP2456_r1.0.pdfIt's 50V and only 0.65EUR a@ MouserLooking back at the datasheet I am not sure why I gave up on it. What do you think about the device? Would it work and is there anything that might trip me up? It's not a popular component so there's very little info on it and the datasheet is not that great.

author: mikerj
author_rank: Super Contributor
timestamp: July 20, 2014, 08:35:32 pm
content: For a very cheap and cheerful switching regulator with high input voltage capability you could do worse than theRoman Blackswitcher.

author: dominicM
author_rank: Regular Contributor
timestamp: July 20, 2014, 08:53:08 pm
content: An interesting solution but if I can I will try to stick with something more common. I am confused enough

author: dannyf
author_rank: Super Contributor
timestamp: July 20, 2014, 09:25:26 pm
content: The output from a resistive divider is load variant. To minimize that, you have to run them at high current levels, which leads to inefficiency.linear regulators can also be configured as switching regulators so check the datasheet for examples.

author: dominicM
author_rank: Regular Contributor
timestamp: July 20, 2014, 09:37:56 pm
content: Saying it woun't work is misleading when it wclearly works just with low efficiency. Either way, efficiency is relatively good compared to linear regulator.I examined the LM317 datasheet with nothing resembling a switching regulator configuration. It's not very useful to say you can do x without any explanation. In any case I don't see why you would have switching regulators if linear regulators can work as switching regulators, it just doesn't make sense. Please explain what you mean.

author: mrpackethead
author_rank: Super Contributor
timestamp: July 20, 2014, 09:57:02 pm
content: Intresting thread, I was looking at the Datasheet for the 34063 Controller which could be a useful thing for many uses. In all the application notes the Input Cap is a 100uF. There was no mention of any calculations for it either.The 100uF seems very arbitrary, but also goes a wee bit in face of what i normally know to be true.I was contemplating a design using a 34063, to deliver 4.2V @ 500mA.http://www.ti.com/lit/an/slta055/slta055.pdf

author: dominicM
author_rank: Regular Contributor
timestamp: July 20, 2014, 10:58:57 pm
content: In my case unfortunately it wouldn't work due to 40V max input. The MP2456 I am considering is 50V but the datasheet is sparse and riddled with errors. Typical application diagram shows additional components not mentioned anywhere, it seems like electronics is a game of guessing sometimes

author: mikerj
author_rank: Super Contributor
timestamp: July 20, 2014, 11:12:50 pm
content: Quote from: dominicM on July 20, 2014, 09:37:56 pmRule of thumb: For the most interesting application notes, always check theNational Semiconductordatasheet for any given part.Also note that the voltage limit for an LM317 is the voltage dropped across the device, not the voltage to ground. You could e.g. use an LM317T to drop the voltage to a safe value for an MC34063 without getting anywhere near the maximum voltage rating. However, you have to make sure that under fault conditions that the output of the LM317 won't get dragged to ground, which would immediately exceed the maximum voltage spec.

author: mrpackethead
author_rank: Super Contributor
timestamp: July 20, 2014, 11:44:33 pm
content: Mmm, Deceide that this Controller isn't something i'm going to use.. Inductor and cap sizes render it uneconomic.

author: dominicM
author_rank: Regular Contributor
timestamp: July 21, 2014, 12:13:01 am
content: @mikerjThat's a useful rule to know. Coincidentally I was actually using national semiconductor datasheet. I was aware that LM317 has no max voltage as such as I mentioned in a previous post. I guess I could use lm317 as a pre-regulator though that would be even less efficient than resistor voltage divider solution though maybe simpler.I am attempting to make MP2456 regulator work. Datasheet is sparse and riddled with errors though.

author: metacollin
author_rank: Contributor
timestamp: July 21, 2014, 11:08:17 am
content: This option isn't the cheapest, but it's cheaper than those arguably suspicious ebay cheapies linked earlier. Try theLT3990. It's a lot like the MP2456, except not shitty. There is a reason the MP2456 is 70¢, because it sucks. Even the datasheet sucks. A lot of people will say this is wasting money, being inefficient, but if you are not accustomed to making switching dc-dc converters, buck or otherwise, then you should begin with the easier, albeit more expensive chips, because the chance of making an expensive mistake, or simply failing to get a working circuit, is much higher with cheap parts with crappy datashits (misspelling of datasheet intentional), and if you can't take the time to become very very confident and have a deep understanding of ac magnetics, noise, current mode vs. voltage mode regulation, continuous mode and discontinuous mode for the topology you are using or whats really inside and going in in a buck converter IC, the best way is to learn on the job, by cutting your teeth on chips that do a little hand holding. Eventually, you'll start to know what you're doing, if you have to build enough switchers. If you really only need to build one this one time, then DEFINITELY use a friendlier IC.And, while the LT3990 is $3.10 in quantity, it needs an inductor that you can get for probably 20 cents or less, tens of cents (or even cents if you up the frequency) of ceramic caps and trivial resistors. You do not need a diode, the LT3990 has them built in. It's definitely going to be cheaper and more reliable than those ebay converters. And the safety margin is great, I would not use a 50V part with 48V unless you also built in transient voltage suppression or other input protection, which of course adds to the bottom line.I am unclear if you are making a pcb or building a hand-made prototype first, but this part comes in both DFN leadless and MSOP-16. If you need hand-buildability, it is possible with an MSOP-16 (0.5mm pitch), you just have to believe in yourself. Well, and not fuck up too, but also believe in yourself. I use 0.5mm chips routinely in a personal, hobbyist,"using perfboards with a bunch of .1" spaced holes and pads" kind of prototyping environment. If you aren't spinning a PCB, or are but want to prototype it at home first, an MSOP16 will require a breakout board. They don't have any on ebay that I can find in that parts lead pitch (0.5mm), except for a large tssop48 0.5mm, which will work, but you gotta ghettofab a pad for the ground pad on the bottom of the chip. I either drill a small hole in the breakout and solder to it that way, or slip a small bit of the tinned ribbon wire stuff, used to connect solar cells in series. Usually chips with that extra ground pad will NOT be pleased if you try, for testing non-production purposes only of course, to use the chip with the pad floating.Obviously, that is sort of a desperate measures method. You should just prototype a proper PCB, but on the other hand, sometimes you need to make sure something works at all before spinning a board and waiting half a month+ for something that never would have worked to begin with. Just depends on your situation. I tend to favor the "make it now by hand and make it work, then you'll have something robust and can improve on it significantly when you do a proper board layout and send it off" philosophy. Though, I seem to be in a small minority. But, if having a pcb made is expensive due to imports or will take ages to arrive, it is 100% possible to use this teeny chip entirely by hand.I attached a screenie of the circuit I mentioned from the data sheet, hopefully it will give you an idea if its suitable or not as simple as you'd like right off the bat, but definitely read the datasheet if it looks useful. Particularly, you can get away with smaller input/output caps and a lower value inductor by ramping up the frequency to 2MHz (the switching frequency is set with a single resistor RT, 51K = 2MHz, couldn't be easier!) and if you are fine with reduced output current (which you are, since you want 100mA out, and it can provide 350mA), you can even reduce the inductor even more, the LT3990 has no problem switching to discontinuous mode. Discontinuous mode will drop your maximum output current to 140mA, which is perfect for you, has that 100mA you need, plus some wiggle room. You can, for 48V input, safely use a 10µH inductor at 2MHz, drop the output cap to 10µF if you are ok with 10mV ripple at worst (the LED controller won't give a crap about 10mV), and 2.2µF input is ideal, but if you don't mind a small reduction in transient response, the 1µF will still do fine. This is all according to the datasheet, not what me, random stranger dude on the interwebs, says should be ok. Ok, and make sure the inductor can handle 2MHz, i.e. is ferrite cored. If it isn't, you'll be building a really bad buck converter but a terrific induction heater with the inductor's core as the work being heated. Which is fine, as long as what you're building is a tiny forge for tiny blacksmiths, rather than a buck converter. Protip: I have yet to find any blacksmiths that wee.There are fixed output versions, so if you get it, make sure it's the adjustable output version. The part you want, for MSOP, is LT3990EMSE#PBF. Or if you're down with DFN leadless packages and not prototyping and just gonna reflow this bitch, the part is LT3990EDD#PBF and 10 cents cheaper.Additional information if prototyping without a board is necessary:I can personally attest, as I have used this very chip in this very, uh, 'less than ideal' way, and breaking out the leads to copper traces then down to .1" pins, THEN to copper pads in the perfboard and the external components, is ok and does not appreciably impact the chips performance. It's not ideal, and if you brought it to the FCC they'd hold you down and shit directly into your mouth, but if you are worried about EMI/FCC/CE/Whatever, you aren't going to be using a breakout board soldered into a perfboard anyway. So it is possible to prototype with it in the ghettoest of fashion. Oh, by Perfboard, I do not mean a breadboard though. No breadboard for switch mode regulators. Every time someone tries to make a switcher on a breadboard, 100 kittens start to cry.

author: richard.cs
author_rank: Super Contributor
timestamp: July 21, 2014, 01:34:27 pm
content: Here's a rather nasty and off-the-wall suggestion.Is your LED driver on all the time and what current do you drive your "High power LEDs" at? If it's a typical series string at a few hundred mA and 35V or so driven by your constant current LED driver you could put your arduino in series at the bottom of the chain with a shunt regulator across it. If your LED chain were 300 mA and the arduino drew 100 mA then the shunt regulator would dissipate (0.3-0.1)*5 = 2W. This is comparable to the other linear regulator case but doesn't need high voltage parts.There are a number of downsides to this approach but if your LED current isn't that much higher than your arduino demands (just at higher voltage) then it may be a better option. There are other ways you could use the LED driver to power the arduino if your LED voltage/current is greatly different, particularly if you have a high-current low-voltage LED array. All these approaches require the LEDs to be powered for the arduino to work.

author: dominicM
author_rank: Regular Contributor
timestamp: July 21, 2014, 03:02:00 pm
content: Quote from: richard.cs on July 21, 2014, 01:34:27 pmI was waiting for someone to make this suggestion but unfortunately the LED is 100W 1050mA and not powered most of the time.

author: dominicM
author_rank: Regular Contributor
timestamp: July 21, 2014, 03:20:20 pm
content: Quote from: metacollin on July 21, 2014, 11:08:17 amThat was entertaining! I agree with most of what you said and yes I was planning to hand solder the parts suspended in the air to make sure it works before designing the pcb (new to CAD also). As far as LT3990 it seems it is not available at mouser and I am not in the US so it's difficult to source less common parts due to shipping cost from US (from more than 1 retailer at a time). It's also expensive like you mentioned and more commonly available maxim or TI chips are about the same so I would probably go with them if cost was irrelevant.Can you elaborate about why MP2456 is "shitty" in particular? The datasheet is terrible but once you get the chip to work that is not much of an issue. The thing is that the arduino has it's own voltage regulator so the quality of power is not as important as if it was powered directly. Only motion detector will be powered from non-arduino regulated voltage but it has wide voltage tolerance (6-20V).I will post a schematic I made and maybe you can give me feedback and clarify a couple of thing. In the end I think I will try to get the cheaper option to work but maybe get a spare higher end chip in case this doesn't work. /delivery is after all the most expensive part so might as well order parts for a fall back option.

author: dominicM
author_rank: Regular Contributor
timestamp: July 21, 2014, 03:36:21 pm
content: Here is the schematic I made with a couple of things unfinished that I wasn't clear about. Maybe someone can clarify.Does EN pin even needs to be connected for the chip to operate? The datasheet shows 500k resistor between 48V input and EN. This reduces the current below 100uA as the chip could be damaged over that value. That I think I understand but what is the resistor from EN to GND for? It also has "NG" as value so I am confused about that.What voltage rating is needed for capacitor between BST and SW? I would guess it should be 16V+ (for 12V output).Using the formulas provided in the datasheet I calculated the inductor value as 166.6. This is very different from the typical application diagram which shows 33uH for 12V output. The inductance value from calculations seems to get lower with higher Vin but still not as low as 33uH at 48Vin. Is the datasheet plain wrong? inductor value can be lower? or my calculations are wrong?The zener diode is optional so I excluded it. There are some additional components that seem useless to me like the extra 0.1uf cap in series with 4.7uF cap at 48V input.

author: mariush
author_rank: Super Contributor
timestamp: July 21, 2014, 03:49:13 pm
content: 1. Get a LM317 with high voltage option (capable of 60v on input) :LM317AHVT :http://uk.farnell.com/fairchild-semiconductor/lm317ahvt/ic-ldo-volt-reg-57v-1-5a-to-220/dp/1564497LM317HVT :http://uk.farnell.com/texas-instruments/lm317hvt-nopb/ic-v-reg-linear-adj/dp/1469086Use this to go down from 48v to about 28v (this way you'll be able to use regulators with max voltage from 30v to 40v)2. Get a MC33063 or MC34063 or something a bit more expensive but still easily available such asap5004orlm2576The switching regulators can produce 7v @ 100mA (low but still high enough for the internal arduino regulator to work fine) with about 75% efficiency. That means to produce 7v @ 100mA = 700mW, the switching regulator will use about 900mW to 1w.If switching regulator gets 28v at input from LM317(hv) that means the current output of lm317hv will only be about 1w/28v = ~ 36mA.So the linear regulator will dissipate (48v-28v) * 0.035 = 0.714w which is manageable with a tiny heatsink.48v - lm317hv - mc34063/33063 -> 7vYes, you can output higher voltage to arduino, but then the linear regulator on the arduino will just waste more power as heat so why do it.-Again, if you can, try to output a lower voltage with your switching regulator. No sense in making that arduino regulator heat up by dropping that 12v - 5v into the pcb heatsink.The ceramic capacitors should be of X5R or X7R or C0G/NPO , nothing else. Voltage rating 35v or higher, preferrably 50v. Even if you have 5v-12v in the circuit, use 35-50v or better. Ceramic capacitors are derated with voltage.The output capacitor could be a bit bigger. 22uF 16v will probably only work if it's a tantalum capacitor. Electrolytic capacitors will probably have too high esr and ceramic capacitors would be very expensive at 22uF (not to mention 22uF 16v ceramic won't be 22uF with 12v on it, it will be maybe 8-10uF).Use 100-220uF 25v electrolytic maybe if you want 12v on output. If you get it down to 7-8v, you could use a 330uF 10v or 16v electrolytic.The inductor.. it depends on switching regulator. Can't say without going into datasheet. Pick inductor with suitable peak current, see what datasheet says about resistance and so on. 160uH is not surprising considering the high Vin-Vout difference and low output current.

author: dominicM
author_rank: Regular Contributor
timestamp: July 21, 2014, 04:28:56 pm
content: Quote from: mariush on July 21, 2014, 03:49:13 pm1. I don't actually need high voltage version, standard LM317 is well within 40V voltage differential to take 48V to ~28V.2. I will do that if using MP2456 doesn't work out.3. You are right I will probably end up using 5V or maybe 6V though arduino regulator never seems to get even warm and the power usage isn't affected noticeably.Well, the datasheet suggests using 4.7uF/50V cap for input and 22uF/16V for output, doesn't this mean the derating is already calculated in this value? Arduino uses 16V caps as well so maybe you are being extremely cautious as opposed to the normal amount of cautious?All caps in my schematic are ceramic as suggested in the datasheet. It does say ceramic because of low ESR as you mentioned. 22uF ceramic caps are indeed pricey at 0.45euro. tantalums are half that. What are the differences between tantalum and ceramic in this scenarion? Can I safely just swap same value tantalum cap for ceramic cap?The datasheet shows 33uH in typycal application diagram at 12V output that's why I have doubts. It also says at low current <100ma higher inductor can improve efficiency, what if I go with higher inductance value like 200uH or even 330uH (have a couple of those)?P.S. I realise now that you were probably talking about C3 cap in the third paragraph from the bottom. I am just not sure what voltage can flow between BST and SW pins. No sense in adding 50V cap for 12V or 5V voltages.

author: tom66
author_rank: Super Contributor
timestamp: July 21, 2014, 04:38:02 pm
content: It's important to note with an LM317, if you have capacitors to ground on the output, you can easily exceed the maximum differential voltage when the capacitor is discharged. Also possibly when the regulator output is shorted.

author: mariush
author_rank: Super Contributor
timestamp: July 21, 2014, 05:34:35 pm
content: QuoteQuoteConclusion: instead of just guessing or risking, just go with it and use a proper regulator rated for more than 48v. hence the lm317hv.QuoteYou will have to go with the 5v + whatever minimum dropout voltage that noname linear regulator has at 100mA. The original micrel chip may have only 1v, the noname one may be 1.5-2v.Besides this minimum voltage, your switching regulator will also have some ripple, maybe 50mV, maybe 100mV. To be on the safe side, just set the regulator to about 7v.QuoteI assume the datasheet is this one:http://www.monolithicpower.com/Page/DownLoad.aspx?ListID=149b7d2b-0bb2-4ba6-a219-a0495acb8b0b&&ItemID=90Pay attention to the language.Use ceramic capacitors with X5R or X7R dielectrics for their low ESRs and small temperature coefficients.For most applications, a 4.7?F capacitor will sufficient.Your application doesn't fit the "most" criteria - most applications will be something like 12v to 1.8v, 5v to 3.3v, 12-24v to 5v etc etc. You are close to the extreme with 48v at the input.X5R and X7R capacitors have to be derated as the voltage on them goes up. Just because a ceramic capacitor is rated for 50v, it doesn't mean the capacitance will be that value at 50v. Dave did a video about this here:https://www.eevblog.com/forum/blog/eevblog-626-ceramic-capacitor-voltage-dependency-download/Let's take for example this Murata 1206 4.7uF 50v ceramic capacitor in 1206 package:http://uk.farnell.com/murata/grm319r61h475ka12d/cap-mlcc-x5r-4-7uf-50v-1206/dp/1845761RLYou download SimSurfing from Murata, you look up the part number for that capacitor and you find this:Even at 10v, the ceramic capacitor already has only 2uF of capacitance! At 40v, the capacitor is only 0.7-0.8uF.Except NPO/C0G types which are expensive or tantalum capacitors, ceramic capacitors are derated at voltages close to their maximum rating.Electrolytics are capable of holding capacitance up to the rated voltage, but they have other flaws, especially when used in high frequency circuits.So 4.7uF 50v ceramic is not good when you have 48v at input. Maybe placing 4 x 4.7-10uF 100v ceramics in parallel at the input will work, but even that is iffy.Now look again in datasheet:The output capacitor impedance should be low at the switching frequency. Use ceramic capacitors with X5R or X7R dielectrics for their low ESR characteristics. For most applications, a 22uF ceramic capacitor will sufficient.Again, for most applications. Doesn't mean yours fits the most. Same story with derating with voltage. Can't use a 22uF 16v ceramic when your output is 9-12v, you won't have 22uF.Here's the 22uF 25v in 0805 from Murata and how the specs look in SimSurfing:At 7v, your 22uF ceramic is actually a 7uF capacitor. They don't say the voltage rating for the ceramics for a good reason, because you have to look up this for your particular capacitor choice.For a "most" application, for example something that converts 5v to 1.8v, this 22uF 25v ceramic will work well at the output. Won't work well with 10-12v on output.Electrolytics may not work well due to the switching frequency, but you can try with a capacitor that's maybe 220-330uF and rated for 25-35v, just to have a reasonably low esr.Tantalum capacitors don't have this problem of being derated with the voltage, but they are not bipolar (they have a plus and minus like electrolytics) and they really hate overvoltage and very rarely they just like to explode with flames, worse than electrolytic capacitors.PS. You don't need a ceramic or tantalum capacitor at the input, you could do with a 10uF 63v electrolytic or something like that. But on the output that pretty much has to be ceramic or tantalum. C3 is ceramic but like i said, it's recommended to use 35-50v rated x5r/x7r/npops2. all these problems are because this regulator works at high frequency, 1.2 mhz. This allows small inductors, small ceramic capacitors etc ...but as you can see, doesn't work well with high voltages.A mc33063 or mc34063 works at 60kHz to 120-150kHz and at these frequencies you can still use electrolytic capacitors but as the freq. is lower, the inductors are bigger and so on.

author: metacollin
author_rank: Contributor
timestamp: July 21, 2014, 07:07:44 pm
content: Sorry, I was being much to strong calling the MP2456 shitty, especially since you got it working. I generally have a strong bias against any part with a poor or worse, actually incorrect, datasheet. Just to prove I'm not some Linear Technology fanboy, Linear themselves has a number of bad datasheets, and at least one with a pretty significant mistake. I generally refer to the infamous LT3080 as shitty as well for the same reason, but it's a perfectly good part for most applications.I actually retract my statement, and as you have a the MP2546 working, and if you're sure 4V on the input is enough headroom (52V max), that's fine. I've realized I kind of jumped the gun and was using my own biases toward board space and most stuff I've worked on has required efficiency be maintained at a wide range of load currents. The LTC3990 blows the MP2456 away on board space, at 2.2MHz, you can make the largest area eater, the inductor, very small indeed. Coupled with that 3x3mm DFN and managing over 60% efficiency with a100µAload is, to me, damned impressive.But none of that has anything to do with what you need and your application. I got a little carried away, hehe. Sorry. Plus its just not worth the expense in your case. And, in case someone happens upon this thread some time in the future and were looking for information: apparently Mouser doesn't carry Linear Technology products at all. All the others do, Digi-key, Arrow, Farnell, Newark...but not mouser. WTF r u doin' Linear. Mouser is probably who I use the most.Anyway, I kind of disagree with my own solution now that I have grown older and wiser (I've aged 4-5 hours at least!), and say you should ignore my younger self from some hours ago and go with the MP2456.

author: richard.cs
author_rank: Super Contributor
timestamp: July 21, 2014, 09:01:20 pm
content: Another thing you could consider would be a simple discrete switcher like these:http://www.romanblack.com/smps/smps.htmFairly low parts count and it's possible to do wide voltage ratios.

author: dominicM
author_rank: Regular Contributor
timestamp: July 21, 2014, 09:58:16 pm
content: Quote from: richard.cs on July 21, 2014, 09:01:20 pmSomeone else beat you to the punch earlier

author: dominicM
author_rank: Regular Contributor
timestamp: July 21, 2014, 10:05:06 pm
content: Quote from: metacollin on July 21, 2014, 07:07:44 pmI don't actually have it working yet, I was just saying if you can get it to work the datasheet become somewhat irrelevant. I havent yet even ordered the partsIn my case a modest increase in efficiency does not outweigh the increased cost.I am not really sure about much in this area but the power supply I use seems stable and never went above 48.1V, it's a bit close but it should work.

author: dominicM
author_rank: Regular Contributor
timestamp: July 21, 2014, 10:28:00 pm
content: Quote from: mariush on July 21, 2014, 05:34:35 pmThat's a lot to considerOne thing I would disagree about is the LM317. It's not guessing, it has no max voltage as such. What it has is a max voltage differential so 48 - 12 = 36. that to me is a reasonable margin especially for a relatively simple part. In any case I probably woun't use it anyways.I am quite confident that arduino clone can handle 5V input. Will test later with 5V input on arduino but for now I will go with 5V as output so let's assume 5V output is fine for my purposes.The datasheet actually does show the voltage ratings of input and output capacitors. refer to: "Figure 5: 12V Output Typical Application Circuit". That's where I got the voltage valuesWith that in mind what would you recommend in this case?

author: David_AVD
author_rank: Super Contributor
timestamp: July 21, 2014, 10:38:49 pm
content: Quote from: dominicM on July 21, 2014, 10:28:00 pmRelying on Vin - Vout being under the max voltage rating may not be the best idea. At the instant the circuit is powered up, Vout is 0V so your 48V Vin will see the spec violated. Others have mentioned this but you don't seem to want to hear it.

author: mariush
author_rank: Super Contributor
timestamp: July 21, 2014, 11:03:40 pm
content: Quote from: dominicM on July 21, 2014, 10:28:00 pmWell, feel free to come back and say i (and a few others) told you so when it blows up.QuoteLooking at the Arduino website, it seems the boards have a "RAW" hole/header, where you can plug/solder a wire and anything will go through the onboard linear regulator, and a Vcc pin that accepts pre-regulated voltage (5v for example). So it's up to you what you use.QuoteDoes it also say CERAMIC? Could be tantalum. They don't mention it so I have no way of knowing if they actually considered that voltage issue with ceramic capacitors - maybe just 4.7uF or 10uF is enough on output but they said 22uF just to consider the derating.. who knows.Anyway, the layout and how you solder everything will be much more important. This regulator can't be done on a breadboard, it has to be soldered on a pcb, with the copper traces as recommended on page 12 if you want it to work right.

author: dominicM
author_rank: Regular Contributor
timestamp: July 21, 2014, 11:11:49 pm
content: Quote from: David_AVD on July 21, 2014, 10:38:49 pmThat wasn't specifically mentioned like that but it does make sense. That being said it doesn't seem correct in practice since I have seen examples that use voltage of 60V. Maybe it's just robust enough to handle it, I woun't pretend to know how it works exactly...

author: mariush
author_rank: Super Contributor
timestamp: July 21, 2014, 11:14:54 pm
content: Thehighvoltage versions of LM317 work with up to 60v. I linked to them on the previous page (a few posts above in this thread).

author: dominicM
author_rank: Regular Contributor
timestamp: July 22, 2014, 02:29:04 am
content: Quote from: mariush on July 21, 2014, 11:03:40 pmThe RAW pin is what I was using. My arduino is 3.3V version so 5V input should leave enough headroom.You are right it could be tantalum though it's most likely ceramic since there is no mention of tantalum at all. That's all assuming the datasheet is to be trusted of course... I any case even if my capacitor values are off due to derating (like you mentioned before) how would it affect the power output and how badly? Also are there a list of steps and calculation that I could use to pick a capacitor without a lot of experience or guesswork?I was planning to just solder parts directly to the pins (no wires where possible) suspended in air. This is just to test and confirm it works before pcb... It's ugly but I think should be possible.