DC-DC Modules: What am I missing here?!? - Page 1
Forum section: electronics / projects

author: IDEngineer
author_rank: Super Contributor
timestamp: October 02, 2017, 03:35:53 pm
content: We're working on a project that requires part of its circuitry to be galvanically isolated. Easy answer: A DC-DC isolation module as made by Murata, Recom, TI, CUI, and many others. They come in various flavors of input voltages (3.3, 5, 12, 15, and 24 are common) and output voltages (same choices) and you can often have mismatched input and output voltages so you can obtain different supply rails than your existing power system. All well and good....except for two things. First, these devices are completely unregulated. I'm not talking about "they're unregulated until you pull some minimum current", I'm talkingCOMPLETELYunregulated. The curves in the spec sheets show a basically linear relationship between output current and output voltage - the very definition of an unregulated source. OK, that sucks, but at least if you know that going in you can add a downstream voltage regulator....except for the second thing: Those output voltage options I mentioned above! If you have an unregulated supply, you're likely to use a voltage regulator. Voltage regulators need some overhead to operate. Even LDO regulators need almost a volt, and traditional regulators need 1.5-2V to be safe. But instead of rating these DC-DC modules with that in mind (say, 7V output voltage so you can safely run a 5V regulator), their nominal output voltages are right at the traditional rails of 3.3, 5, 12, etc. Leaving no headroom to run a regulator.If they wanted to produce a supply device at traditional voltages, they could have included actual regulation. But they didn't. Conversely, if they wanted to keep them simple and not have regulation, fine - they could have increased the nominal output voltages to make it easier to use a downstream regulator. But they didn't. Instead, they took the worst of both worlds - the least optimal of every option - and built that.Getting back to the title: I must be the one missing something here, because virtually all of the DC-DC module manufacturers build them this way. So what is the typical way to use these? I've had precious few projects where the supply rails could just vary unpredictably all over the place with no ill effects. At the moment our plan is to go to the next highest secondary voltage (12V) and use a downstream 5V regulator, but that dissipates 7V across the regulator and dramatically reduces the output current rating for a given DC-DC module package size and cost. And it could all have been so easily avoided.What am I missing here?

author: GreggD
author_rank: Regular Contributor
timestamp: October 02, 2017, 03:58:30 pm
content: I have used many of these. First off they require at least a 10% minimum load. This keeps them closer to there specified output voltage but they can malfunction without that load. There output, say 5V is close enough to drive RS485 ics "75176".

author: 0xdeadbeef
author_rank: Super Contributor
timestamp: October 02, 2017, 04:13:48 pm
content: There are actually also types with regulation. Still, most need 10% load and might create twice the specified voltage or so when no valid load is connected.When I used them and wanted to avoid this, I added some shunt regulator to draw a bit of current until the voltage was somewhat in spec:

author: IDEngineer
author_rank: Super Contributor
timestamp: October 02, 2017, 04:15:52 pm
content: Quote from: GreggD on October 02, 2017, 03:58:30 pm"Closer to their specified output voltage" isexactlythe problem.QuoteOK, yes, that application probably doesn't care about voltage regulation. But most projects sort of rely on stable supply rails. I guess these modules were not intended for such applications, though I'd think they'd be intentionally designing themselves OUT of the majority of possible design wins. Weird.

author: IDEngineer
author_rank: Super Contributor
timestamp: October 02, 2017, 04:18:01 pm
content: Quote from: 0xdeadbeef on October 02, 2017, 04:13:48 pm"somewhat in spec"Again, I have no problem with them being unregulated - IF they'd chosen voltages that left headroom for a separate regulator. It's like they suboptimized them on purpose.

author: lem_ix
author_rank: Regular Contributor
timestamp: October 02, 2017, 05:39:01 pm
content: Depending on your requirements this might be relevant. Allows for more flexibility on your design.http://www.ti.com/product/sn6501-q1Probs similar solutions from other manufacturers.

author: H.O
author_rank: Frequent Contributor
timestamp: October 02, 2017, 06:09:10 pm
content: I understand your point but it's not like there aren't any regulated DC/DC converter modules available if that's what you need. I don't think you mentioned what sort of power you're looking for but Traco have various models from 1W and up. Murata MEF1 seems to be another option in the 1W range and their NCS3 is an isolated, regulated 3W job and CUI Inc has a whole bunch of them as well.

author: IDEngineer
author_rank: Super Contributor
timestamp: October 02, 2017, 06:25:53 pm
content: Quote from: H.O on October 02, 2017, 06:09:10 pmTotal current requirement is around 10mA at 5V (e.g. 50mW). We've been looking at the 1W units because they're plentiful, not because we need the power.Yes, there are internally regulated units but we are trying to stick with what passes for an industry standard here in terms of packages and pinouts, so we have multiple sources. When you get into the internally regulated units things start varying a bit more.And even then... since regulated units ARE available, why have the UNregulated units work at the target output voltages instead of leaving some headroom? If you already don't care about your voltage going all over the place, why not center that variation a couple of volts higher so your modules can be used in a huge number of additional situations?

author: chris_leyson
author_rank: Super Contributor
timestamp: October 02, 2017, 06:50:02 pm
content: I agree regulation isn't too good with these small DC-DC converters. However, there is another issue to watch out for and that's input voltage risetime, if your input supply comes up too slowly then some of them won't startup and just sit there getting hot until they cook themselves. It's in the small print in some of the data sheets. Input supply needs a fairly fast risetime for them to startup properly.

author: 0xdeadbeef
author_rank: Super Contributor
timestamp: October 02, 2017, 07:01:52 pm
content: Quote from: IDEngineer on October 02, 2017, 06:25:53 pmWell, it doesn't really help you to complain. I was in the same boat, but I don't see what's so terrible about the shunt regulator approach I suggested.Don't get upset too much about that "somewhat in spec" phrase of mine. For the actual use case, I wanted to get 5V on the isolated side of a 5V to 5V isolated DC/DC.The DCDC converters I used had a +-5% output voltage accuracy - which was OK for me. It was just that this accuracy was only guaranteed at >=10% load.Since I didn't want to waste energy by adding a dummy load resistor, I came up with that shunt regulator thing which just drains current if needed and also doesn't limit the output current.The shunt regulator has a better accuracy than the +-5% of the DC/DC but I didn't need it, didn't waste to waste energy and therefore set it to just 5.25V instead of 5.1V or whatever.This is what I meant with "somewhat in spec" (for the USB voltage spec I wanted to meet). Actually with the shunt regulator, the DC/DC operates fully in its +/-5% spec.

author: IDEngineer
author_rank: Super Contributor
timestamp: October 02, 2017, 07:17:22 pm
content: Quote from: 0xdeadbeef on October 02, 2017, 07:01:52 pmYes, I agree your shunt circuit is elegant and clever. What frustrates me is that is should be unnecessary!With regards to +/-5% accuracy above 10% load... I don't know which manufacturer and model you were using, but every spec sheet I've reviewed has a straight line inversely relating current to voltage. That means there is no voltage regulation at all, unless you HOLD your current consumption at a constant level. Your shunt circuit may inadvertently have a higher duty cycle than you intended!If their spec sheets can be believed, that is. Perhaps a more accurate expression would have been "output voltagerepeatability" (above 10% load), since based on those curves they don't seem to provide any sort of actual regulation at all.You're right, complaining doesn't improve things. But it's frustrating when their design choices don't seem to make sense, and in this case actually get in the way of using their parts. That's why I presumed up front I must be missing something, that there was some good reason they emit unregulated voltages so close to standard target supply rails.

author: Someone
author_rank: Super Contributor
timestamp: October 02, 2017, 08:40:14 pm
content: Quote from: IDEngineer on October 02, 2017, 07:17:22 pmThere are many many designs that don't need regulation on the power rails, these parts meet that market even if it doesn't match your requirements. If a reference voltage is needed then add a reference.Or as suggested above there are also well regulated isolated DC-DC supplies available from multiple sources in common through hole SIP "modules".

author: IDEngineer
author_rank: Super Contributor
timestamp: October 02, 2017, 11:55:37 pm
content: Quote from: blueskull on October 02, 2017, 11:47:20 pmYes, but their output voltage varies with output CURRENT (read: load), no matter how well regulated the input power may be. This is straight from their spec sheets.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: October 03, 2017, 12:18:40 am
content: Quote from: IDEngineer on October 02, 2017, 11:55:37 pmQuote from: IDEngineer on October 02, 2017, 03:35:53 pmAt least a couple of links to part numbers or spec sheets.

author: David Hess
author_rank: Super Contributor
timestamp: October 03, 2017, 05:13:05 am
content: They are intended for applications where the load regulation is not critical. This includes most analog circuits and many higher voltage digital circuits and includes applications where a floating bias supply is needed.There are ways to regulate the secondary side output voltage from only the primary side and compensate for resistive and rectifier losses but I rarely see this done. It is too bad they do not give access to the primary side signals to do this but it would mean extra pins that most users would ignore.Coilcraft and others make small high frequency transformers intended for isolated power supply applications so you could roll your own. Gate drive transformers can also work for this.Quote from: blueskull on October 03, 2017, 12:02:46 amOften the only thing which is necessary is an RLC filter to reduce residual noise and this might be needed to isolate one circuit from another anyway. Clever circuits or at least not the wrong circuits can have considerable power supply rejection.A soft switched inverter using one of the transformers I mentioned can have low output noise and this fits well with primary side regulation.

author: IDEngineer
author_rank: Super Contributor
timestamp: October 03, 2017, 03:15:01 pm
content: Quote from: David Hess on October 03, 2017, 05:13:05 amUnfortunately, analog circuits that involve A/D's or D/A's - including MCU's that incorporate such devices - can be sensitive to (a lack of) regulation.QuoteWe've checked into that, but it's hard to beat the price, second sourcing, and footprint of those little modules. So our present plan is to move to a 5V in, 12V out isolated module and use a downstream 5V regulator. We don't need much current so the reduction in a 1W package from 200mA out to 83mA out isn't a problem, and low current means low dissipation from the regulator despite the 7V drop across it.In our specific application, we can get away with going to the next highest secondary voltage. But it's still a shame that doing so is necessary. If, as everyone says, these things are intended for unregulated applications where the absolute voltage doesn't matter all that much then it wouldn't hurt to have the output voltage be (say) 8V instead of 5V... which would allow existing insensitive applications to work just fine AND allow downstream regulation if needed without such a huge loss of output current.

author: tszaboo
author_rank: Super Contributor
timestamp: October 03, 2017, 03:55:04 pm
content: Look, the standard ones are good enough to run a RS485 or CAN transciever off it, even a micro. Just place a Zener on the output, and it will limit the voltage, and provide the 10% ish load.If you need regulation, you can still use a ie. 12S09 instead the 12S05 and place a simple 7805 afterwards. You can literally run precision analog components on these, if you clean up the power supply, and they cost almost nothing. There was this one-off circuit, where the input voltage was 24V, and I've just used one of these to drop it down to 5V, because I couldn't be bothered to design an SMPS to do it, and linear would have been silly.

author: mariush
author_rank: Super Contributor
timestamp: October 03, 2017, 04:21:46 pm
content: If the power requirements are low, maybe you could just drop in a tiny 5v LDO ... there's plenty out there which have a voltage drop as low as 30mV to 100mV ... so you have 5..8v in, 4.95v out or something like that.exlp3985 ( 6v in, 100mV drop at 150ma out ) :http://uk.farnell.com/texas-instruments/lp3985im5-5-0-nopb/ldo-reg-0-2vdo-0-15a-5v-5sot23/dp/2436533lp2980 , lp2981, lp2985 (up to 16v in, 120 to 300 mV at 50 .. 150mA depending on ic )mcp1711 (6v in 150ma)etc

author: Damianos
author_rank: Frequent Contributor
timestamp: October 04, 2017, 09:59:19 am
content: I have a cargo-container full of magnetic material. I want to transport it with my scooter but the manufacturer has not available any accessory for that purpose! The suggestions are to use a truck, a ship, a train... Why they don't help me to use my scooter that is cheap, economic in fuel consumption and environmentally friendly?Sorry if the above is seeming offensive,but:Quote from: IDEngineer on October 03, 2017, 03:15:01 pmThe question is: why they don't change the specifications of million devices, to do my work, even if I have selected an improper device?The next step will be something like: why they don't make the desk-lamp's power supplies stable enough and noiseless, so as to use them as voltage references in 40-bit ADCs and to have the last digit stable?To answer the question of the title and the initial post: you have to realize that these devices are meeting specifications that were established years ago. Is it the time to change them?If you insist to use them in your application, I think that you have to follow one of the proposals that are given to you. Otherwise you have to select a proper device.

author: IDEngineer
author_rank: Super Contributor
timestamp: October 04, 2017, 02:21:44 pm
content: Quote from: Damianos on October 04, 2017, 09:59:19 amMy warmest thanks for your helpful and supportive comment.Nowhere did I suggest they should change the specs. I was simply wondering why they didn't spec them to be more flexible from the start. And my last post before yours specifically outlined our present plan to accommodate their existing specs.Engineering is an art of compromise. Everything is a tradeoff. My most recent post described how we intend to balance the specs to make these parts work, with both their advantages and disadvantages. How your post then improved things for this community isn't clear to me, but I'll go back and read it again in case I missed some useful nugget of technical insight.Thanks again.

author: David Hess
author_rank: Super Contributor
timestamp: October 04, 2017, 03:01:42 pm
content: Quote from: IDEngineer on October 03, 2017, 03:15:01 pmADCs and DACs have power supply rejection as well. Except in ratiometric application, using the power supply as the reference is asking for trouble.One thing I am always surprised about is that designs rarely use a simple operational amplifier, discrete transistor, and voltage divider to make precision remote sense references or regulators as needed.QuoteI noticed the same thing looking through Coilcraft's selection of suitable transformers however there are a lot of analog parts now which cannot tolerate voltages higher than the absolute maximum of their parent digital process; they work fine at lower voltages but not higher ones.Regulators are so inexpensive now unlike in the past that secondary side regulation is probably the way to go but that conflicts with the above so they cannot use the same floating output inverters.

author: IDEngineer
author_rank: Super Contributor
timestamp: October 04, 2017, 03:15:37 pm
content: Quote from: David Hess on October 04, 2017, 03:01:42 pmAgreed, so we have a conundrum. Absolute output voltage and stability don't matter - except when they do.With modern analog parts some form of regulation is required, since based on the spec sheet curves the output can vary from -6% to +10% based on load.And lest we forget: "Lower than 10% minimum loading will result in an increase in output voltage, which may rise to typicallydouble the specified output voltageif the output load falls to less than 5%." While other respondents herein have posted helpful shunt circuits to overcome this, handling such variation is quite literally why voltage regulators exist.Hence our plan to have a downstream regulator... cheap, low parts count, and excellent regulation performance. Of course, almost anything is better than -6% to +100%!

author: IDEngineer
author_rank: Super Contributor
timestamp: October 04, 2017, 03:23:32 pm
content: Quote from: David Hess on October 04, 2017, 03:01:42 pmI concur but will offer one reason I've seen: Because no spare opamp section was available, and there was no real estate to add another package and its attendent discretes.BTW, I resolved this in one design by using an internal bandgap reference on the MCU. Measuring that known voltage with the A/D yielded a value whose difference from the predicted value gave a correction factor. That correction factor could then be applied to other measurements to improve accuracy. It worked amazingly well. I maintained a filtered average of the bandgap conversion value to further remove noise and normal regulation variability, and did the correction in floating point with proper rounding (had lots of spare CPU cycles available). On paper it was troubling not to have a formal hardware reference, but given the constraints of the design it worked quite nicely.

author: Sceptre
author_rank: Regular Contributor
timestamp: October 05, 2017, 02:40:41 am
content: I see a half-dozen 6V 1W isolated modules in stock at both Mouser and Digikey (for example, 1470-2978-5-ND, 580-MTE1S0506MC); if you can't find an LDO that works with that, there are plenty of 9V modules as well (1470-1399-5-ND, 490-PDS1-S5-S9-M). There's also a 7V module from Bel (DSP1N5S7), but it's pricey. So, unless I'm missing something, I'd say your issue is sub-optimal search functionality.

author: IDEngineer
author_rank: Super Contributor
timestamp: October 05, 2017, 02:49:54 am
content: Quote from: Sceptre on October 05, 2017, 02:40:41 amAs I said, it's a compromise. All of the parts you mentioned are more expensive than the ones we've been talking about, and don't have as many second sources for the same specs. And as I and others here have noted, regulators are cheap. Hence our plan to go up in secondary voltage and then regulate down.You can usually find an expensive solution to a one-off problem. But for volume production, the trick is to find the optimal solution that balances cost, multiple sources, PCB real estate, etc. It's not just about whether a part can be found... it's about finding part(s) that work in volume.Thanks!

author: edavid
author_rank: Super Contributor
timestamp: October 05, 2017, 03:01:10 am
content: OP, I don't know why people are being so hostile. Your question is completely reasonable.Have you looked into driving the input of the cheap modules with a higher than rated voltage?If you monitor the supply current, you should be able to tell if the transformer is saturating.ETA: You could also try under-driving a 12V-12V module.

author: IDEngineer
author_rank: Super Contributor
timestamp: October 05, 2017, 03:14:57 am
content: Quote from: edavid on October 05, 2017, 03:01:10 amThanks! I'm trying to not let their behavior affect mine.QuoteThat is a very interesting idea. Since they are obviously completely unregulated, I wonder if they just loosely shoot for a primary-secondary ratio... in which case your idea would work! Need to monitor dissipation too, since running it "hot" might yield precisely that. Our current draw isn't great so no problem there, but if the transformer drivers are running close to their voltage specs we could get some mortality issues.Excellent idea, thanks!

author: Sceptre
author_rank: Regular Contributor
timestamp: October 05, 2017, 04:14:31 am
content: Quote from: edavid on October 05, 2017, 03:01:10 amI'll take exception to that. One of the OP's original complaints is that modules with intermediate output voltages don't exist:QuoteI thought I was being helpful by pointing the OP to available 6V and 9V modules (and even a 7V module, albeit expensive). Then I find out that there are stringent cost and second source constraints. And for some reason the OP thought I needed volume production design tradeoffs explained to me. All in all, not much incentive to take the time and effort to help out.

author: IDEngineer
author_rank: Super Contributor
timestamp: October 05, 2017, 05:38:44 am
content: Jeez, I hate it when simple questions turn into pissing matches.1) When I asked about DC-DC modules, I was talking about the majority of them. Yes, there are specials with different voltages, but the majority of pin and size interchangeable parts are at the traditional supply rails that I mentioned.2) As a long time lurker here, I've noticed that when someone is doing a one-off they tend to specifically note that ("I'm building a such-and-such for my car", etc.). So I presumed when that isn't mentioned, folks understand we're talking about Engineering for production. Mea culpa.3) Since we ARE talking about Engineering for production, things like cost and compatibility and second sources matter. Since your examples didn't seem to take those things into account, I then mentioned these criteria. Again, I didn't mention them before because I presumed we are mostly a community of professional Engineering people who work in Engineering for a living and thus default to the criteria that matter in such an environment.I continue to maintain that it's odd for high volume parts, that target applications where regulation very clearly doesn't matter, to set their nominal outputs right at common rail values rather than a bit higher to capture regulation sensitive applications too. Yes, such output voltages are available at higher cost and lower compatibility but it's strange that the majority would design themselves out of such a market when there doesn't appear to be a technical reason to do so.I apologize if I've offended you in some way, that was never my intention. I don't know why my question elicited such an emotional response from you but that too was not my intention.Can we put the emotion behind us, please? Thanks.

author: Damianos
author_rank: Frequent Contributor
timestamp: October 05, 2017, 09:04:12 am
content: Quote from: IDEngineer on October 04, 2017, 02:21:44 pmIf you widen a little your view of the things, it will become both helpful and supportive!In an attempt to be more helpful I will try to explain a little more.From your first post here and in every next to that you are wondering why these specific devices have these specifications and not something else. As my little knowledge says, they satisfy the specifications that were established about half a century ago. These have not changed up to today and probably not in the near future (at least). There may be reasons for that! Think about it.It seems that we have a little different opinion of what is engineering. Lets describe what I mean, by taking as example the present case:I have to put down what I have and what I have to do. This includes things such as inputs, outputs, environment etc.Then I have to translate the above in specifications.Search for parts that can satisfy my specifications.Evaluate what I have found in all of their characteristics; Including cost, availability, reliability etcIf in what I found there is a part that satisfies all of my needs, then the problem is solved. If not I have to continue ...Examine if a ready made part can be used with an adaptation, so as to match their specifications with mine.If the above fail, I have to plan to design something ... ...The above are some draft points, that I think that is a proper way to design a system/device/product.By the way, you speak about engineering and professionalism but you found great the idea to abuse a little a device! What about your reputation if you have a lot of returns of your product, soon or a little later? Also what you are stated is similar to the trend, when a circuit is not operating as expected, many people are referring to fake components without examining the design and implementation.To conclude, all of my effort is to attract the attention to thespecifications, because, in my opinion, engineering has to do with them.PS1: to clarify things; I do not know who you are, such as any member of this forum, so there is not something personal. Also take in account that English is not my native language, so if my wording seems offending it is not by intention but from bad translation of my thinking to a language that I barely handle!PS2: Sorry for the long and seeming as philosophical comment, but it is an attempt to describe how I act practically in the last about forty years.

author: IDEngineer
author_rank: Super Contributor
timestamp: October 05, 2017, 02:14:15 pm
content: Quote from: Damianos on October 05, 2017, 09:04:12 amFirst of all, thank you for the polite tone of your comments. That is sincerely appreciated, and it keeps the forum much more civil.Secondly, please note that my first comment regarding the "out of spec operation" suggestion was to worry about reliability (running it "hot"). You're absolutely correct about the threat of returns, and my personal #1 Rule is "prevent failures in the field".I viewed the suggestion similar to the technique of offseting the reference pin of a three-terminal voltage regulator... at first glance you're running the device at a "higher" voltage but strictly speaking it's not being run out of spec. That was the reason for my comment regarding their possible ratiometric design, where the output is a certain percentage of the input - it may be (subject to testing, as I mentioned) that the device would be perfectly fine operating that way, as long as overall power dissipation or max input voltage wasn't exceeded. I wouldn't commit to a design like that without a call to the manufacturer's Applications Engineering staff to discuss what was really going on in those little packages. Your caution is well warranted and thank you for emphasizing it for future readers of this thread.

author: Sceptre
author_rank: Regular Contributor
timestamp: October 06, 2017, 04:40:16 am
content: Quote from: IDEngineer on October 05, 2017, 05:38:44 amOK, let's look at those parts on Mouser a bit more. Here are the first 20 normally-stocked isolated 1W, 4.5V min Vin, 6/9/12V Vout converters (onesies prices, but higher volumes track):Mean Well SBT01L-12 $3.57Mean Well SBT01L-09 $3.57CUI PDM1-S5-S12-S $4.24Mean Well SFTN01L-12 $4.27CUI PDS1-S5-S12-M-TR $4.43Mean Well SMU01L-12 $4.46Mean Well SMU01L-09 $4.46Mean Well SMA01L-12 $4.46Mean Well SMA01L-09 $4.46Delta PB01S0509A $4.58Delta PB01S0512A $4.58Mean Well SFT01L-09 $4.64CUI PEM1-S5-S12-S $4.84Delta PA01S0509A $4.91Delta PA01S0512A $4.91Cincon EC1TA02N $4.91Murata MEU1S0509ZC $5.08Murata MEU1S0512ZC $5.08Delta PE01S0512A $5.20Delta PE01S0509A $5.20Notice how there are seven 9V converters that, far from being specials, are simply variants of the 12V devices? And that they are priced the same as their 12V counterparts? Only five 12V converters in that list don't have identically priced 9V brethren. What about 6V? Well, those are indeed rarer: down the list is the Murata MTE1S0512MC/MTE1S0506MC at $6.68.QuoteI understood that just fine ("We're working on a project here..."), but 'Engineering for production' can mean many different things.QuoteTo varying degrees, depending on circumstances. We have no idea what this product's volume, sale price, development cycle time, market lifetime, margin, etc. are, so it's unrealistic to expect us to know what level of BOM cost sensitivity you face. And there are organizations, believe it or not, that have loose or even no second source requirements (or have exceptions that may cover these modules as well as FPGAs, micros, and certain high-performance analog devices).QuoteI agree that we are mostly a community of professional Engineering people who work in Engineering for a living, but your presumption that we should all default to the criteria that matter to you in your particular environment is spurious.

author: IDEngineer
author_rank: Super Contributor
timestamp: October 06, 2017, 03:34:18 pm
content: And if you dig just a bit deeper into the spec sheets, you find the following packaging and pinout varieties:SBT01L-09/12: SMD, pins 1, 2, 4, 5, qty on hand 50 and 15PDM1-S5-S12-S: Thruhole, SIP, pins 1,2,4,6, qty on hand 60SFTN01L-12: SMD, pins 1, 2, 5, 8, qty on hand 0, 16 weeks outPDS1-S5-S12-M-TR: SMD, pins 1, 2, 4, 5 (at last, something compatible), qty on hand 262SMU01L-09/12: Thruhole, pins 1, 2, 3, 4, qty on hand 16 and 22...and that's just across your first seven examples.Why care about package and pinout compatibility? Because you'll notice that none of these parts are deeply stocked. Even a very small production run of 500 units can't be satisfied by any one of your first seven part numbers, even IF they were compatible. In fact, there aren't 500 pieces in stock of ALL of your first seven examples, despite them being different in packages, voltages, and pinouts.There's more to selecting parts than just matching a few of the numbers on the spec sheet. Unless you're doing a one-off, that is. But we've already discussed that.Are you done now? I'd like to be. Thanks.

author: tszaboo
author_rank: Super Contributor
timestamp: October 06, 2017, 04:16:38 pm
content: So what if there is no stock? Either your application is not cost sensitive, or you wait for production times. Its really either or. It doesnt work, that you want to make something and order all of them from digikey, and somehow tell yourself that it is cost sensitive. 16 weeks of lead time is quite normal recently. MOQ will be low for these kind of devices. I tell my bosses, that either accept it, or go into the bakery business, there everything is ready in 30 minutes.If it is a critical component, and you want 500-ish production numbers, then just simply stock it at your favorite distributor. Its really not rocket science. You just need to understand how the sourcing of electronics components work.

author: IDEngineer
author_rank: Super Contributor
timestamp: October 06, 2017, 05:06:54 pm
content: I understand that, but we're getting off topic. That's a discussion of stocking, specifically "safety stock", which you can set up anywhere in the supply chain from the manufacturer to your own stockroom. Instead, my most recent post was addressing the assertion that there are any number of equally applicable part numbers available with various output voltages. The off-rail voltages are out there, but just because you can compile a list of various voltages doesn't mean the parts themselves are mix-and-match interchangeable.On your topic: Another useful thing you can do on some distributor's websites is an inverse sort by stock on hand. This is a clue to the consumption that particular parts enjoy. Highly popular parts are more likely to be stocked in higher quantities, stocked by more distributors, and less likely to be near end of life, all of which buys you some insurance for your designs.