solved:paralleled LM317's without resistors share current during 25ms transient? - Page 1
Forum section: electronics / beginners

author: incf
author_rank: Frequent Contributor
timestamp: April 13, 2025, 04:09:42 pm
content: I have an application where I need to provide 3.2 amps for 25 milliseconds once a day. (to provide standby power to a bus while a relay switches power source supplying the bus)I'd like to know if it would be possible parallel 3x LM317's without resistors. They implement current limiting internally, and the load will only exist for 25ms. After the transient they will provide 0 amps for 24 hours before being subjected to the next 25ms transient load. (over a wide range of input voltages up to 30V)Additionally, if one did use series resistors, what is the minimum value that one could get away with using?edit1 & 2: clarified

author: PGPG
author_rank: Frequent Contributor
timestamp: April 13, 2025, 04:26:17 pm
content: I don't think relay switching need precise regulated voltage to use linear regulators.

author: themadhippy
author_rank: Super Contributor
timestamp: April 13, 2025, 04:43:10 pm
content: Why not just use an lm338? good for 5A

author: incf
author_rank: Frequent Contributor
timestamp: April 13, 2025, 04:50:05 pm
content: Quote from: themadhippy on April 13, 2025, 04:43:10 pmDidn't really consider it. I thought the 338 was obsolete, or nearly obsolete?I don't know how I feel about using it in a new commercial design.

author: bdunham7
author_rank: Super Contributor
timestamp: April 13, 2025, 05:24:25 pm
content: Quote from: incf on April 13, 2025, 04:50:05 pmThe answer to your actual question is yes, that should work. The LM317 doesn't use foldback current limiting so until thermal issues arise it should put out >1.5A. Just make sure you don't get counterfeit ones! However, I don't think this is a very elegant solution.TI shows the LM338T as "active" and recommended for new designs, IDK why you think it is going obsolete. I would use it without hesitation.If you want to feel more modern you can look at LDO designs like the LM1084,or AZ1084 that comes in a DPAK form factor.However I wouldn't be shocked if 20 years goes by and the LM338T is still available and the LM1084 is not.Edit: I missed your 30V requirement.

author: xvr
author_rank: Frequent Contributor
timestamp: April 13, 2025, 05:31:29 pm
content: QuoteWhy not use a strorage capacitor?

author: tooki
author_rank: Super Contributor
timestamp: April 13, 2025, 05:37:20 pm
content: Quote from: incf on April 13, 2025, 04:09:42 pmMaybe this is a dumb question, but couldn’t you just use some beefy capacitors to sustain the standby bus during switchover? A single regulator could then recharge the capacitor at a leisurely pace.

author: incf
author_rank: Frequent Contributor
timestamp: April 13, 2025, 06:31:27 pm
content: Quote from: tooki on April 13, 2025, 05:37:20 pmQuote from: xvr on April 13, 2025, 05:31:29 pmGoal is is 1V drop during the 25ms switchover.That's a *very* big capacitor. My application would require 0.1F at ~30V. It would cost $30. I don't have the space or budget for that. The LM317 is much cheaper and uses less space.Maybe I could "fudge" the spec and have a 2.5V drop during switchover - still the capacitor would be too big and expensive.

author: David Hess
author_rank: Super Contributor
timestamp: April 13, 2025, 06:49:38 pm
content: 3 x 317 in parallel will work for that, but only because one of them will be overloaded allowing the others to conduct. A 3 amp LM350 or 5 amp LM338 would be better and are drop in replacements.

author: tooki
author_rank: Super Contributor
timestamp: April 14, 2025, 05:15:27 pm
content: Quote from: incf on April 13, 2025, 06:31:27 pmYou said your input voltage is up to 30V. So your output voltage can't be 30V, you need a few volts dropout.What's youractualoutput voltage?

author: incf
author_rank: Frequent Contributor
timestamp: April 14, 2025, 05:40:54 pm
content: Adjustable and around 24V (part of a diode and relay ORing operation of different power sources)

author: UnijunctionTransistor
author_rank: Frequent Contributor
timestamp: April 14, 2025, 08:18:32 pm
content: The thread header mentions “Solved”.What was the solution?

author: incf
author_rank: Frequent Contributor
timestamp: April 14, 2025, 09:35:26 pm
content: Quote from: UnijunctionTransistor on April 14, 2025, 08:18:32 pmto parallel the LM317's without resistors

author: MrAl
author_rank: Super Contributor
timestamp: April 14, 2025, 11:43:18 pm
content: Quote from: incf on April 13, 2025, 04:09:42 pmHi,Why not use series resistors for current sharing. The minimum value depends on the tolerance of the two LM317's. If the tolerance is 1 percent for example, then at 24v you would have to drop 0.24 volts roughly. That's 0.075 Ohms. With 0.1 Ohms, it would drop 0.32 volts. These drops only happen if only one chip supplies current for some reason, but most likely they will share a little bit at least.Of course if the tolerance was 2 percent, then you'd have the possibility of 0.48 volts difference, but that's also probably with the full temperature range. If you don't expect a huge temperature shift you might get by with lower value resistors. A narrower temperature range means a lower tolerance, and so first you have to figure out what tolerance you want to use. If you have a very wide temperature swing, you also have to take into account the temperature swing of the resistors.I've been involved in power electronics for many years either professionally or as teaching and also as a hobby. If I was going to design a circuit for a company that needed 3.2 amps for 1 to 100ms, I would never use two regulators in parallel, it's just not good practice. There must be alternates out there that are still being made that can handle the entire current for umpteen minutes. If you can't find an LM- version perhaps you can find an LT- version.About the capacitor, if we look at the definition:i=C*dv/dtand with your requirements, yes the required value of the cap would be 80000uf. That may come out kind of big physically. I would think offhand that there should be some sort of capacitor though with low ESR and as big as you can fit. The filter caps should also have low ESR or else probably nothing we talked about so far would work.Transients like this can cause all kinds of crazy stuff to happen. Whatever you do has to be tested really well, and with all the components that will be used with the end product. It might even be wise to create a jig that cycles the prototype so that it can be tested over and over again without human intervention. If something goes wrong with the product in the field, it could mean a huge number of recalls.

author: bdunham7
author_rank: Super Contributor
timestamp: April 15, 2025, 12:19:26 am
content: Quote from: MrAl on April 14, 2025, 11:43:18 pmDo you mean on the output of the regulator (trio single larger model) or are you talking about something else entirely? I would think that a large, low-ESR capacitor on the output would be the very last thing you'd ever want to do. I also think that the OP's entire problem can be solved for $5 or less in 5 minutes or less with almost no chance of anything going wrong. The most expensive and challenging aspect would be if he wanted to add a heat sink to enable the circuit to work continuously in case of some fault instead of relying on the 25ms designed on-time to keep things cool.

author: bdunham7
author_rank: Super Contributor
timestamp: April 15, 2025, 12:22:25 am
content: Quote from: incf on April 14, 2025, 05:40:54 pmI hope this isn't too silly a question, but the operating voltage of the main system is well above 24V, right?

author: incf
author_rank: Frequent Contributor
timestamp: April 15, 2025, 12:23:02 am
content: Thought 1: perhaps the LM317's can be biased so that one always take priority over the other for the sake of consistency.Thought 2: perhaps I can drive a beefy N-channel FET in linear mode using a LM324 and a TL431?--I'd prefer to use cheap surface mount commodity components.The 338 would do it too, and I could use it if there was a sufficient justification.Quote from: bdunham7 on April 15, 2025, 12:22:25 amYes.

author: bdunham7
author_rank: Super Contributor
timestamp: April 15, 2025, 12:51:52 am
content: Quote from: incf on April 15, 2025, 12:23:02 amIf you want to stick to SMT parts for assembly cost reasons, then perhaps 3X LM317 isn't so crazy. I saw an article once about someone that did this and according to them if the regulators are from the same manufacturer and they are thermally bonded, they share the current reasonably well. However, if you want an officially sanctioned way to parallel 3 LM317s, look at p16 of this datasheet:https://www.ti.com/lit/ds/symlink/lm317.pdfI suspect that could be simplified further if you just wanted a fixed 24V and weren't overly concerned about the exact voltage.

author: incf
author_rank: Frequent Contributor
timestamp: April 15, 2025, 01:03:21 am
content: I don't really care of they share current - they self limit. And it's only 25ms. Same for the voltage at the output, so long as it is within 10% or so, it's fine.My concerns were more around oscillation or other weird failure modes.

author: MrAl
author_rank: Super Contributor
timestamp: April 15, 2025, 01:46:38 pm
content: Quote from: bdunham7 on April 15, 2025, 12:19:26 amHi,There should be a low ESR at least on the input or else it is very possible that 3 amps will not be possible over short time periods because cheap capacitors will not be able to deliver that kind of current level as fast as needed.A low ESR cap on the output would be possible when there are series resistors to help with current sharing, but of course everything has to be sized correctly.I also recommend a single unit that would handle the entire current at least for 25ms (see that part of the post), and I did not question why only 25ms was needed. It is of course very possible that other solutions would be an option. If you have another solution you should post it if you didn't already.

author: MrAl
author_rank: Super Contributor
timestamp: April 15, 2025, 01:51:53 pm
content: Quote from: incf on April 15, 2025, 12:23:02 amAre you saying you must go with two of something rather than one IC chip?One IC takes up less room than two or three. The reliability is then there, with almost no question about it.

author: bdunham7
author_rank: Super Contributor
timestamp: April 15, 2025, 02:01:50 pm
content: Quote from: incf on April 15, 2025, 01:03:21 amKeep in mind that you'd be in almost completely uncharted territory here--I've only read the one thing about parallel operation and that was by no means a rigorous test. Using the current limiting might just cause a "weird failure mode", especially if your load is reactive in some way such as having a lot of decoupling capacitance. From an online discussion athttps://community.element14.com/technologies/power-management/f/forum/43594/lm317-in-parallel---answered---lm2596-circuit-questionI invented the Lm317. It is NOT designed to be paralleled. If you parallel devices, one will supply all the current until it thermally or current limits and then the other one will come on. This not reliable. The data sheet show circuitry that allow paralleling by forcing current sharing. A new device, the LT3081 is designed for easy sharing. Also the LT 3080Bob DobkinI have no way of knowing whether the poster actually was Bob Dobkin and even if so, they weren't specifically discussing transient operation or thermally-bonded matching devices. However, adding a 0.1R 1206-sized resistor to each LM317 output would only increase your BOM by a few cents.

author: MrAl
author_rank: Super Contributor
timestamp: April 15, 2025, 02:17:24 pm
content: Quote from: bdunham7 on April 15, 2025, 02:01:50 pmThere is almost nothing to discuss here. No seasoned designer would ever parallel two devices unless they were forced to due to some other constraint.

author: bdunham7
author_rank: Super Contributor
timestamp: April 15, 2025, 03:00:05 pm
content: Quote from: MrAl on April 15, 2025, 02:17:24 pmI think the BOM cost and use of a surface mount device are reasonable constraints. Can you suggest a cheap single SMT device that would work just as well? As for parallel operation, the datasheet expressly tells you how to do it properly and simpler designs can be adapted from that example.

author: Gertjan
author_rank: Frequent Contributor
timestamp: April 15, 2025, 03:37:15 pm
content: LT3080 and LT3081 were on my mind from the start of this topic. These are "next generation LM317", specially designed to use in parallel. (The idea behind the design is to parallel regulators, so heat sinking can be spread over the PCB)In this case, I think LT3081 would be most appropriate.regards, Gertjan.

author: MrAl
author_rank: Super Contributor
timestamp: April 16, 2025, 12:57:01 pm
content: Quote from: bdunham7 on April 15, 2025, 03:00:05 pmHi,Well the choice is always yours.I would think the LT devices would be more expensive, but how much more you could check out. Each manufacturer has a catalog online that you can browse and that means you can look over 10's to 100's of devices and make a decision.In any case it would be necessary to test this really good. I don't know what the production run count would be, 10, 100, 1000, 100000, etc. The more expected the better the design should be sort of.When we move up in current we probably loose the desired packaging. This may require a TO220 or similar. If that size is a problem, then we're back to lower current devices.The LM350 might work too, but it's TO220 or even worse TO3 (ha ha).

author: David Hess
author_rank: Super Contributor
timestamp: April 16, 2025, 05:02:25 pm
content: Quote from: bdunham7 on April 15, 2025, 02:01:50 pmThe 317, and every other power IC that I am aware of, was never designed or intended to routinely operate while thermal limiting. This would count as operating at an absolute maximum rating, leading to poor reliability. This is what Bob Dobkin referred to.This is a separate issue from current limiting which could be happen on every startup.QuoteThe original Linear Technology datasheets discuss parallel operation, but Analog Devices removed this from newer ones.

author: bdunham7
author_rank: Super Contributor
timestamp: April 16, 2025, 05:22:02 pm
content: Quote from: MrAl on April 16, 2025, 12:57:01 pmThey're about 10X as much in volume as far as I can tell. The 3x LM317 solution can probably be done for under $1BOM + 1 square inch or so of PCB.QuoteOne of the redeeming characteristics of the LM317 is that any moron with a basic understanding of the LM317 characteristics (even me) can sketch up a barbarically simple circuit on the back of an envelope and it will probably work just fine. With any reasonable layout this one should work just fine.

author: MrAl
author_rank: Super Contributor
timestamp: April 16, 2025, 06:46:31 pm
content: Quote from: bdunham7 on April 16, 2025, 05:22:02 pm

author: bdunham7
author_rank: Super Contributor
timestamp: April 16, 2025, 07:17:28 pm
content: Quote from: MrAl on April 16, 2025, 06:46:31 pmThis is not a generic design recommendation, it is a custom envelope sketch tailored for the OP's requirements--cheap, simple, voltage regulation doesn't need to be precise, provides standby power with a diode to prevent reverse power flow. I think using a Nexperia BZT52H-B24,115 for VR1 would keep the voltage at about 24.5+/-1V over a +/- 20C temperature range without any adustment or testing. And I've made an obvious error with that output diode as it is not available in any SMT package. A Diotec S3D or similar would be appropriate.

author: David Hess
author_rank: Super Contributor
timestamp: April 16, 2025, 09:58:55 pm
content: Quote from: bdunham7 on April 16, 2025, 05:22:02 pmYour circuit will work, with or without VR1, but it is the wrong arrangement to preserve the best load regulation, even with a 317.The reason that the 317 is not suitable for paralleling is that its loose tolerance requires too high of a ballast resistance for good load regulation, but if the compromised load regulation is acceptable, it can still be done. The LT1083 datasheet describes the problem in two different examples, shown below. They apply just as well to the 317 as the LT1083 which may be treated as an 317 with much better tolerance.First the top of the divider should be connected to remove as much of the lead and ballast resistance as possible for best load regulation. This circuit configuration is also shown in the very old 317, 350, and 338 datasheets from Linear Technology; they used to be a second source for these parts.Then the ballast resistance is selected based on the voltage tolerance to allow reasonable current sharing. This works much better with the LT1083 series because their output voltage tolerance is so much better.The third example below shows how to use an operational amplifier to enforce current sharing with no compromise in load regulation from ballast resistors.

author: bdunham7
author_rank: Super Contributor
timestamp: April 16, 2025, 11:34:09 pm
content: Quote from: David Hess on April 16, 2025, 09:58:55 pmAgreed, but this was for a known, fixed ~3.2A load. I'm pretty sure the regulation will at least meet what I specified--24.5V +/- 1.0V. I drew in zener because the OP wanted less idle current and didn't mind the voltage floating up at no load. Using VR1 minimizes this because the current through R4 will rise much more quickly in response to increased output voltage than if VR1 was just an appropriately sized resistor. If the floating up at all is a problem, then R4 could be reduced to as little as 47R which would result in about 26mA zener current (the p/n I specified will just handle that but I'd go bigger) and enough load to keep the regulators regulating. Also, using a zener means that it really doesn't matter much where I connect the upper end of R4 as far as load regulation goes.Looking at the datasheet tolerances of the LM317 doesn't reflect the reality that thermally bonded or at least proximate devices from the same manufacturer (and perhaps batch, but at least process) will behave much better than 3 random devices from the scrap bin.