Low quiescent alternative to LM317 - Page 1
Forum section: electronics / projects

author: Ghydda
author_rank: Regular Contributor
timestamp: March 08, 2016, 10:01:15 am
content: Hi everyone,I am having difficulties coming up with suitable search terms in Google for locating an alternative part.The part I would like to replace is the old and trusty (crusty) LM317 in a SO8 package.In my application it is reducing a 12V supply rail to 3.3V, and the load is appr. 25mA.For my application the LM317 excels in a few areas:no thermal shutdown protectionvery low costcommon as mud (multiple distributors and manufactures)did I mention it is cheap?However, for my application the LM317 has these undesired traits:a high minimum load to maintain regulationsomewhat high-ish reference pin currentIs there such a part, which has better performance than the LM317 regarding the undesired traits, yet does not compromise (too much) on the positive traits?Cheers!

author: MK14
author_rank: Super Contributor
timestamp: March 08, 2016, 10:14:27 am
content: Quote from: Ghydda on March 08, 2016, 10:01:15 amI've NOT checked the price, or the details very carefully.But what about the LM317L, which is something like a tenth of the (min) power consumption, all round.Example:http://www.ti.com/lit/ds/symlink/lm317l.pdfIt's NOT clear from your original post, what the maximum output current needs to be.For the LM317L, it is around 100ma. Which may NOT be enough ?

author: Ghydda
author_rank: Regular Contributor
timestamp: March 08, 2016, 12:25:43 pm
content: QuoteWell since I mentioned the SO8 package type and the 25mA load scenario, I kinda figured it was implicit understood I was talking about the 100mA version of the various LM317 types. Oh well.My backup plan is to build the supply with a reference+opamp+pass transistor should no qualifiable part (is this even a word?) surface on this thread.I have no need for current limiting, as it is provided the upstream circuitry.However, a discrete solution is a route I would prefer to take, due to it being a more complex design.Cheers!

author: MK14
author_rank: Super Contributor
timestamp: March 08, 2016, 12:33:52 pm
content: Quote from: Ghydda on March 08, 2016, 12:25:43 pmI missed the SO8 bit. I thought the 25mA you said, was the MINIMUM (current value). I did not realize that was the maximum, sorry.I thought you meant that the LM317, was using so much min load current (25mA), that it was bringing down your 12V rail (which I assumed was VERY weak), down to 3.3V.

author: 8of10
author_rank: Newbie
timestamp: March 08, 2016, 12:44:07 pm
content: Hey thereIf I may ask , is your application battery or mains supply ? Have you thought of using a buck regulator ?Those you can get with a stand by current of a few micro Amps and they are over all more efficient. Most don't even need cooling ? I use them for various applications .Best regards

author: zlymex
author_rank: Frequent Contributor
timestamp: March 08, 2016, 01:48:34 pm
content: What about LT1763?http://www.linear.com/product/LT1763No min. load current, ground pin current <75uA when no load, ADJ pin current <0.1uA, and is available in SO8 package.

author: MK14
author_rank: Super Contributor
timestamp: March 08, 2016, 02:07:59 pm
content: Quote from: zlymex on March 08, 2016, 01:48:34 pmNice find but...Early indications are that one off, the price is about $5 each!I know I linked to the WRONG case style, but it is just an example:http://uk.farnell.com/linear-technology/lt1763cs8-1-5-pbf/ldo-reg-0-5a-1-5v-8soic/dp/2102598$4.78 here:http://www.digikey.com/product-detail/en/linear-technology/LT1763CS8-5%23TRPBF/LT1763CS8-5%23TRPBFCT-ND/3885224

author: mariush
author_rank: Super Contributor
timestamp: March 08, 2016, 02:09:10 pm
content: MCP1702-33 :http://uk.farnell.com/microchip/mcp1702t-3302e-cb/v-reg-ldo-250ma-smd-sot-23-3/dp/13314902uA quiescent currentLP2985-33DBVRhttp://uk.farnell.com/texas-instruments/lp2985-33dbvr/ldo-fixed-3-3v-0-15a-sot-23-5/dp/2395925max 150mA output, max 0.85 mA quiescent at 150mA loadLM2936MP-3.3 :http://uk.farnell.com/texas-instruments/lm2936mp-3-3-nopb/ic-v-reg-linear-3-3v-smd/dp/146906215uA @ 100uA, ~ 0.2 mA at 10mAetc etc.. i mean is it so hard to crawl digikey or farnell/newark and pick a regulator and check its datasheet ?If you mean you really want it in the same soic-8 package (you don't want to redesign your pcb) , just dig up some moreAnd I agree with others, you could make a switching regulator in pretty much the same footprint. Get something like a LM2734 and a couple of ceramic capacitors and diodes and an inductor and a couple of resistors to set the voltage to 3.3v and you're done

author: zlymex
author_rank: Frequent Contributor
timestamp: March 08, 2016, 02:39:01 pm
content: HT7133, Very cheap, current consumption only 4uA typical, in SO89 package.

author: MK14
author_rank: Super Contributor
timestamp: March 08, 2016, 03:05:56 pm
content: Quote from: zlymex on March 08, 2016, 02:39:01 pmDo they still make that chip ?I can't seem to find it for sale, in the main channels. The part you quoted, was discontinued in 2002, apparently. There was a replacement, with a longer part number. But that seems to have disappeared as well.Is it for sale in Digikey (USA), or a similar company ?One of the discontinuation notices:http://www.holtek.com/english/literature/EOL-71XX.pdfThe replacement part the datasheet mentions, seems to have disappeared as well. Or I failed to find it. Sorry!EDIT: Correction, the datasheet (new), is at least still valid.http://www.holtek.com/pdf/consumer/ht71xx_1v230.pdfI've found it for sale (available quantity=0, from China ...), as long as you don't want production quantities.http://www.ebay.com/itm/5pcs-HT7133-HT7133-1-30mA-7133-1-Three-Terminal-Voltage-Regulator-IC-SOT-89-/200982522276

author: bseishen
author_rank: Contributor
timestamp: March 08, 2016, 06:23:56 pm
content: We use a bunch of MCP1703A, but will require a board respin. SOT-89, SOT-223, 2x3 DFN, SOT-23A packages.

author: Ghydda
author_rank: Regular Contributor
timestamp: March 09, 2016, 08:15:00 am
content: Quote from: 8of10The entire design is powered by an external source between 15 and 62 volt dc.However as mentioned the regulator in question runs off a local 12 volt rail.And there is no room in the budget for another switching converter; hence a cheap and cheerful linear regulator is needed.Also, most cheap controllers have a temperature shutdown feature, which is a no-go.Quote from: zlymexAll nice specs, but it cost a fortune and has temperature protection (the datasheet is kinda vague on what actually takes place past 125 degC junction temperature).Quote from: mariushMCP1702-33: "typical shutdown threshold of 150°C" --> not good enoughLP2985-33DBVR: "Standard regulator features, such as overcurrent and overtemperature protection" --> now the datasheet does not state what actually happends past trip point, but since the part has a dedicated shutdown pin, I stongly suspect they use this built in feature to yank the outpout to zero - hence no good.LM2936MP-3.3: "Internal Thermal Shutdown Protection" --> no goodWell you are right, it is not that difficult to plough through endless lists of datasheets on Mouser/Farnell/Digikey and so on. However, one would think reading the thread before answering would be easy too.On the top of the LM317 pro-list was "no thermal shutdown protection".Finding a alternate part which fullfills this requirement kinda makes it a +3 month task to check every datasheet on earth; time this project does not have.I certainly do not expect you to do the actual crawling for me. But I had hope some other chap might know of a similar part, with the same pro-list and less of the con-list for me to consider.Quote from: zlymexNow this looks promising.I'll order some samples and do some testing to verify the performance.Quote from: MK14Damn! The availability of the part (HT7133-1) is very limited, not many part vendors carry Holtek. This could end up being a purchasing issue even if the part passes tests.Quote from: bseishenNice part. A board spin is a non-issue, as the layout phase starts next week if all goes to plan. It is bit expensive, but that could turn out to be alright - but the deal breaker is the thermal protection.My external design limits are governed by available board space - so anything larger than an SO8 + bypass caps is a no-go.Next up is ambient temperature. This thing needs to pass a building fire test. That means board temperature in excess of 200 degC.Good news is, the unit has to stay alive only for the first 5 minutes of said test. Yes this is a run to destruction application. Any thermal protections schemes kicking in at sub 200 degC does not stand a chance of passing the test.The old LM317 series regulators does throttle the maximum output current available once they heat up, but they never shut down to zero - and thus from previous designs they have worked good enough for the task. But this time, LM317's use to much quiescent current.Oh well, I should mentally prepare myself for a discrete solution.Thanks for chipping inCheers!

author: Siwastaja
author_rank: Super Contributor
timestamp: March 09, 2016, 09:08:36 am
content: Quote from: Ghydda on March 09, 2016, 08:15:00 amI didn't understand this. In which way, exactly, it is better that the regulator burns short circuit, usually passing Vin directly to Vout, instead of shutting down and recovering?You are understanding that the thermal shutdown works just below the threshold where the chip would self-destruct catastrophically otherwise, yes?

author: ivan747
author_rank: Super Contributor
timestamp: March 09, 2016, 01:33:34 pm
content: Quote from: Ghydda on March 08, 2016, 12:25:43 pmThe word you're looking for is ”qualifying”

author: rob77
author_rank: Super Contributor
timestamp: March 09, 2016, 03:02:22 pm
content: and what's wrong with 1117 ? the most common LDO nowadays... and it's available in SO8 as well.

author: macboy
author_rank: Super Contributor
timestamp: March 09, 2016, 03:44:46 pm
content: Reconsider LM317L. The quiescent current is probably not as high as you fear. The ADJ pin current is only typically 0.2 uA, 5 uA max for loads under 100 mA. The minimum load (with <15 V input) is 1.5 mA typical/ 2.5 mA max. So if you can guarantee that the load will always be at least 2.5 mA, then you do not need to consume this minimum load via the output setting resistors, and can use much higher values here to reduce quiescent current. If you fear going unregulated due to low load at startup (for example), then an additional shunt such as a zener could be used to limit the voltage. (numbers here are from TI datasheet).

author: Ghydda
author_rank: Regular Contributor
timestamp: March 09, 2016, 04:46:24 pm
content: Quote from: Siwastaja on March 09, 2016, 09:08:36 amI think you would find the answer is provided. If not, please allow me to clarify:You light match, you accidentally drop it in a gasolin soaked couch. Flames appear, growing bigger. Rapidly even.After 5 minutes the fire alarm inside the building kicks into gear and alerts everyone and everything of the danger. Escape doorways and windows open up, the HVAC system goes into reverse sucking air, clearing the areas of dangerous fumes and smoke as best they can, thus facilitating an easier and safer getaway for personnel, than otherwise possible. Secondly, it also make the life of the firefighters easier once they arrive.Now, by that time the fire-signal comes in, the circuit board is already at +150 degC and rapidly climbing.To rephrase your wording: In this exact way, it is better that the regulator runs until short circuit, regardless if it passes Vin directly to Vout, because there will be no place in time to do recovering in anyway!This is arun-till-you-dieapplication. The device needs only to function once, because afterwards there will be nothing left of it to salvage.Or would you prefer safety gear in general throw in the towel and says to you: Your on your own mate! Good luck, cause I'm outta here!"Quote from: blueskull#1 Tell me about it!#2 There is no need to go times 100 on the BOM. This device will undergo a fire test performed by an independent certified laboratory which will determine if the device as adequately built for the task. This is no different than any other approval situation like UL or EMC.#3 Fail it will. That is certain. But it will keep going as long as it can.Quote from: rob77Hmmm... not sure. I think I assumed it would just shut off completely. I'll check.... ...Nationalsdatasheet does not specifically say thermal shutdown - and the block schematic suggests the thermal management circuitry manipulates the output stage the same way as the current limiter.OnSemion the other hand uses the wording "thermal shutdown". So doesDiodes Inc. AndTIaswell.Linearuses the wording "internal thermal limiting circuitry", and further states "For continuous normal load conditions however, the maximum junction temperature rating of 125°C must not be exceeded."Not surprisingly, the Linear part cost a small fortune.But it is interesting Linear implicitly say the part will run above the recommended/rated temperature (albeit with a reduced lifespan).Quote from: macboyIhaveconsidered it for days. In most circumstances you would be right. I do expect my load to be minimum 3-4 mA at all times. However, the part that makes me most uncomfortable is the reference pin current. Usually it is quite low as you mention. But what happens after the device heats up beyond ABS-MAX ratings?No manufacturer has graphs that dare to speculate on the behavior beyond those limits, as they will implicitly adopt some responsibility for customer designs using said graphs and data. Therefore I assume the worst and have found the LM317L too unsuitable for this particular design.In a previous design (for another product) the power consumption was not an issue and thus I used the LM317L (with quite low values for the output setting resistors) and the design passed the fire test.I will be closing the design sprint on this sub-circuit now, it is time to focus on other aspects of the product.Once the board has gone through layout, been populated and in-house tested, I may reopen the design as needed. But for now I shall go the discrete components way.Thanks and cheers to all!

author: mmagin
author_rank: Frequent Contributor
timestamp: March 09, 2016, 10:19:28 pm
content: Quote from: Ghydda on March 09, 2016, 04:46:24 pmUh, wait, you are really cost sensitive, want this to run at high temperatures, and it involves safety of life? Choose two.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 09, 2016, 10:38:12 pm
content: So,You are designing a safety-relatedproductand seemingly have no clue what you are talking about, let alone some logical thinking?I'm horrified.If the product needs to run as long as possible in a fire, consider designing it so, properly (look for thermal insulation and evaporative cooling, for example)! Just having no thermal protection in the regulator allows it to run only very slightly hotter before self-destructing. The protection level is very near to the thermal runaway and self-destruction. The stuff behind the regulator will likely stop working anyway. As you are running low current, the regulator won't heat up internally much so die temperature will be close to ambient temperature, and the thermal protection will work at temperatures above the rating of most microcontrollers, for example. So your circuit has most likely already failed at the time the ambient heats up the regulator over the protection threshold.Sure, you may notneedthermal protection in the regulator, but not having one will not help you the slightest, as you seem to think. I think this "lack of thermal protection" thing is giving you a false sense of security. Sorry.

author: Howardlong
author_rank: Super Contributor
timestamp: March 09, 2016, 10:51:33 pm
content: I am using MC78LCxx series in battery operated devices nowadays that need always on (these are not the old 78xx/78Lxx!)1.1uA quiescent (I don't know of any regulators better than that)12V Vin max (in your application your might want a series diode to drop another 0.7V to keep it comfortably in spec)80mA ImaxSOT89 or SOT23-530mV dropout voltage<$0.20 in 3ku volumeGetting a characterised regulator at 200C will be a challenge, I doubt you'll have much luck for something off the shelf.

author: Ghydda
author_rank: Regular Contributor
timestamp: March 10, 2016, 12:32:41 pm
content: Quote from: blueskull on March 09, 2016, 10:37:56 pmThis is along the right lines.Cost is always a factor. In any design.Quote from: SiwastajaDepending on the application your point is absolutely valid. However most thermal protection schemes are designed to ensure the integrity of the parts for thousands of hours during operation near or at the thermal protection limit.For plastic molded parts this means keeping the epoxy resin near the die below 150-160 degC (the limit depends on the exact formula used and the desired lifetime).The silicon inside usually runs fine at and above this limit. However, higher means shorter lifespan. It is a trade-of thing.Try this on for size:Sure fire-proofing could extend the life of any electronics device for hours, even in the worst of blazes. But why would you build such a device? Your competitors do not. That only makes your product too expensive in the marketplace; thus nobody buys it.Fire prevention/extinction/warning/alarming gear for buildings need to live up to the local building codes. Nothing more. Those contracting an office building or a warehouse are on a budget too, and the buy the cheapest option which satisfies the local building code.This is real life.This is not Utopia or any other glorified world.Just about every technical solution engineers come up with has to be good enough. Nothing more.More means extra bulk/cost/time/fuel/manpower/energy/resources. Not needed. In most cases even not wanted.Quote from: SiwastajaStay horrified.I'll stay clueless.Cheers!