Anyone found a ~40Vmax 1A adjustable reg with sensible thermal shutdown temp? - Page 1
Forum section: electronics / general

author: Psi
author_rank: Super Contributor
timestamp: August 10, 2025, 01:58:49 am
content: Has anyone come across a ~40Vmax ~1A adjustable voltage regs with a sensible thermal shutdown temp that won't ever cause chip damaged from long term trip state at max temp, or cycling in/out of it? A simple 3 terminal jellybean reg like a LM317 but with a safe thermal limit.Like a 80C, 90C or 100C trip instead of the ~150C you normally get plus a warning in the datasheet that continued use of thermal trips will damage the chip.Obviously you can add some extra thermal protection yourself with a NTC, but it would be nice to use a chip with this built in.Anyone come across one?I'm aware some of the larger I2C controllable regulators have a programable shutdown temp. I'm more after simple cheap jellybean regs.

author: Zoli
author_rank: Frequent Contributor
timestamp: August 10, 2025, 05:46:07 am
content: LM317HV: jellybean 317 extended to 60V drop.

author: Psi
author_rank: Super Contributor
timestamp: August 10, 2025, 07:46:14 am
content: Quote from: Zoli on August 10, 2025, 05:46:07 amNot sure what the 60V drop has to do with shutdown temp.The LM317HV doesnt mention what the thermal shutdown temp actually is.I'm not just wanting a lower thermal shutdown to keep the chip safe, it's also to stop the rest of the PCB from overheating.So a chip that can run safe at 150C isn't really the best solution.

author: voltsandjolts
author_rank: Unknown
timestamp: August 10, 2025, 08:30:20 am
content: Quick and dirty solution could be a resetable thermal smd fuse placed adjacent (thermally coupled-ish) to smd regulator.Or on heatsink if going that way.Might give slow startup and shutdown behaviour.

author: pmx
author_rank: Contributor
timestamp: August 10, 2025, 08:46:32 am
content: Hi,I'm afraid that the combined functions : voltage regulator (I suppose linear) and specific temp monitoring and switching only exist in specialty chips, maybe computer energy management (?).From you post, it seems that the temperature shutdown feature is only required for this negative voltage.As I suppose that the -40V in only one part of the circuit voltage, you may have some 3.3V or 5V available somewhere.All I can suggest is to use a "resistor programmable, SOT-23-6, temperature switch" like this microchip MCP9509, plus a regulator with an "ENABLE" input (may require voltage translator in the case of a negative regulator).https://www.microchip.com/en-us/product/mcp9509If an ENABLE pin is not available a dual N+P Mosfet in SOT-23-6 (for the voltage translation + actual switching) would do the trick. A few more R and C would even allow to control the current surge at startup, if necessary.I used this many times in commercial products with many controllable, or sequenced, symmetrical low intensity power supplies.BTW, the cost of a vanilla regulator + a temperature switch is probably much less than a specialized regulator... if you can find one!Potential manufactured of this kind of beasts, like Analog Devices for example, are not on the cheap side...For a one-off product, this may be less problematic.A side though, I'm sure you will agree :I don't think you can rely on the regulator's temp protection for what you want to do.At least not without some experiments, and very little control over the final temperature trip point :The temperature inside the regulator chip (=> the temperature that would trigger the shutdown), depends on the current in the regulator, the voltage drop (YES, in this case the voltage drop matters a lot !), and the regulator cooling / heathsink / air flow.You see what I mean.That said, once tuned, and if you get what you want, it should be repeatable.But not something I would do.My 2 (euro-)cents...

author: pmx
author_rank: Contributor
timestamp: August 10, 2025, 08:49:16 am
content: Quote from: voltsandjolts on August 10, 2025, 08:30:20 amCertainly a very good solution !

author: tom66
author_rank: Super Contributor
timestamp: August 10, 2025, 08:51:20 am
content: Depends on the device. Some regulators are rated for operation into thermal shutdown, whereas others recommend not going into it. For instance LT1185 datasheet states that it's ok to enter thermal shutdown without 'damaging' the device, but choose the metal cased version because the plastic case may melt. I've found PMICs that have an "alert" flag when temperature is high, and a separate "shutdown" flag, but usually all choose to shutdown at a very high die temperature.How many units are you making? How regularly will these be driven into thermal overload? I will take a bet if your answer is "one to a few" and "rarely" that the regulator will survive as many thermal overload events as needed. The thermal shutdowns on regulators are designed to protect the part, so they will be set before the destruction threshold. And whilst it is probably the case that the regulator has a lifetime in perhaps only thousands of hours at that temperature, unless the design is constantly pushing it there, it will not noticeably shorten its lifetime.

author: Psi
author_rank: Super Contributor
timestamp: August 10, 2025, 09:44:24 am
content: It's pretty rare that it will ever get into thermal shutdown.I'm not too worried about this, there are many options to fix it.I was considering just adding a cheap nFET shorting ADJ to ground and use a voltage divider on the gate with NTC. So it turns down/off the output voltage once the temp gets to a point where the gate voltage reaches the gate threshold. It would be pretty cheap parts wise. But it got me thinking if there were VRegs with a more sane thermal overload shutdown temp, and I couldn't find any after doing some searching.Voltsandjolts's thermally coupled polyfuse idea is another option. However I'm pretty sure polyfuses have the same issue. They're not really designed to cycle in/out of trip state a lot.It just seems kind of silly that so many regulators have a thermal shutdown feature but only ever at a temp massively higher than their max safe operating temp.Perhaps it's just a very inaccurate threshold with a wide spread which makes it impossible to do at a specific temp.Or maybe it's more a side effect of the circuitry you get for free once things get crazy hot, and they just call it a thermal shutdown feature. Dunno.

author: tom66
author_rank: Super Contributor
timestamp: August 10, 2025, 11:11:45 am
content: I suspect it is set so high so you can get the maximum amount of current out of the minimum heatsink. If it were set at 100C, then some customers wouldn't be so happy because they couldn't push it to the absolute limit. Note that this can just be a seasonal/environmental effect. For instance on the kit we design, 99% of the time it will operate in an air conditioned room at 21C. But there will be an occasion it will be operated in the middle of the desert in UAE somewhere after having been left to cook on the dashboard of a Jeep parked up in the sun all day. So when we do our design calculations we can assume lifetime of operation at 21C but the device still needs to function with internal temperatures at 65C. I would be miffed if the only reason I could not do that was an abnormally low thermal shutdown temperature, even if I was still within the junction rating of the part.

author: Psi
author_rank: Super Contributor
timestamp: August 10, 2025, 11:42:57 am
content: Quote from: tom66 on August 10, 2025, 11:11:45 amUnderstandable, just seems odd that it's not even an option to get some variants/models that shutdown at like 90C.With tightly packed designs sometimes you'd rather it powered down or regulated it's own temp rather than having to deal with the fact that things on the PCB might be sitting at 150C pumping heat all over the PCB and into other components.

author: voltsandjolts
author_rank: Unknown
timestamp: August 10, 2025, 12:11:33 pm
content: Quote from: Psi on August 10, 2025, 09:44:24 amQuote from: Psi on August 10, 2025, 09:44:24 amWell, is it rare or is it a lot?There are small bimetallic thermal switches rated for thousands of cycles at full load current of several amperes.

author: Psi
author_rank: Super Contributor
timestamp: August 10, 2025, 01:40:16 pm
content: Quote from: voltsandjolts on August 10, 2025, 12:11:33 pmIt's rare but it might get stuck in the state for 8 hours and that seems to long to hold any die at 150-200C. But yeah, the poly fuse idea would work for my use case.But the point of the thread isn't so much to solve my problem, it's trivial to solve with some extra parts. The thread is more to find regulators with safe thermal limiting and/or discuss it. It just seemed strange that they don't seem to exist.

author: Siwastaja
author_rank: Super Contributor
timestamp: August 10, 2025, 02:03:10 pm
content: Quote from: Psi on August 10, 2025, 01:40:16 pmI have wondered the same. The difference between unsafe and safe thermal limiting is not very big. If the limit triggered 10-20 degC earlier, it would not affect the regulator's capability to supply inrush currents (e.g. to charge capacitors or start DC motors) in any meaningful way. It would reduce the maximum long-term current of course, but that's the whole point.I see no point in having this marginal maybe-safe-for-some-time limit. What purpose does it serve that you can use, say for example, 1A regulator to supply roughly 2A safely for some hours (maybe tens of hours), which you then need to micromanage by proving by design that you don't exceed this not-exactly-disclosed time limit, when they could have spent exact same budget (transistor count, design complexity etc.) with a tad lower limit which could then, in same conditions, supply e.g. 1.5A indefinitely with no worries whatsoever.Why do a botched halfway thermal protection when it costs as much as a real protection?Can someone explain why that is the norm, instead of a limit that can be trusted indefinitely? What Psi and I are missing?

author: Zero999
author_rank: Super Contributor
timestamp: August 10, 2025, 02:53:25 pm
content: Quote from: Psi on August 10, 2025, 01:40:16 pmHave you done any tests? Are you sure it does that? It might have some hysteresis.

author: voltsandjolts
author_rank: Unknown
timestamp: August 10, 2025, 03:07:46 pm
content: Lower thermal shutdown temperatures would likely eliminate that device from the automotive under-hood market.Maybe other industrial environments to some extent.It's funny, I'm often designing for ambient temperature of 150C and above, where built-in thermal shutdown is a PITA for me. I'm all for adjustable, 80C to 220C

author: thm_w
author_rank: Super Contributor
timestamp: August 11, 2025, 09:43:59 pm
content: Operating the die for a few hours or days at 150C won't instantly kill it, we are talking months or years of use.So, it makes sense that they put in a shutdown of 150, the whole point is to protect the IC, not protect your PCB.But as you say, most do excuse themselves from liability saying not to do it. Micrel doesn't have that liability note in the datasheet I looked at."Although operation at the verge of thermal shutdown is not recommended, Micrel has tested LDOs for several million ON/OFF thermal cycles without undue die stress. In fact, during reliability testing, regulators are burned-in at the thermal shutdown-cycle limit."https://ww1.microchip.com/downloads/en/devicedoc/ldobk.pdf

author: Psi
author_rank: Super Contributor
timestamp: August 12, 2025, 12:36:06 am
content: There's a decent number of products where if any IC is at over 100C then something has gone wrong.It's a lot easier to engineer your design to handle occasional thermal situations caused by parts siting at 100C than parts sitting at 150-200C. So a Vreg that shuts down or limits to 100C would be a better choice here.Most PCBs you get are TG150 or less, so right off the batt you shouldn't have things at 150C anyway.Yes I know it's 150C die temp and the outer case will be less, but with modern packages the Junction-To-Board/Case difference can be quite small. And some chips don't shutdown until near 200C

author: thm_w
author_rank: Super Contributor
timestamp: August 12, 2025, 12:58:39 am
content: You are right 200C is quite high, but for 150C shutdown, the board temp can theoretically be quite low:SOT23 - Tj 150C - Rjb 57C/W, 0.6W, Tboard = 116CSOT89 - Tj 150C - Rjc 15C/W, 2.6W, Tcase = 111CI would not want to limit to 100C, that would not give much headroom for industrial temperature range (-40 to 85C).

author: Zero999
author_rank: Super Contributor
timestamp: August 12, 2025, 09:13:35 am
content: Quote from: Zero999 on August 10, 2025, 02:53:25 pmI haven't done any temperature tests on the LM317, but I do know that when overloaded, it does trip into a current limited state and doesn't return to normal, until the load is disconnected. I could be wrong, but I doubt it just sits there at a high temperature continuously. Testing is required to confirm this.

author: Psi
author_rank: Super Contributor
timestamp: August 12, 2025, 09:27:29 am
content: Quote from: Zero999 on August 12, 2025, 09:13:35 amI've not tested that either, but I would imagine it's still possible to hold the reg at a temp just under thermal cutoff continuously. So I'm not sure it matters if the thermal trip causes an equilibrium state with lower temp/current and doesn't return to normal until the load is removed.If it's possible a semi-short can hold it at 149C then the product should handle that without damage to the reg or other parts on the PCB. So we're back to addressing the situation unless we're ok with some tiny % of failures and I kind of reject that on principle.Maybe I'm being too much of a perfectionist.