Reverse power flow through step-down DC-DC - Page 1
Forum section: electronics / projects

author: Unixon
author_rank: Frequent Contributor
timestamp: January 29, 2021, 09:12:40 pm
content: Hi, I guess I need an advice on how to deal with this...There's a device with two power inputs +12V DC and +5V DC (USB) and a TPS565208 step-down DC-DC converter circuit between them.It is possible to power this device from either input or both inputs, however when +5V source is connected and +12V source is not connected,a weird failure may occur if +5V rail is not stable enough -- DC-DC starts oscillating its input node to a voltage it can't handle and then it destroys itself.Is there an good and/or easy (minimal modification) way to solve this?

author: Unixon
author_rank: Frequent Contributor
timestamp: January 30, 2021, 07:26:00 am
content: There are significant number of assembled boards which have to be modified in some way so I'm limited in options.I wonder if a dummy input load will work to drain charge from input caps when +12V source is not connected...

author: ajb
author_rank: Super Contributor
timestamp: January 30, 2021, 07:46:58 am
content: It'd be a lot easier to help you with a schematic. Also if you're looking for advice on rework strategy, a PCB layout would help with that.I'm not sure I'm following your description correctly. Is the output of the converter directly connected to the 5V input to the device?

author: Unixon
author_rank: Frequent Contributor
timestamp: January 30, 2021, 09:03:04 am
content: Quote from: ajb on January 30, 2021, 07:46:58 amI'm sorry, I think I can't can post whole schematic, only DC-DC part of it, but there's nothing special about it.Quote from: ajb on January 30, 2021, 07:46:58 amNot yet. I'm looking for an idea on now to prevent this behavior of DC-DC converter in principle.Quote from: ajb on January 30, 2021, 07:46:58 amAlmost, and that is the part of the problem.We have two boards, one 3rd party board with USB input and our 2nd board that contains among other things a step-down DC-DC.When boards are connected, our board with DC-DC supplies +5V rail to the board with USB.It is up to an end user to decide which combination of power supplies to use.Recently we found out that if USB power supply is connected and +12V supply is not, sometimes DC-DC blows up for no obvious reason.It was hard to replicate this problem but eventually I discovered that if 5V from USB is noisy enough,TPS565208 starts boosting voltage on its input (transferring power back-to-front) to a higher level that it can withstand.Initially we had a Schottky diode between these 5V rails on 2nd board, but it dropped too much voltage and we had to remove it and replace with a jumper or PTC.In recent revision of 2nd board there's an "ideal diode" made out of MOSFET and BJT current mirror and we don't have this problem.But we still have to do something to already manufactured boards of that old revision that should have had a diode.

author: eblc1388
author_rank: Frequent Contributor
timestamp: January 30, 2021, 09:32:16 am
content: Quote from: Unixon on January 30, 2021, 09:03:04 amWould it helps if you add the Schottky diode in the following manner(see image)

author: Unixon
author_rank: Frequent Contributor
timestamp: January 30, 2021, 10:28:18 am
content: Quote from: eblc1388 on January 30, 2021, 09:32:16 amProbably yes, but we can't fit that onto existing boards without badly affecting their appearance.High frequency DC-DC layout is very tight and the board in question is already populated as hell.Whatever the modification is it must look nice not turn the product into garbage.

author: capt bullshot
author_rank: Super Contributor
timestamp: January 30, 2021, 12:31:10 pm
content: The auto destruction mechanism is "as expected" for a synchronous DC/DC converter.You need to disable the converter if your circuit is fed from the 5V input, or decouple the converter from the 5V rail. If a schottky diode drop is too much of a voltage, one could consider adding another (smaller) schottky diode in series with R7, or simply adjust R7/R6 to provide a somewhat higher voltage to compensate for the diode drop.You might try to set a higher turn-on voltage for the converter by adding a resistor from R5 / IC3 Pin 5 junction to GND.

author: Unixon
author_rank: Frequent Contributor
timestamp: January 30, 2021, 02:12:08 pm
content: Quote from: capt bullshot on January 30, 2021, 12:31:10 pmWhy is it even turning on? The output voltage from 5V rail is about the same as its own set voltage, IMO it shoudn't be switching at all.Quote from: capt bullshot on January 30, 2021, 12:31:10 pmI would say it must not enable itself when there's no power at input connector, i.e. the correct way of doing this is to teach it to distinguish between input voltage occurring from a connected power supply and a voltage occurring from reverse power flow.Quote from: capt bullshot on January 30, 2021, 12:31:10 pmCompensation was one of ideas, eventually it didn't take off because we have onboard circuits coupled with DC-DC without a diode and compensating decoupled rail would drive coupled rail above allowed voltage range.Quote from: capt bullshot on January 30, 2021, 12:31:10 pmThis may actually help... Should this turn-on voltage be higher than any voltage the converter can boost itself to? Just higher than 5V rail + noise?

author: capt bullshot
author_rank: Super Contributor
timestamp: January 31, 2021, 08:53:34 am
content: As soon as there's high enough a voltage on its input, it'll turn on.And once it is on, the control loop cannot work, since the output voltage is "fixed" and not under control of the converter. So depending on the actual 5V "input" voltage, the (reverse) "output" voltage will rise in an uncontrolled manner once the converter is on (this applies to synchronously rectified converters, typically not to ones that have a diode rectified output).The internal high side switch will conduct in reverse through the mosfet body diode. So if you have 5V at the output, you'll have at least 4.3V at the input, even if the converter is disabled. Some special purpose converters add another (back-to-back connected) mosfet for reverse current protection in this case.IMO, you should set the turn-on threshold as high as possible for your circuit to work as desired when the external 12V supply is connected.Anyway, it's always a good idea to decouple (active-"ORing") power supplies if they are intended to power the same rail at the same time. You could damage your supplying 5V USB in case the 12V converter is on, as it might supply reverse current into the USB supply. Depends on that supply what will happen. Or what just happened to you, the USB supplied 5V will damage the 12V converter. Some supplies / converters can handle reversely applied power, others not. In general it's safe to assume they can't and take measures to prevent.

author: nali
author_rank: Frequent Contributor
timestamp: January 31, 2021, 09:23:59 am
content: The TPS565208 enable pin threshold is 1.6V, so you could maybe tack a resistor from there to ground to form a potential divider, so the regulator only works with a higher voltage say 10V.

author: Mau5
author_rank: Newbie
timestamp: January 31, 2021, 10:00:04 am
content: Hi,I don't think that this solution will help you on an already manufactured board..But I once designed a mosfet switch, that allows you to distinguish between two sources and prevents the described behavior.The use case was exactly the same. I had a 5V supply from a Buck-Converter and another 5V supply from USB.The mosfet switch network ensures that only one source is active at a time and has a small voltage drop.An image of the schematic is attached.Maybe it would help to just insert a pin-header that either connects the 5V net to the USB or the DC-DC input.I know this is very cheap but maybe it is the only feasible thing that you can do on a populated board.

author: Unixon
author_rank: Frequent Contributor
timestamp: January 31, 2021, 10:32:34 am
content: Rising UVLO threshold didn't help, it just insignificantly delays a moment when input caps are charged to a higher voltage.It looks like 5V rail noise pushes current through the inductor and slowly charges input capacitors until voltage is high enough to overcome UVLO and then DC-DC turns on, works for some time, then it turns off (or doesn't).This cycle can happen on and off many times, when DC-DC is on I see about 25V on input caps and this is already their rated voltage.Loading unconnected 12V input with a resistor kind of helped, but required resistor value makes this idea ridiculous -- static heat dissipation by this dummy load ruins everything.

author: poorchava
author_rank: Super Contributor
timestamp: January 31, 2021, 12:17:24 pm
content: You can try bodging an N-MOSFET at the output of the DC/DC, source to dcdc, drain to 5V rail, gate to 12V supply, +resistor between gate and source. This way the dcdc will be isolated from the 5v rail unless there is voltage at the 12V port.I haven't simulated this, but i think it should work (came up with this just now, not sure if I haven't made some logical mistake). Using a normal threshold mosfet (ie not logic level type) you should get a voltage headroom of maybe 3...4 Over the 5V rail before the fet turns on. Kind of the reversed version of pmos reverse polarity protection.

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 31, 2021, 04:19:58 pm
content: Quote from: Unixon on January 31, 2021, 10:32:34 amHow about a zener diode, something like 18V?

author: langwadt
author_rank: Super Contributor
timestamp: January 31, 2021, 05:55:15 pm
content: maybe change the voltage divider so the nominal output from the switcher is slightly above 5V, so it won't try to pull down the USB ~5Vput a ~15V zener on the 12V input

author: Unixon
author_rank: Frequent Contributor
timestamp: January 31, 2021, 06:40:56 pm
content: Quote from: NiHaoMike on January 31, 2021, 04:19:58 pmHmm, yes, I've been thinking about this too... I'll try this next.My concern was that reverse flow might be energetic enough to blow either zener / TVS or converter itself.Since nothing bad happened with a resistor, I guess a zener diode will also be fine.p.s. 100R worked for dummy load, 1k didn't, max workable value must be somewhere between them, but hundreds of ohms is too small resistance for this purpose.

author: Unixon
author_rank: Frequent Contributor
timestamp: January 31, 2021, 06:48:54 pm
content: Quote from: langwadt on January 31, 2021, 05:55:15 pmIt's very difficult to set a fixed voltage with that high precision. Typical allowed range is 5% and DC-DC precision is about 2% even with good feedback divider. We've tried that approach in order to compensate for Schottky diode loss but we couldn't come up with a repeatable result. For both supplies the specs are 5V+/-5% and this make it nearly impossible to be both slightly above another supply and stay withing specs at the same time.Quote from: langwadt on January 31, 2021, 05:55:15 pmYes, sure.

author: Unixon
author_rank: Frequent Contributor
timestamp: January 31, 2021, 08:42:06 pm
content: Well, a zener diode at input caps mostly works fine, at least it's the best solution for a nice looking mod that saves the board.A downside is pretty high energy consumption during self-excitation phase, the inductor and the zener diode (I've used 15V one) are pretty hot, the converter IC -- not so much.Nothing blows up in short time frame (minutes), don't know if it holds for really long time like days/weeks/months.Definitely not recommended for battery powered devices...

author: capt bullshot
author_rank: Super Contributor
timestamp: February 01, 2021, 06:31:29 am
content: Try a zener diode and a transistor:Zener from input node to base of transistor base, transistor emitter to GND and collector to feedback node of the regulator. This should draw the control loop into the correct direction to stop reverse energy flow.Or connect to collector to the enable pin, to turn off the converter if the input voltage gets too high.

author: Unixon
author_rank: Frequent Contributor
timestamp: February 01, 2021, 08:31:07 am
content: Quote from: capt bullshot on February 01, 2021, 06:31:29 amOK, I'll see what works best. Unfortunately, there's no nice way to budge this into an assembled board.We have 200+ boards with this issue and after installing a mod they must look clean like they've been initially assembled that way.

author: fcb
author_rank: Super Contributor
timestamp: February 01, 2021, 08:58:24 am
content: Wire the EN (enable) pin to the +EXT input (via 10K), you'll only switch on the converter when the external 12V is present.You'd have to do it before the reverse-polarity protection diode, and you might need a resistor across the EXT input to ensure it doesn't creep up (diode leakage).

author: gnuarm
author_rank: Super Contributor
timestamp: February 02, 2021, 05:18:55 am
content: Quote from: fcb on February 01, 2021, 08:58:24 am+1Seems like the best idea yet unless you can add the FET like poorchava suggests. Heck, this is so simple it might be better anyway.To do fcb's hack if you can cut the trace from resistor to the diode cathode without disturbing the rest of that trace you can use a wire to connect the resistor pad to the diode anode. That may show up more than you'd like. You can also remove R5 and make the connection with a leaded resistor making it look like it belongs there. I did this on boards that were Rev 1 in order to make them compatible with Rev 2 operation. Most of the changes were in the FPGA on board, but this was a required hardware mod. I could have told them I couldn't do it and make $200 per board rather than $50 per board for the upgrade, but I felt I should do this. Later the customer had rules that precluded even that sort of mod. No more than one white wire allowed.

author: Unixon
author_rank: Frequent Contributor
timestamp: February 02, 2021, 08:51:23 am
content: fcb, gnuarm, thanks, good idea. I know this solution, but it can't be nicely applied to a production board.I will try to integrate this kind of protection into a new board revision.To illustrate the problem a little further, I've attached relevant part of PCB layout.As you can see all power traces are very thick and necessary pads are pretty far away.Wire budges are prohibited for production boards, all mods like that are only for prototypes.So, from all solutions suggested until now, there are two definitely applicable:1) raise UVLO threshold - OK - a 0603 resistor could be nicely soldered between R5 and R6;2) clamp VIN+ with a zener diode - OK - a SOD80C zener could be nicely soldered on top of C3,C4 or between them, this would allow for better heat transfer away from the zener;Of course, an experienced person would immediately know this is a budge, but it still looks good enough.These mods don't solve the problem entirely, but they can buy enough time to rethink one's power supply connections.

author: fcb
author_rank: Super Contributor
timestamp: February 02, 2021, 11:02:13 am
content: +1 for thoroughly testing BEFORE production

author: gnuarm
author_rank: Super Contributor
timestamp: February 02, 2021, 11:14:41 am
content: Here's something that's bugging me. Looking at the schematic it looks like the 12V input polarity is backwards. I know these jacks are not standard, but I've never seen one before that wasn't positive on the inner conductor and negative on the outer sleeve or ring as they call it in telephone work.I'm wondering if you've been testing using X2 and not found a bug with X1 that could be very important.So is X1 wired in ok?

author: Unixon
author_rank: Frequent Contributor
timestamp: February 02, 2021, 11:22:08 am
content: Quote from: gnuarm on February 02, 2021, 11:14:41 amSchematic symbol may be wrong, the connector itself is wired as center-positive.

author: langwadt
author_rank: Super Contributor
timestamp: February 02, 2021, 11:54:01 am
content: Quote from: Unixon on February 02, 2021, 08:51:23 amwho came up with the silly rule that a mod that doesn't quite work is preferable to a wire budge that works? if that is really the rules the fix is scrap the boards

author: Unixon
author_rank: Frequent Contributor
timestamp: February 02, 2021, 01:41:28 pm
content: Quote from: langwadt on February 02, 2021, 11:54:01 amMod wires and other functionally good but ugly looking fixes are OK for boards that go inside an enclosure and don't affect the appearance of a product,but if a board is a product by itself, you just can't sell an obviously repaired item as brand new, probably can't even give it away.

author: langwadt
author_rank: Super Contributor
timestamp: February 02, 2021, 01:54:05 pm
content: Quote from: Unixon on February 02, 2021, 01:41:28 pmbut you can sell an item that doesn't quite work?

author: Unixon
author_rank: Frequent Contributor
timestamp: February 02, 2021, 02:52:05 pm
content: Quote from: langwadt on February 02, 2021, 01:54:05 pmThis board shouldn't be used that way in the first place and it is written in big red letters all over the manual.However, a user still can physically connect supplies in the unrecommended way and get an unexpected result.What we have to do is to make sure nothing goes horribly wrong even in that case.Intermittent high power consumption is bad but probably acceptable, auto-destructing converter is definitely not.I still hope there's a better solution that could be implemented in this layout.

author: Unixon
author_rank: Frequent Contributor
timestamp: February 02, 2021, 04:43:56 pm
content: I have a little bit alternative idea about UVLO circuit implementation.What if UVLO threshold is set by a zener diode and a resistor rather than a purely resistive divider?It's kind of the same but may push EN pin into ground better at low voltages.Or maybe it is worth doing even crazier thing like differential zener-capacitor divider for better noise immunity / hysteresis...I still don't know how it creeps from off-state into full on-state... if it is possible to stop this intermediate mode then the problem may be solved.

author: gnuarm
author_rank: Super Contributor
timestamp: February 03, 2021, 09:23:46 am
content: Quote from: langwadt on February 02, 2021, 11:54:01 amYou know what the rule is and you know why. The rule is to not allow wire modified boards in the field. Wire modifications are not up to snuff in many regards including reliability and EMI for starters.That's why I thought the leaded resistor was a good idea. Technically it's not a wire, it's a component with a bit of insulation added to one lead. /;)

author: fcb
author_rank: Super Contributor
timestamp: February 03, 2021, 11:11:57 am
content: Quote from: gnuarm on February 03, 2021, 09:23:46 amWhat tosh. A wire is as much a component as a leaded resistor. Well applied 'bodge' wire will have little effect on reliability - costs more in production and aesthetics for sure.You'd have to take a view on whether it is likely to affect EMC, but you wouldn't normally have gone through final EMC without a fully tested product would you??

author: gnuarm
author_rank: Super Contributor
timestamp: February 03, 2021, 11:57:51 am
content: Quote from: fcb on February 03, 2021, 11:11:57 amNot sure how you can claim a wire is as reliable as any other component. That's a big reason why we have printed circuits and not wired chassis. A leaded resistor may not be any more reliable, but it might serve as a way past the "rules" was my point.As to EMC, bugs can be found at any time. There is no such thing as a "fully tested product". As we all know, testing can show the presence of errors, but not the absence.I would expect any seasoned engineer to understand that. Tosh indeed!

author: fcb
author_rank: Super Contributor
timestamp: February 03, 2021, 03:40:52 pm
content: Quote from: gnuarm on February 03, 2021, 11:57:51 amI think you might have gotten the wrong end of the stick - here's some slightly condescending tosh:A bodge/mod wire should be thought of as a component. It will have resistance, inductance and may introduce undesirable effects (noise pick-up, radiated noise, etc), but a well applied wire is no more or less reliable than a leaded resistor.We have printed circuit boards to allow greater complexity and speed up manufacturing. In certain products they still use hand-wired components (valve/tube amps, high-voltage systems, heck even the insides of fairly modern washing machines have relays, sensors that form part of the 'circuit' that are off-board).Disagree with the "EMC bugs can be found anytime" argument. A well tested product (stable casing/hardware/firmware/software) should test the same regardless of when it was made and where it was tested. So if it's compliant in Lab A on Day B it should be compliant in Lab X on Day Y - there is no spooky action. As usual, the devil is in the detail, and the quality/thoroughness of the testing should ensure this. If the device fails at a later point it should be possible to do a root-cause-analysis and discover what as changed.

author: langwadt
author_rank: Super Contributor
timestamp: February 03, 2021, 05:26:55 pm
content: https://workmanship.nasa.gov/lib/insp/2%20books/links/sections/302_jumper%20wires.html

author: gnuarm
author_rank: Super Contributor
timestamp: February 03, 2021, 08:56:55 pm
content: Quote from: fcb on February 03, 2021, 03:40:52 pmThere's the problem. You keep comparing a wire to a leaded resistor. No one is saying a leaded resistor applied in the manner of a fixup wire is a reliable component. Please read what I write.QuotePWBs allow much higher reliability in the end product. When I was a kid Zenith advertised their TVs were highly desirable because they were "hand crafted" and "no printed circuits", but the reality was that "hand crafting" was less reliable and printed circuits dominated after just a few years of commercial production.In mil/space applications printed circuits very quickly became the rule, again because of the reliability.There is literally no comparison. The lower production cost is just a bonus.QuoteNot what I said. i talked about bugs being discovered after EMC testing was complete.QuoteEMC results can be changed by any mechanical or electrical changes. I see EMC issues fixed this way, they can also be caused this way. The point is a bug in the system can be found after EMC testing is complete and often is. Adding a wire to fix a bug is much more likely to impact EMI than is a change to the PWB. Traces on the PWB are in close proximity to the power/ground planes so as to minimize radiation and pickup. A flying wire is much less so.I think we have beaten this issue to death.

author: fcb
author_rank: Super Contributor
timestamp: February 03, 2021, 10:48:06 pm
content: Quote from: gnuarm on February 03, 2021, 08:56:55 pm"The point is a bug in the system can be found after EMC testing is complete and often is" - then your EMC testing is now practically worthless if you think that in fixing a bug you may have caused an EMC issue. Or put another wayif you change the design (in any way shape or form) AND you think it might have some bearing on EMC compliance THEN YOU SHOULD RETEST for EMC.Otherwise what is going out the door might not be compliant.So regardless of what form a bodge/mod may take (wire/resistor/whatever), it's affect on a products EMC must be considered.

author: penfold
author_rank: Frequent Contributor
timestamp: February 04, 2021, 12:48:26 am
content: Whilst I'm not aware of any standard that explicitly prevents the selling or use of PCBs with a bodge wire as people seem to be alluding to, all standards that I have worked to, however, definitely do state that the modification should be fully documented... which includes the usual design change process, FMEA (meaning 'impact assessment' not MTBF calc before someone misinterprets me!) and with that, EMC and electrical safety should be considered, the mod wire should also be present in the latest schematic and PCB/maufacturing data pack which should include instructions on how to implement said modification...

author: Zero999
author_rank: Super Contributor
timestamp: February 04, 2021, 10:50:19 am
content: Quote from: penfold on February 04, 2021, 12:48:26 amYes, I don't see the problem with selling a PCB with a few wire modifications on, but it depends on the expectations of the customer. It certainly wouldn't bother me, if I asked for custom made product and the first run of PCBs had a few bodge wires on. In fact I would expect it to be the case for the first run. I'd certainly prefer it over an inferiour/marginal design.Has the customer explicitly stated they don't want PCBs with modifications? If not, then just do it, in the neatest, most reliable way possible. Otherwise, scrap the boards and get new ones made. If that means making a loss, then learn from it and remember to build it into the cost, the next time a customer says they won't accept modified PCBs.

author: mikerj
author_rank: Super Contributor
timestamp: February 04, 2021, 02:15:04 pm
content: Quote from: gnuarm on February 03, 2021, 09:23:46 amWhich rule would that be? It meets IPC standards if done correctly.

author: Unixon
author_rank: Frequent Contributor
timestamp: February 04, 2021, 07:42:35 pm
content: UVLO via EN pin is finally working as plannedLast time it didn't work properly because I've targeted wrong threshold voltage (VIN UVLO instead of EN V_hi).3k resistor from EN pin to ground (10k to VIN) nicely stops the converter from starting up at 6.8V typically occurring at the converter input when it's reverse powered,15V zener diode at input caps works as 2nd line of defense just in case UVLO levels aren't precisely met for a particular board.

author: gnuarm
author_rank: Super Contributor
timestamp: February 05, 2021, 12:09:33 am
content: Quote from: mikerj on February 04, 2021, 02:15:04 pmCustomers overrule IPC. You know we are talking about customers' or company's internal rules. IPC simply sets standards for levels of work quality. They don't tell anyone what to ship or not ship. That's one reason why they have multiple levels to their standards.

author: Unixon
author_rank: Frequent Contributor
timestamp: February 10, 2021, 07:30:13 am
content: Quote from: capt bullshot on February 01, 2021, 06:31:29 amI would probably go with the following schematic for new revision: [see attached]This will keep DC-DC off at all times unless it is powered from an input connector with correct minimum voltage.Obviously, EN pin divider should be different than 10k/3k, I'll find optimal values later.It takes a little bit more parts to build this, but all of parts are low current and come in tiny packages.

author: gnuarm
author_rank: Super Contributor
timestamp: February 10, 2021, 07:45:35 am
content: Excuse my ignorance. I don't recall all the details of this design. Why can't R2 be connected directly to Vin rather than going through the transistor? If the threshold is too low the Zener can be applied, no? I'm not getting a full understanding of the operation.

author: Unixon
author_rank: Frequent Contributor
timestamp: February 10, 2021, 09:59:28 am
content: Quote from: gnuarm on February 10, 2021, 07:45:35 amVIN or VIN+?It can't be connected to VIN+ because this node could be supplied with reverse current.But you're right about a different thing - this transistor could be omitted and R2 connected directly to D3 anode.Now I don't know why I wanted this BJT... I guess I don't need R1 and R2 either.

author: fcb
author_rank: Super Contributor
timestamp: February 10, 2021, 10:27:14 am
content: The EN pin will have an ESD structure on it, it'll look like a pair of diodes limiting the the voltage on the EN pin between VIN+0.6V and GND-0.6V. So if you have apply -12V through a 10K to the EN pin, you'll only passing a smidge over 1mA. This won't hurt the device.Worst case and the buck converter VIN is still at 12V when you reverse the polarity (unlikely), your going to be perhaps 2.4mA. Again this will likely not damage the part.The EN pin operates between 2.4V (ON) and 0.8V (OFF) according to the datasheet, so putting a zener (and resistor) in series with it would be OK if you need a more accurate turn ON point, the EN pin impedance to GND is 120K to 400K. So test your zener for leakage at high temperatures if you need a more accurate switching, or perhaps add a second 10-50K between EN and GND.

author: Unixon
author_rank: Frequent Contributor
timestamp: February 10, 2021, 10:30:27 am
content: Oh, I'm overcomplicating this again...I don't need a zener diode either, let's go back to classic divider.So, only 1 new part + 1 low side resistor that should have been there in the first place.

author: langwadt
author_rank: Super Contributor
timestamp: February 10, 2021, 10:31:29 am
content: Quote from: Unixon on February 10, 2021, 09:59:28 amI see no need for a zener, a resistor will do to divide the input voltage down to the 1.6V EN treshhold. Might not need the diode either, the series resistor will limit any reverse current

author: Unixon
author_rank: Frequent Contributor
timestamp: February 10, 2021, 10:34:28 am
content: Quote from: langwadt on February 10, 2021, 10:31:29 amI'm not sure it is a good thing to leave EN pin open to this challenge...