The right way to protect analog MCU ADC input - Page 1
Forum section: electronics / projects

author: zero0d
author_rank: Contributor
timestamp: February 04, 2021, 11:39:13 am
content: Dear all,please can you suggest a ESD, overvoltage, overcurrent and reverse polarity protection for the two versions of analog ADC input of MCU?1) picture1i would preferer this scheme with protection2) picture2how to select the right opamp for analog buffer ? What protection do we need to add if opamp buffer is used?3) also i attached picture 3 -> protection of solution 1 , please can you comment what do you think is needed or not needed and what has to be changed.Best Regards,0

author: penfold
author_rank: Frequent Contributor
timestamp: February 04, 2021, 02:47:35 pm
content: Hi,If you could share a little about the target application, maybe what the sensor/input actually is and the target environment, I could give a more specific answer... but... what you've posted as a candidate protection scheme is okay, maybe add a series resistance between the TVS and ESD diode and do some maths to find the right resistances to keep currents within limits (just noting that under fault, 8V clamp voltage on the TVS causes 4mA or so to flow into the power rail, I would personally take steps to avoid that or at least add add some filtering to the 3V3 line on the PCB).There's probably a more optimised protection method but it would require a bit more knowledge of the system. Also consider the likelihood of EMI emissions from your board which may warrant a common mode choke.An opamp buffer on its own won't really help out much in this case and it won't necessarily be any more robust than the MCU's ADC input and would still need protection. It would come down to what else is going on the PCB and what level of EMI disturbance you can tolerate,A PTC or polyswitch fuse would be useful if you're anticipating long duration faults, maybe being accidentally connected to power source, otherwise I wouldn't bother, but I would make sure the input resistance or ferrite could handle the current pulse you would get from a transient (and if you're dealing with repetitive fast transients, check the average power dissipation is up to it).

author: zero0d
author_rank: Contributor
timestamp: February 05, 2021, 06:46:02 am
content: Hi penfold,thank you for your answer.On the sensors side, there is actuality a 1k res (0402), and the target environment is industrial automation.i think to use this kind of fuse 0ZCJ0020FF2E.what ferrite would you choose?Regarding resistance, i think i can't go more than 2.5k because the PIC specifies recommended: Recommended Impedance ofAnalog Voltage Source Max 2.5k"

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 05, 2021, 08:22:26 am
content: Use 3, the left half.Clamp diodes: probably avoid schottky, they have too much leakage for most analog applications. Might not be bad with the 1kohm resistances here.Likely candidates BAT54S (when you do want schottky), BAV99, 1N4148W, etc.Move the filter cap to the clamp diode. Don't worry about fusing, TVS, etc. Or if you use a TVS or zener, you can remove the clamp diodes.Ferrite beads don't do much compared to resistors. A ferrite bead might have a peak impedance of 1kohm at 100MHz for example; the resistor has 1kohm flat from DC to a lot of Hz.They are, however, a good idea when you need a low resistance path, like for output pins. Then, any impedance is welcome, and the fact that it's still low resistance at DC preserves that low resistance path.Tim

author: Zero999
author_rank: Super Contributor
timestamp: February 05, 2021, 09:04:12 am
content: The diodes divert spikes into the power supply, which can cause the voltage to rise. This can be mitigated with an oversized decoupling capacitor and zener diode across the power supply.

author: zero0d
author_rank: Contributor
timestamp: February 05, 2021, 09:10:33 am
content: Hi Tim,thank you for the answer.1) Regarding clamp diodes, so for analog signals is better to use fast switching diodes vs schottky diodes because of leakage, and why is better to use schottky for digital signals?also CDSU4148-HF is then a good choice? simillar tohttps://www.vishay.com/docs/85748/1n4148w.pdf2) filter cap before or after clamp diode?3) about fusing, TVS diode ( the high power diode), EDS diodeMy thought was , please comment3a) use TVS PTVS3V3S1UR for complying to:IEC 61000-4-5 for 10/1000 us pulses(this one doesn't have IEC 61000-4-4 EFT Protection)3b) use ESD TPD1E1B04 for complying to :IEC 61000-4-2 Level 4 ESD Protection ±30-kV Contact Discharge and Air Gap Discharge,IEC 61000-4-4 EFT Protection – 80 A (5/50 ns)IEC 61000-4-5 Surge Protection 8/20 us pulses3c) i tough the TVS and ESD diodes should handle the ESD protection while the clamp diodes should handle if the line is accidently connected to 24V or 48V- also the PTC fuse 0ZCJ0020FF2E should then trip and protect the system4)because i already have 1k before the analog input and another 1k before the clamp diodes ( as you suggested) to limit the current through the clamp diodes,and i want to have a filter on input i would like to use LC filer ( ferrite bead , capacitor) in order not to load ADC input pin (PIC18F67J60) ?but i'm not shore of ferrite bead selection regarding to current spec of ferrite bead.5) regarding reverse voltage i think here will the negative clamp diode and fuse help? or am i wrong?-> 6:006) should i filter the 3V3 rail that is connected to clamp diode and pull-up resistor?Best Regards,0

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 05, 2021, 10:32:00 am
content: Does it really need 10/1000 surge?If so, I would be more concerned with the ground return path as well as the signal itself. That's a LOT of current to sink through a signal wire, or its ground return (or through whatever else is in your circuit, if a ground-return path can be found in that direction!) Perhaps an isolated sense circuit should be used instead. (Maybe you already have this, I don't know; we're only seeing a fragment here, not the whole system.)The PTVS is rated for 30kV ESD and will definitely handle EFT. No need to use two. Conversely the ESD diode probably handles surge (though isn't rated for 10/1000, but anyway you haven't given what test condition shall be used to test for 10/1000, who knows).Using a TVS makes the clamp diodes somewhat redundant. You can put the TVS towards the connection, then series resistor, then clamp diode, for somewhat better robustness. The main thing is to have resistance between the main ESD/surge clamping device, and the MCU, so that its internal ESD clamp diodes have limited current into them.And yes, the CDSU is fine, presumably the "4148" means it's the same thing just in a smaller package. There are duals and quads of those too, if you're interested.Filter cap in same place as clamp diode.Quote from: zero0d on February 05, 2021, 09:10:33 amOhhh there's a cross-wire requirement too? Why didn't you say so!PTCs aren't great, because they respond very slowly, 100s of ms. A TVS may not sustain fault current for that long. Would be something to test.Also, is this like a RTD sensor or something? Would it be better with 3 or 4 wire sensing? Different protection circuits can apply for that case too.Tim

author: penfold
author_rank: Frequent Contributor
timestamp: February 05, 2021, 11:37:39 am
content: Quote from: zero0d on February 05, 2021, 06:46:02 amDo you mean to say that the signal source is an analogue voltage with a source impedance of 1k? Just, from the way you've drawn the diagram (and with the 1k pull-up on your side) makes it look very much like a "resistive" sensor such as a thermistor.

author: Benta
author_rank: Super Contributor
timestamp: February 05, 2021, 12:06:29 pm
content: I suggest you look around at some of the semiconductor suppliers to see their solutions and get inspiration. A lot of ready-made protection devices are available for this kind of application. Here's an example (no affiliation):https://www.onsemi.com/products/isolation-protection-devices

author: zero0d
author_rank: Contributor
timestamp: February 05, 2021, 12:10:14 pm
content: I was looking at 10/1000 surge eg. IEC 61643-321 and i don't think we need this ( i think this is for telecommunications ).I attached the new schematic, if you can please comment.I'm not shore is i need 1206 1k series resistor or 0603 could be used?I wanted to ask is it is a good idea to place resistor or ferrite bead on GND path?@penfold, probably, i look at this as a simple resistor divider, with 1k pull-up on my side, and 1k pull-down on other side.

author: zero0d
author_rank: Contributor
timestamp: February 05, 2021, 12:12:12 pm
content: Hi Beneta,i was looking through this, do you have a specific device on your mind?

author: Benta
author_rank: Super Contributor
timestamp: February 05, 2021, 03:36:45 pm
content: Quote from: zero0d on February 05, 2021, 12:12:12 pmNot off the top of my head, but companies like ON, ST, TI, NXP etc. have comprehensive selections.

author: jkostb
author_rank: Regular Contributor
timestamp: February 05, 2021, 08:15:27 pm
content: You cannot connect sensor from 30m directly to ADC. Most ADC have internal capacitors which must be charged fast enough. That is why you need a buffer circuit which can supply the current and drive the ADC. So you need to go for circuit 2. If you buffer then you have a high impedance input. So the easy protection method is then to add resistance in the line to block the currents from the line. If you put at the input TVS protection, RC filtering and a common mode choke you can easily make the circuit pass during EMC immunity tests.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 05, 2021, 10:33:38 pm
content: It's just an MCU ADC, it's not one of those big obsolete bastards that needs low-ohms drivers. kohms is likely fine.Tim

author: gnuarm
author_rank: Super Contributor
timestamp: February 06, 2021, 01:44:52 am
content: Quote from: zero0d on February 05, 2021, 09:10:33 amI think this video is the typical sort of information that seems like it says a lot, but it doesn't give enough quantitative information to actually design anything. This guy basically threw the kitchen sink at the problem without actually explaining just how to size or calculate any details of the circuit.For example, he talks about a unidirectional TVS only being useful for unsymmetrical powered circuits and requiring two such devices. This ignores the circuit he is showing where the two power connections are +V and ground! Clearly the unidirectional TVS diode is a good match to this circuit using only one per signal.However, this ignores the fact that diodes other than Schottky diodes have too high a clamp voltage in the forward direction (clamping signals going negative) which typically has a minimum of -0.3V. The TVS is going to clamp at a voltage more like 0.7V not taking into account the potentially high currents.This is why I don't bother with TVS/zener diodes. I use Schottky diodes to the rails which come four diodes in two series strings per package, very small and cheap. Add a series resistor and you are in like Flynn. I've never seen a power supply rail that needed more capacitance to limit the voltage transient on the power rail from ESD.Am I doing this wrong?

author: nctnico
author_rank: Super Contributor
timestamp: February 06, 2021, 01:55:33 am
content: Quote from: Zero999 on February 05, 2021, 09:04:12 amAgreed. Never use the power supply to dump excess current into. What gnuarm is overlooking is that every IC has internal clamping / TVS diodes on the input (that is a gross over-simplication as IC input/output design is an art in itself but it suffices for a simple model). The only thing you need to do is limit the amount of current (energy) into a pin. Decent datasheets show the maximum current that may be forced into a pin. That is the current handling capability of the internal clamping circuitry; use that to your advantage.To the OP:Protecting ADC inputs is tricky because leakage current in the protection devices will cause a drop across any series resistance.Zeners typically have a large leakage current so not a good idea if you want to have some precision from the ADC. The simplest form is to have a series resistor at the input (say 100 Ohms to 1k Ohms), a TVS diode (low leakage) and then another series resistor connected to the ADC input. This way the ADC input is protected against excessive currents. The ADC input is also a good place to put a capacitor to ground forming an RC filter. If you expect high voltages then size the resistor at the input accordingly and adding a PTC fuse can help to protect the circuit against excessive currents.

author: coppice
author_rank: Super Contributor
timestamp: February 06, 2021, 01:56:01 am
content: Quote from: T3sl4co1l on February 05, 2021, 08:22:26 amAlways watch for those leakage issues with protection diodes. I see so many people just throw in a 1N4148, test their design, and pass it to production..... Then they find that results are terrible when the product reaches 50C, because the protection diodes are leaking like a sieve.

author: gnuarm
author_rank: Super Contributor
timestamp: February 06, 2021, 02:26:18 am
content: Quote from: nctnico on February 06, 2021, 01:55:33 amYou are talking about the parasitic diodes in CMOS devices, NOT every IC. These diodes are actually part of a parasitic NPNP structure which will form an SCR with the parasitic diode as the control gate. Dump much current into it and the part shorts between power and ground with the obvious consequences. That is why the spec on I/Os is 0.3V beyond the rails. Much more and the diodes are forward biased and the current starts to flow. Not good. That's why Schottky diodes are used to the rails. As someone else said, if you need it, add capacitance. That will absorb a lot of ESD. Otherwise add a single Zener to the power rail rather than one on each incoming line. On the incoming like the Zener won't stop the I/O from going past the 0.3V spec. On the power rail it limits the rail voltage which is much less sensitive.I'm happy to hear about well thought out ESD techniques. I think ESD protection is mostly designed like clock crossing circuits, "It seems to work". If I'm all wet I'm happy to hear a clear, factual, rationale why.BTW, it is very rare indeed that ICs have TVS style protection built in. Perhaps on some special devices. I have seen them on the gates of MOSFET discretes, some, not nearly universal.

author: nctnico
author_rank: Super Contributor
timestamp: February 06, 2021, 02:34:48 am
content: Quote from: gnuarm on February 06, 2021, 02:26:18 amNo, I'm talking about the input protection circuit which every IC has (except some very rare cases) in order to survive ESD and other events where too much current can flow into a pin (up to some level). The goal of external protection is to stay within the safe limits of the chip's input protection. The effect you are describing is a latch-up but AFAIK it is well known how to mitigate that and thus taken care of by the design of the input circuits.See this report for more information on how IC input protection is implemented:https://www.researchgate.net/publication/2978331_ESD_protection_design_on_analog_pin_with_very_low_input_capacitance_for_high-frequency_or_current-mode_applications/link/0f31753212829d9bd5000000/download

author: coppice
author_rank: Super Contributor
timestamp: February 06, 2021, 04:05:29 am
content: Quote from: Zero999 on February 05, 2021, 09:04:12 amMost zeners are not that tightly specified. If the zener turns on at a high enough voltage to be guaranteed to not affect the normal operation of the power rail, it might be too high to offer useful clamping of that rail. If the power regulator is only supplying a small amount of power, you might consider using a shunt regulator, which will tightly control the power rail when energy is dumped into it.

author: ebclr
author_rank: Super Contributor
timestamp: February 06, 2021, 04:19:34 am
content: That's isn't a nice approach, will be a better way to use the ADC remotely and communicate digitally with rs/ 422 /485 / canbus, which will be much more reliable, If you really insist on the analog way, a 4..20 ma loop will be less worse

author: xavier60
author_rank: Super Contributor
timestamp: February 06, 2021, 09:56:07 am
content: This issue might not apply in this case.I have noticed with PIC ADC inputs that if the input protection diodes become forward biased, significant crosstalk will be applied to other ADC input channels.

author: Psi
author_rank: Super Contributor
timestamp: February 06, 2021, 10:15:48 am
content: Ok, he's what i do for MCU protection, Not saying this is the best way, but I've never had any issues with it.I start by puting a 10k resistor in series. Since the ADC is usually designed for 10k input impedance.After doing that you can handle whatever voltage will cause 30mA into the IO pin.So 0.030 A * 10000 ohms = sustained 300V before you're over driving the pin.Then I put a TVS diode to ground on the MCU side to take care of any spikes higher than 300V.To prevent the TVS loading down the analog 0-5V signal I use a much higher voltage TVS than 5V. Something like 12 or 24V.But this creates a problem, if the TVS needs to clamp a load it's clamping voltage might be 30V which is too high for the 5V ADC pin.So I add an extra series resistor to make sure this clamping voltage cant overload the pin.30V clamp / 0.030A = 1kSo to sum up.MCU ADC pin ---- 1k ------+---- 10k ----- to voltage for sampling|24V TVS diode|GNDYou could argue that the 10k should in fact be 9k to get a total of 10K but it really doesn't matter.If for some reason I need more protection than this I go to a linear analog optoisolator

author: poorchava
author_rank: Super Contributor
timestamp: February 06, 2021, 10:25:54 am
content: I typically use (mostly custom industrial/building automation and some power electronics) a resistor in series that limits the current through clamping diodes. If need be some external diodes to mcu rails can be added. Beware though, Schottky types has considerable leakage. Silicon diodes are ineffective because of too high of a voltage drop. A front end TVS might be good, but for transients rather than permanent overvoltage, as their maximum clamping voltage is typically significantly higher than their working voltage.Adding significant resistance in series forces you to use longer sampling times, single the internal sampling capacitor of the ADC needs to charge up.

author: Psi
author_rank: Super Contributor
timestamp: February 06, 2021, 10:28:42 am
content: Quote from: poorchava on February 06, 2021, 10:25:54 amtrue, I've just never had a situation were 10k caused a problem. But i only really work on 8 or 10bit ADCs

author: poorchava
author_rank: Super Contributor
timestamp: February 06, 2021, 11:41:45 am
content: I often face situation where I have to sample voltage at switching frequency (say, up to 50kHz), but i need it settled fast. I those cases this makes a difference and requires a fast opamp as a front end together with some compensation of dividers. And I generally have to rely on at most 12-bit converters integrated inside MCUs.Otherwise, it's true, that usually adding an RC network like 10k + 1n/10n is sufficient. Sampling capacitance is usually in 10s of pf range anyway.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 06, 2021, 11:44:27 am
content: Quote from: gnuarm on February 06, 2021, 01:44:52 amSchottky are often not as great as they seem, and you may actually be making things worse anyway.Consider, the 0.3V or whatever voltage drop, is only at low currents. We're talking surges here, easily 10A or more (depending on condition and waveform). A signal schottky might be a low-value resistor under such conditions, if it survives at all (the surge rating even at t_p = 1us might not be enough).For example -- hmm, having trouble finding any with meaningful pulse ratings. A quick survey of manufacturers finds I_FSM from 100mA to 1A for BAS70 for example, with t_p ranging from 10ms to 1s. Typically the 1us rating isn't that much more than the 10ms rating, but these are pretty small diodes so maybe this is into the thermal-mass asymptote already, and we shouldn't expect much more current for shorter pulses?Friends like BAT54 and BAT85 are larger, the latter having a 10ms 5A rating at least.But compare BAT85 (200mA 30V 10pF) to BAV99 (70V 200mA 1.5pF, I_FSM = 8A at 300us (ON Semi)) and you see the performance per area (area as judged by capacitance) is so much better for PN diodes. Both only plot up to 1A for Vf(If) so it's hard to judge how much their internal resistances compare, that is to say, which has the lower Vpk(Ipk) under surge conditions.Anyway, what I meant by "making things worse" is, the schottky part typically has fairly high internal resistance, and is almost always paired with a PN diode ("guard ring") anyway. So, at high Vf, you're getting the combination of both, and despite the guard ring being a smaller area, it may end up doing most of the work! So you're really disadvantaging yourself, when you could commit all that area to a PN junction instead, and get lower Ri and Cjo as a result.Case in point, I don't recall any schottky based TVS arrays offhand, but PN diode types -- often with vanishingly small capacitances (giving passbands into the low GHz), are the norm. And unlike all these discrete diodes, they do publish surge ratings, and Vpk(Ipk) gives us a typical figure for Ri.In summary, it might be worth using them for different applications:1. Schottky are good at signal clamping, where you can guarantee the IC's ESD diodes won't be forward-biased, at currents up to some 10s, maybe 100s of mA.2. PN diodes are good at surge clamping, where the Vf will be rather high regardless (5, 10 even 20V), and only that much higher with schottky (say for given capacitance or leakage). So, if you need to guarantee the condition in (1) even under surge conditions, consider using two stages of clamp diodes, with a series resistor between them? For typical ESD handling (where momentary malfunction is permitted, or even a power cycling is required), the excess voltage isn't as much of a problem, just making sure the bulk of the surge goes through the protection device.And for purpose (2), zener/TVS are perfectly adequate -- the peak clamping voltage will again be some amount in excess of nominal, the point is to prevent destruction.We can combine this categorization with another one: time.a. Very short pulses (ESD and EFT) carry very little charge (10s A, 50ns or thereabouts), so are effective to divert into supply rails where only a few uF are needed to control it. It's very unlikely that the PRF (pulse repetition rate) will be high enough to raise the DC voltage.b. Surges carry a lot of charge (10s, potentially 100s or 1000s of A, 10-1000 us), so would be difficult to bear in in all but the largest supplies. Which, in fact, that's a good way to do it, if you're doing a power supply for example -- the bulk filter cap can absorb a lot of charge, give or take source impedance (which can be controlled to some extent with say, CMC resistance for smaller supplies, or inductance for larger ones) and rating. (Indeed you can even buy surge protectors that are little more than a few hundred uF hanging off a FWB -- they offer potentially unlimited life in comparison to MOVs, the downside being they only work for surges up to a certain amplitude and duration.)For everything else, surges need to be clamped, and for that, there's the range of TVSs, MOVs, GDTs, SIDACs and so on. (Note that TVSs and MOVs respond ~instantaneously, and do not latch. GDTs and SIDACs are latching types, and have some delay: GDTs delayed depending on peak overvoltage, and SIDACs taking low to fractional µs to fire. Note, latching types are unsuitable where continuous power is available.)c. Very long surges, or continuous overvoltage (cross-wiring, etc.). This might not even be clamped at all.Automotive load dump is a very long surge (~100ms), so has enough energy to melt just about anything. TVSsareavailable -- and they're expensive! (For that matter, mains surge TVSs are available too, and even more expensive.)For continuous duty, whatever the current and voltage are limited at, is that much power that must be dissipated. This gets intensive, very quickly.So, crowbarring a fuse is a good strategy, if fuses can be replaced. Polyfuses can allow automatic recovery, as long as it's not extreme (they're most effective at small ratings, say under 40V 5A nominal) and not too repetitive (they're a protection system, not a limiting system -- typically they're only good for 100s, maybe 1000s of cycles).Current limiting circuits, like depletion MOS, can be very useful. Or a regulator or switch at the power input, to provide ride-through or self-disabling under high voltage conditions. Note that these have high incremental resistance (i.e., a CCS characteristic), or even negative resistance (turning off past a threshold), which can cause the input voltage to rise even further; and these will be implemented with semiconductors that have a definite avalanche voltage, so it's most likely going to come back to some more basic kind of voltage clamping and/or current limiting device in the end, for example a MOV and fuse.QuoteSo, as explained above -- if what's being protected is ESD, that's a short pulse with little charge, so it's easily absorbed by most supply rails. Perfectly fine.Some applications might vary, say if you've got a mix of analog and digital supplies, or asymmetrical supplies, or need the input clamped to some particular range. It could be better to use a 5V zener/TVS instead of clamping to a very small 5V supply.Of course, signal clamping is best done with a dedicated stage, simply because discrete devices are relatively sloppy. You can avoid the "merely" exponential transfer curve of a diode, using an op-amp or two -- or more rather, magnifying that curve to the point where it looks abrupt enough "for government work".Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 06, 2021, 12:01:19 pm
content: Quote from: gnuarm on February 06, 2021, 02:26:18 amKeep in mind, there's good reason why they publish current limits (well, sometimes they do..). 74HC for example is rated to 100mA into the ESD diodes without latchup -- they do indeed latch at currents not much higher than that, but you're pretty safe otherwise.MCUs often have rating specified by pin, for example the pins with ADC stuff on them might be limited to +/-0mA for proper operation -- presumably, the ESD diodes would leak charge into nearby analog stuffs, causing malfunction. Maybe the ADC readings get skewed, maybe it affects the minimum sampling clock (i.e. the signal leaks away faster), who knows. It's probably not latchup, but all the more reason to avoid such operation, after all.Cool fact, the ESD diodes, are effectively thyristors as mentioned. And the two back-to-back transistors making up that thyristor? Typically, the pin connects with the emitters of the respective BJTs. The bases of which are tied to the respective supply. When you draw current, you're actually forward-biasing the B-E junction. The collector? It draws current from the opposite rail! The hFE of these transistors is low (~1), but that still means the alpha is significant (~0.5), so a sizable fraction of current drawn in/out the pin, is also drawn as supply current. Which means it's a whole lot harder to overvoltage an IC this way -- it's not diverted into the supply, it's largely diverted to GND. This also means the power dissipation goes way up!There's also an even smaller hFE between adjacent pins, at least for multiple-input gates -- this is on the order of 3%, pretty small, but does increase the leakage current noticeably. That is, measuring the leakage of pin 1A while dumping current into pin 1B.(At least, this is the behavior of the 74HC family. YMMV.)QuoteI think this is true for most generic CMOS devices -- from traditional logic to ASICs. The special cases you'll find otherwise, are special pins, or certain families, where input voltage range is not referenced to supply voltage, or where the output is effectively disabled when unpowered. 74LVC is a classic example, with I think 6-7V zeners on the inputs (or maybe they're snapback diodes, I don't know), and disabled-when-unpowered outputs.Some MCUs and FPGAs offer 5V-tolerant pins (inputs, sometimes open-drain outputs too), which must have similar protection. Sometimes they're still referred to VDD, but with a higher offset; I wonder how that's done (maybe a low voltage snapback diode?). STM32s are like this, for example.Tim

author: Zero999
author_rank: Super Contributor
timestamp: February 06, 2021, 07:44:35 pm
content: Quote from: coppice on February 06, 2021, 04:05:29 amGood point. What about speed? A shunt regulator might be too slow. Perhaps both a zener diode as well as a shunt regulator could be used together.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 07, 2021, 12:01:09 am
content: If you need very tight supply, adequate bypass to fill in for short periods, and an active shunt regulator for the long term, is a fine way to go. The regulator will have limited current capacity, so another solution should be found to withstand larger surges.Tim

author: Zero999
author_rank: Super Contributor
timestamp: February 07, 2021, 03:18:24 pm
content: Quote from: T3sl4co1l on February 07, 2021, 12:01:09 amI got my wires crossed. I thought he meant adding the shunt regulator for overvoltage protection, when it should have been obvious the idea was to use it as a voltage regulator, in which case it should work well, as the decoupling capacitors will take care of the spikes.

author: coppice
author_rank: Super Contributor
timestamp: February 07, 2021, 04:09:47 pm
content: Quote from: T3sl4co1l on February 06, 2021, 12:01:19 pmThe increasing number of devices with complex power rail arrangements, as core voltages decrease while external voltages remain high, seems to have brought intersting new ways for protection circuitry to do funky things, like lock up at the wrong moment. This is especially true during power up and power down sequences. I think the TVS approach is getting more common, even though it takes more die area.

author: zero0d
author_rank: Contributor
timestamp: February 08, 2021, 08:59:38 am
content: Dear all,what do you think of my proposed schematic ?Also can you comment why is better to place TVS after ferrite bead and not before ( to protect the Ferrite bead and input cap )?Current configuration -> Ferrite bead - Cap - Fuse - TVSSecond configuration -> Fuse - TVS - Ferrite bead - CapWhich is better and why ?Best Regards.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: February 10, 2021, 01:20:30 pm
content: I have already suggested several times in this forum to use an analog optocoupler with two combined LEDs. Take a look at the HCNR200-500E in the datasheet there are good use cases.https://datasheet.lcsc.com/szlcsc/1811141238_AVAGO-Broadcom-HCNR200-500E_C23785.pdf