Backfeeding a linear voltage regulator - Page 1
Forum section: electronics / projects

author: leonerd
author_rank: Regular Contributor
timestamp: September 26, 2018, 02:38:15 pm
content: What happens if a 3.3V linear regulator (e.g. the ubiquitous AMS1117) sees 5V on its output pin? Anything bad?I'm wanting a switchable 3.3V or 5V power rail. I wonder if I can do this by putting a bypass pFET in parallel with the regulator, and feed 5V in.FET off: regulator outputs 3.3VFET on: output is full 5VBut does the regulator mind this setup? It seems that most LDO regulators are a pFET pass transistor plus some control logic on its gate, so I can't imagine the pass transistor getting upset that now Vs=Vd. But perhaps any of the output feedback / control circuitry, that normally never sees more than 3.3V.

author: CJay
author_rank: Super Contributor
timestamp: September 26, 2018, 02:51:18 pm
content: Why not just jack up the ground pin on the regulator with three diodes in series then short them out with the FET when you need the 3.3V?A tried and true method of increasing the output of a linear regulator.

author: leonerd
author_rank: Regular Contributor
timestamp: September 26, 2018, 02:55:17 pm
content: It seems simpler to just add that parallel pass transistor - it's one single extra component. If it works, that isOtherwise I have to pull my 5V output through the regulator anyway and incur its own dropout voltage. With a parallel pass pFET I only have channel resistance to contend with.

author: leonerd
author_rank: Regular Contributor
timestamp: September 26, 2018, 03:05:00 pm
content: Oh - tiny added complication: the switching transistor is controlled by a microcontroller which itself is powered by the output of this arrangement. It allows the MCU to boot on 3.3V, but switch up to 5V if required. So any solution has to be capable of self-starting in that manner.

author: CJay
author_rank: Super Contributor
timestamp: September 26, 2018, 03:07:27 pm
content: Try it then, it might work, but personally, I'd be concerned about longevity especially if there isn't a specific rating in the datasheet for it.I don't think this will be necessary for only 1.7V but on the linear regulator boards we used to build we fitted a diode from output to input to prevent the regulator being damaged by reverse current flow.

author: leonerd
author_rank: Regular Contributor
timestamp: September 26, 2018, 03:10:42 pm
content: Quote from: CJay on September 26, 2018, 03:07:27 pmI've not yet seen any datasheet make a comment on the subject of overvoltage into the output pin. It's something manufacturers don't seem to make a comment on.QuoteI don't think there'll be any reverse flow, because Vout=Vin here. The input will be permanently 5V powered, and the output can't get any higher than that.

author: CJay
author_rank: Super Contributor
timestamp: September 26, 2018, 03:31:10 pm
content: Like I say, try it.I tend to avoid doing things that aren't specified in the data sheets though I suspect you'll be OK.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 26, 2018, 03:37:19 pm
content: Is the regulator supplied by 5V, and being bypassed, then?The datasheet will say if it is acceptable to overdrive the output voltage, and by how much. Usually the limit is Vin+0.3 max, which you will not exceed with a bypass switch.Tim

author: leonerd
author_rank: Regular Contributor
timestamp: September 26, 2018, 04:12:19 pm
content: Quote from: T3sl4co1l on September 26, 2018, 03:37:19 pmYes. That's the situation.QuoteI was hoping it would say something about that situation, but so far the data sheets I've looked at didn't appear to make comment. Do you have a reference to some specific wording as an example I can look for?

author: David Hess
author_rank: Super Contributor
timestamp: September 26, 2018, 04:15:53 pm
content: Bipolar regulators with an emitter follower output should tolerate an output voltage which is a couple volts higher than the input voltage.If you want to be sure, add a series diode to a regulator which uses remote sense. Or add remote sense to an existing regulator using an operational amplifier.

author: langwadt
author_rank: Super Contributor
timestamp: September 26, 2018, 04:25:49 pm
content: Quote from: leonerd on September 26, 2018, 03:10:42 pmplenty of datasheet that recommend a diode from output to input to avoid damaging the regulator ifyou short the input and thus discharge the bypass capacitor backwards through the regulator

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 26, 2018, 05:44:51 pm
content: Quote from: leonerd on September 26, 2018, 04:12:19 pmWell.. the one you're looking at is pretty shit:http://www.advanced-monolithic.com/pdf/ds1117.pdfAbs max input voltage 15V. No other limits. Uh. So we can put, say, 600V on the output, or the ADJ, and it's fine, right?...yeah...sure.They don't say. Doesn't get a design win from me.They do go on later to describe a bit,QuoteOkay, so they've got internal diodes. Would've been nice if they documented that in the abs max ratings (namely, "max output <= input + 0.3V" would do).For example, this one specifies:http://ww1.microchip.com/downloads/en/DeviceDoc/20001826D.pdfwell, "all inputs with respect to"...nothing? Doesn't make much sense, but presumably they're talking about any other pin (besides VDD). Which, being a three-terminal part, is just the output pin, within the range shown.That leaves VDD negative unspecified, unless that's included as well because only VDD positive was specified. Who knows.Design rules usually work that way, carving out only the condition that they specify and leaving other cases to default to another lower precedence rule. Which means VDD negative would default to the >= VSS - 0.3V condition. Rules like this, it's logic, it's not inconsistent, it's just annoying and tedious.They also specify the body diode on the schematic (and don't get it backwards, which I've seen before...), so we should also expect this one to behave when its output is pulled up, until the diode kicks in (beyond VDD + 0.3V).I've seen some regulators that claim operation with the output above input (no protection diode needed, within the ratings that is), although I don't remember what type that was.Anyway, for this case, it should be okay, but that AMS1117 datasheet is badly written.Tim

author: Siwastaja
author_rank: Super Contributor
timestamp: September 27, 2018, 10:46:52 am
content: Some blow up. Others don't. Some include internal diode from OUT to IN, so while the regulator doesn't blow up, it backfeeds into the input, which might be something you don't want.Datasheet should tell you about this, but often won't. If not explicitly disallowed, you still need to test.