dropout voltage of linear regulators when Vin is too low - Page 1
Forum section: electronics / projects

author: JeanF
author_rank: Regular Contributor
timestamp: August 05, 2012, 11:51:06 pm
content: Hello ! This is my first post to this great forum. After reading some great threads here, I couldn't resist to subscribe ! I'm an engineering student in France, by the way. I'm interested in electronics, even if it isn't my school's specialty... I think I'm still a beginner, but probably not a complete newbie. Who knows !I've a question which I didn't manage to solve with Google so far, but it's probably because I don't know the correct terms, so I'm sure someone here will be able to make it clear.How does the majority of linear regulators behave when their input voltage is lower than Vout + nominal additional voltage ? It's not in all datasheets. I've made some experiments with LM2940CT-9, which will output (nearly) Vin if Vin < 9.5v, and this behavior is indeed specified in the datasheet. The same with LP3965-adj that I used in another project ; it outputs Vin minus a very small drop at low Vin voltages... and suddenly falls to 0 when the input voltage is really too low.78xx series seem to drop a considerable voltage in these situations, i've measured 0.8 to 2.4v of drop, even without load. I couldn't find info on this topic for the more powerful LT1085, for example.So, what's the most common situation ? regulators which behave like that : (from 2940ct datasheet)or regulators which behave like 7805's, with a great voltage drop ?Thank you very much !JF

author: Rufus
author_rank: Super Contributor
timestamp: August 06, 2012, 12:04:21 am
content: Quote from: JeanF on August 05, 2012, 11:51:06 pmThere is no 'majority' of linear regulators. Regulators which are designed to operate with little voltage across them are know as LDO (Low Drop Out) regulators.Data sheets for LDO regulators will tend to specify what happens with low voltages across them because that is one of their main design features. Data sheets for non-LDO regulators like most data sheets will tell you under what conditions the device functions correctly and not go into detail about how it functions incorrectly outside those conditions.

author: JeanF
author_rank: Regular Contributor
timestamp: August 07, 2012, 08:46:00 pm
content: Thank you very much. I didn't make connection between this sort of low voltage behavior and the fact for regulators to be LDOs. That's a shame, because both of regulators I've tested which showed this feature were, actually, LDOs.In fact I'm looking for such a thing, to make regulated 12v (or the highest possible voltage if it's <12v) out of a lead battery, which will vary between 14v and 11.5v depending on its charging state.Let's look into more LDOs datasheets !! :-DDo you have a preference when it comes to the brand of linear regulators ?

author: mariush
author_rank: Super Contributor
timestamp: August 07, 2012, 09:05:21 pm
content: The most common very low drop linear regulators can require very low voltage difference.For example, MIC2941 needs something as low as 40 mV at low loads and up to 350 mV at close to its maximum of 1A of current:http://www.micrel.com/_PDF/mic2940.pdfSo as long as your battery will give it at least 12.35v it should output a clean 12v.You won't be able to get 12v when you battery goes below 12v, well.. unless you resort to something like voltage doubling circuits or step-up (boosting) regulators.If your battery will actually be inside a car, then it's not recommended to go this route (ldos) because at some times when you start the car or maybe when you start the lights at nights, you might get voltage from battery as low as 9-9.5 volts.Consider using a step-up (boost) voltage regulator.

author: JeanF
author_rank: Regular Contributor
timestamp: August 07, 2012, 09:54:29 pm
content: thanks, I didn't know Micrel.Well, I won't need any voltage boost. It is very unlikely that the battery will ever drop below 12v : it will be a small sealed battery set a floating voltage (around 13.8v) to make a rudimentary uninterruptible power supply for 12v equipment (say, a small ethernet router, etc) and I don't expect power cuts long enough to discharge completely the battery (I'll design some sort of auto cut-off to preserve it, eventually). I could probably power directly these devices with 14v and they'd survive, because there must be onboard step-down regulators to make 5v, 3.3v, 1.6v or whatever, but I don't want to open the cases (and void warranties) to check the circuitry at the moment :-)

author: tom66
author_rank: Super Contributor
timestamp: August 07, 2012, 11:21:34 pm
content: Also, don't forget good input filtering (large ~100µF cap, fuse and TVS -minimum-), if you are hoping to get this anywhere near an automotive electrical system...