Is it necessary for providing Reverse Current protection for an LDO ?? - Page 1
Forum section: electronics / projects

author: madhu.wesly01
author_rank: Regular Contributor
timestamp: June 28, 2018, 06:58:13 am
content: Hi,I want to use TPS7A8300 in my design. My application is Battery powered so there will be the minimum voltage of even 2V(or Less) at Vin pin of the LDO.Since this LDO does not have Reverse current protection, will there be any issue if I do not provide Reverse current protection externally???One more doubt is will there be any chance of going VOUT higher than VIN in the above application?? This LDO doesn't Boost at any case right??TPS7A8300 Datasheet:http://www.ti.com/lit/ds/symlink/tps7a8300.pdfThanks in advance.

author: capt bullshot
author_rank: Super Contributor
timestamp: June 28, 2018, 07:39:35 am
content: Depends ...In most use cases you won't need that protection.E.g. figure you short the input pin to GND, and you've got a large capacitor at the output pin. This capacitor discharges then through the LDO, and this is one case you want the reverse protection. If you ensure the input pin won't be shorted when someone disconnects the battery, no protection is required.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 28, 2018, 07:46:26 am
content: Input short is surprisingly common, especially in development products, or any product which is going to be powered by the end user from anything else than a specific supply provided by you as the manufacturer.While people tend to expect that a short circuit can damage the device that outputs the shorted voltage, it's going to be very difficult to many to grasp thatdownstreamdevices may blow up, as well.Also, capacitance mismatch between in/out does happen quite easily; if you are not adding the basic protection, you actually need to analyze this!After a few unexpected failures in cases where it "shouldn't have happened", I started designing in reverse current protection diodes for the regulators that lack it, (almost) every time, except when absolute cost reduction and miniatyrization is needed, in which case, more time can be spent in design analysis.To me, the lack of the protection shows the regulator IC designers are not doing the level of integration that should be expected from an integrated regulator. It makes little sense to add external diodes when it's much cheaper to just integrate it in the IC process, along with the other power silicon devices on the die, but if the regulator lacking this protection otherwise performs better than the other, properly integrated products, then you have little choice. (In some cases, the lack of protection might be to avoid noise coupling through the diode capacitance, or something like that. If you need these specs, then adding the protection might negate some of the reasons you are using the regulator for.)

author: digsys
author_rank: Unknown
timestamp: June 28, 2018, 07:55:27 am
content: I'd add a polyfuse at the battery input, and a clamp zener (on the circuit side of the poly). That will cover the most serious possibilities.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 28, 2018, 08:03:01 am
content: Quote from: digsys on June 28, 2018, 07:55:27 amWhile this provides reverse voltage and (limited energy) overvoltage protection, this doesn't offer the reverse current protection at all, which is the subject of this thread, and the seriousness of which is similar or even worse, because this failure mode is often totally unexpected.

author: digsys
author_rank: Unknown
timestamp: June 28, 2018, 08:17:43 am
content: I've rarely ever seen a "higher Vop than Vip" cause any issues. The reverse Vip <> Vop diode takes care of that.In a normal case, the bridge rectifier halts any more progress of "reverse" flow. However, in this case, we have a battery at Vip, which should ALWAYS beprotected. I was aware of the original question, and it was pretty much covered in a previous post ! I added an important addition.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 28, 2018, 11:20:21 am
content: Quote from: digsys on June 28, 2018, 08:17:43 amThat diode isexactlythe protection we are talking about.Of course you are not seeing reverse current issues if you have that protection in place.Many regulator ICs have this diode built-in, but some don't.

author: tszaboo
author_rank: Super Contributor
timestamp: June 28, 2018, 11:32:51 am
content: This was required for linear regulators powered by 230V and transformers, where there was 1000s of uFs on the output to get rid of 50Hz ripple. Then those capacitors could blow the regulator. But I hardly find this relevant anymore.

author: Peabody
author_rank: Super Contributor
timestamp: June 28, 2018, 02:19:32 pm
content: You may be able to use a P-channel MOSFET installed "backwards" for polarity protection. That would provide much lower voltage drop, and power loss, than a diode. You would need to make sure the MOSFET will turn on fully at your lowest battery voltage, and handle the max current. Here's an excellent video on this method:And if you don't need the gate Zener, then it's only the MOSFET, no additional parts, and the cost probably isn't materially more than a Shottky diode.Edit: Sorry. Looks like I answered the wrong question.

author: David Hess
author_rank: Super Contributor
timestamp: June 28, 2018, 02:55:06 pm
content: Reverse protection is usually not needed and some regulators include it anyway at moderate voltages and currents. The times that it is needed include:1. Unusually large amounts of output capacitance.2. An input connected crowbar circuit.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 28, 2018, 03:18:17 pm
content: Quote from: David Hess on June 28, 2018, 02:55:06 pmI might want to list a few more:3. User-accessible input on a module-type product, where an accidental short circuit can occur. Seen this blowing regulators about 2-3 times.4. A shared input bus, where a short can occur due to other devices.5. Other devices share the input, and consume non-negligible current. Seen this blowing regulators about dozen of times.6. The regulator is used to charge a supercap or backup battery (extension to point 1). Seen this blowing regulators about 2-3 times.Especially #5 happensall the timein many real-world non-trivial projects that are bigger than simple LED blinkers. These projects can be even fairly small, think about the input supplying a higher voltage display backlight, and the regulator supplying an MCU. Now you need to analyze the MCU consumption (which can be minimal) from the output caps versus backlight consumption from the input caps so that the voltages ramp in the correct order in all cases, including corner cases...This requires careful analysis of power sequencing - or the protection diode, integrated or external. This diode, of course, needs to be sized to handle the reverse current.The assumption that the regulator input only goes to the regulator and nothing else can consume current from it is insane. It's truesometimes, but not something you can assume about a generic project. If you don't actuallyprovethe opposite, you should definitely add the protection.Which is why most regulators integrate at least some level of reverse current protection, so I'm not surprised people find it's rare to see any problems. The conclusion is just wrong - it's not rare because the protection is notneeded- it's rare exactly because the protectionisneeded and hence integrated, even when you don't realize it by using a modern regulator IC.All this is very important when dealing with those regulators that won't provide a meaningful level of reverse current protection.

author: SiliconWizard
author_rank: Super Contributor
timestamp: June 28, 2018, 03:22:23 pm
content: Quote from: Siwastaja on June 28, 2018, 03:18:17 pmYep. Again, just shorting the input can lead to problems and depending on the design, is not at all unlikely to happen. Even if it's just during the development phase.Back when we were using LM317's a lot, I was always adding the extra protecting diode. Failing to do so was usually a bad idea.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 28, 2018, 03:23:49 pm
content: Quote from: NANDBlog on June 28, 2018, 11:32:51 amAre you serious?These 100/120Hz filtering bulk caps are normally placed at theinputof the regulator. This way, the regulator keeps over the minimum drop-out voltage all the time. If they were at the output, the regulator wouldn't be regulating most of the time.As they are in the input, they are effectively offering someprotectionagainst the reverse current case, meaning fewer of those conditions listed above come true. Input short is still a concern.

author: tszaboo
author_rank: Super Contributor
timestamp: June 28, 2018, 08:00:07 pm
content: Quote from: Siwastaja on June 28, 2018, 03:23:49 pmI didnt write they were god designs.You really need to learn reading between the lines.There. Random crap from the interwebs.

author: David Hess
author_rank: Super Contributor
timestamp: June 30, 2018, 08:35:08 am
content: That is interesting. The collector-base junction of the PNP transistor will protect the regulator; it makes an excellent high power diode.That output filter is insane.