5V regulator outputting 5.9V and more. - Page 1
Forum section: electronics / beginners

author: Simon
author_rank: Unknown
timestamp: December 08, 2019, 04:21:18 pm
content: I made a small board with a µC and a 5V regulator. On power up i was surprised to find that it outputs 5.9V. I thought maybe it does not have enough load so i put a 510 ohm resistor on its output as the datasheet says that all the specs given are at 10mA. But now it outputs 6.1V.is it a dodgy part? did I overheat it on re-flow? It's a LM2931ADT-5.0RKG

author: Wimberleytech
author_rank: Super Contributor
timestamp: December 08, 2019, 04:51:07 pm
content: Quote from: Simon on December 08, 2019, 04:21:18 pmMeasure the voltage on all three pins (right at the pins of the regulator). Let us know what you read.

author: Simon
author_rank: Unknown
timestamp: December 08, 2019, 05:03:40 pm
content: that is the voltage on the regulator pin. There is a 330nF capacitor either side as close as practical.

author: andy3055
author_rank: Super Contributor
timestamp: December 08, 2019, 05:06:15 pm
content: Can you post the schematic you used?

author: Wimberleytech
author_rank: Super Contributor
timestamp: December 08, 2019, 05:08:52 pm
content: Quote from: Simon on December 08, 2019, 05:03:40 pmI would like to know that pin 2 (the ground pin) is actually zero volts.You should also look at the pins with an oscilloscope to be sure that there are no oscillations.

author: andy3055
author_rank: Super Contributor
timestamp: December 08, 2019, 05:09:54 pm
content: I assume you have this:https://www.onsemi.com/pub/Collateral/LM2931-D.PDF

author: Simon
author_rank: Unknown
timestamp: December 08, 2019, 05:11:01 pm
content: Well relative to that 6V output the ground pin is 0V I took the measurement across the regulator. I'll shove the scope on it.

author: Simon
author_rank: Unknown
timestamp: December 08, 2019, 05:17:31 pm
content: Quote from: andy3055 on December 08, 2019, 05:09:54 pmYes

author: andy3055
author_rank: Super Contributor
timestamp: December 08, 2019, 05:23:48 pm
content: So, which circuit did you use?

author: thinkfat
author_rank: Unknown
timestamp: December 08, 2019, 05:43:39 pm
content: It might oscillate. Try with a larger output capacitor.

author: Simon
author_rank: Unknown
timestamp: December 08, 2019, 05:59:27 pm
content: Quote from: andy3055 on December 08, 2019, 05:23:48 pmRegulator, 330nF either side

author: Simon
author_rank: Unknown
timestamp: December 08, 2019, 06:12:28 pm
content: I see they don't suggest any value of capacitor, but some example diagrams have what appears to be 100µF

author: wraper
author_rank: Unknown
timestamp: December 08, 2019, 06:19:25 pm
content: Quote from: Simon on December 08, 2019, 06:12:28 pmReally? Sometimes it's worth to read the datasheet besides simply looking at example schematics. Also note that LDO are not 78xx. You cannot simply slap few hundred nF on output and expect them working properly.

author: james_s
author_rank: Super Contributor
timestamp: December 08, 2019, 06:20:54 pm
content: 330nf is much, much smaller than I would typically use. I bet if you scope it you'll find that it's oscillating, I ran into that once when I used too small capacitors on the input and output of a 7815.

author: Simon
author_rank: Unknown
timestamp: December 08, 2019, 06:21:12 pm
content: OK, fair copI never saw that way of explaining it before.

author: wraper
author_rank: Unknown
timestamp: December 08, 2019, 06:32:13 pm
content: If you look at max ESR value, It's probably best to use MLCC or LOW ESR tantalum. Keep in mind that if you choose electrolytic, ESR most likely will be above mentioned 0.4 Ohm for 10-22 uF capacitors even if they are LOW ESR type.

author: Simon
author_rank: Unknown
timestamp: December 08, 2019, 06:52:35 pm
content: what abut my existing 330nF with an electrolytic?

author: james_s
author_rank: Super Contributor
timestamp: December 08, 2019, 07:02:22 pm
content: It's a common technique to pair an electrolytic with a ceramic capacitor, not as necessary as it used to be but it will still work.

author: Simon
author_rank: Unknown
timestamp: December 08, 2019, 07:18:12 pm
content: Just remembered, i have some nice 150µF polymer capacitors at work.

author: floobydust
author_rank: Super Contributor
timestamp: December 08, 2019, 10:15:12 pm
content: LM2931 is an old 1st gen LDO, finicky about the output capacitance (value and ESR) and I would use something else. Ensuring it stays stable over temperature can be a challenge when including the O/P capacitor's behaviour. 150uF solid polymer may be outside the stable range for ESR at room temp. alone.

author: MagicSmoker
author_rank: Super Contributor
timestamp: December 08, 2019, 11:24:16 pm
content: Quote from: floobydust on December 08, 2019, 10:15:12 pmYep, this. I like the NCP5501 series of LDOs because they are pin-compatible with the 78xx series and have much more relaxed ESR/min. capacitance requirements. The LM2931 and xx1117 series should be left to the surplus bins and stocked only for replacement purposes.

author: james_s
author_rank: Super Contributor
timestamp: December 09, 2019, 04:17:07 am
content: There's nothing really wrong with them, regardless of what part you use it's prudent to study the datasheet carefully. All components have limitations and this is a classic case where RTFM would have avoided the whole issue.

author: floobydust
author_rank: Super Contributor
timestamp: December 09, 2019, 04:45:15 am
content: The LM2931 has more traps than usual. Noobs use a MLCC or polymer output capacitor and have troubles, especially in cold weather.Jim Williams did a nice write up on using short PCB traces a few mm long to add ESR, have to dig for that LT app note.

author: wraper
author_rank: Unknown
timestamp: December 09, 2019, 05:11:57 am
content: Apparently LM2931 from different vendors are not equal. Say according to ON semi datasheet it does not have any problems with extremely low ESR (MLCC). But according to TI, it should not be to low. On the other hand, TI can tolerate double ESR on the high side. IC schematic in the datasheets has some differences too.

author: wraper
author_rank: Unknown
timestamp: December 09, 2019, 05:18:32 am
content: Quote from: floobydust on December 09, 2019, 04:45:15 amMore like little problems with half decent MLCC (given added series resistance or using LM2931 from ON semi) and big problems with aluminium electrolytic at low temperature according to those charts. It actually recommends poly cap (OS-CON).

author: floobydust
author_rank: Super Contributor
timestamp: December 09, 2019, 05:44:09 am
content: MLCC has a huge voltage coefficient of capacitance which makes themuselessat the 100uF 5V level. You're lucky to get 22-30uF at room temp in 1206 i.e.TDK C3216JB1A107 datasheetThe ST datasheet graph is even worseI hate parts like this because a second-source actually ends up being a terrible mistake in production, when supply chain assumes it's the same part number.

author: james_s
author_rank: Super Contributor
timestamp: December 09, 2019, 07:42:10 am
content: You really have to be careful with MLCC, many people are blissfully unaware of how dramatically the capacitance drops with DC across them. I was also not aware of this trait until it bit me a few times, fortunately in one-off hobby projects. It usually works fine to sprinkle on a bunch of 100n decoupling caps, but if you actually try to calculate the needed capacitance it will really get you if you don't account for that.

author: Simon
author_rank: Unknown
timestamp: December 09, 2019, 07:55:02 am
content: well my choice of part was dictated by the load dump protection. So a normal electrolytic might be better? will have to check the specs on those caps I have at work. I just need to get this working, do a demo for my course work and move on.

author: Simon
author_rank: Unknown
timestamp: December 09, 2019, 07:55:33 am
content: Quote from: james_s on December 09, 2019, 07:42:10 amYea, that's why you use 25-50V parts even at 5V

author: wraper
author_rank: Unknown
timestamp: December 09, 2019, 10:15:51 am
content: Quote from: floobydust on December 09, 2019, 05:44:09 amLOL, it's almost like ESR requirements for ON semi and ST are mutually exclusive.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 09, 2019, 10:32:46 am
content: Yeah, while old-schoolnon-low-drop linear regulators such as 7805 are still fairly OK, avoid most old low-drop regulators, as they tend to have strict requirements for the exact types and values of output capacitors.Market is full of regulators specified to be stable with ceramic output capacitors; use them whenever possible. Also try to find one which is happy with lower capacitance, allowing smaller solution area and possibly lower total cost. Some are OK with 1uF while others require 10uF.Using specific types and values of output capacitors in your BOM tends to eat away the price benefit you get from a cheap, old-tech regulator.

author: Simon
author_rank: Unknown
timestamp: December 09, 2019, 11:47:06 am
content: I thought it was a newish regulator as it has the load dump capability, in the past I only ever found 5V regulators that needed wrapping in protection circuitry and rarely accepted auto range voltages. Seems like they took and old design and beefed it up but it's still the old design. I will remove it from my parts library, while the input will withstand the auto environment clearly as an overall part it will not survive the full range of conditions.

author: MagicSmoker
author_rank: Super Contributor
timestamp: December 09, 2019, 12:37:53 pm
content: Quote from: Simon on December 09, 2019, 11:47:06 amI would be very wary of trusting any regulator whose datasheet says it can withstand "load dump" unless the manufacturer certifies compliance with ISO 16750-2 (or the older 7637-2) and specifies the minimum series resistance needed to do so.Good practice with any kind of automotive product is to add at least a 24V nominal TVS diode, preferably in a SMC or larger package, to provide the bulk of load dump protection. That said, whaddya need an LDO for to provide 5V with a 12V nominal input? If it's to maintain operation even during engine cranking that is not something I would entirely entrust to minimizing the dropout voltage; a blocking diode and some bulk capacitance before the regulator would be much more reliable (and give better load-dump tolerance to boot).

author: Simon
author_rank: Unknown
timestamp: December 09, 2019, 12:40:13 pm
content: TVS is for spikes, I don't know what they mean by load dump buh I take it to mean the surges that should not exceed 40V. I have a 33V TVS

author: MagicSmoker
author_rank: Super Contributor
timestamp: December 09, 2019, 12:44:26 pm
content: Quote from: Simon on December 09, 2019, 12:40:13 pm"Load dump" is the term for the voltage overshoot from an alternator that occurs when a heavy load is suddenly disconnected (usually the battery while it is being charged, but it could be any heavy load like electric power steering or the like). An "okayish" article on load dump is here:https://www.electronicdesign.com/power/eliminate-those-automotive-load-dump-circuit-protection-headachesYou definitely want to get a copy of ISO 16750-2 if you are making commercial products for on-vehicle use.

author: james_s
author_rank: Super Contributor
timestamp: December 09, 2019, 04:55:19 pm
content: Quote from: Simon on December 09, 2019, 07:55:33 amThat won't help you. The reduction in capacitance is more closely related to physical package size, higher voltage rating does not really affect this. Pull up some datasheets and examine them closely and you'll see what I mean. This is generally a good habit to get into.

author: Simon
author_rank: Unknown
timestamp: December 09, 2019, 05:42:39 pm
content: Well what else do you do? i have seen some capacitors with a graph for capacitance versus voltage but few have them.

author: james_s
author_rank: Super Contributor
timestamp: December 09, 2019, 06:41:31 pm
content: If you want to use MLCC for DC filtering you have to find a datasheet with that graph and choose the value accordingly. If you can't find the data for the exact capacitor you are considering you ought to be able to find one for a part with the same dielectric. It's a pitfall of this type of capacitor and something to be aware of. Tantalum, electrolytic and dry polymer are other viable options.

author: mikerj
author_rank: Super Contributor
timestamp: December 09, 2019, 10:00:41 pm
content: Quote from: Simon on December 09, 2019, 05:42:39 pmThe decent manufacturers will have these characteristics available somewhere, though finding them can sometimes be difficult IME. I really like Murata's "SimSurfing" web app that provides numerous characteristic graphs for every part, and also Spice models for them.

author: Simon
author_rank: Unknown
timestamp: December 09, 2019, 10:05:23 pm
content: this is a kemet part. I thought they were decent.

author: james_s
author_rank: Super Contributor
timestamp: December 09, 2019, 11:22:54 pm
content: You could also test a sample if you have an oscilloscope and a resistor. With the addition of a function generator you can lash up a little low pass filter and observe the characteristics under realistic conditions. I think Kemet is decent stuff but I can't speak for their documentation offhand. It's possible that this data is buried in some other document instead of the individual datasheets. As I mentioned previously it's not nearly as widely known or as well publicized as it ought to be. I think a LOT of people get snared in that trap, but in many cases the reduced capacitance doesn't cause obvious problems so it is never noticed.

author: floobydust
author_rank: Super Contributor
timestamp: December 09, 2019, 11:56:55 pm
content: MLCC selection is much more dangerous now due to manufacturers "race to the bottom" for smallest size. The push is to go down a size group with the "same" ratings. 1210 obsolete in favour of 1206, 0805 for 0603 etc.So the voltage coefficient of capacitance effect is much greater now. 0603 is only about 33%of rated C at 5V for a 25V part, whereas 1206 is around 90% - you want the physically biggest part.TDK MLCCallows you to sort by the capacitance with DC bias, which helps a lot.Taiyo Yudenalso allows DC bias sort and check.

author: wraper
author_rank: Unknown
timestamp: December 10, 2019, 12:05:36 am
content: Quote from: floobydust on December 09, 2019, 11:56:55 pmDepends on capacitance for size. Say if you take 4.7uf and 22uF part with same size and 6.3 V rating, there are chances that at 5V actual capacitance is almost the same for both.

author: floobydust
author_rank: Super Contributor
timestamp: December 10, 2019, 12:43:07 am
content: Yes, there seems to be a density where it makes more sense to use a lower value part. Temperature and aging are also worth looking at.Example 0603 6.3V at 5V:47uF gives 9.4uF -80%22uF gives 4.4uF -70%10uF gives 3.0uF -70%4.7uF gives 3.3uF -30%

author: viperidae
author_rank: Frequent Contributor
timestamp: December 10, 2019, 04:43:16 am
content: That 4.7 is an x7r dielectric and the others are x5r. I think that's why it's better

author: Simon
author_rank: Unknown
timestamp: December 10, 2019, 07:53:34 am
content: Well 25V on 5V is surely not bad. it's 1206. Yes as soon as i chose X7R all the 0805 parts disappeared from the list.