Negative LDOs with high PSRR? - Page 1
Forum section: electronics / projects

author: SiliconWizard
author_rank: Super Contributor
timestamp: December 07, 2022, 04:40:36 am
content: Looks like those are pretty rare and expensive.I'm looking for something with <= -16V input voltage, -12V output (can be adjustable of course), PSRR >= 65 dB @ 1 kHz, output current max ~200 mA.Very few or pretty expensive (> $6). Anything that would be more around $2 with these specs? Or any idea?

author: magic
author_rank: Super Contributor
timestamp: December 07, 2022, 09:20:21 am
content: What do you mean by "<= -16V input voltage"?Literally, -16V or less means -16V or -17V or so on. That's 4V headroom, definitelynotrequiring an LDO.It seems that LM337 with ADJ bypass could just meet your 65dB@1kHz spec.editOK, on second reading I guess what you meant is inputs between -12V and -16V.Well, nevermind then

author: spostma
author_rank: Regular Contributor
timestamp: December 07, 2022, 06:39:04 pm
content: TPS7A3001 would do IMHO, but costs about 3,50 (25+)the datasheet says that when regulating at -5V @200mA, PSSR is 68dB @ 1kHz.https://nl.mouser.com/c/semiconductors/power-management-ics/voltage-regulators-voltage-controllers/ldo-voltage-regulators/?q=TPS7A3001&sort=pricing

author: SiliconWizard
author_rank: Super Contributor
timestamp: December 07, 2022, 07:29:02 pm
content: Well, <= -16V literally means what it says. I know negative bounds can look confusing sometimes.That means the input must be able to handle -16V or lower (negative bound, not in absolute value.)Which doesn't mean that I can tolerate a large dropout voltage. But I didn't mention it. Typical input would be somewhere between -16V and -13V, so it must handle -16V or lower, but max dropout should not exceed about 1V.That, and the requirement of the LM337 for a min. load current of 10mA is not going to fit my requirements. Otherwise, albeit old, the LM337 is pretty good in terms of PSRR.I had seen the TPS7A3001 and while a bit expensive, it should be ok, so it could be a match. Nice specs. If only it was available. Earlier availability is feb 2023, and I don't even trust that.Note that the LM337 in SMD package is also currently unobtainium as far as I've looked.Now the LM337 approach for getting a high PSRR tells me that I might be able to achieve the same using some adjustable LDO with lower PSRR and improve it bypassing one feedback resistor?

author: magic
author_rank: Super Contributor
timestamp: December 07, 2022, 08:47:03 pm
content: It's better than not bypassing it if you choose an adjustable regulator, but also no guarantee that a well designed fixed regulator couldn't do better.The benefit is of course that the regulation loop essentially runs unity gain at AC, and whatever gain selected by resistors at DC. More feedback, better performance. You will never get better than the same adjustable regulator set for its minimum voltage, 1.2V or whatever it is.

author: SiliconWizard
author_rank: Super Contributor
timestamp: December 07, 2022, 08:50:51 pm
content: Yeah, but bypassing will also make the regulation loop slower, won't it?Otherwise, for a dual positive/negative supply with good PSRR and low noise, the LT3032 could be nice. It's fricking "expensive" though. (As with most LT and AD parts, the unit price per 1000 that AD shows on their website is nowhere near what is shown through usual distributors, which prices especially on LT parts can be as much as 50% higher or more that the suggested price. Not sure how easy it is to buy from AD direct now? I used to buy direct from LT back when it was still LT.)

author: magic
author_rank: Super Contributor
timestamp: December 07, 2022, 08:53:39 pm
content: It makes startup slower because the cap needs to charge. There are potential gotchas when input/output gets shorted and the bypass cap discharges through protection diodes in the regulator.Otherwise it's faster. The cap is basically a constant voltage, and the regulator runs like a 1.2V regulator regulating with respect to the cap voltage rather than ground. Unity gain.

author: mzzj
author_rank: Super Contributor
timestamp: December 07, 2022, 09:47:24 pm
content: Room for capacitance multiplier "preregulator" ?Quick fooling around on ltspice gives 60db attenuation at 1khz and about 0.5v voltage drop with circuit built from 2SAR574 PNP, 470uF 16V electrolytic and 700 ohm resistor, total BOM <1 USD

author: langwadt
author_rank: Super Contributor
timestamp: December 07, 2022, 09:56:50 pm
content: how bad is an LM7912?

author: Someone
author_rank: Super Contributor
timestamp: December 07, 2022, 10:21:23 pm
content: Quote from: langwadt on December 07, 2022, 09:56:50 pmA *79*12 part can meet the requirements but outside the guaranteed specifications (and is model specific) so it is an option but needs validation testing.As usual, too many undefined dimensions/requirements to make any real suggestions.

author: SiliconWizard
author_rank: Super Contributor
timestamp: December 08, 2022, 05:36:45 am
content: Quote from: langwadt on December 07, 2022, 09:56:50 pmDepends too much on vendor. It's from pretty mediocre to good in terms of PSRR. The SMD variants in smaller packages (thus rather the L versions) tend to be worse, and are only rated for up to 100mA which isn't quite enough. Otherwise you get pretty large packages.

author: SiliconWizard
author_rank: Super Contributor
timestamp: December 08, 2022, 05:38:17 am
content: Quote from: mzzj on December 07, 2022, 09:47:24 pmSomething to consider, but area cannot be wasted too much so large capacitors are not that great.

author: magic
author_rank: Super Contributor
timestamp: December 08, 2022, 07:52:10 am
content: Hmm, at -13V input you could afford to use an emitter follower, and place it in the feedback loop of... something?7912 won't work from -13V, forget it.

author: Swainster
author_rank: Frequent Contributor
timestamp: December 08, 2022, 11:40:32 am
content: Am I going mad or was Magic correct with his original interpretation of "<=", and SiliconWizard the one with it back to front?

author: SiliconWizard
author_rank: Super Contributor
timestamp: December 08, 2022, 07:05:12 pm
content: Quote from: Swainster on December 08, 2022, 11:40:32 amUh. If you need some part that can accept down to -16V input, it must handle a minimum of -16V. So <= -16V.If it were a positive voltage, I would have said >= 16V.It's basic maths and basic physics.

author: langwadt
author_rank: Super Contributor
timestamp: December 08, 2022, 07:29:03 pm
content: Quote from: Swainster on December 08, 2022, 11:40:32 amthere was no difference both were correct, just wasn't clear whether the <=-16V was the available supply or the minimum rating of the regulator

author: SiliconWizard
author_rank: Super Contributor
timestamp: December 08, 2022, 07:46:39 pm
content: magic's point was about not necessarily requiring an LDO, and I gave complementary info afterwards.Anyway, even if not strictly requiring a low dropout, that wouldn't help much as the offering is very limited: 29x12 are not a match as I explained above, and the LM337, beside its highish dropout voltage, is unobtainium in SMD packages at the moment (or the larger DPACK stuff). So that's pretty much it.Good matches have been mentioned: TPS7A3001, LT3032 (I could use the positive regulator in it to replace one other regulator in my design), but they are either unobtainium or pretty expensive. Now given the LT3032 would be two regulators, its price is not that awful though, so that might be my pick. Still a bit pricy. The LT1964 could also be a match, but it's expensive and its PSRR is not that great. Now I could add some pre-filtering to improve the PSRR if I find a good match otherwise, as there aren't many at all anyway.There are tons of matches for positive LDOs with similar characteristics (and even more with lower positive input voltage, in terms of PSRR and low noise), but extremely few for negative LDOs. I guess there is little market. And the current state of shortage doesn't help.

author: spostma
author_rank: Regular Contributor
timestamp: December 08, 2022, 10:53:29 pm
content: Maybe you could make a discrete regulator with a LM3411 and a transistor?The LM3411 is a kind of 'negative TL431' with reference tied to V+.

author: spostma
author_rank: Regular Contributor
timestamp: December 08, 2022, 11:04:19 pm
content: or a LM4051 (adjustable 3-pin negative shunt regulator)

author: nctnico
author_rank: Super Contributor
timestamp: December 08, 2022, 11:07:16 pm
content: Quote from: SiliconWizard on December 08, 2022, 07:05:12 pmIMHO the wrong way around. For positive voltage you'd say supply voltage is up to 16V. So the regulator would need to be able to deal with +16V or less (<= +16V). Or Vmax = 16V. Same for negative. Down to -16V so -16V or higher ( >= -16V). Or Vmin = -16V. OTOH <=-16V means -16V or less (-20V? -100V?). How much less is unclear in that case which has thrown quite a few people off including me.Anyway, it looks like component shortage is causing you trouble finding a good part.LCSC seems to have some LM337 in stock but not many and quite expensive. I ordered parts from them before and received real parts so at least they are legit.https://www.lcsc.com/product-detail/Linear-Voltage-Regulators-LDO_Texas-Instruments-LM337KVURG3_C2871286.html

author: langwadt
author_rank: Super Contributor
timestamp: December 08, 2022, 11:17:29 pm
content: Quote from: nctnico on December 08, 2022, 11:07:16 pmis the right way around, just some confusion on whether it was max rating for the regulator or supplyfor negative you need a regulator that can handle <= -16V when running on supply that is >= -16Vfor positive you need a regulator that can handle >= 16V when running on supply that is <= 16V

author: SiliconWizard
author_rank: Super Contributor
timestamp: December 08, 2022, 11:28:06 pm
content: Quote from: langwadt on December 08, 2022, 11:17:29 pmYep.

author: SiliconWizard
author_rank: Super Contributor
timestamp: December 08, 2022, 11:29:15 pm
content: Quote from: spostma on December 08, 2022, 10:53:29 pmGiven the problematic offering, I might go this route of rolling my own. As long as it doesn't take up too much space.

author: edavid
author_rank: Super Contributor
timestamp: December 08, 2022, 11:34:28 pm
content: Quote from: SiliconWizard on December 08, 2022, 11:29:15 pmIf you have a positive supply, you don't need a negative reference, just an op amp and a PNP.(Assuming you don't need fancy current limiting/thermal protection, which you shouldn't at 200mA.)

author: SiliconWizard
author_rank: Super Contributor
timestamp: December 09, 2022, 05:10:57 am
content: Quote from: edavid on December 08, 2022, 11:34:28 pmYou mean by using the positive rail as a reference?

author: langwadt
author_rank: Super Contributor
timestamp: December 09, 2022, 08:34:01 am
content: Quote from: SiliconWizard on December 09, 2022, 05:10:57 amkinda, you know that midpoint between +/- supply is gndhttp://www.seekic.com/uploadfile/ic-circuit/s201159215141626.gif

author: magic
author_rank: Super Contributor
timestamp: December 09, 2022, 09:07:46 am
content: Quote from: spostma on December 08, 2022, 10:53:29 pmThis is an interesting part, but the internal feedback network with ±25% tolerance is not helping when you need custom closed loop voltage. Without this rubbish it could be a viable negative TL431 substitute.There is supposed to exist adjustable LM385, which is more like a true complement of 431. But it's fairly low voltage only, so further parts would be required to deal with that.Quote from: SiliconWizard on December 09, 2022, 05:10:57 amLooks like the cheapest way.Make sure the opamp isn't doing something stupid when the positive rail collapses (or either of the upstream supplies disappears) and inputs may go outside normal common mode input range.Watch out for PSRR of the opamp itself.

author: SiliconWizard
author_rank: Super Contributor
timestamp: December 09, 2022, 08:18:46 pm
content: Quote from: magic on December 09, 2022, 09:07:46 amYes if we already have a positive supply and a regulated positive rail, we don't need an extra reference.I have played a bit with simulation to see what could be achieved. What I did here was to set up the opamp as an inverter rather than follower and use the positive rail as a reference. It works fine.The potential issues are the usual - possible oscillation, PSRR and what you mentioned above.And, if we want reasonable protection against overcurrent, it adds another layer. Did I mention I was looking to save some space.But it's interesting to work on.

author: edavid
author_rank: Super Contributor
timestamp: December 09, 2022, 09:00:39 pm
content: Quote from: SiliconWizard on December 09, 2022, 05:10:57 amRight!