Negative high voltage reg using LM337 - Page 1
Forum section: electronics / beginners

author: knudsenuk
author_rank: Contributor
timestamp: October 28, 2018, 06:05:10 pm
content: I wondered if someone could kindly comment on the circuit below. I built it and it works a treat. I wanted to build a complimentary negative/PNP version but can't get it to work the same. I must be doing something wrong. I have substituted Q1 and Q2 for PNP devices and have the collectors on the negative rail. I have reversed the direction of the diodes and the LM329 and swapped the LM317 for a 337. Am I missing something??Any help much appreciated.

author: mvs
author_rank: Frequent Contributor
timestamp: October 28, 2018, 08:25:31 pm
content: LM337 is not 1:1 implementation of LM317 for negative supply. It might be less stable in this circuit. Try to get it working with more basic circuit (Figue 1, same app. noteLB-47).PS I hope you do not use LowESR caps (MLCC, Mylar, etc) without series resistor at the output.

author: iMo
author_rank: Super Contributor
timestamp: October 28, 2018, 09:10:19 pm
content: 1. double-check your wiring2. I would try with an 8.2V D1.

author: knudsenuk
author_rank: Contributor
timestamp: October 29, 2018, 07:28:58 pm
content: A bit of a breakthrough: reversing the potential divider........Quote from: imo on October 28, 2018, 09:10:19 pmPlease elaborate on why you recommend this higher value diode?

author: spec
author_rank: Frequent Contributor
timestamp: October 30, 2018, 12:28:57 am
content: + knudsenukYour approach of changing NPN transistors to PNP transistors and reversing diodes is correct.But as mvs says in reply #1, the LM337 is not an exact compliment of the LM317.One important area is frequency stability. The LM317 does not need an input or output capacitor. The LM337 does not require an input capacitor either but it does require an output capacitor. Although both regulators require an input capacitor if they have more than 12 inches of wire (6inches feed and 6 inches return) between their input and the raw power supply reservoir capacitor. But, it is good practice to fit input and output capacitors to both devices, and this is the approach used in most applications.The LM317 high voltage circuit, described in National LB47, can't use an input capacitor because that would cause excess voltage across the LM317 if the LM317 output were shorted. An input capacitor may cause other problems, I suspect.So the bottom line is that the more tolerant LM317 is stable in the HV circuit, whereas the less tolerant LM337 is not. As a result the LM337 is oscillating, which would give all sorts of weird behaviors.Of course, this is just a theory and the problem may lie elsewhere, but you could try connecting a 1uf low ESR capacitor from the output of the LM337 to the 0V line.You could also try connecting a capacitor with the same characteristics from the input of the LM337 to the 0V line.CORRECTION 2018_11_01 (see following posts about suitable capacitors)Suitable capacitors would be 2uF, or higher, X7R dielectric ceramic types or 1uF, or higher, metal film polypropylene types.Be interesting to hear what happensDatasheets:LM317http://pdf.datasheetcatalog.com/datasheet/nationalsemiconductor/DS009063.PDFLM337http://www.ti.com/lit/ds/symlink/lm137.pdf

author: David Hess
author_rank: Super Contributor
timestamp: October 31, 2018, 07:41:44 pm
content: Quote from: spec on October 30, 2018, 12:28:57 amThe low ESR of a film or ceramic capacitor is exactly the wrong thing to use with a 337. A capacitor on the output is required for stability but it must have a relatively high ESR.If I was doing this, I would make some network analysis measurements on the output to quantify the stability. This is a good idea for any regulator but critical for voltage and current boosted designs.Do not forget that the LM317 and LM337 have different pinouts. This is because the substrate (middle pin and tab of the TO-220 package) of the IC must be maintained at or below the most negative voltage which is the input for the LM337 and the adjust pin for the LM317.

author: knudsenuk
author_rank: Contributor
timestamp: October 31, 2018, 08:07:01 pm
content: Thanks, I had read about this too. People do seem to think low ESR is a panacea. I also understand that a series resistor (low value) can work with an output capacitor.

author: spec
author_rank: Frequent Contributor
timestamp: October 31, 2018, 08:40:56 pm
content: Quote from: David Hess on October 31, 2018, 07:41:44 pmWhere do you get this information from?The front page of the January 2016 TI datasheet for the LM337 says:†C1 = 1μF solid tantalum or 10μF aluminum electrolytic required for stability*C2 = 1μF solid tantalum is required only if regulator is more than 4′′ from power-supply filter capacitorOutput capacitors in the range of 1μF to 1000μF of aluminum or tantalum electrolytic are commonly used to provide improved output impedance and rejection of transientsSolid tantalum capacitors are low ESR and if you fit an aluminum electrolytic capacitor the data sheet says to fit 10x the capacitance value than for a tantalum type. This is clearly in order to get a low ESR.

author: spec
author_rank: Frequent Contributor
timestamp: October 31, 2018, 09:02:29 pm
content: Quote from: knudsenuk on October 31, 2018, 08:07:01 pmGood decoupling is very important to get many circuits to function correctly, voltage regulators being a case. Time and again problems with circuits are corrected by using low ESR capacitors. I don't know about panacea, but all too often people do not bother with decoupling at all.I would also be interested to learn of your source recommending the use of high ESR capacitors with the LM337.

author: David Hess
author_rank: Super Contributor
timestamp: November 01, 2018, 12:00:26 am
content: Quote from: spec on October 31, 2018, 08:40:56 pmI got it from the same source you did, the datasheets.Solid tantalum capacitors are *not* low ESR compared to film, ceramic, or polymer electrolytic capacitors which is what makes them so useful at the output of an NPN negative regulator or any low dropout regulator.

author: spec
author_rank: Frequent Contributor
timestamp: November 01, 2018, 02:32:03 am
content: You are rightI got this from the ON LM337 datasheet, page 7:Using the classical tantalum or aluminum electrolytic capacitor types with non−reduced ESR (Equivalent Series Resistance) value is necessary. Low−ESR or similar capacitor types with reduced ESR value and ceramic capacitors can cause instability or continuous oscillations in the application.https://www.onsemi.com/pub/Collateral/LM337-D.PDF

author: eV1Te
author_rank: Regular Contributor
timestamp: May 06, 2020, 07:34:34 pm
content: I just ran into problems with the LM337 and thought I would add a comment here for future engineers.The LM337 was oscillating like crazy, but not causing too much ripple on the output, instead it caused most ripple on the input rail. The board was filled by many OpAmps hence I of course assumed it was one of those first, then the input power rail, lastly after desoldering all op-amps I discovered the source was the LM337.Initially I had ceramic caps, so I added an electrolytic without change, I replaced the ceramic caps with tantalums with no change!When I saw the ON datasheet listed by spec I started checking the ESR on my tantalum caps and they were low ESR types (100 mOhm)I added a 1 ohm resistor in series with the largest cap and that solved the issue! (I still have small 0.1 uF caps close to ICs without series resistor, they don't seem to cause any issues).In the future I will try to stay away from LM337 if possible, too little documentation about its ESR requirements to be safe.Can anyone recommend a "better" negative linear regulator that do not require special ESR caps?

author: David Hess
author_rank: Super Contributor
timestamp: May 07, 2020, 09:06:35 pm
content: Quote from: eV1Te on May 06, 2020, 07:34:34 pmAlmost all if not all integrated regulators use NPN output devices so negative integrated regulators will always suffer from greater instability because the common emitter configuration has voltage gain which depends on load impedance much more than the common collector configuration of a positive regulator.