Lm317 question - Page 1
Forum section: electronics / beginners

author: masterx81
author_rank: Frequent Contributor
timestamp: March 14, 2020, 12:21:23 pm
content: Hi! In the tipycal lm317 schematic i often find a 10uf cap between adj pin and ground. This for sure can reduce the ripple and stabilize the output in normal conditions, but what about transient response? I think that can be slowed a lot as the capacitor tries to maintain the voltage at the adj pin.I'm correct? Thanks!

author: StillTrying
author_rank: Super Contributor
timestamp: March 14, 2020, 12:43:49 pm
content: what about transient response? I think that can be slowed a lot as the capacitor tries to maintain the voltage at the adj pin.I'm correct?No, the regulator tries to keep its output 1.25 V above the adj pin, so maintaining the voltage at the adj pin as steady as possible is what you want.

author: masterx81
author_rank: Frequent Contributor
timestamp: March 14, 2020, 01:32:48 pm
content: So actually the adj pin isn't a reference input. And why not simply put a zener so that the voltage will be rock solid?

author: janoc
author_rank: Super Contributor
timestamp: March 14, 2020, 02:21:43 pm
content: Quote from: masterx81 on March 14, 2020, 01:32:48 pmThen why not buy and use a 78xx regulator instead? Because that's exactly what you have made. Except with much worse performance (zeners don't make great references) and needing more components.

author: StillTrying
author_rank: Super Contributor
timestamp: March 14, 2020, 02:29:51 pm
content: Yes you could replace the bottom resistor with a zener, but adjusting the zener to give adjustable or exact voltages on the LM317's output would be difficult.https://electronics.stackexchange.com/questions/56506/problems-controlling-an-lm317-with-an-op-amp

author: masterx81
author_rank: Frequent Contributor
timestamp: March 14, 2020, 05:53:25 pm
content: I will try some variants, in the hope that my scope can catch the transientThanks!

author: Vovk_Z
author_rank: Super Contributor
timestamp: March 15, 2020, 06:17:04 pm
content: Quote from: masterx81 on March 14, 2020, 12:21:23 pm- transients become better too. There is some pictures at some datasheets on this topic.

author: wraper
author_rank: Unknown
timestamp: March 15, 2020, 06:27:00 pm
content: Quote from: masterx81 on March 14, 2020, 01:32:48 pmBecause it will be rock solid with resistive divider and less solid with usual zener. Only downsides with no upsides. Current through resistive divider is automatically adjusted to keep stable 1.25V voltage between out and adj. Hence voltage across adj to GND resistor becomes stable too due to stable current.

author: IDEngineer
author_rank: Super Contributor
timestamp: March 15, 2020, 06:46:01 pm
content: Hmm... this raises the prospect of using an adjustable regulator to intentionally inject noise into a DC rail for R&D purposes. Anyone tried that? I haven't looked at the regulator spec sheets to see what sort of frequency response they'd tolerate.

author: magic
author_rank: Super Contributor
timestamp: March 16, 2020, 05:43:22 am
content: Quote from: wraper on March 15, 2020, 06:27:00 pmI don't think so. Replacing the bottom resistor with a zener would decrease dynamic output impedance of the "divider" and presumably improve stability against ripple and load transients.That being said, zeners typically have worse tolerance than the internal reference of LM317 and I'm not sure how their thermal drift compares either. Also, zeners are only available in a number of selected values.

author: wraper
author_rank: Unknown
timestamp: March 16, 2020, 11:32:02 am
content: Quote from: magic on March 16, 2020, 05:43:22 amFor that you connect large enough capacitor between adj and gnd.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: March 16, 2020, 01:16:19 pm
content: A lot IC's can be used in different ways, sometimes they work excellently in ways not even thought of by the designers.The LM317 has a very basic rule in that it tries to keep it's output voltage (approx) 1.25V above the voltage on the adjust pin.The adjust pin also has a fixed current which is needed to keep the LM317 working. (A few uA I believe, have not checked).A fun way to experiment with the LM317 is to build an audio amplifier with it:The lower LM317 is a constant current sink, while the upper LM317 is used as a glorified output buffer for the opamp.The 1.25V voltage referential is "nullified" by the feedback resistors around the opamp. This can also learn you a bit about the workings of opampsI found this image via a picture search on startpage/com for "lm317 class A amplifier"

author: exe
author_rank: Unknown
timestamp: March 16, 2020, 01:47:52 pm
content: TLDR if lm317lz has enough current and power capability, I suggest use it instead of plain lm317.JFYI I found that behaviour of lm317 and lm317lz quite different. In my measurements lm317lz (tried from ti, st, and on semi) behaved much better than lm317 (sorry, don't remember which vendor was that, nor year of production). The reference voltage was dead-on despite input voltage (though I didn't do load testing, load was in the order of 5 or 10mA, so about a minimum load). PSRR was good too. I tested it without output cap to see performance of the part itself. I'd recommend LM317LZG part from onsemi, it was best overall. Although, it was a close competition between vendors, any other aformentioned would do the job almost as good.