solved: Is the LM317 still "king" for 35V Vin_abs_max applicaitions? - Page 1
Forum section: electronics / projects

author: incf
author_rank: Frequent Contributor
timestamp: April 13, 2025, 03:48:41 pm
content: Have adjustable linear regulators advanced enough in the past ~40 years to offer considerable advantages over the humble and ever faithful LM317? (for applications requiring Vin_abs_max >= 35V)If so, what might those advantages be?

author: Siwastaja
author_rank: Super Contributor
timestamp: April 13, 2025, 05:56:53 pm
content: Biggest advances are lower and lower dropout voltages combined with better output stability (without requiring weird medium-ESR capacitors). But if you are going to drop a lot of voltage anyway, this has no value for you. Old-generation high-dropout parts like 78xx and LM317 have decent output stability characteristics and don't require massive amount of output capacitance damped with series resistance.Old-school "low-drop-out" parts like 1117 are technically obsolete since they are not very "low drop out" in modern terms but still suffer greatly with the back-then compromise of output stability vs. drop-out voltage. But this does not apply to LM317.Beware of large (in modern terms) minimum load requirement - satisfied by the recommended ADJ pin resistor divider network, of course. But this is a showstopper for most battery-powered designs.

author: bdunham7
author_rank: Super Contributor
timestamp: April 13, 2025, 06:00:46 pm
content: What improvements might there be and what would be the point? A linear regulator, LDO or not, in a circuit with a huge difference between input and output is wasteful and thermally challenging. You wouldn't deliberately design something that way anymore unless it were for a very small or intermittent current. So other than the higher-current LM 338 or the higher voltage LM317HV types, I don't think anyone has pursued any improvements in this area. There is the very high voltage TL783 (125V) if you want an extreme example. But other than the higher currents and voltages, what improvements are you looking for? If you have large voltage differentials then LDOs are irrelevant and your thermal issues are the same no matter what the design is.

author: David Hess
author_rank: Super Contributor
timestamp: April 13, 2025, 06:30:57 pm
content: The LT1086 (1.5A) and LT1083/4/5 (3A/5A/7A) from Linear Technology are improved replacements for the 317 with better precision and lower dropout, but have 25 and 30 volt maximum voltages.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: April 13, 2025, 07:28:32 pm
content: Of course things have advanced.The 317 is still ‘alive’ because it has huge volumes and low prices.Tolerance, quiescent current, poor dropout, noise, lack of power good, and tab not GND, are all compromises, but often price wins.

author: Konkedout
author_rank: Frequent Contributor
timestamp: April 13, 2025, 10:58:04 pm
content: I have tremendous respect for the original LM317. A couple of successful production designs which I did with it:1) A medical grade battery charger which regulated the output using SCR phase control. This unit had a mains frequency transformer powering everything. The LM317 powered the control circuitry AND provided the pot-adjusted 12.0V reference used by the output regulator. The LM317 is not intended as a voltage reference but this worked really well.2) An AC/DC flyback 48V output switcher which used an LM317 as a linear regulator in the output. The switcher feedback loop regulated the INPUT-OUTPUT differential across the LM317 with diodes in series with the feedback optocoupler LED. This design did need protection for the LM317 in the form of maybe 8-10 1N4004 diodes around it. A film capacitor from the adjust pin to ground gave it very low output noise for telecom applications.I do wonder whether the 2nd sources (ST and ON for example) work as well as the original National > TI part?

author: porter
author_rank: Regular Contributor
timestamp: April 14, 2025, 12:12:06 am
content: A little off topic...Back in the 90s, I had a question about using a LM317 as a current limiter. I called National Semiconductor.Soon enough, Bob Pease picked up the phone and answered my question as nice as can be(and not the most intelligent question on my part).I noticed a similar discussion regarding the LM317 as a current limiter on the TI forum posted 10 months ago.Maybe not a king, but it deserves some title.

author: UnijunctionTransistor
author_rank: Frequent Contributor
timestamp: April 14, 2025, 01:04:58 am
content: The LM317/ 337 satisfies the vast majority of users requiring a simple to use adjustable regulator. For the remaining applications, Linear Tech, now ADI, introduced improved devices as others have already mentioned.But they may be 4X or 5X the price and not nearly as ubiquitous. And if I recall correctly, they do require a significant amount of capacitance for stability.