LM317 vs LM317L: The 1.5A Workhorse Is the Wrong Part Half the Time

The Real Differences Are Not What You’d Guess

The obvious one is current: 100mA versus 1.5A, with the LM317M’s 500mA sitting in between. Fifteen times the current, twenty times the package.

The surprise is minimum load. The big LM317 needs 10mA worst case (3.5mA typical) or the output drifts up unloaded. The LM317L needs only 5mA worst case — the little part is the one that behaves on light rails.

A 240Ω R1 pulls 5.2mA — fine on the L, marginal on the big one.

Then the corners: the LM317L spans −40°C to +125°C where the TO-220 part starts at 0°C, and the SOIC-8’s 625mW limit barely matters when the whole point is low current.

Four Places the LM317L Is the Right Part

• 4–20mA transmitter current source. Iout = 1.25/R — one resistor sets the loop, and linearity comes in better than 0.02%. No heatsink, no drama.
• Op-amp and bias rails. A 30mA analog rail with a 5mA worst-case minimum load stays stable without burn resistors.
• Battery portables. The −40°C floor and the SOIC-8 footprint fit where a tabbed TO-220 never will.
• Cramped boards. Eight pins beat a tabbed TO-220 when the rail is a support act, not the star.

So what’s not on that list? Anything above 100mA. That’s the whole game.

And Three Places It Isn’t

Dropout doesn’t get better. Both parts are quoted up to 2.5V worst case — the E2E forum settles it: the L is no lower-dropout than the big one. At 24V down to 5V you’re burning the difference either way.

The current-limit spec is a band, not a value: 100/200/300mA across the L family. Sample it, don’t design to it — a 150mA load may sit in regulation on some parts and sag on others.

And above 100mA, the LM317M owns the middle. 500mA, same family, no reason to push the L past its envelope.

Side by Side: The Numbers That Matter

The Decision in One Picture

The Short Version

Rail under 100mA, light loads normal? The LM317L does the job in an SOIC-8 with a 5mA worst-case minimum load and a −40°C floor.

Need real current or a heatsink? Step up to the LM317M at 500mA or the full 1.5A LM317. But don’t reach for the big one out of habit — the little one is the same regulator, trimmed for the job.