80dB PSRR at 120Hz — the ripple-rejection baseline for a clean rail
The LM317AEMPX/NOPB is a positive adjustable linear regulator from the classic LM317 family, specified for 1.5A continuous output and a 1.25V to 37V adjustable output range. Its power-supply rejection ratio (PSRR) is rated 80dB ~ 65dB at 120Hz — the 80dB floor at mains frequency means the part attenuates full-wave-rectifier ripple by a factor of 10,000, which is the design entry point for noise-sensitive analog rails where a switching regulator would inject too much hash.
1.5A in SOT-223-4 — thermal budget is the real constraint
The 1.5A current rating is the electrical limit, but in the SOT-223-4 package (TO-261-4 equivalent) the practical output at high ambient temperatures is derated by the junction-to-ambient thermal resistance. At 125°C junction maximum and a typical theta-JA of 60°C/W on a standard PCB footprint, every watt of dissipation costs 60°C rise. A 1.5A load with a 5V input-to-output differential dissipates 7.5W — that exceeds the package capability without a heatsink or forced air. For continuous 1.5A operation, plan a larger copper pad or a lower dropout scenario.
Protection features and operating range
Built-in over-current, over-temperature, and short-circuit protection are included — the regulator folds back current and shuts down on die overtemperature rather than letting the pass transistor run away. The input voltage range extends to 40V maximum, and the output can be set from 1.25V up to 37V with two external resistors. The 10 mA quiescent current is typical for a three-terminal adjustable regulator of this class; it flows through the output and adds to the load current, so the minimum load to maintain regulation is about 10 mA.