TO-3 steel package — the mounting reality
The LM317K STEEL ships in the TO-204AA (TO-3) steel can, a through-hole package that requires a chassis or heatsink mount. The two-lead TO-3-2 configuration means the metal tab is the output (Vout) connection — the mounting screw hole is electrically common with the output, so an insulating washer and bushing are needed if the heatsink is grounded. The steel can provides better mechanical ruggedness than the plastic TO-220 variant, at the cost of a larger footprint and a dedicated mounting hole pattern.
1.5 A output with built-in protection
Rated for 1.5 A continuous output with over-current, over-temperature, and short-circuit protection built in. The protection circuitry folds back current during a fault and shuts down the pass transistor if the die temperature exceeds the safe limit — no external crowbar or current-limit resistor needed for basic fault tolerance. The 10 mA quiescent current is the overhead the regulator draws regardless of load; it matters most in always-on or battery-backed rails where every milliamp counts.
The 80 dB to 65 dB power-supply rejection ratio at 120 Hz is the spec that matters for analog supply rails fed from a full-wave rectifier. 80 dB means the 120 Hz ripple on the input is attenuated by a factor of 10,000 — a 1 V pk-pk ripple on the input becomes 100 µV at the output. That level of rejection keeps hum out of audio preamp rails, ADC reference supplies, and sensitive instrumentation circuits. The PSRR rolls off above 120 Hz; for higher-frequency noise a post-filter or a low-dropout regulator with wider bandwidth would be needed.
Adjustable output from 1.25 V to 37 V
Output voltage is set by two external resistors using the classic LM317 topology — the reference voltage between the output and adjust pins is 1.25 V. The output can be adjusted from 1.25 V up to 37 V, limited by the 37 V maximum input. The 37 V input ceiling means the part works from a 24 VAC rectified rail (about 34 V unloaded) or a 28 VDC supply, but not from a 48 V telecom bus. For higher input voltages, the LM317HV family extends the input range to 60 V.
The steel TO-3 variant remains in the manufacturer's current portfolio alongside the bulk-packaged LM317KSTEEL. The 'STEEL' suffix indicates the tray-packaged TO-3 version — the die and electrical specs are identical to the bulk-packaged sibling, only the shipping medium differs.