1.5 A adjustable regulator in D²Pak — what the ratings mean
The LM317D2T is a positive adjustable voltage regulator from the LM317 family, delivering up to 1.5 A output current from a 1.2 V to 37 V adjustable range. It accepts an input voltage up to 40 V and provides line and load regulation typical of the three-terminal adjustable regulator topology.
40 V input ceiling and 1.5 A load — where the headroom lives
The 40 V maximum input voltage sets the upper rail the part can survive — design the input bus below that with margin for transients. The 1.5 A output rating is continuous; the actual current the part can deliver at a given output voltage depends on the input-output differential and the thermal dissipation in the D²Pak. The 10 mA quiescent current is drawn regardless of load, so at light loads the efficiency drops — this matters for always-on or battery-backed rails. PSRR is specified at 80 dB to 65 dB at 120 Hz, which is the full-wave rectified line frequency and its second harmonic. For a 50 Hz mains, the 100 Hz ripple rejection sits in that band — the 80 dB floor at light load gives about 10,000:1 attenuation of input ripple, dropping to around 1,800:1 as the load pulls the output down. That is adequate for most analog supply rails but not for precision references without post-filtering.
Protection features and operating range
Built-in protection includes over-current limiting, over-temperature shutdown, and short-circuit current limiting — standard for the LM317 family. The single regulator output is configured as positive adjustable, with the adjustment pin setting the output via an external resistor divider.