Dropout and PSRR — what the numbers mean for the rail
The LP38691DT-3.3/NOPB: At lower currents dropout is lower, but the 0.55 V ceiling governs worst-case headroom. PSRR is 55 dB at 120 Hz — this is the ripple rejection at the fundamental of a full-wave rectified line. Quiescent current is 55 µA typical, with a maximum supply current of 100 µA. This Iq is low enough for always-on or battery-backed rails where the regulator's own draw matters for standby power.
Temperature grade and package fit
Input voltage maximum is 10 V — the part can run from a 5 V or 9 V rail, but the dropout headroom and power dissipation in the package set the practical ceiling. At 500 mA with 5 V input, the power dissipated is (5.0 - 3.3) × 0.5 = 0.85 W; the TO-252-3's thermal resistance (junction-to-ambient, ~80 °C/W with a 1-inch² pad) keeps the junction below 125 °C in still air.
Protection and sourcing posture
The LP38691DT-3.3/NOPB includes over-temperature protection — the regulator shuts down when the junction exceeds the thermal limit, then auto-recovers after cooling. No over-current protection is listed, so the output current is limited by the pass-element SOA and the thermal foldback.