The LM2931CT/LF04 is a positive adjustable low-dropout regulator from the LM2931 family, delivering up to 100 mA with a maximum dropout voltage of just 0.3 V at full load. That dropout figure means it can maintain regulation from a 5 V rail down to about 3.3 V output with only a few hundred millivolts of headroom — useful in battery-backed or post-switcher rails where every volt of overhead costs efficiency. The input can go as high as 26 V, so it handles unregulated supplies from 12 V or 24 V buses without a pre-regulator.
With 55 dB of power-supply rejection at 120 Hz, this LDO attenuates full-wave-rectified ripple by a factor of about 560. That is enough to keep a 1 Vpp ripple on the input down to under 2 mVpp on the output — important for feeding sensitive analog loads like op-amp rails or ADC reference buffers from a simple transformer-and-capacitor supply. Quiescent current is 1 mA typical, rising to 15 mA maximum under heavy load or fault conditions. The 1 mA Iq is competitive for the era and keeps battery drain modest in always-on automotive or industrial modules.
Built-in protection covers over-temperature shutdown, reverse polarity on the input, and output short-circuit current limiting. These three features make the part survivable in automotive and industrial environments where supply faults are common — reverse-battery events, hot-plug transients, or a downstream short. The TO-220-3 through-hole package with its metal tab can dissipate several watts with a heatsink, though at 100 mA output the thermal margin is generous even in free air.