What this 5V LDO does — and where the 75dB PSRR matters
The LM2931BDT50 is a 5V fixed-output low-dropout regulator from onsemi's LM2931 series, delivering up to 100mA with a maximum dropout voltage of 0.6V at full load.
Dropout headroom and quiescent current — the real-world trade-offs
With a 0.6V dropout maximum at 100mA, the input rail needs at least 5.6V to maintain regulation. That 1 mA quiescent current is the price of keeping the pass element biased — it's not a micropower LDO, but the 30 mA supply maximum suggests the internal reference and protection circuits draw very little overhead. Over-current and over-temperature protection are built in — the part shuts down before the die exceeds the 125°C limit, which is a practical safeguard for a regulator that might see a shorted load in a production unit.
The DPAK (TO-252-3) package with two leads and a tab is a common footprint, so if you are evaluating a replacement, look for a 5V fixed-output LDO in the same DPAK with a similar dropout and PSRR profile. The 75dB PSRR at 120Hz is the spec most likely to differ between candidates — a generic 78L05 will not match it.