5V fixed LDO in a TO-92-3 — what the scorch mark tells you
The STMicroelectronics LM2931BZ50 is a 5V fixed-output low-dropout regulator in a TO-92-3 through-hole package, rated for 100 mA continuous output. It delivers a clean 5V rail with a dropout voltage of 0.6 V at full load and PSRR of 75 dB at 120 Hz, which makes it a solid choice for powering analog front-ends or microcontroller rails in noisy industrial environments.
Dropout and PSRR — sizing the input rail
The 0.6 V dropout at 100 mA means the input must stay above 5.6 V to maintain regulation. If the pre-regulator rail sags under load, the output drops out of spec — a common failure signature on boards where a 5 V rail is fed from a 6 V or 7 V supply that dips during a transient. The 75 dB PSRR at 120 Hz knocks down line-frequency ripple by a factor of about 5600, which is enough to keep a 100 mVpp input ripple from reaching the output as more than a few tens of microvolts.
Temperature range and protection — where it survives
Built-in over-current and over-temperature protection means a shorted load or a stalled fan won't instantly kill the regulator — it folds back or shuts down, and the board comes back when the fault clears.
The LM2931BZ50 is listed as obsolete.