Fixed 2.5 V rail — the output that defines the BOM position
The STMicroelectronics LD1117D25 is a single-output, positive-voltage LDO regulator that delivers a fixed 2.5 V output at up to 800 mA. It comes in an 8-SOIC narrow-body surface-mount package, with a maximum dropout voltage of 1.2 V at full load. The 75 dB PSRR at 120 Hz is the figure that matters when this regulator sits downstream of a full-wave rectifier — it tells you how much 120 Hz ripple the regulator attenuates before it reaches the 2.5 V rail.
Dropout headroom and load budget — sizing the input rail
At 800 mA the dropout voltage can reach 1.2 V maximum, meaning the input must stay above 3.7 V to keep the 2.5 V output in regulation. The quiescent current is 10 mA — this is the current the regulator burns just to run, and it adds to the total load on the upstream supply. Protection features include over-current and over-temperature shutdown, which guard against a shorted output or a thermal runaway condition on a crowded board.
No official successor part is listed on the ST record for this exact order code, so a buyer evaluating a replacement should look at the broader LD1117 family for a pin-compatible fixed 2.5 V variant in the same 8-SOIC package.
Operating range and environment
The maximum input voltage is 15 V, so the part can run from a 12 V bus or a 9 V battery rail with headroom to spare. The 8-SOIC supplier device package uses the standard narrow-body footprint — the copper area on the PCB under the die pad sets the effective thermal resistance.