Fixed 2.5 V rail — what the 1 A rating and dropout mean for the BOM
The LD1117AS25TR is a fixed 2.5 V output, positive-voltage LDO regulator from STMicroelectronics, rated for 1 A continuous output current. The 1.3 V maximum dropout at full load means the input rail must stay above 3.8 V to maintain regulation — budget that headroom when the upstream supply is a 5 V bus or a 3.6 V battery that sags under load.
80 dB PSRR — where the ripple rejection matters
With 80 dB of power-supply rejection at 120 Hz, this part attenuates rectified mains ripple by a factor of 10,000. That makes it a good fit for cleaning up a post-rectifier rail feeding a sensitive analog front-end, an audio codec supply, or a low-noise sensor bias. The 10 mA quiescent current is typical for a 1 A LDO of this generation — not a low-Iq part for battery-idle designs, but acceptable where the load current dominates the power budget.
SOT-223 package — reflow and footprint reality
The part ships in a SOT-223 (TO-261-4) surface-mount package. For 1 A continuous at 1.3 V dropout, expect about 1.3 W dissipation; a 1-square-inch copper pour on the top layer is the minimum to keep the junction below 125°C at 25°C ambient.
STMicroelectronics lists the LD1117AS25TR as obsolete. No last-time-buy window remains; the part is sourced as available inventory.