The STMicroelectronics LDL1117S33R is a 3.3 V fixed-output low-dropout linear regulator rated for a full 1.2 A of output current, packaged in a four-lead SOT-223 (TO-261-4/TO-261AA). It is built as a single positive regulator and accepts an input supply up to 18 V, which gives design margin for 5 V, 9 V, or even a lightly loaded 12 V rail once headroom is checked against dropout. The maximum dropout is specified at 0.6 V at the full 1.2 A load, so a 5 V input holds regulation comfortably down to roughly 3.9 V at the input pin before the pass device starts to saturate — that margin is what makes the part forgiving in systems with loosely regulated front-end supplies or long PCB traces.
Where the LDL1117 earns its place on the BOM is power-supply rejection. It holds 87 dB of PSRR at 120 Hz, decaying to 65 dB by 100 kHz, which makes it a practical choice for post-regulating a noisy DC rail ahead of audio codecs, AFE reference buffers, and mixed-signal loads sensitive to mains-frequency ripple and low-frequency switching residue. The quiescent current is 500 µA — not a battery part, but reasonable for a 1.2 A-capable LDO, and it buys a wide-bandwidth PSRR curve. Built-in protection includes over-current limiting, over-temperature shutdown, and an input under-voltage lockout, so the part is self-protecting across the specified -40 °C to 125 °C junction-temperature range. When you select it, confirm the SOT-223 copper area can handle 1.2 A continuous load and verify the input-to-output dropout budget across your operating corners; with a fixed 3.3 V output, a 0.6 V dropout envelope, and an 18 V absolute input limit, it is a drop-in-capable workhorse for the 5 V-to-3.3 V rail that needs quiet output at real current.