The LD1117S50TR is a single-output, fixed 5 V positive LDO from STMicroelectronics, rated for 800 mA continuous output. The 1.2 V maximum dropout at full load means the input rail must stay above 6.2 V to hold regulation — a common constraint when deriving 5 V from a 6.5–12 V bus. PSRR is 75 dB at 120 Hz, which knocks down mains-frequency ripple from a bulk capacitor by a factor of about 5600; that matters when the 5 V rail feeds an analog front-end or a codec. The SOT-223 package (TO-261-4) is a three-terminal tabbed surface-mount part. The tab is the output pin and the primary heat path — the copper area under it on the PCB sets the effective thermal resistance. No enable pin, no power-good flag; this is a basic three-terminal regulator, so the bill of materials stays lean.
Dropout, PSRR, and the thermal envelope
Dropout is specified at 1.2 V maximum at 800 mA output. That is the worst-case headroom needed; at lighter loads the dropout voltage drops, but the BOM should be sized for the max figure. The 10 mA quiescent current is typical for a 800 mA LDO of this generation — not a low-Iq part, so it is not ideal for battery-powered always-on rails. The lower bound excludes cold-crank automotive or outdoor telecom cabinets, but the 125 °C ceiling covers most industrial enclosures and power-supply hot zones. Over-current and over-temperature protection are built in — the part folds back before the silicon reaches destructive temperatures.
Because the part is active, there is no last-time-buy clock. The base product number LD1117 covers multiple fixed and adjustable voltage variants in SOT-223 and DPAK packages — confirm the exact suffix (S50TR for 5 V fixed, tape-reel) when ordering.