3.3V rail, 100 mA, and what the PSRR buys you
The LD2981ABU33TR is a single-output positive LDO from STMicroelectronics, fixed at 3.3 V and rated for 100 mA continuous output. The dropout voltage maxes out at 0.375 V at full load — that means the input rail needs to stay above 3.675 V to keep the output in regulation across all loads and temperatures. PSRR is 63 dB at 1 kHz — this is the rejection of ripple on the input rail. That matters for analog front-ends or RF supplies where the noise floor of the rail directly sets the signal-to-noise ratio.
Operating envelope and protection
The input range spans up to 16 V, so it can take a 5 V or 12 V intermediate bus without a pre-regulator. In a battery-powered system drawing 100 mA from the output, the LDO's own consumption adds 0.15% overhead — negligible for a continuous load but worth budgeting in always-on low-power states. Built-in protection covers over-current and over-temperature. The current limit folds back the output when the load exceeds the rating, and the thermal shutdown prevents die damage if the ambient exceeds the dissipation capability of the SOT-89-3 package.
SOT-89-3 footprint and board-fit
The tab on the bottom is the ground plane — it needs a decent copper pour on the PCB to pull heat out of the die. At 100 mA with 0.375 V dropout, dissipation is only 37.5 mW, so thermal performance is not a constraint in most layouts. Cut Tape works for prototyping or small-batch builds.