Fixed 2.1 V rail for noise-sensitive loads
The STMicroelectronics LD39020ADTPU21R is a fixed-output positive LDO regulator delivering 2.1 V at up to 200 mA from a maximum input of 5.5 V. Its PSRR starts at 80 dB at 1 kHz and rolls off to 55 dB at 100 kHz, which means it cleans up ripple on the input rail well into the switching-frequency range — useful for powering analog front-ends, sensor bias, or low-noise RF stages where a clean 2.1 V rail is needed. The 40 µA quiescent current keeps the idle draw low for always-on or battery-backed sections.
Maximum dropout is 0.35 V at the full 200 mA load. Protection features include over-current, over-temperature, and soft-start — the soft-start limits inrush when the enable pin pulls the output up, which matters if the load has a large decoupling cap.
Package — 4-DFN (1x1 mm)
The part comes in a 4-lead XDFN with exposed pad, body size 1x1 mm. That is a tiny footprint — no room for a human hand with a soldering iron. It is a reflow-only package; plan for a stencil and a hot-air or oven process. Not a field-swap part — this goes on the pick-and-place reel.