Fixed 5V rail with 63 dB PSRR — analog supply noise floor
The STMicroelectronics LD2981CU50TR is a fixed-output positive LDO regulator delivering 100 mA at 5 V from a maximum 16 V input rail. The 0.375 V dropout at full load (100 mA) leaves only 375 mV of headroom between the input rail and the regulated 5 V output. A 5 V rail fed from a 5.5 V supply has 125 mV of margin above dropout — fine for a clean bus, but marginal if the input sags under transient load. Budget the dropout into the rail-stack design.
150 µA quiescent — always-on and battery-backed rails
Quiescent current is 150 µA typical, with a maximum supply current of 2.6 mA. For a microcontroller's always-on domain or a real-time clock supply that stays powered when the main rail is off, the 150 µA Iq keeps the battery drain low enough that a 100 mAh coin cell runs the regulator for over 27 days before the quiescent draw alone depletes it. The industrial temperature range (-40°C to 125°C) covers engine-bay electronics, outdoor telecom enclosures, and factory-floor sensor modules. The SOT-89-3 package (TO-243AA) has a large center tab that conducts heat to the board copper — the thermal resistance to ambient depends on the pad area, so a solid copper pour under the tab is the difference between 100 mA continuous and derating at high ambient.