3.3V fixed rail, 500mA capacity — the BOM-fit decision
The BD33KA5WF-E2: The BD33KA5WF- is a single-output positive linear regulator from ROHM, delivering a fixed 3.3V at up to 500mA. That output current covers a modest digital rail — a small MCU plus a few peripherals — without needing a switcher's inductor and output capacitor pair. Input voltage maxes at 5.5V, so it takes a 5V rail or a 3.7V Li-ion directly. The 0.2V dropout at 200mA means the input must stay above 3.5V to hold regulation at that load — a 3.3V rail from a 3.6V battery has 0.1V headroom, which is below the dropout and will push the output out of spec.
PSRR at 120Hz — what 50dB actually buys you
The 50dB PSRR at 120Hz is the rejection of ripple from a full-wave rectified mains supply. 50dB attenuation means the ripple at the output is about 0.3% of the ripple at the input — a 100mV p-p input ripple becomes roughly 0.3mV p-p at the output.
Quiescent draw and protection — always-on and fault-tolerant
Quiescent current is 550 µA max — not an ultra-low-Iq part for battery-sleep applications, but acceptable for a continuously-powered rail where the load current is in the tens of mA. The enable pin lets a GPIO or supervisor shut the output off, dropping the supply current to near zero when the rail is not needed. Built-in over-current and over-temperature protection means the regulator folds back or shuts down under a short or excessive ambient heat — it does not blow up, it just stops delivering. For a prototype or a field unit that might see a marginal load, that saves a board spin.
Package, temperature grade, and compliance
The 8-SOIC (0.173", 4.40mm width) body is the standard narrow SOIC footprint — the same land pattern as a 74HC or an op-amp in SOIC-8. The supplier device package is listed as 8-SOP, which is the same mechanical outline. Reflow profile follows the standard JEDEC MSL level for this package family; no special bake required unless the reel has been exposed to humidity beyond the floor life. ROHS3 compliant, no exemptions expiring.