1.8 V fixed LDO — parametric fit for 150 mA rail
The LP3990MFX-1.8/NOPB is a fixed 1.8 V output LDO from Texas Instruments, rated for 150 mA continuous output with a maximum dropout of 0.2 V at full load. Quiescent current is 80 µA typical, with a maximum supply current of 120 µA. For a battery-powered sensor node that stays awake 1 % of the time, the regulator's own draw adds about 1 µA average to the system budget — negligible against a 150 mA load pulse but significant if the rail powers a deep-sleep domain where every microamp counts.
PSRR across the audio band
Power-supply rejection ratio is specified at 55 dB at 1 kHz and 35 dB at 10 kHz. At 1 kHz, that 55 dB attenuates a 100 mVpp ripple on the input to about 180 µVpp at the output — clean enough for an analog front-end or a codec supply. The 20 dB roll-off by 10 kHz means switching-regulator ripple above that frequency passes through with less rejection; a 1 MHz switcher's fundamental ripple will see roughly 30 dB of attenuation, so a post-filter LC stage ahead of this LDO is worth considering if the load is noise-sensitive.
Package, temperature grade, and protection
Housed in a SOT-23-5 (SC-74A) package, the LP3990MFX-1.8/NOPB occupies about 3 mm × 3 mm on the board — a standard footprint for 150 mA-class LDOs, reflow-compatible with lead-free profiles. On-chip protection includes over-temperature shutdown and short-circuit current limiting. The short-circuit limit folds back the output current, so a hard short on the 1.8 V rail won't exceed the package's thermal capacity before the thermal trip engages. Enable control lets the system power down the regulator, dropping quiescent current to near zero — useful for sequencing or sleep-mode power gating.
The device is ROHS3 compliant, covering the EU RoHS exemption set including lead in high-melting-temperature solder.