The STMicroelectronics LDK320M30R is a fixed 3 V output, 200 mA positive LDO in a SOT-23-5 package. Its headline PSRR of 88 dB at 120 Hz drops to 48 dB at 10 kHz, which means the regulator knocks down low-frequency ripple from a mains-rectified rail or a DC-DC converter's fundamental very effectively, but the rejection rolls off well before the switching harmonics. For a 400 kHz switcher, the bulk of the attenuation happens at the fundamental; the 10 kHz figure is where you start to see the residual ripple on the output. The 0.35 V max dropout at 200 mA sets the input headroom — a 3.3 V rail needs to stay above 3.35 V to hold regulation at full load. Quiescent current sits at 90 µA, making this a candidate for always-on or lightly loaded rails where standby draw matters.
Dropout and quiescent — the BOM-fit numbers
The 0.35 V dropout maximum at 200 mA is the number to budget against the input rail's minimum during cold-crank or brownout. If the input is a 3.6 V rail, you have about 250 mV of margin before the output drops out. The 90 µA quiescent current is typical at no load; the supply current maximum is 100 µA. Enable control is provided, so the regulator can be gated off by a GPIO to cut the 90 µA when the load is shut down.
Protection features include over-current and over-temperature shutdown — no reverse-current or reverse-voltage protection, so the output should not be driven above the input.
No official second-source or pin-compatible alternate is listed by ST, but the LD39020DTPU33R (3.3 V fixed, 200 mA, SOT-23-5) shares the same package and output current rating, though its PSRR is specified at 80 dB to 55 dB over 1 kHz to 100 kHz and its input voltage max is 5.5 V versus 18 V for the LDK320M30R.