a low-dropout (LDO) regulator
/ LDO = EL-dee-OH /
A low-dropout regulator is a linear regulator built to work with only a whisker of voltage between input and output. An ordinary 78xx needs about 2 V of headroom; an LDO can regulate with as little as 0.1 to 0.3 V to spare. That is precious when your input is barely above your output — say, making a clean 3.3 V from a 3.6 V battery, or 5.0 V from a sagging 5.2 V USB rail — where an old-style regulator would simply give up.
The trick is the pass element. A classic regulator uses an NPN/Darlington follower, which always needs roughly a base-emitter drop or two of headroom to stay in control. An LDO instead uses a PNP transistor or, more often, a P-channel MOSFET as the pass device, and that arrangement can be driven nearly fully on, dropping only its on-resistance times the current. Example: an LDO pass FET with 0.1 ohm on-resistance passing 0.5 A drops just 0.1 times 0.5 = 0.05 V, so a 3.35 V input can still deliver a regulated 3.3 V. The tradeoff is that this topology is fussier about stability and usually demands a specific output capacitor with the right value and ESR (equivalent series resistance).
Why it matters: LDOs are the workhorses of battery and post-regulation power, where every wasted volt is wasted runtime and the input-output gap is deliberately tiny. Note the honest point that low dropout does not mean high efficiency by itself — efficiency is still Vout/Vin. An LDO is efficient only because you run it with a small drop; run an LDO from a much higher input and it wastes just as much heat as any linear regulator.
Running a 3.3 V LDO from a single lithium cell that drops from 4.2 V down to 3.5 V as it discharges, the regulator keeps 3.3 V right down to nearly empty because it only needs ~0.2 V of headroom. A 78xx-style part would have stopped regulating once the cell fell below ~5.3 V — useless from a single cell.
Tiny headroom keeps a battery rail alive far longer than an old-style regulator.
Many LDOs are picky about their output capacitor: too little capacitance or the wrong ESR can make them oscillate. Always use the type and value the datasheet specifies, not whatever cap is in the drawer.