the dropout voltage
The dropout voltage is the minimum amount by which a linear regulator's input must exceed its output for it to keep regulating. Think of it as the headroom the regulator needs to do its job: give it enough and it holds the output rock-steady; squeeze the input too close to the output and the regulator runs out of room, the output follows the input down, and regulation collapses.
It exists because the pass transistor inside cannot drop to zero volts — it always keeps a little voltage across itself to stay in control. For a standard 78xx that minimum is about 2 V; for a low-dropout (LDO) part it might be 0.1 to 0.3 V. Crucially, dropout usually rises with load current, because more current through the pass device means more voltage across it. Example: a 5 V regulator with 2 V dropout needs at least 7 V in. If your unregulated supply ripples down to 6.5 V at the bottom of each cycle, that valley dips into dropout and you get hum on the 5 V rail even though the average input looks fine.
Why it matters: dropout is one of the most common silent killers of a supply design. People size the average input voltage correctly but forget the ripple valley, or forget that dropout grows under load and with temperature. The fix is to make sure the lowest instantaneous input — average minus half the peak-to-peak ripple, at full load — still clears the dropout with margin. When your input genuinely sits just above your output, an LDO with its tiny dropout is the only linear choice; otherwise a switching regulator sidesteps the problem entirely.
A 12 V regulator with 2.5 V dropout fed from a transformer supply whose ripple valley is 13 V will work; lower the input to 14 V average with 2 V peak-to-peak ripple and the valley is 13 V — still just OK. But add load so dropout rises to 3 V, and now you need the valley above 15 V; the once-fine supply starts to hum under load.
Headroom the regulator needs — check the ripple valley at full load, not the average.
Dropout is not a single number: it grows with load current and often with temperature, and the datasheet usually quotes it at one specified current. Design for the worst case (highest current, hottest, ripple valley), not the typical figure.