The following tutorial nugget comes courtesy of Greg Waterfall of Texas Instruments, one of our experts.
In the low-dropout (LDO) regulator, the pass element (Figure 1) can be a single PNP bipolar transistor (Figure 2) or a MOSFET (Figure 3). The big advantage of the LDO is that the PNP pass transistor can maintain output regulation with very little voltage drop across it.
heat and reduces efficiency.
VDROP = VSAT (LDO Regulator)
very low dropout voltages.
With the bipolar PNP transistor, full-load dropout voltages of less than 500 mV are typical. At light loads, dropout voltages can fall as low as 10-20 mV. Using a MOSFET pass element, in some cases, will bring dropout voltages an order of magnitude lower than a bipolar transistor pass element.