Classic three-terminal voltage regulators (also known as standard voltage regulators) use an NPN-type transistor or an N-channel MOSFET as an output transistor.
For these regulators to work, an input-output voltage differential called a dropout voltage (VDO) is necessary as shown below. In the case of NPN-type voltage regulators, the minimum value of VDO must satisfy VIN - VOUT > RIN x IIN + 2 × VBE.
In the case of MOSFET-based voltage regulators, the minimum value of VDO must satisfy VIN - VOUT > RIN × IIN + VGS. Suppose RIN = 1 kΩ, IIN = 1 mA, VBE = 0.7 V, and VGS = 1 V. Then, the minimum input voltage required to generate a 5-V output is calculated to be 7.4 V in the case of NPN-type voltage regulators and 7 V in the case of MOSFET-based voltage regulators.
In contrast to these regulators, LDO regulators mainly use a PNP transistor or a P-channel MOSFET as an output transistor. The minimum dropout voltage of such LDO regulators is determined by the collector-emitter voltage (VCE(sat)) and the drain-source voltage (VDS = RDS(ON) × ID). Therefore, LDO regulators can operate with a smaller dropout voltage than three-terminal voltage regulators.
There is also a new type of LDO regulator that provides an even smaller dropout voltage, which uses an N-channel MOSFET with a smaller on-resistance than a P-channel MOSFET as an output transistor and has a power supply pin dedicated to the control circuit (including the MOSFET drive circuit).