This slide shows why the dropout voltage in dual supply LDOs are superior to single supply LDOs. In general, on-resistance of an N-channel MOSFET is lower than that of P-channel MOSFETs of the same size. However, N-channel MOSFETs need an adequate gate-to-source voltage to reduce on-resistance and thus reduce the dropout voltage. A conventional single supply LDO uses a P-channel MOSFET because it does not have access to a second voltage source. The dual supply LDO provides the second voltage source and can achieve lower dropout voltages, biasing the error amplifier and providing adequate Vgs to an N-channel MOSFET.