Integrated circuit (IC) linear voltage regulators use an active pass element to reduce the input voltage to the regulated output voltage. Some devices use bipolar transistors and others use metal-oxide semiconductor field effect transistors (MOSFETS). The use of voltage-controlled sources enables IC linear voltage regulators to force a fixed voltage to appear at the output terminal. Control circuitry monitors the output voltage and adjusts the current source accordingly. Generally, the control circuitry contains a pass device such as pair of NPN transistors driven by a PNP transistor. NPN transistors contain a P region between two N regions. PNP transistors contain an N region between two P regions.
For IC linear voltage regulators, the regulator type determines the configuration of the pass device as well as the dropout voltage, the voltage at which the output falls out of regulation. Standard voltage regulators use an NPN Darlington configuration and have a dropout voltage between 1.5V and 2.2 V. Low dropout (LDO) regulators differ from standard regulators in that the pass device consists of a single PNP transistor. The dropout voltage equals the voltage across the PNP transistor and ranges from 100 mV to 300 mV. Quasi-LDOs are similar to standard regulators, but use NPN and PNP transistors as the pass device. Dropout voltages for quasi-LDOs are higher than for LDOs, but lower than for standard regulators.
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