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A low dropout or LDO regulator is a DC linear voltage regulator which has a very small input-output differential voltage (Figure 1). The main components are a power FET and a differential amplifier (error amplifier). One input of the differential amplifier monitors a percentage of the output, as determined by the resistor ratio of R1 and R2. The second input to the differential amplifier is from a stable voltage reference (bandgap reference). If the output voltage rises too high relative to the reference voltage, the drive to the power FET changes so as to maintain a constant output voltage.
Regulation
A regulator's dropout voltage determines the lowest usable supply voltage. An LDO may be specified to provide a fixed 3.3V output with a 3V to 5.5V input. If, for example, said LDO has a dropout voltage around 700mV (0.7V), a 3.3V output would require the input to be at least 4V.
An LDO's output voltage variation is due primarily to a variation in the temperature of the constant voltage reference source and the differential amplifier characteristics, as well as the sampling resistor tolerance (R1 and R2).
Some LDOs employ a control or bias voltage that provides the ability to supply lower output voltages.
Quiescent Current
Among other important characteristics is the quiescent, or ground current (the current flowing through the system when no load is present), which creates a difference between the input and output currents. The series pass element, topologies, and ambient temperature are the primary contributors to quiescent current. Quiescent current and input/output limit the efficiency of LDO regulators and should thus be minimized.