Low dropout regulator
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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. 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.
[edit] Regulation
A regulator's dropout voltage determines the lowest usable supply voltage. If, for example, the LDO has a dropout voltage around 700mV (0.7V), a 3.3V output would require the input to be at least 4V. Thus the LDO may be specified to provide a fixed 3.3V output with a 4V to 5.5V input.
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.
[edit] 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.
