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Voltage regulators are used to provide a stable power supply voltage independent of load impedance, input-voltage variations, temperature, and time. Low-dropout regulators are distinguished by their ability to maintain regulation with small differences between supply voltage and load voltage. For example, as a lithium-ion battery drops from 4.2 V (fully charged) to 2.7 V (almost discharged), an LDO can maintain a constant 2.5 V at the load.
The increasing number of portable applications has thus led designers to consider LDOs to maintain the required system voltage independently of the state of battery charge. But portable systems are not the only kind of application that might benefit from LDOs. Any equipment that needs constant and stable voltage, while minimizing the upstream supply (or working with wide fluctuations in upstream supply), is a candidate for LDOs. Typical examples include circuitry with digital and RF loads.
A low-dropout or LDO regulator is a DC linear voltage regulator which can operate with 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 the fraction of the output 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 to maintain a constant output voltage.
The output capacitor and its equivalent series resistance (ESR) often limit the stability of a conventional low dropout regulator (LDR). A CMOS LDR with dynamic zero compensation is presented to tolerate the wide range of the output capacitor and the ESR. The stability constraints for the output capacitor with the ESR are derived. The measured LDR is stable for the output capacitor 2nF-47uF with ESR of 0.1-50Omega. The maximum quiescent current of this LDR with a bandgap reference is 43.2uA and its maximum output current is 150 mA for the output voltage of 1.8V.
To choose the right regulator for a specific application, the type and range of input voltage (e.g., the output voltage of the dc-to-dc converter or switching power supply ahead of the regulator), needs to be considered. Also important are: the required output voltage, maximum load current, minimum dropout voltage, quiescent current, and power dissipation. Often, additional features may be useful, such as a shutdown pin or an error flag to indicate loss of regulation.
The source of the input voltage needs to be considered in order to choose a suitable category of LDO. In battery-powered applications, LDOs must maintain the required system voltage as the battery discharges. If the dc input voltage is provided from a rectified ac source, the dropout voltage may not be critical, so a standard regulator—which may be cheaper and can provide more load current—could be a better choice. But an LDO could be the right choice if lower power dissipation or a more precise output voltage is necessary.
The regulator should, of course, be able to provide enough current to the load with specified accuracy under worst-case conditions.
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