Tutorial
Single-Output Low Dropout (LDO) DC-DC Controller ICs
Low dropout (LDO) voltage regulators are linear ICs with the topology shown in Fig. 1. The main components are a power semiconductor and a differential amplifier (error amplifier). One input of the differential amplifier monitors a percentage of the output as determine the resistor ratio of R1 and R2. The second input to the differential amplifier is from a stable voltage reference (VREF). If the output voltage tends to rise relative to VREF, the drive to the power semiconductor changes to maintain a constant output voltage.
Low dropout refers to the difference between the input and output voltages that allows the IC to regulate the output voltage. That is, the LDO regulates the output voltage until its input and output approach each other at the dropout voltage. Ideally, the dropout voltage should be as low as possible to minimize power dissipation and maximize efficiency.
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 specified 3V to 5.5V input. However, the 3.3V output requires about a 4V input with any typical dropout voltage. That is, although the specs show a broad input voltage range, there must be a higher enough input voltage to obtain the required output voltage.
Some LDOs employ a control, or bias, voltage that provides the ability to supply lower output voltages. This additional voltage is used for the output stage, which can either be a MOSFET or bipolar transistor. The majority of LDOs employ an integrated output stage, usually a PMOS or PNP transistor.
Among other important characteristics is the quiescent, or ground current: the difference between their 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. For high efficiency, dropout voltage and quiescent current must be minimized. The quiescent, or ground current, is the operating current of the device.
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).
Most LDOs provide a nominal output voltage, which can vary according to the specific application. Therefore, there is usually a minimum and maximum output voltage. Nominal values usually anywhere from 1.5V to 5V.
Besides low input-to-output voltage conversion, LDOs require tight output voltage regulation. The output voltage specification includes input voltage change, output load current change, temperature change and output load current transient response. This requires total tolerances as low as ±2%. This is only ±30mV for a 1.5V output. Transient response is a critical component as output current can go from zero to amps in tens of nanoseconds, which requires an accurate, high-speed regulator.
Available LDOs provide either an adjustable or fixed output voltage. Fixed output types typically exhibit anywhere from a ±2% to ±6% output voltage variation. They usually provide outputs in the 1V to 5V range. Adjustable LDOs typically provide a ±50% variation around a center voltage.
Some LDO families provide a full range of outputs in 100mV or 50mV steps, for example, 2V to 6V in 100mV steps. These output voltages are made possible by laser trimming during the manufacturing process.
LDO power supply rejection ratio (PSRR) is also important because it affects the LDO's ability to prevent output voltage fluctuations caused by input voltage variations.<