lowdropout

Low dropout (low-dropout regulator), commonly abbreviated as LDO, refers to a type of linear voltage regulator that can regulate an output voltage with a small difference between the input and output voltages. LDOs are integrated circuits used to provide a stable supply rail in space-constrained and battery-powered systems. They typically include a reference voltage, an error amplifier, a pass element, and often features such as enable, current limit, and thermal shutdown.

Operation and dropout: The regulator compares the reference to a scaled version of Vout and adjusts the pass element to keep Vout constant. The dropout voltage is the minimum input–output differential needed to maintain regulation. In practice, dropout can range from a few tens of millivolts at light loads (especially in PMOS-based LDOs) to a few hundred millivolts at higher currents. The choice of pass element, often PMOS or a bipolar transistor, influences dropout characteristics and load regulation.

Characteristics and design considerations: LDOs offer high efficiency when Vin is close to Vout, low output noise, and good power-supply rejection ratio (PSRR). They typically require an output capacitor with suitable stability characteristics; many modern designs tolerate or require low ESR ceramics. Quiescent current (ground current) is a key metric affecting battery life. Many LDOs include features such as enable pins, adjustable outputs via resistor dividers, and protection mechanisms like thermal shutdown and current limiting.

Applications and limitations: LDOs are widely used in battery-powered devices, microprocessors, and analog/RF circuits needing a clean, stable supply with minimal noise. They are simple and compact but are less efficient than switching regulators when the input-output voltage difference is large, since heat dissipation grows with the voltage drop.