LDO

A low-dropout regulator (LDO) provides a clean, regulated voltage from a noisier or higher supply, using a pass transistor (in linear, not switching, operation) controlled by a feedback amplifier that compares a divided-down output voltage against a reference. Because the pass device operates as a variable resistor rather than switching, LDOs are inherently low-noise and fast-responding compared to switching regulators, making them ideal for powering noise-sensitive analog blocks like PLLs, VCOs, and precision references, at the cost of efficiency (proportional to VOUT/VIN, since the dropout voltage is dissipated as heat).

The feedback loop's stability is a central design challenge: the pass transistor's gate capacitance and the output capacitor (with its ESR) create poles that interact with the load current, which varies the output impedance of the pass device over orders of magnitude. PMOS pass transistors in common-source configuration provide low dropout but create a low-frequency pole at the output that moves with load current, traditionally requiring a specific ESR range in the output capacitor for stability — a constraint that capacitor-less (CL) LDO designs eliminate through internal compensation and fast local feedback loops capable of responding to load transients without relying on a large external capacitor.

Key specifications include dropout voltage (minimum VIN-VOUT for regulation), PSRR across frequency (critical when powering sensitive analog blocks from a noisy digital supply), transient response (output voltage droop/overshoot during fast load steps, governed by loop bandwidth and output capacitance), quiescent current (especially important in always-on or battery-powered applications), and line/load regulation accuracy. Advanced LDOs use adaptive biasing, multiple feedback paths (fast AC path plus slow DC path), and digitally-assisted calibration to meet stringent transient and PSRR specs simultaneously.

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