Voltage Regulators - Linear, Low Drop Out (LDO) Regulators

Linear voltage regulators are power-management ICs that maintain a stable DC output voltage despite changes in input voltage or load current, within their specified operating limits. They use a series pass element controlled by a feedback circuit and are available with fixed or adjustable outputs. Typical applications include powering logic, amplifiers, sensors, and RF circuits. Linear regulators are particularly useful when the input-to-output voltage difference and load current are modest, or when simplicity and low output noise justify the power loss.

LDO stands for low-dropout regulator, a type of linear regulator designed to operate with a small difference between input and output voltage. Dropout voltage is the minimum input-to-output voltage difference required to maintain regulation under specified conditions. An LDO can be useful when the supply voltage approaches the required output voltage, as in battery-powered systems. A conventional linear regulator may be sufficient when more voltage headroom is available. Low dropout does not, by itself, guarantee low noise or strong rejection of input ripple.

The primary disadvantage of a linear regulator is power dissipation. The regulator dissipates approximately (VIN − VOUT) × IOUT as heat, excluding additional losses associated with its operating current. For example, reducing 12 V to 5 V at 100 mA dissipates approximately 0.7 W in the regulator. A device’s rated output current does not guarantee that current is available at every input-to-output voltage difference: package, PCB layout, ambient temperature, and cooling can impose a lower practical limit. When these losses become excessive, a switching regulator or converter may provide substantially better efficiency.

When selecting a linear regulator or LDO, check input and output voltage ranges, load current, dropout voltage, and thermal performance first. Then consider quiescent current for battery life, power supply rejection ratio (PSRR) for rejection of upstream ripple, output noise for sensitive circuits, and transient response for rapidly changing loads. Evaluate PSRR at the relevant ripple frequencies and voltage headroom. Follow the datasheet’s capacitor requirements for stable operation, and check whether the regulator requires a minimum load current. Some designs combine a switching converter with an LDO, using the switching stage for efficient voltage conversion and the LDO for additional regulation or noise reduction.

DigiKey’s LM317 voltage regulator design guide brings these fundamentals to the workbench with circuit diagrams, resistor-selection calculations, and a measured output-voltage example. It explains how feedback sets the output voltage and examines practical pitfalls, including overheating, insufficient minimum load, and potentiometer-wiper failures.