Voltage Regulators: LDOs and Switching Regulators

A low-dropout regulator (LDO) is a linear voltage regulator that controls the output voltage through a series pass transistor, whose control voltage is generated by an error amplifier comparing a reference voltage with a fraction of the output voltage taken from a resistive divider. The term low-dropout refers to the minimum voltage difference between input and output at which the pass transistor is just barely kept out of saturation and regulation breaks down; modern LDOs achieve dropout voltages of only 100 to 300 mV at load currents of several hundred milliamperes.

Because the pass transistor operates as a controllable resistor, the power dissipated in the regulator always equals the product of the voltage difference and the load current, regardless of the ratio of input to output voltage. The efficiency of an LDO therefore roughly equals the ratio V_out/V_in – converting from 3.3 V to 1.8 V, for example, gives a theoretical efficiency of only about 55 %. This purely linear operation makes LDOs particularly low-noise and fast in regulation, but inefficient at larger voltage differences.

A switching regulator, by contrast, generates the output voltage not by continuously dissipating power in a series transistor but by periodically turning a power switch on and off at typical switching frequencies of 300 kHz to several megahertz, with its duty cycle controlled by a feedback loop. In the widely used buck converter, the switch charges an energy-storage inductor from the input voltage during the on-time; during the off-time, a freewheeling diode or a synchronous rectifier keeps current flowing through the inductor, so that the output capacitor sees a smoothed output voltage that, on average, is lower than the input voltage.

Because the switching transistor ideally passes through only two states – fully on or fully off – almost no power is dissipated in the switching element itself, allowing switching regulators to reach efficiencies of 85 to over 95 %, well above linear regulators, especially at large voltage differences between input and output.