Among regulators, the simplest regulator circuit is for a low-dropout (LDO) voltage regulator whose topology is shown in Fig. 1. As a linear voltage regulator, its main components are a pass transistor, error amplifier, voltage reference, and output power MOSFET. One input to the error amplifier, set by resistors R1 and R2, monitors a percentage of the output voltage. The other input is a stable voltage reference (VREF). If the output voltage increases relative to VREF, the error amplifier changes the pass-transistor’s output to maintain a constant output voltage (VOUT).

Fig. 1. An LDO’s low dropout voltage and low quiescent current make it a good fit for portable and wireless applications.

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.

The major advantage of an LDO IC is its relatively “quiet” operation because it does not involve switching. In contrast, a switch-mode regulator typically operates between 50 kHz and 1 MHz, which can produce EMI that affects analog or RF circuits. LDOs with an internal power MOSFET or bipolar transistor can provide outputs in the 50 to 500 mA range. The LDO’s low dropout voltage and low quiescent current make it a good fit for portable and wireless applications.

An LDO regulator’s dropout voltage determines the lowest usable input supply voltage. That is, although specs may show a broad input voltage range, the input voltage must be greater than the dropout voltage plus the output voltage. For a 200 mV dropout LDO, the input voltage must be above 3.5 V to produce a 3.3 V output.

With an LDO, the difference between input voltage and output voltage may be small, and the output voltage must be tightly regulated. Plus, transient response must be fast enough to handle loads that can go from zero to tens of amperes in nanoseconds. Further, output voltage can vary due to changes in input voltage, output load current, and temperature. Primarily, these output variations are caused by the effects of temperature on LDO voltage reference, error amplifier, and its sampling resistors (R1 and R2).