Basic knowledge of low dropout regulator

1 What's inside an LDO?

As shown in Figure a, a low dropout regulator (LDO) consists of a reference voltage, an error amplifier, a feedback voltage divider, and a series pass element (usually a bipolar or CMOS transistor).

The output current is controlled by a PMOS transistor, which in turn is controlled by an error amplifier. This amplifier compares the feedback voltage from the output with a reference voltage and amplifies the voltage difference.

Figure a: LDO can provide the required voltage regulation from input voltage to output voltage with a low voltage difference, that is, a small voltage change occurs between Vin and Vout.

If the feedback voltage is lower than the reference voltage, the gate of the PMOS device will be pulled down, allowing more current to flow and increasing the output voltage. Conversely, if the feedback voltage is higher than the reference voltage, the gate of the PMOS device will be pulled up, allowing less current to flow and reducing the output voltage.

This closed-loop system is based primarily on two poles, an internal pole of the error amplifier/pass transistor and an external pole of the output capacitor equivalent series resistance (ESR).

ADI's LDOs are designed to operate stably over the specified operating temperature and voltage ranges when the recommended capacitors are used. The ESR of the output capacitor affects the stability of the LDO control loop. To ensure control loop stability, a minimum ESR of 1Ω or less is recommended. The LDO's response to rapid changes in load current, known as transient response, is also affected by the output capacitor. Using a larger value of output capacitor improves the LDO's transient response, however this results in a longer startup time.

2 Why use LDO?

The purpose of an LDO regulator is to obtain a low output voltage from a main power source or battery. Ideally, the output voltage is stable to line and load variations, and does not change with external temperature and time.

The voltage difference between the LDO input and output voltage should be as low as possible. For example, in a battery-powered design, a 2.8V LDO is connected using a Li-Ion battery. The battery voltage drops from 4.2V (fully charged) to 3.0V (depleted) and provides a constant 2.8V output to keep the LDO voltage drop below 200mV.

In some systems, LDOs are used for post regulation.

In this case the LDO is connected to the output of a high efficiency switching regulator to provide noise filtering and a constant and regulated output voltage.

3 What are the new features of high-performance LDO?

The high-performance LDO includes an enable input, a power-good indicator, undervoltage lockout, reverse polarity protection, a sense input, and a soft-start function, as shown in Figure b.

Figure b: Block diagram of a high-performance LDO with new features