As Figure 8-1 shows, an LDO functions as a class-B amplifier in which the input signal is the internal reference voltage (V_REF). V_REF is designed to have a very low bandwidth at the input to the error amplifier through the use of a low-pass filter (V_NR/SS).

As such, the reference can be considered as a pure dc input signal. The low output impedance of an LDO comes from the combination of the output capacitor and pass element. The pass element also presents a high input impedance to the source voltage when operating as a current source. A positive LDO can only source current because of the class-B architecture.

This device achieves a maximum of 0.75% output voltage accuracy primarily because of the high-precision band-gap voltage (V_BG) that creates V_REF. The low dropout voltage (V_DO) reduces the thermal power dissipation required by the device to regulate the output voltage at a given current level, thereby improving system efficiency. These features combine to make this device a good approximation of an ideal voltage source.

V_OUT = V_REF × (1 + R₁ / R₂).

Figure 8-1 Simplified Regulation Circuit