A low-dropout regulator is a special voltage regulator that maintains a steady output at an input voltage level only slightly higher than the output voltage. Standard voltage regulators require an input voltage that is higher by more than about 5 volts (V) than the output voltage. The low-dropout regulator will be able to maintain tight voltage regulation even when the input is higher only by 1 V or less. In general, voltage regulators are needed because almost all electronic circuitry needs a steady direct current (DC) voltage. The linear voltage regulator uses a series pass device, a bipolar junction transistor (BJT) or a field-effect transistor (FET), that regulates the load voltage.
The voltage regulator is the closest approximation to an ideal voltage source. By definition, the ideal voltage source has zero internal resistance, but real-world voltage sources have a significant amount of internal resistance. If a 15 V power supply has an internal resistance of 1 ohm at no load, it looks like an ideal power supply. At a load current of 1 ampere (A), the output becomes 14 V, at 2 A the output is 13 V, and so on. A low-dropout regulator can be connected to the mentioned power supply and will be able to provide a steady output of 9 V with load current ranging from 0 to about 5.8 A.
Quiescent current, which is needed to maintain the reference and comparison circuitry that maintains the regulated voltage, can be measured at "no load” condition. A lower quiescent current means a more efficient regulator. A non-low-dropout regulator ends up with high quiescent current because the reference circuitry needs a relatively higher standby current to maintain regulation at higher load currents. It will have a reference circuitry that may not allow an input voltage too close to the regulated output voltage. Non-low-dropout regulators are simplified regulators that usually have higher quiescent currents but lower efficiency ratings.
The low-dropout regulator usually uses the open collector for the BJT or open drain configuration for an FET version. In an open collector circuit, the regulated output appears at the collector of a BJT. This output voltage is sampled by a resistor divider where the scaled-down voltage is compared to a reference that has also been scaled-down by the same factor as the output voltage. With the very high gain of operational amplifiers, the scaled-down samples are compared and tracked extremely rapidly. The result is a highly accurate output voltage with high short-term and long-term stability.