Overview
A linear regulator converts a stable or unstable input voltage into a stable output voltage. When operating correctly, the output voltage remains stable even if the input voltage varies significantly.
Principle of Operation
Most linear regulators use closed-loop control. To obtain the desired output voltage, the resistor ratio R1/R2 is adjusted so that the amplifier's inverting input equals the reference voltage at the non-inverting input. The error amplifier feedback loop forces the voltage difference between the two inputs toward zero.
In operation, if the load decreases or the input voltage increases, the output voltage tends to rise. The amplifier's inverting input becomes higher than the non-inverting input, so the error amplifier output swings negative. That reduces the base/gate drive of the pass device, causing the pass device output to fall and restoring the output voltage. Conversely, if the load increases or the input voltage falls, the inverting input falls below the non-inverting input, the error amplifier output increases, the pass device drive increases, and the output voltage is driven back up. The feedback loop compensates for both line regulation (input voltage changes) and load regulation (load current changes).
Low-Dropout Linear Regulators (LDO)
Conventional linear regulators often use bipolar transistors as the current amplifier. Because they can form Darlington configurations, a certain voltage drop is required across the pass device.
Replacing the bipolar pass transistor with a P-channel MOSFET yields a low-dropout regulator (LDO). An LDO typically requires only a few hundred millivolts of dropout voltage.
Advantages, Disadvantages, and Applications
Advantages
- Simple circuit and low cost
- Low output noise
- Good isolation from input noise
- Fast transient response
Disadvantages
- Requires a minimum voltage drop to regulate, so it can only be used for step-down applications
- Low efficiency when the voltage drop is large; excess power is dissipated as heat, which can affect board stability and reliability
- When designed for high power, components can operate near full load even when the load does not require large current, producing significant heat
- Insufficient dropout voltage can lead to increased output ripple
Typical Applications
- Analog circuits and clock generation circuits with strict supply noise requirements
- Digital circuits with low current demand where power-conversion efficiency is not critical
