Technical Analysis of Several Linear Regulators

Each type of linear regulator has its own advantages and disadvantages, and it is ultimately up to the designer to determine whether a certain type of regulator is suitable for the device based on requirements such as voltage drop, ground current, and stability compensation method.

The voltage difference and ground current values ​​are mainly determined by the linear regulator's pass element. Once the voltage difference and ground current values ​​are determined, the type of equipment the regulator is suitable for can be determined. The five major linear regulators currently in use each have different pass elements and unique performance, and are suitable for different equipment.

The advantage of a standard NPN regulator is that it has a stable ground current that is approximately equal to the base current of a PNP transistor and is fairly stable even without an output capacitor. This regulator is better suited for devices with higher voltage differences, but the higher voltage difference makes this regulator unsuitable for many embedded devices.

For embedded applications, the NPN bypass transistor regulator is a good choice because of its small dropout voltage and ease of use. However, this regulator is still not suitable for battery-powered devices with very low dropout requirements because its dropout voltage is not low enough. Its high-gain NPN bypass tube can stabilize the ground current to a few mA, and its common emitter structure has a very low output impedance.

The PNP bypass transistor is a low dropout voltage regulator in which the bypass element is the PNP transistor. Its input-output voltage difference is generally between 0.3 and 0.7V. Because of the low voltage difference, this PNP bypass transistor regulator is very suitable for battery-powered embedded devices. However, its large ground current will shorten the life of the battery. In addition, the PNP transistor has a low gain and will form an unstable ground current of several milliamperes. Due to the common emitter structure, its output impedance is relatively high, which means that an external capacitor with a specific range of capacitance and equivalent series resistance (ESR) is required to work stably.

P-channel FET regulators are widely used in many battery-powered devices today because of their low dropout voltage and ground current. This type of regulator uses a P-channel FET as its pass element. The voltage dropout of this regulator can be very low because it is easy to adjust the drain-source impedance to a low value by adjusting the FET size. Another useful feature is low ground current because the "gate current" of the P-channel FET is very low. However, since the P-channel FET has a relatively large gate capacitance, it requires an external capacitor with a specific range of capacitance and ESR to work stably.

N-channel FET regulators are very suitable for devices that require low voltage drop, low ground current and high load current. The N-channel FET is used for the bypass tube, so the voltage drop and ground current of this regulator are very low. Although it also requires external capacitors for stable operation, the capacitance value does not need to be large and the ESR is not important. N-channel FET regulators require a charge pump to establish the gate bias voltage, so the circuit is relatively complicated. Fortunately, the size of the N-channel FET can be up to 50% smaller than the P-channel FET at the same load current.