Each type of linear regulator has its own advantages and disadvantages, and ultimately it is up to the designer to determine whether a certain type of regulator is suitable for use in the device based on requirements such as dropout voltage, ground current, and stability compensation methods.
The voltage difference and ground current values are mainly determined by the pass element of the linear regulator. After the voltage difference and ground current values are determined, the type of equipment suitable for the voltage regulator can be determined. The five mainstream linear regulators currently in use each have different pass elements and unique properties, which are suitable for different equipment applications.
The advantage of a standard NPN regulator is that it has a stable ground current approximately equal to the base current of the PNP transistor, which is fairly stable even without the output capacitor. This kind of voltage stabilizer is more suitable for the equipment with higher voltage difference, but the higher voltage difference makes this kind of voltage stabilizer not suitable for many embedded devices.
For embedded applications, NPN bypass transistor regulators are a good choice because of their low dropout and ease of use. However, this regulator is still not suitable for battery-powered equipment with very low dropout requirements, because its dropout is not low enough. Its high-gain NPN bypass tube can stabilize the ground current to a few amps, and its common emitter structure has very low output impedance. A PNP bypass transistor is a low dropout voltage regulator in which the bypass element is a PNP transistor. Its input and output voltage difference - generally between 0.3 to 0.7V. Because of the low dropout voltage, this PNP bypass transistor regulator is ideal for battery-operated embedded devices. However, its large ground current will shorten the life of the battery. In addition, the lower gain of the PNP transistor can lead to unstable ground currents of several milliamps. Due to the use of a 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 now widely used in many battery-powered devices due to their low dropout voltage and ground current. This type of regulator uses a P-channel FET as its pass element. The voltage drop of such a regulator can be very low because it is easy to adjust the drain-source impedance to a lower value by adjusting the size of the FET. Another - one has a FET as its pass element. The voltage drop of such a regulator can be very low because it is easy to adjust the drain-source impedance to a lower value by adjusting the size of the FET. Another - a capacitor with a specific range of capacitance and ESR to work stably.
N-channel FET regulators are ideal for devices that require low dropout voltage, low ground current and 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 an external capacitor to work stably, the capacitor value does not need to be large, and ESR is not important. N-channel FET regulators require a charge pump to establish the gate bias voltage, so the circuit is relatively complex. Fortunately, N-channel FETs can be up to 50% smaller than P-channel FETs for the same load current.
