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. Each of the five major linear regulators in use today has a different pass element and unique properties, suitable for use in different devices.
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 type of regulator is more suitable for devices with higher dropout voltages, but the higher dropout voltage makes this type of regulator unsuitable for many embedded devices.
For embedded applications, an NPN bypass transistor regulator is a good choice because of its low dropout and ease of use. However, this regulator is still not suitable for battery-operated equipment with very low dropout requirements, because its dropout is not low enough. Its high-gain NPN bypass tube stabilizes ground currents to a few milliamps, and its common-emitter structure has low output impedance.
A PNP bypass transistor is a low dropout regulator in which the bypass element is a PNP transistor. Its input and output voltage difference is 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. Also, the lower gain of the PNP transistors can lead to unstable ground currents of several milliamps. Due to the common emitter structure, its output impedance is relatively high, which means that an external capacitor with a specific range of capacity and equivalent series resistance (ESR) is required to work stably.
P-channel FET regulators are now widely used in many battery-operated 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 useful feature is low ground current because the "gate current" of a P-channel FET is low. However, since a P-channel FET has a relatively large gate capacitance, it requires an external capacitor with a specific range of capacitance and ESR for stable operation.
N-channel FET regulators are ideal for devices that require low dropout voltage, low ground current, and high load current. An N-channel FET is used for the bypass, so this regulator has low dropout and grounds current. Although it also needs an external capacitor to work stably, the capacitor value does not need to be very 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 complex. Fortunately, N-channel FETs can be up to 50% smaller than P-channel FETs at the same load current.
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