Each linear regulator has its own advantages and disadvantages, and ultimately by the designer under the pressure, ground current and stability compensation methods to determine whether a certain type of regulator is suitable for equipment use. The voltage difference and the ground current are mainly determined by the linear regulator's pass element. Once the voltage difference and the ground current have been set, the type of device to which the regulator is applied can be determined. Each of the five mainstream linear regulators currently in use has a different pass element and unique properties, each for a different device. The advantage of a standard NPN regulator is that it has a stable ground current that is approximately equal to the base current of the PNP transistor, which is quite stable even with no output capacitance. This regulator is more suitable for equipment with higher voltage difference, but higher voltage drop makes this regulator unsuitable for many embedded devices. For embedded applications, NPN bypass transistor regulator is a good choice, because of its small pressure drop, and very easy to use. However, this regulator is still not suitable for use with battery-operated devices that have very low dropout requirements because of its low differential pressure. Its high-gain NPN bypass tube stabilizes the ground current to a few milliamps and its common emitter structure has a very low output impedance. PNP bypass transistor is a low-dropout regulator, which is the bypass element PNP transistor. Its input and output pressure is generally between 0.3 to 0.7V. Because of the low dropout voltage, this PNP bypass transistor regulator is ideal for battery-powered embedded devices. However, its large ground current will shorten the battery life. In addition, PNP transistor gain is lower, will form a few milliamps of unstable ground current. Due to its common emitter structure, it has a high output impedance, which means that a capacitor with a specific range of capacitance and equivalent series resistance (ESR) is required to operate stably. P-channel FET regulators are currently widely used in many battery-operated devices due to their lower voltage drop and ground current. This type of regulator uses a P-channel FET as its bypass element. The voltage difference of this regulator can be very low because it is easy to adjust the FET size to adjust the drain-source impedance to a lower value. Another useful feature is low ground current because the gate current of the P-channel FET is low. However, because P-channel FETs have a relatively large gate capacitance, they require external capacitors with a specified range of capacities and ESRs for stable operation. N-Channel FET regulators are ideal for devices that require low dropout voltage, low ground current, and high load current. For the bypass tube is the use of N-channel FET, so the voltage regulator and the ground current is very low. Although it also requires an external capacitor to work stably, the capacitance is not 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 more complicated. Fortunately, N-Channel FETs can be up to 50% smaller than P-Channel FETs at the same load current.
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Technical Analysis of Several Linear Regulators