Why use a linear regulator

In the ECU of the automobile, the single chip microcomputer and other devices of the system need a reliable and stable power supply with a voltage lower than the battery voltage (such as 3.3V or 5V). The power supply needs to work in the whole temperature range from - 40 ° C to 150 ° C. Discrete device power supply because its voltage is affected by load current (e.g. resistance frequency divider) or temperature (e.g. zener diode), the discrete device solution can not meet the requirements of power supply in automotive applications.

A linear regulator converts a DC input voltage (e.g., battery voltage) to a lower DC output voltage (e.g., 5V). As long as the input voltage is greater than the output voltage, the output voltage can remain stable even if the input voltage fluctuates.

Linear voltage regulator is the most commonly used power solution in automotive applications.


How does the linear regulator work

The linear regulator consists of internal reference voltage, error amplifier, feedback voltage divider and power transistor. The conducting element transmits the output current through the control of the error amplifier. The error amplifier compares the reference voltage with the output feedback voltage.

If the output feedback voltage is lower than the reference voltage, the error amplifier makes more current flow through the power device to increase the output voltage.

Conversely, if the feedback voltage is higher than the reference voltage, the error amplifier reduces the current flowing through the power transistor, and the output voltage decreases.

 

Linear regulator comparison

Technical analysis of common linear voltage regulator

The advantage of the standard NPN regulator is that it has a stable ground current approximately equal to the base current of PNP transistor, which is quite stable even without output capacitance. This voltage regulator is more suitable for devices with high voltage difference, but the high voltage difference makes this voltage regulator not suitable for many embedded devices.

For embedded applications, NPN bypass transistor regulator is a good choice because of its small differential voltage and easy to use. However, this voltage regulator is still not suitable for battery powered equipment with very low differential pressure requirements, because its differential pressure is not low enough. Its high gain NPN bypass tube can stabilize the grounding current at several Ma, and its common emitter structure has very low output impedance.

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

Because p-channel FET regulator has low voltage difference and grounding current, it is widely used in many battery powered devices. This type of regulator uses the p-channel FET as its bypass element. The voltage difference of this regulator can be very low because it is easy to adjust the drain source impedance to a lower value by adjusting the FET size. Another useful feature is the low ground current because the "gate current" of the p-channel FET is very low. However, because the p-channel FET has a relatively large gate capacitance, it needs to be externally connected with a capacitance with a specific range of capacity and ESR in order to work stably.

N-channel FET regulator is very suitable for equipment requiring low differential voltage, low grounding current and high load current. N-channel FET is used for bypass pipe, so the voltage difference and grounding current of this regulator are very low. Although it also needs external capacitors to work stably, the capacitance value is not very large, and ESR is not important. The n-channel FET regulator needs a charging pump to establish the gate bias voltage, so the circuit is relatively complex. Fortunately, under the same load current, the size of n-channel FET can be 50% smaller than that of p-channel FET.