The Basic Knowledge Analysis of Linear Regulators

[maryzhu]

I read an electronic magazine in which there is a report on the basic knowledge analysis of linear regulators, http://www.kynix.com/Detail/163586/ADP1 ... EVALZ.html. After reading, I became interested in linear regulators, so I searched some related materials and information about this topic on net. Today I come to this forum to turn to you to discuss my understandings are appropriate.

Here are my personal understandings:

For a long time, linear regulators have been widely used in the industry. In the switch mode power supply in the last century after the 60's became mainstream, the linear regulator was the basis of the power industry. Even today, linear regulators are still widely used in a wide range of applications. Here we come to the basic knowledge of the linear regulator for a related introduction.

1st, the basic concept of linear regulator

Linear Regulator uses a transistor or FET running in its linear region to subtract excess voltage from the applied input voltage to produce a regulated output voltage. Its products are small package, with excellent performance, and provide thermal overload protection, safety limit and other value-added features, turn-off mode can significantly reduce power consumption.

2nd, the linear regulator works

We start with a simple example. In an embedded system, a 12V bus voltage rail can be provided from the front supply. On the system board, a 3.3V voltage is required for an operational amplifier (op amp) powered. The easiest way to generate a 3.3V voltage is to use a resistor divider from the 12V bus, as shown in Figure 1. Is this good? The answer is often "no". Under different operating conditions, the op amp's VCC pin current may change. If a fixed resistor divider is used, the IC VCC voltage will vary with the load. In addition, 12V bus input may not be well adjusted. In the same system, there may be a lot of other load sharing 12V rails. Due to bus impedance, the 12V bus voltage will change as the bus load changes. Therefore, the resistor divider can not provide a 3.3V stabilized voltage for the op amp to ensure proper operation. Thus, a dedicated voltage regulation loop is required. As shown in Figure 2, the feedback loop must adjust the resistance of the top resistor R1 to dynamically adjust 3.3V on VCC.

The features for linear voltage regulators

The so-called anti-short circuit capability requirements, refers to the relevant material in the short-circuit conditions, the regulator is not damaged. The short circuit resistance of the regulator includes two aspects: the heat resistance of the short circuit and the dynamic stability of the short circuit.

After the differential and ground current values ​​are set, the type of device to which the regulator is suitable can be determined. The five major linear regulators each have different bypass components (pass element and unique performance, voltage difference and ground current values ​​are mainly determined by the linear regulator of the bypass element (pass element.) Suitable for different devices.

Even if there is no output capacitor is quite stable, it is more suitable for devices with higher voltage differentials. The advantage of the NPN regulator is that it has a stable ground current equal to the base current of the PNP transistor. But the higher pressure differential makes this regulator not suitable for many embedded devices.

NPN bypass transistor regulator is a good choice, for embedded applications, because it is small pressure, easy to use. However, this regulator is still not suitable for use with very low pressure differential battery-powered equipment, because it is not low enough pressure. The high gain NPN bypass tube stabilizes the ground current at several milliamps, and its common emitter structure has a very low output impedance.

One of the bypass components is the PNP transistor. Input and output differential pressure is generally between 0.30.7V. Because the differential pressure is low, the PNP bypass transistor is a low dropout regulator. So 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 low, will form a few milliamperes of unstable ground current. Because of the use of a common emitter structure, its output impedance is relatively high, which means that an external range of specific capacity and equivalent series resistance are required (ESR capacitor is stable enough to work.

Ps: Excuse me if I was wrong in words or expressions as I am a green hand in the field of linear voltage regulators. I need continual learnings.

What is your idea ? Do you agree with my ideas ? Any of your ideas would be highly appreciated.

May someone would like to help ?

thanks in advance.