For a long time, linear regulators have been widely adopted by the industry. Before switching mode power became mainstream in the 1960s, linear regulators used to be the basis of the power supply industry. Even today, linear regulators are still widely used in many applications. Here we come to the basic knowledge of linear regulators for one by one related to the introduction.
First, the basic concept of linear regulator
Linear Regulator Transistors or FETs operating in their linear region subtract the excess voltage from the applied input voltage to produce a regulated output voltage. Their products are small packages, with excellent performance, and provide thermal overload protection, safe current limiting and other value-added features, shutdown mode can also significantly reduce power consumption.
Second, the working principle of linear regulator
Let's start with a simple example. In embedded systems, a 12V bus voltage rail can be provided from the front end power supply. On the system board, a 3.3V voltage is needed to power an op amp (op amp). The easiest way to generate 3.3V is to use a resistor divider from the 12V bus, as shown in Figure 1. Is this good? The answer is often "No." The op amp's VCC pin current may change under different operating conditions. If a fixed resistor divider is used, the ICVCC voltage will vary with load. In addition, 12V bus input may not be well adjusted. In the same system, there may be many other loads sharing the 12V rail. Due to the bus impedance, the voltage of 12V bus will change with the change of bus load. Therefore, the resistor divider can not provide the op amp with a 3.3V regulated voltage to ensure proper operation. Thus, a dedicated voltage regulation loop is required. As shown in Figure 2, the feedback loop must adjust the value of the top resistor R1 to dynamically adjust 3.3V at VCC.
Such a variable resistor can be implemented using a linear regulator, as shown in Figure 3. Linear regulators allow a bipolar or field-effect power transistor (FET) to operate in its linear mode. In this way, the transistor functions as a variable resistor in series with the output load. Conceptually, to build a feedback loop, a sampling resistor network (RA and RB) can be used by an error amplifier to sense the DC output voltage and then compare the feedback voltage, VFB, with a reference voltage, VREF. The error amplifier output voltage drives the base of a series power transistor through a current amplifier. When the input VBUS voltage drops or the load current increases, the VCC output voltage drops. The feedback voltage VFB will also drop. Therefore, the feedback error amplifier and the current amplifier generate more current and are input to the base of the transistor Q1. This will reduce the voltage drop VCE and thus restore the VCC output voltage, thus VFB = VREF. On the other hand, if the VCC output voltage rises, the negative feedback circuit takes a similar approach to increase VCE to ensure accurate regulation of the 3.3V output . In summary, any change in VO is diminished by the VCE voltage of the linear regulator transistor. Therefore, the output voltage VCC is always constant and in good regulation.
Third, the characteristics of linear regulators
The so-called anti-short circuit capacity requirements, refers to the relevant materials in the short circuit conditions, the regulator does not damage. The short-circuit resistance of the regulator includes withstand short-circuit heat capacity and withstand short-circuit dynamic stability of two aspects.
After the pressure difference and the grounding current value are set, the applicable device type of the regulator can be determined. Each of the five mainstream linear regulators has a different bypass element (passelement and unique performance, voltage difference and ground current values ​​are mainly determined by the linear regulator bypass element (passelement, respectively, suitable for different equipment use.
Even without the output capacitor is quite stable, it is more suitable for equipment with higher voltage difference, NPN regulator specification has the advantage of roughly equal to the PNP transistor base current of a stable ground current. However, the higher voltage drop makes this regulator not suitable for many embedded devices.
NPN bypass transistor regulator is a good choice, for embedded applications, because of its small pressure drop, easy to use. However, this regulator is still not suitable for battery-powered devices with very low dropout requirements because of its low enough dropout voltage. The high gain NPN bypass tube stabilizes the ground current to a few milliamps and its common emitter structure has a very low output impedance.
The bypass element is the PNP transistor. Input and output pressure is generally between 0.30.7V. The PNP bypass transistor is a low-dropout regulator because of the low dropout voltage. Therefore, this PNP bypass transistor regulator is ideal for battery-powered embedded devices. However, its large ground current will shorten the life of the battery In addition, PNP transistor gain is low, will form a few milliamps of unstable ground current. Because of the common emitter structure, it has a high output impedance, which means that it is necessary to add a specific range of capacitance and an equivalent series resistance (ESR capacitance is enough for stable operation).
Fourth, the advantages and disadvantages of linear regulator
Linear regulators use transistors or FETs running in their linear region to subtract the excess voltage from the applied input voltage to produce a regulated output voltage. Their products are small packages, with excellent performance, and provide thermal overload protection, safe current limiting and other value-added features, shutdown mode can also significantly reduce power consumption.
For a long time, linear regulators have been widely adopted by the industry. Before switching mode power became mainstream in the 1960s, linear regulators used to be the basis of the power supply industry. Even today, linear regulators are still widely used in many applications.
Advantages of linear regulators
In addition to ease of use, linear regulators also have other performance advantages. Power management vendors have developed many integrated linear regulators. Typical integrated linear regulators require only VIN, VOUT, FB, and optional GND pins. Figure 4 shows a typical 3-pin linear regulator LT1083, which Linear Technology developed more than 20 years ago. The device requires only one input capacitor, output capacitor, and two feedback resistors to set the output voltage. Almost all electrical engineers can use these simple linear regulators to design the power supply.
2. Linear regulator shortcomings analysis
Linear regulators consume a lot of power. One of the major drawbacks of using a linear regulator is that its series transistor Q1 running in linear mode suffers from excessive power dissipation. As mentioned earlier, the linear regulator is conceptually a variable resistor. Since all the load current must go through a series resistor, its power dissipation is PLOSS = (VIN-VO) IO. In this case, the efficiency of the linear regulator can be given by
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