The role of a voltage regulator is to convert an unstable DC voltage into a stable DC voltage. The integrated voltage regulator has a wider and wider range of applications due to its advantages such as small size, good voltage regulation performance and perfect maintenance function. There are two types of integrated regulators: linear power regulators and switching power supply regulators. Because switching power supply regulators are analogue, digital hybrid circuits, we will introduce them later, and only analogue integrated regulators will be described below. There are two types of analog integrated regulators: three-terminal and multi-terminal.
(1) Three-terminal regulator
The three-terminal regulator is divided into two types: non-adjustable and adjustable.
(1) The regulator cannot be adjusted at three ends. The three-terminal non-adjustable voltage regulator is the most widely used voltage regulator at present. There are two main types of three-terminal non-adjustable regulators: 78×× series and 79×× series. In the meantime, the 78×× series regulators output a positive voltage, while the 79×× series regulators output a negative voltage.
a. Classification. The three-terminal non-adjustable regulator can be divided into 10 types according to the output voltage, taking the 78×× series regulator as an example, including 7805 (5V), 7806 (6V), 7808 (8V), 7809 (9V), 7810 (10V), 7812 (12V), 7815 (15V), 7818 (18V), 7820 (20V), 7824 (24V). The three-terminal non-regulating regulator can be divided into many kinds according to the output current.
b. The operating principle of the three-terminal non-adjustable regulator. The following takes the 78×× series three-terminal non-regulating regulator as an example to introduce the operating principle of the three-terminal non-regulating regulator. The 78×× series three-terminal non-regulating regulator is composed of a starting circuit (constant current source), a sampling circuit, a reference voltage composition circuit, an error amplifier, an adjustment tube, and a maintenance circuit.
When there is a normal power supply voltage input at the input terminal of the 78×× series three-terminal non-regulating regulator, the voltage is not only added to the collector of the adjustment tube, but also the reference voltage is generated by the reference voltage to form the circuit. After the reference voltage is added to the error amplifier, the error amplifier supplies the reference voltage to the base of the adjustment tube, so that the emitter output voltage of the adjustment tube is limited by R1, and then outputs through the output terminal of the three-terminal non-regulating regulator to supply power to the load.
When the input voltage drops or the load becomes heavier, causing the output voltage Uo of the three-terminal unregulated regulator to drop, the voltage decreases after sampling by the adjustable resistors RP and R2. The voltage is added to the inverting input of the error amplifier, and after comparing with the reference voltage of the input at the non-inverting input terminal, the output voltage increases, and the conduction degree of the adjustment tube is strengthened due to the increase of the base input voltage, so that the Uo rises to the specified value and completes the voltage regulation control. When the output voltage rises, the regulation control process is reversed.
When the load is abnormal, the overcurrent of the adjustment tube is detected by the overcurrent maintenance circuit, and the adjustment tube stops the operation to avoid the overcurrent damage of the adjustment tube, and the overcurrent maintenance is completed. In addition, when the overcurrent of the adjustment tube is overcurrent, the temperature will rise significantly, and after being detected by the overheating maintenance circuit in the chip, the adjustment tube will also stop working, so as to avoid the overheating damage of the adjustment tube and complete the overheating maintenance.
(2) Three-terminal adjustable voltage regulator. The three-terminal adjustable regulator is developed on the basis of the three-terminal non-adjustable regulator, and its biggest advantage is that the output voltage can be adjusted continuously within a certain range. Like the three-terminal non-regulating regulator, it also has a positive voltage output and a negative voltage output.
a. Classification. The three-terminal adjustable regulator can be divided into 4 types according to the output voltage: the first is the output voltage of 1.2-15V, such as LM196/396; the second is the output voltage of 1.2-32V, such as LM138/238/338; The third is the output voltage of 1.2-33V, such as LM150/250/350; The fourth is the output voltage of 1.2-37V, such as LM117/217/337.
The three-terminal adjustable regulator is divided into 0.1A, 0.5A, 1.5A, 3A, 5A, 10A and other types according to the output current. The output current of the regulator with the letter "L" after the regulator type is 0.1A, for example, LM317L is the regulator with a maximum current of 0.1A; The output current of the regulator with the letter "M" after the regulator type is 0.5A, for example, LM317M is a regulator with a maximum current of 0.5A; The output current of a regulator without a letter after the regulator type is 1.5A, such as the LM317, which is a regulator with a maximum current of 1.5A. The LM138/238/338 is a 5A regulator and the LM196/396 is a 10A regulator.
b. Principle of operation. The three-terminal adjustable voltage regulator consists of a constant current source (engine circuit), a reference voltage constituent circuit, a regulator (adjustment tube), an error amplifier, and a maintenance circuit.
When there is a normal supply voltage input at the input end of the voltage regulator LM317, the voltage not only supplies power to the regulator, but also supplies power to the reference amplifier through a constant current source, and the reference voltage is generated by it. When the reference voltage is added to the non-inverting input of the error amplifier, the error amplifier supplies the on-voltage to the regulator, so that the output voltage at the beginning of the regulator is supplied to the load after the output voltage is output through the output terminal.
When the input voltage decreases or the load becomes heavier, causing the output voltage of LM317 to drop, the input voltage of the inverting input terminal of the error amplifier decreases, the voltage supplied by the error amplifier to the regulator increases, and the output voltage of the regulator increases, and finally the output voltage rises to the specified value to complete the voltage stabilization control. When the output voltage rises, the regulation control process is reversed.
The output voltage of the 1.25V reference circuit in the LM317 is controlled by the input voltage of the ADJ at the regulator terminal, and when the input voltage of the ADJ terminal increases, the output voltage of the reference voltage constituent circuit increases, and the output voltage of the error amplifier increases due to the increase of the voltage at the non-inverting input terminal, which increases the output voltage of the regulator. Conversely, the control process is reversed. In this way, the output voltage of the LM317 can be changed by adjusting the voltage of the ADJ terminals.
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