There are two main types, one output voltage is fixed, called fixed output three-terminal voltage regulator, the other output voltage is adjustable, called adjustable output three-terminal voltage regulator, the basic principle is the same, all use series type voltage regulator circuit. In the linear integrated voltage regulator, because the three-terminal voltage regulator has only three leading terminals, it has the advantages of few external components, easy to use, stable performance and low price, so it is widely used