3 17 the principle of adjusting the output voltage is that the voltage between pin 2 and pin 3 is constant,1.25v. After connecting a resistor between this constant voltage, pin 2 will get a constant grounding current. This constant current will flow through the regulating resistor you connect, so that the voltage of No.2 against the ground will change constantly.
That is to say, 3 17 adjusts the ground voltage of pin 2 by adjusting the resistance connected to pin 2, thus adjusting the output voltage.
This system controlled by single chip microcomputer controls the ground voltage of 3 17 with single chip microcomputer, thus adjusting the output voltage.
The adjustment process is as follows: if the voltage is set to 3.25 volts, the single chip microcomputer will output a command of 2 volts, which will be converted into analog 2 volts through the "digital-to-analog conversion" circuit DAC0832, amplified by the operational amplifier and output to the 2 pins of 3 17, and then the output voltage of 3.25 volts will be obtained after being superimposed with the 2 pins.
But why choose 3 17? Because 3 17 can output the lowest voltage of 1.25 volts, that is to say, this system can finally output the lowest voltage of 1.25 volts (this is theory). If you choose 7805(5 V fixed three-terminal regulated integrated circuit), this system can also be realized, but it may only output 5 V at least. Although it can be used, the lowest voltage that can be output is much higher, and its practicability is reduced.
The purpose of using single chip microcomputer is to realize "numerical control".