The project is a DC-DC converter from 5 V to 4 V with consumer switching in case of disconnection of the main power supply to the backup power supply (e.g. Li-Ion). The maximum current is up to 2 A.

The power supply is controlled by two switching transistors: NPN bipolar Q1 and MOSFET Q2.

The main requirement for the converter is the ability to reduce the input voltage from 5 V to 4 V and stabilize it at the output. Also important is its ability to withstand a passing current of 2 A. The LMS1587CT-ADJ was chosen as the voltage regulator - a positive stabilizer with a low voltage drop and an output load current of 3 A, which sets the output voltage through two connected external resistors.

This circuit guarantees automatic switching from mains operating voltage to battery voltage and vice versa. The advantages of such a system are the independence of switching from the voltage value of the main source and a fairly low energy consumption of the backup in the static quiescent mode. But, the disadvantage is the vulnerability to connecting a backup source without observing the correct polarity, which will cause a breakdown of the transistor Q2.