It steps down DC voltage from 70 V to 20 V. It measures the output voltage and compares with reference voltage. Their difference is passed to PI controller which gives current to be drawn to maintain output voltage to reference value. This calculated reference current is compared with actual inductor current. If actual inductor current is less than calculated reference current, then switchin mosfet S is turned on so that inductor current increases. If actual inductor current is greater than calculated reference current, then switchin mosfet S is turned off so that inductor current decreases. In this way, output voltage is regulated.