Re: TB3155 What is this symbol?
The buck converter senses voltage at the output using OPA1 and DAC1. Current in the inductors is sensed by the CMP1 and CMP2, which provide half-bridge PWM to the MOSFETs. Being a buck converter, the input voltage is always higher than the output voltage, so there would never be reverse current through the body diode of the series MOSFETs, except maybe if input power was removed.
You are right, though, that a normal PWM with the lower MOSFET ON would backfeed through it. But the current sensor should turn it off when current stops flowing. It is being used as a synchronous rectifier to improve efficiency over using the body diode for commutation.