A buck-boost converter is ostensibly just a combination of a buck controller and a boost controller. In buck mode, the transistor Q4 acts as a closed switch and Q3 acts as an open switch. Q1 and Q2 modulate based upon the required load. In boost mode, Q1 is in the closed position, Q2 is open, and Q3 and Q4 modulate depending upon the load. When V_IN is close to the same value as the required V_OUT, then the device may alternate between bucking and boosting the circuit as shown in the next slide.