Figure 21 shows that the MC34167 ca

Figure 21 shows that the MC34167 can be configured as a step−up/down converter with the addition of an external power MOSFET. Energyis stored in the inductor during the ON time of transistors Q1 and Q2. During the OFF time, the energy is transferred, with respect to ground,to the output filter capacitor and load. This circuit configuration has two significant advantages over the basic step−up converter circuit. The firstadvantage is that output short circuit protection is provided by the MC34167, since Q1 is directly in series with Vin and the load. Second, theoutput voltage can be programmed to be less than Vin. Notice that during the OFF time, the inductor forward biases diodes D1 and D2, transferringits energy with respect to ground rather than with respect to Vin. When operating with Vin greater than 20 V, a gate protection network is requiredfor the MOSFET. The network consists of components RG, D3, and D4.