2 Inverting Buck-Boost Concept

For the standard buck converter, the inductor is connected to V_OUT and the switch pin (SW). To change a standard buck converter to an inverting buck-boost, reassign the buck converter V_OUT to system ground, and the old buck system ground to - V_OUT. The input capacitor needs to be reconnected to the new system ground, and a new bypass capacitor, C_IO, is needed between V_IN and -V_IN.

The positive input and the feedback resistors remains the same as in the buck converter. To adjust the output of the inverting buck-boost, calculate the feedback resistor values as if the feedback resistor was a buck converter. The schematics in Figure 2-1 show the changes that have to be made when configuring the standard buck converter as an inverting buck-boost converter. This inverting topology allows the output voltage to be inverted and always lower than the ground.

Figure 2-1 Converting From Buck to Inverting Buck-Boost Topology

The circuit operation is different in the inverting buck-boost topology than in the buck topology. Figure 2-2 shows that the output voltage terminals are reversed, though the components are wired the same as a buck converter. As Figure 2-3 shows, during the ON-time of the control MOSFET, the inductor is charged with current while the output capacitor supplies the load current. The inductor does not provide current to the load during this time.

During the OFF-time of the control MOSFET and the ON-time of the synchronous MOSFET shown in Figure 2-4, the inductor provides current to the load and the output capacitor. These changes affect many parameters, which the following subsections describe in further detail.

Figure 2-2 Inverting Buck-Boost Configuration

Figure 2-3 ON-Time of IBB Configuration

Figure 2-4 OFF-Time of IBB Configuration