The TPS6293x is a high-efficiency, easy-to-use synchronous buck converter with a wide input voltage range of 3.8 V to 30 V, and supports up to 2-A (TPS62932) and 3-A (TPS62933 and TPS62933x) continuous output current. The device employs fixed-frequency peak current control mode for fast transient response and good line and load regulation. The optimized internal loop compensation eliminates external compensation components. This application report describes the TPS62933 device in an inverting buck-boost topology, for use in low-current negative rails for an operational amplifier, optical module biasing, or line drivers and other low-power applications. This application report also discusses how to choose an output LC filter in the buck-boost topology to achieve applicable transient and steady performance.
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The inverting buck-boost topology is similar to the buck topology. In the buck configuration, shown in Figure 1-1, the positive connection (VOUT) is connected to the inductor, and the return connection is connected to the integrated circuit (IC) ground (GND). However, in the inverting buck-boost configuration, shown in Figure 1-2, the IC GND is used as the negative output voltage pin. What was the positive output in the buck configuration is used as the GND. This inverting topology allows the output voltage to be inverted and always lower than the GND.
The circuit operation in the inverting buck-boost topology is different from the buck topology. Figure 1-3 (a) shows that the output voltage terminals are reversed, though the components are wired the same as a buck converter. During the on time of the control MOSFET, shown in Figure 1-3 (b), the inductor is charged with current while the output capacitor supplies the load current. The inductor does not provide current to the load during that time. During the off time of the control MOSFET and the on time of the synchronous MOSFET, shown in Figure 1-3 (c), the inductor provides current to the load and the output capacitor. These changes affect many parameters as described in the upcoming sections.