One common design challenge for space applications is the need to generate a negative voltage rail. This application note illustrates how to design an inverting buck-boost using the radiation tolerant TPS7H4010-SEP. Radiation tolerant devices are needed to shield against single event effects and dose effects in LEO™ applications. The TPS7H4010-SEP is rated for 30-krad (Si) for total ionizing dose and 43 MeV*cm2/mg for singe event latch-up immunity. This topology takes a positive input voltage and create a regulated negative output voltage. This application note details design considerations and simulation results, so that designers can scale the design to different requirements. The document also includes experimental results and configuration instructions to regulate voltage rails of -12 V, -6 V, and -1.8 V from a 12-V input and a 5-V input for the -6 V and -1.8 V rails.
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Implementing the TPS7H4010-SEP as an inverting buck-boost is similar to buck topology. In the buck topology, the positive connection of the output is connected to the inductor and the return is connected to the integrated circuit (IC) ground. In the inverting buck-boost configuration however, the IC ground is used as the negative output voltage and what used to be the positive output is connected to ground. The inverting buck-boost topology allows the output voltage to be inverted and lower than ground because Vin is typically referred to ground, but in the inverting buck-boost topology, Vin is referred to Vout. The input voltage range of the TPS7H4010-SEP in the inverting buck-boost is 3.5-V to 32-V (+ Vout), where Vout is a negative value. Since the input is switching from Vin all the way to Vout instead of ground, remember to pick adequately rated capacitors for the input. The maximum output current is reduced when going from buck to inverting buck-boost configuration. For this application, 3 A was the maximum current tested as exceeding 3 A is not recommended. Figure 1-1 shows the schematic for the inverting buck-boost topology.