A Texas Instruments application note (PDF!) on using the TPS5430 as an inverting buck–boost converter:
The wide input voltage range SWIFT™ (Switcher With Integrated FET) dc/dc converters are typically used as step-down converters where the derived output is a positive voltage less than the input voltage source. In some cases, it may be required to generate a negative voltage from the input voltage source. In such instances, it is possible to configure the TPS5430/20/10 devices in an inverting buck–boost topology, where the output voltage is negative with respect to ground.
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