App note from ON semiconductors about repurposing a buck converter to produce negative voltages. Link here (PDF)
Applications in the electronics industry ranging from sensor−based designs to power amplifiers are periodically faced with the requirement to generate a negative voltage rail. Although many transformer−based designs, charge pumps and other methods have been used to meet such a requirement, the inverting buck−boost topology stands out as simple to design and can save on power and board space too.
With power budgets in many applications already stretched, and PCB real estate limited due to the high levels of functionality incorporated in many new products, power devices that use an inverting buck−boost topology can prove extremely valuable to systems designers.
A Buck regulator can be reconfigured to generate a negative output voltage from a positive input voltage using the inverting buck−boost topology. Unlike a buck regulator, the Inverting buck−boost transfers energy to the output through the output diode during the ’Off’ time. For this reason, users must keep in mind that the average output current is always less than the average inductor current.