Generating a negative output voltage rail from a positive input voltage is possible by reconfiguring an ordinary buck regulator into an inverting buck-boost (IBB) topology. Rather than operating considerations such as voltage and current stresses, the main challenge associated with this transformation arises from differences in the small-signal closed-loop characteristics. The most critical distinction is the presence of a right-half-plane (RHP) zero in the control-to-output transfer function of the IBB, which introduces non-minimum-phase behaviour, reduces the achievable phase margin, and may lead to instability and poor load transient response. For conventional PI controllers, the buck and IBB converters resulted in settling times of 5.0 ms and 6.8 ms, with overshoots of 13.65% and 8.46%, respectively, thus demonstrating the performance limitations of fixed-gain designs. As a solution, an auto-tuned PI controller with adaptive features has been proposed, which operates effectively in both the buck and IBB topologies. The proposed scheme achieves a significantly faster settling time of 2.27 ms with overshoot and undershoot below 2%, thereby ensuring superior robustness and dynamic performance compared to conventional approaches.