Making a Voltage Inverter from a Buck (Step-Down) DC-DC Converter
When using a step-down DC-DC converter as an inverter, there are some limitations. The voltage difference between the input and the negative output must be less than the step-down DC-DC converter's maximum operating input voltage. In essence, a step-down DC-DC converter with maximum input of 12V can be used to convert 5V to -7V, but not more.
In addition, the minimum input voltage of the inverting circuit must be greater than the minimum operating voltage of the step-down IC, minus one diode drop. That is because on start-up, there is initially no negative output voltage. Consequently, the step-down IC is biased only by the input voltage, less a diode forward drop.
Figure 1. Block diagram shows a generic step-down DC-DC converter as both a normal step-down (Figure 1a) and connected as a negative-output voltage inverter (Figure 1b).