Switch Mode Regulator Provides -12V Negative Supply from Positive Supply

This is a circuit that can provide -12 V negative supply from positive supply. The output voltage is Vo= -Vin· D/(1-D). The maximum output current is Iout=1·(1-D). Here is the circuit :

In this circuit, the ground of U1 is connected to negative output voltage. The input voltage of U1 must be lower than maximum operating input voltage, 36V, because the U1’s input voltage equal to Vin-Vo. [Source: STMicroelectronics Application Note]