Figure 1. Autotransformer T1 limits the voltage across IC1, allowing use of a high-efficiency chip (with 21V absolute-maximum voltage) in this inverting dc-dc regulator.
Autotransformer T1 (a center-tapped inductor with 1:1 turns ratio) offers a design alternative. In the circuit shown, LX flies back to 1/2VOUT plus a diode drop, or approximately -6V. V+ remains at 12V, producing an 18V maximum between V+ and LX that is well within the 21V limit.
Because IC1 drives the gate of its internal MOSFET between the V+ and OUT voltages, you normally connect OUT to VOUT to ensure sufficient gate drive (in a typical application, the chip inverts 5V to -5V). In this circuit the 12V input provides adequate gate drive, so OUT is connected to ground.