The step-up dc/dc converter in Fig 1 trades a slight loss in efficiency and a transformer in place of a less expensive inductor for two important advantages:

• The inductor offers low shutdown current because the transformer eliminates a leakage path.
• The circuit stays in regulation, even if the input voltage rises above its nominal figure.

Note that in Fig 1, the input voltage is negative with respect to the output's ground. Transistor Q₁ shifts the output voltage's level down to a level compatible with IC₁'s feedback pin.

Powering IC₁ from V_IN + V_OUT minimizes internal losses in IC₁ by maximizing the gate drive to IC₁'s internal MOSFET. When this MOSFET turns off, the energy stored in T₁'s primary flows to the V₊ output, generating a voltage across the primary equal to V₊ plus a diode drop.

Similarly, the secondary winding of T₁ generates the 5V output voltage. Because of T₁'s 1:1 turns ratio, the negative output tracks the positive output.

Fig 2 shows that the conventional configuration employing an inductor has two faults: When the input voltage rises above the output voltage (plus a diode drop), the output voltage follows the input voltage. The dashed line shows the leakage path through the circuit that is active when the regulator IC is in shutdown mode. In contrast, the shutdown current for the circuit in Fig 1 is 26 µA. (DI #1628)