SEPIC generates 5V at 100 mA

Dongyan Zhou, Linear Technology Corp, Milpitas, CA -- EDN, 5/3/2001

Some applications require an input voltage higher than the breakdown voltage of the IC supply pin. In boost converters and SEPICs (single-ended primary-inductance converters), you can separate the V_IN pin of the IC from the input inductor and use a simple zener regulator to generate the supply voltage for the IC. Figure 1 shows a SEPIC that takes a 4 to 28V input and generates 5V at 100 mA.

In this application, Q₁ and Q₂ generate the supply voltage for IC₁ because the supply voltage exceeds IC₁'s maximum input voltage. The circuit uses Q₁ in place of a zener diode to save cost. The emitter-to-base breakdown voltage gives a stable 6V reference. The follower, Q₂, provides the supply voltage for the IC. This circuit demonstrates an inexpensive way to extend the input range of the IC.

This SEPIC can step up or step down the input voltage. Because the flying capacitor, C₂, breaks the input-to-output dc path, the output disconnects from the input when you shut down the device, inhibiting any possible load current in shutdown mode, which is important for portable applications and which prevents the input voltage from appearing at the output.