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 VIN 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 28 V input and generates 5 V at 100 mA.
|Figure 1.||Q1 stands in for a zener diode in this SEPIC with a wide input-voltage range.|
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 6 V 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.