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LM3478

SNVS085X – JULY 2000 – REVISED DECEMBER 2017

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Typical Applications (continued)

8.2.2 Typical SEPIC Converter

Figure 34. Typical SEPIC Converter

Since the LM3478 controls a low-side N-Channel MOSFET, it can also be used in SEPIC (Single Ended Primary

Inductance Converter) applications. An example of a SEPIC using the LM3478 is shown in Figure 34. Note that

the output voltage can be higher or lower than the input voltage. The SEPIC uses two inductors to step-up or

step-down the input voltage. The inductors L1 and L2 can be two discrete inductors or two windings of a coupled

inductor since equal voltages are applied across the inductor throughout the switching cycle. Using two discrete

inductors allows use of catalog magnetics, as opposed to a custom inductor. The input ripple can be reduced

along with size by using the coupled windings for L1 and L2.

Due to the presence of the inductor L1 at the input, the SEPIC inherits all the benefits of a boost converter. One

main advantage of a SEPIC over a boost converter is the inherent input to output isolation. The capacitor CS

isolates the input from the output and provides protection against a shorted or malfunctioning load. Hence, the

SEPIC is useful for replacing boost circuits when true shutdown is required. This means that the output voltage

falls to 0V when the switch is turned off. In a boost converter, the output can only fall to the input voltage minus a

diode drop.

The duty cycle of a SEPIC is given using Equation 34.

(34)

In Equation 34, VQ is the on-state voltage of the MOSFET, Q, and VDIODE is the forward voltage drop of the

diode.

8.2.2.1 Design Requirements

To properly size the components for the application, the designer needs the following parameters: input voltage

range, output voltage, output current range, and required switching frequency. These four main parameters affect

the choices of component available to achieve a proper system behavior.

For the power supply, the input impedance of the supply rail should be low enough that the input current

transient does not drop below the UVLO value. The factors determining the choice of inductor used should be

the average inductor current, and the inductor current ripple. If the switching frequency is set high, the converter

can be operated with very small inductor values. The maximum current that can be delivered to the load is set by

the sense resistor, RSEN. Current limit occurs when the voltage generated across the sense resistor equals the

current sense threshold voltage, VSENSE. Also, a resistor RSL adds additional slope compensation, if required.

The following sections describe the design requirements for a typical LM3478 boost application.