LM2735XMFSLASHNOPB datasheet(34/59 Pages) TI

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R ¸

§ D

IL1

and

D =

V +

VIN

LM2735

LM2735, LM2735-Q1

SNVS485G – JUNE 2007 – REVISED AUGUST 2015

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8.2.18 SEPIC Converter

Figure 39. SEPIC Converter Schematic

8.2.18.1 Detailed Design Procedure

The LM2735 can easily be converted into a SEPIC converter. A SEPIC converter has the ability to regulate an

output voltage that is either larger or smaller in magnitude than the input voltage. Other converters have this

ability as well (CUK and Buck-Boost), but usually create an output voltage that is opposite in polarity to the input

voltage. This topology is a perfect fit for Lithium Ion battery applications where the input voltage for a single-cell

Li-Ion battery will vary from 3 V to 4.5 V and the output voltage is somewhere in-between. Most of the analysis of

the LM2735 Boost Converter is applicable to the LM2735 SEPIC Converter.

8.2.18.1.1

SEPIC Design Guide

SEPIC Conversion ratio without loss elements:

(17)

Therefore:

(18)

8.2.18.1.2

Small Ripple Approximation

In a well-designed SEPIC converter, the output voltage, input voltage ripple, and inductor ripple is small in

comparison to the DC magnitude. Therefore, it is a safe approximation to assume a DC value for these

components. The main objective of the Steady State Analysis is to determine the steady state duty-cycle, voltage

and current stresses on all components, and proper values for all components.

In a steady-state converter, the net volt-seconds across an inductor after one cycle will equal zero. Also, the

charge into a capacitor will equal the charge out of a capacitor in one cycle.

Therefore:

(19)

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