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LT8362

8362fa

For more information www.linear.com/LT8362

Due to the current limit of its internal power switch, the

LT8362 should be used in a SEPIC converter whose maxi-

mum output current (IO(MAX)) is:

IO(MAX) < (1 – DMAX) • (2A – 0.5 • ∆ISW) • η

where

η (< 1.0) is the converter efficiency. Minimum

possible inductor value and switching frequency should

also be considered since they will increase inductor ripple

current ∆ISW.

Given an operating input voltage range, and having cho-

sen ripple current in the inductor, the inductor value (L1

and L2 are independent) of the SEPIC converter can be

determined using the following equation:

L1 = L2 =

VIN(MIN)

0.5 • ΔISW • fOSC

• DMAX

For most SEPIC applications, the equal inductor values

will fall in the range of 2.2µH to 100µH.

By making L1 = L2, and winding them on the same core,

the value of inductance in the preceding equation is

replaced by 2L, due to mutual inductance:

L =

VIN(MIN)

ΔISW • fOSC

• DMAX

This maintains the same ripple current and energy storage

in the inductors. The peak inductor currents are:

IL1(PEAK) = IL1(MAX) + 0.5 • ∆IL1

IL2(PEAK) = IL2(MAX) + 0.5 • ∆IL2

The maximum RMS inductor currents are approximately

equal to the maximum average inductor currents.

Based on the preceding equations, the user should choose

the inductors having sufficient saturation and RMS cur-

rent ratings.

Similar to Boost converters, the SEPIC converter also needs

slope compensation to prevent subharmonic oscillations

while operating in CCM. The equation presented in the

Boost converter section defines the minimum inductance

value to avoid sub-harmonic oscillations when coupled

inductors are used. For uncoupled inductors, the minimum

inductance requirement is doubled.

SEPIC Converter: Output Diode Selection

To maximize efficiency, a fast switching diode with a low

forward drop and low reverse leakage is desirable. The

average forward current in normal operation is equal to

the output current.

It is recommended that the peak repetitive reverse voltage

rating VRRM is higher than VOUT + VIN(MAX) by a safety

margin (a 10V safety margin is usually sufficient).

The power dissipated by the diode is:

PD = IO(MAX) • VD

where VD is diode’s forward voltage drop, and the diode

junction temperature is:

TJ = TA + PD • RθJA

The RθJA used in this equation normally includes the RθJC

for the device, plus the thermal resistance from the board,

to the ambient temperature in the enclosure. TJ must not

exceed the diode maximum junction temperature rating.

SEPIC Converter: Output and Input Capacitor Selection

The selections of the output and input capacitors of the

SEPIC converter are similar to those of the boost converter.

APPLICATIONS INFORMATION

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