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LT8365

Rev. A

For more information www.analog.com

APPLICATIONS INFORMATION

and the peak switch current is:

ISW(PEAK) = 1 +

⎛

⎝

⎜

⎞

⎠

⎟ • IO(MAX) •

1 − DMAX

The variable c in the preceding equations represents the

percentage peak-to-peak ripple current in the switch,

relative to ISW(MAX)(AVG), as shown in Figure 7. Then, the

switch ripple current ∆ISW can be calculated by:

∆ISW = χ • ISW(MAX)(AVG)

The inductor ripple currents ∆IL1 and ∆IL2 are identical:

∆IL1 = ∆IL2 = 0.5 • ∆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.

BymakingL1=L2,andwindingthemonthesamecore,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.

SimilartoBoostconverters,theSEPICconverteralsoneeds

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

inductorsareused.Foruncoupledinductors,theminimum

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.

8365 F07

ΔISW = χ • ISW(MAX)(AVG)

ISW

DTS

ISW(MAX)(AVG)

Figure 7. The Switch Current Waveform of the SEPIC Converter

The inductor ripple current has a direct effect on the

choice of the inductor value. Choosing smaller values of

∆IL requires large inductances and reduces the current

loop gain (the converter will approach voltage mode).

Accepting larger values of ∆IL allows the use of low in-

ductances, but results in higher input current ripple and

greater core losses. It is recommended that c falls in the

range of 0.5 to 0.8.

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

LT8365 should be used in a SEPIC converter whose

maximum output current (IO(MAX)) is:

IO(MAX) < (1 – DMAX) • (1.5A – 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.

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