LT8331 datasheet(20/30 Pages) AD | Low IQ Boost/SEPIC/Flyback/Inverting Converter with 0.5A, 140V Switch

LT8331

Rev. C

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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

inductances, 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

LT8331 should be used in a SEPIC converter whose maxi-

mum output current (IO(MAX)) is:

IO(MAX) < (1 – DMAX) • (0.5A – 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.

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 4.7µH to 220µH.

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.

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.

SEPIC Converter: Selecting the DC Coupling Capacitor

The DC voltage rating of the DC coupling capacitor (CDC,

as shown in Figure 10) should be larger than the maxi-

mum input voltage:

VCDC > VIN(MAX)

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LT8331 Datasheet(HTML) 20 Page - Analog Devices

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