LT8331 datasheet(17/24 Pages) AD

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LT8334

Rev. 0

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and the peak switch current is given by Equation 23.

ISW(PEAK) = 1 +

⎛

⎝

⎜

⎞

⎠

⎟ • IO(MAX) •

1 − DMAX

(23)

The constant c in the preceding equations rep-

resents 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

with Equation 24.

∆ISW = χ • ISW(MAX)(AVG)

(24)

The inductor ripple currents ∆IL1 and ∆IL2 are identi-

cal (Equation 25).

∆IL1 = ∆IL2 = 0.5 • ∆ISW

(25)

8334 07

ISW =  • 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

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

LT8334 should be used in a SEPIC converter whose max-

imum output current (IO(MAX)) is given by Equation 26.

IO(MAX) < (1 – DMAX) • (5A – 0.5 • ∆ISW) • η (26)

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 with Equation 27.

L1 = L2 =

VIN(MIN)

0.5 • ΔISW • fOSC

• DMAX

(27)

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 Equation 27 is replaced by 2L,

due to mutual inductance (Equation 28).

L =

VIN(MIN)

ΔISW • fOSC

• DMAX

(28)

This maintains the same ripple current and energy stor-

age n the inductors. The peak inductor currents are given

by Equation 29.

IL1(PEAK) = IL1(MAX) + 0.5 • ∆IL1

IL2(PEAK) = IL2(MAX) + 0.5 • ∆IL2

(29)

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 9 pre-

sented in the Boost Converter: Switch Duty Cycle section

defines the minimum inductance value to avoid subhar-

monic oscillations when coupled inductors are used. For

uncoupled inductors, the minimum inductance require-

ment 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.

APPLICATIONS INFORMATION

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