LT8330 datasheet(14/24 Pages) LINER

DMAX

1−DMAX

IL2(MAX)(AVE) =IO(MAX)

In a SEPIC converter, the switch current is equal to IL1 +

IL2 when the power switch is on, therefore, the maximum

average switch current is defined as:

ISW(MAX)(AVE) =IL1(MAX)(AVE) +IL2(MAX)(AVE)

= IO(MAX) •

1−DMAX

and the peak switch current is:

ISW(PEAK) = 1+

⎛

⎝

⎜

⎞

⎠

⎟ •IO(MAX) •

1−DMAX

The constant c in the preceding equations represents

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

relative to ISW(MAX)(AVE), as shown in Figure 6. Then, the

switch ripple current ∆ISW can be calculated by:

∆ISW = c • ISW(MAX)(AVE)

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

∆IL1 = ∆IL2 = 0.5 • ∆ISW

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 0.2 to 0.6.

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

LT8330 should be used in a SEPIC converter whose

maximum output current (IO(MAX)) is less than the output

current capability by a sufficient margin (10% or higher

is recommended):

IO(MAX) < (1 – DMAX) • (1A – 0.5 • ∆ISW) • (0.9)

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 • ∆ISW • fOSC

•DMAX

For most SEPIC applications, the equal inductor values

will fall in the range of 1µH to 47µH.

BymakingL1=L2,andwindingthemonthesamecore,the

value of inductance in the preceding equation is replaced

by 2L, due to mutual inductance:

L =

VIN(MIN)

∆ISW • 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.

applicaTions inForMaTion

Figure 6. The Switch Current Waveform of the SEPIC Converter

8330 F06

∆ISW = χ•ISW(MAX)(AVE)

ISW

DTS

ISW(MAX)(AVE)

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