LT8335 datasheet(14/20 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)

It is recommended to have a ∆ISW of 600mA or more.

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

LT8335 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) • (2A – 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 • ΔISW • 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)

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.

APPLICATIONS INFORMATION

Figure 6. The Switch Current Waveform of the SEPIC Converter

8335 F06

ΔISW = χ • ISW(MAX)(AVE)

ISW

DTS

ISW(MAX)(AVE)

LT8335 Datasheet(PDF) 14 Page - Linear Technology

LT8335 Datasheet(HTML) 14 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Datasheet Download

Link URL