LT8331 datasheet(24/34 Pages) AD

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LT8357

Rev. 0

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relative to ISW(MAX), as shown in Figure 11. Then, the

switch ripple current ΔISW can be calculated by:

�ISW = χ • ISW(MAX)

The inductor ripple currents ΔIL1 and ΔIL2 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

ΔIL requires large inductances and reduces the current

loop gain (the converter will approach voltage mode).

Accepting larger values of ΔIL allows the use of low

inductances, but results in higher input current ripple and

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

range of 0.2 to 0.4.

8357 F11

ΔISW = χ • ISW(MAX)(AVE)

ISW

DTS

ISW(MAX)(AVE)

Figure 11. The Switch Current Waveform of the SEPIC

Converter

Given an operating input voltage range and having chosen

the operating frequency and ripple current in the induc-

tor, 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 •f

•DMAX

For most SEPIC applications, the equal inductor values

will fall in the range of 1μH to 100μH. By making L1 = L2,

and winding them on the same core, the value of induc-

tance in the preceding equation is replaced by 2L, due to

mutual inductance:

APPLICATIONS INFORMATION

L =

VIN(MIN)

ΔISW •f

•DMAX

This maintains the same ripple current and energy storage

in the inductors. The peak inductor currents are:

IL1(PEAK) =IL1(MAX)+0.5•ΔIL1

IL2(PEAK) =IL2(MAX)+0.5•ΔIL2

The RMS inductor currents are:

IL1(RMS) =IL1(MAX)• 1+

χ2L1

where:

χL1=

ΔIL1

IL1(MAX)

IL2(RMS) =IL2(MAX)• 1+

χ2L2

where:

χL2 =

ΔIL2

IL2(MAX)

Based on the preceding equations, the user should choose

the inductors having sufficient saturation and RMS cur-

rent ratings.

In a SEPIC converter, when the power switch is turned on,

the current flowing through the sense resistor (ISENSE) is

the switch current ISW. Set the sense voltage at ISW(PEAK)

to be less than the maximum SENSE current limit thresh-

old of 60mV (typical). The sense resistor value can then

be calculated as:

RSENSE =

60mV

ISW(PEAK)

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