LT3757 datasheet(21/32 Pages) LINER

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LT3759

3759fb

The inductor ripple currents ΔIL1 and ΔIL2 are identical:

DIL1=DIL2 = 0.5• DISW

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:

VIN(MIN)

DISW •fOSC

•DMAX

RSENSE •VIN(MIN)

0.01V • fOSC

•DMAX

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

the current flowing through the sense resistor (ISENSE) is

the switch current.

Set the sense voltage at ISENSE(PEAK) to be minimum of

the SENSE current limit threshold with a 20% margin. The

sense resistor value can then be calculated to be:

RSENSE =

40mV

ISW(PEAK)

SEPIC Converter: Power MOSFET Selection

For the SEPIC configuration, choose a MOSFET with a

VDC rating higher than the sum of the output voltage and

input voltage by a safety margin (a 10V safety margin is

usually sufficient).

The power dissipated by the MOSFET in a SEPIC converter

is:

PFET = I

SW(MAX) •RDS(ON) •DMAX

+ (VIN(MIN)+ VOUT)2 •ISW(MAX)

•CRSS •

fOSC

The first term in this equation represents the conduction

losses in the device, and the second term, the switching

loss. CRSS is the reverse transfer capacitance, which is

usually specified in the MOSFET characteristics.

Figure 8. The Switch Current Waveform of a SEPIC Converter

3759 F08

ISW =

ISW(MAX)

ISW

DTS

ISW(MAX)

APPLICATIONS INFORMATION

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 in-

ductances, 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.

Choose an inductor value based on operating frequency,

input and output voltage to provide a current mode ramp

on SENSE during the switch on-time of approximately

10mV magnitude. The inductor value (L1 and L2 are

independent) of the SEPIC converter can be determined

using the following equation:

= L2 =

VIN(MIN)

0.5 •

DISW •fOSC

•DMAX

RSENSE •VIN(MIN)

0.5 • 0.01V • fOSC

•DMAX

For most SEPIC applications, the equal inductor values

will fall in the range of 1μH to 100μH.

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