LT8301 datasheet(23/38 Pages) AD

23 / 38 page

LT3757/LT3757A

Rev. G

For more information www.analog.com

Given an operating input voltage range, and having cho-

sen the operating frequency and ripple current in the

inductor, the inductor value (L1 and L2 are independent)

of the SEPIC converter can be determined using the fol-

lowing equation:

= 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 inductance in the preceding equation is

replaced by 2L, due to mutual inductance:

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 • ∆IL1

IL2(PEAK) = IL2(MAX) + 0.5 • ∆IL2

The RMS inductor currents are:

IL1(RMS) = IL1(MAX) • 1+

χ2 L1

where:

χL1 =

ΔIL1

IL1(MAX)

IL2(RMS) = IL2(MAX) • 1+

χ2 L2

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.

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

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

The sense resistor value can then be calculated to be:

RSENSE =

80 mV

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 = I2SW(MAX) • RDS(ON) • DMAX

+ 2 • (VIN(MIN) + VOUT)2 • IL(MAX) • CRSS • f/1A

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.

For maximum efficiency, RDS(ON) and CRSS should be

minimized. From a known power dissipated in the power

MOSFET, its junction temperature can be obtained using

the following equation:

TJ = TA + PFET • θJA = TA + PFET • (θJC + θCA)

TJ must not exceed the MOSFET maximum junction

temperature rating. It is recommended to measure the

MOSFET temperature in steady state to ensure that abso-

lute maximum ratings are not exceeded.

APPLICATIONS INFORMATION

LT8301 Datasheet(PDF) 23 Page - Analog Devices

LT8301 Datasheet(HTML) 23 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38

Datasheet Download

Link URL