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LTC1871

APPLICATIO S I FOR ATIO

It is worth noting here that SEPIC converters that operate

at high duty cycles (i.e., that develop a high output voltage

from a low input voltage) can have very high input cur-

rents, relative to the output current. Be sure to check that

the maximum load current will not overload the input

supply.

SEPIC Converter: Inductor Selection

For most SEPIC applications the equal inductor values will

fall in the range of 10

µH to 100µH. Higher values will

reduce the input ripple voltage and reduce the core loss.

Lower inductor values are chosen to reduce physical size

and improve transient response.

Like the boost converter, the input current of the SEPIC

converter is calculated at full load current and minimum

input voltage. The peak inductor current can be signifi-

cantly higher than the output current, especially with

smaller inductors and lighter loads. The following formu-

las assume CCM operation and calculate the maximum

peak inductor currents at minimum VIN:

L PEAK

O MAX

IN MIN

L PEAK

O MAX

IN MIN

()

(

)

()

(

)

()

••

















The ripple current in the inductor is typically 20% to 40%

(i.e., a range of ‘χ’ from 0.20 to 0.40) of the maximum

average input current occurring at VIN(MIN) and IO(MAX)

and

∆IL1 = ∆IL2. Expressing this ripple current as a

function of the output current results in the following

equations for calculating the inductor value:

IN MIN

MAX

∆

()

•

where

O MAX

MAX

••

–

()

∆= χ

By making L1 = L2 and winding them on the same core, the

value of inductance in the equation above is replace by 2L

due to mutual inductance. Doing this maintains the same

ripple current and energy storage in the inductors. For

example, a Coiltronix CTX10-4 is a 10

µH inductor with two

windings. With the windings in parallel, 10

µH inductance

is obtained with a current rating of 4A (the number of turns

hasn’t changed, but the wire diameter has doubled).

Splitting the two windings creates two 10

µH inductors

with a current rating of 2A each. Therefore, substituting 2L

yields the following equation for coupled inductors:

IN MIN

MAX

∆

()

••

•

Specify the maximum inductor current to safely handle

IL(PK) specified in the equation above. The saturation

current rating for the inductor should be checked at the

minimum input voltage (which results in the highest

inductor current) and maximum output current.

SEPIC Converter: Power MOSFET Selection

The power MOSFET serves two purposes in the LTC1871:

it represents the main switching element in the power

path, and its RDS(ON) represents the current sensing

element for the control loop. Important parameters for the

power MOSFET include the drain-to-source breakdown

voltage (BVDSS), the threshold voltage (VGS(TH)), the on-

resistance (RDS(ON)) versus gate-to-source voltage, the

gate-to-source and gate-to-drain charges (QGS and QGD,

respectively), the maximum drain current (ID(MAX)) and

the MOSFET’s thermal resistances (RTH(JC) and RTH(JA)).

The gate drive voltage is set by the 5.2V INTVCC low

dropout regulator. Consequently, logic-level threshold

MOSFETs should be used in most LTC1871 applications.

If low input voltage operation is expected (e.g., supplying

power from a lithium-ion battery), then sublogic-level

threshold MOSFETs should be used.

The maximum voltage that the MOSFET switch must

sustain during the off-time in a SEPIC converter is equal to

the sum of the input and output voltages (VO + VIN). As a

result, careful attention must be paid to the BVDSS speci-

fications for the MOSFETs relative to the maximum actual

switch voltage in the application. Many logic-level devices

are limited to 30V or less. Check the switching waveforms

directly across the drain and source terminals of the power

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