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LTC1871

1871fe

APPLICATIONS INFORMATION

It is worth noting here that SEPIC converters that operate

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

age from a low input voltage) can have very high input

currents, 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 10μH 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 signiﬁcantly

higher than the output current, especially with smaller in-

ductors and lighter loads. The following formulas assume

CCM operation and calculate the maximum peak inductor

currents at minimum VIN:

IL1(PEAK) = 1+

•IO(MAX) •

VO + VD

VIN(MIN)

IL2(PEAK) = 1+

•IO(MAX) •

VIN(MIN) + VD

VIN(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

ΔIL1 = ΔIL2. Expressing this ripple current as a function of

the output current results in the following equations for

calculating the inductor value:

VIN(MIN)

IL • f

•DMAX

where:

IL = •IO(MAX) •

DMAX

1– DMAX

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 10μH inductor with two

windings. With the windings in parallel, 10μH inductance is

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

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

ting the two windings creates two 10μH inductors with a

current rating of 2A each. Therefore, substituting 2L yields

the following equation for coupled inductors:

=L2=

VIN(MIN)

2• IL • f

•DMAX

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))versusgate-to-sourcevoltage,thegate-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

speciﬁcations 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 MOSFET to ensure the VDS remains below

the maximum rating for the device.

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