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LTC1871X

1871xf

For more information www.linear.com/LTC1871X

APPLICATIONS INFORMATION

from another converter could cause substantial input

voltage ripple, and this could interfere with the opera-

tion of the LTC1871X. A few inches of PC trace or wire

(L ≈ 100nH) between the CIN of the LTC1871X and the

actualsourceVINshouldbesufficienttopreventcurrent

sharing problems.

SEPIC Converter Applications

The LTC1871X is also well suited to SEPIC (single-ended

primaryinductanceconverter)converterapplications. The

SEPIC converter shown in Figure 15 uses two inductors.

The advantage of the SEPIC converter is the input voltage

may be higher or lower than the output voltage, and the

output is short-circuit protected.

and size. All of the SEPIC applications information that

follows assumes L1 = L2 = L.

SEPIC Converter: Duty Cycle Considerations

For a SEPIC converter operating in a continuous conduc-

tion mode (CCM), the duty cycle of the main switch is:

VO + VD

VIN + VO + VD

⎛

⎝⎜

⎞

⎠⎟

where VD is the forward voltage of the diode. For convert-

ers where the input voltage is close to the output voltage

the duty cycle is near 50%.

The maximum output voltage for a SEPIC converter is:

VO(MAX) = VIN + VD

(

)

DMAX

1– DMAX

– VD

1– DMAX

ThemaximumdutycycleoftheLTC1871Xistypically92%.

SEPIC Converter: The Peak and Average Input

Currents

The control circuit in the LTC1871X is measuring the input

current (either using the RDS(ON) of the power MOSFET

or by means of a sense resistor in the MOSFET source),

so the output current needs to be reflected back to the

input in order to dimension the power MOSFET properly.

Based on the fact that, ideally, the output power is equal

to the input power, the maximum input current for a SEPIC

converter is:

IIN(MAX) =IO(MAX) •

DMAX

1– DMAX

The peak input current is:

IIN(PEAK) = 1+

⎛

⎝⎜

⎞

⎠⎟

•IO(MAX) •

DMAX

1– DMAX

The maximum duty cycle, DMAX, should be calculated at

minimum VIN.

The constant ‘

χ’ represents the fraction of ripple current in

the inductor relative to its maximum value. For example, if

30% ripple current is chosen, then

χ = 0.30 and the peak

current is 15% greater than the average.

The first inductor, L1, together with the main switch,

resembles a boost converter. The second inductor, L2,

together with the output diode D1, resembles a flyback or

buck-boost converter. The two inductors L1 and L2 can be

independentbutcanalsobewoundonthesamecoresince

identical voltages are applied to L1 and L2 throughout the

switching cycle. By making L1 = L2 and winding them on

the same core the input ripple is reduced along with cost

Figures 15. SEPIC Topology and Current Flow

•

COUT

VOUT

VIN

15a. SEPIC Topology

•

VOUT

VIN

15c. Current Flow During Switch Off-Time

•

VOUT

VIN

15b. Current Flow During Switch On-Time

1871x F15

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