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LT8603

8603f

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APPLICATIONS INFORMATION

The lower of these frequencies is the maximum switching

frequency for the SEPIC converter.

SEPIC: Inductor and RSENSE Selection

Choosing the inductor ripple current, ∆IL, has a direct

impact on the choice of the inductor value and the con-

verter’s maximum output current capability. Choosing

smaller values of ∆IL increases the converter’s output

current capability but requires larger inductors. Choosing

larger values of ∆IL provides faster transient response and

allows the use of smaller inductors but results in higher

input current ripple, greater core losses, and lower output

current capability. In addition, larger values of ∆IL at high

duty cycle may result in sub-harmonic oscillation.

Given an operating input voltage range and operating fre-

quency, the inductor value is given by:

L =

VBATT(MIN) • DMAX

∆IL • fSW

where,

∆IL =

χ • IOUT(MAX) • DMAX

1– DMAX

χ in the above equation represents the percentage peak-

to-peak ripple current in the inductor relative to the maxi-

mum average inductor current. The typical range of χ is

20% to 40% though careful evaluation of system stability

should be made to ensure adequate design margin.

For coupled inductors, L1 = L2 and the effective induc-

tance is doubled due to the mutual inductance. The value

of each equal winding is given by:

L1 = L2 =

VBATT(MIN) • DMAX

2 • ∆IL • fSW

The maximum input current of the SEPIC converter is cal-

culated at the minimum input voltage and full load current.

The peak inductor current can be significantly higher than

the output current. The following equations assume CCM

operation and calculate the maximum peak inductor cur-

rent at minimum VBATT:

Figures 7. SEPIC Topology and Current Flow

•

COUT

VOUT

7a. SEPIC Topology

•

VOUT

7c. Current Flow During Switch Off-Time

•

VOUT

VIN

7b. Current Flow During Switch On-Time

8603 F07

VBATT

SEPIC: Duty Cycle and Frequency

With the SEPIC configuration operating in continuous

conduction mode (CCM), the duty cycle is given by:

D =

VOUT + VD

VBATT + VOUT + VD

















The maximum switching frequency of the SEPIC con-

verter is limited by both maximum and minimum duty

cycles. The maximum duty cycle is set by VBATT(MIN). The

maximum switching frequency, fSW(MAX) at VBATT(MIN) is

given by:

fSW(MAX)@VBATT(MIN) = 1 –

VOUT + VD

VBATT(MIN) + VOUT + VD













tOFF(MIN)

The minimum duty cycle is set by VBATT(MAX). The maxi-

mum switching frequency at VBATT(MAX) is given by:

fSW(MAX)@VBATT(MAX) =

VOUT + VD

VBATT(MAX) + VOUT + VD













tON(MIN)

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