LT8331 datasheet(19/30 Pages) LINER

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LT8331

8331fa

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APPLICATIONS INFORMATION

In a SEPIC converter, no DC path exists between the input

and output. This is an advantage over the boost converter

for applications requiring the output to be disconnected

from the input source when the circuit is in shutdown.

SEPIC Converter: Switch Duty Cycle and Frequency

For a SEPIC converter operating in CCM, the duty cycle

of the main switch can be calculated based on the output

voltage (VOUT), the input voltage (VIN) and the diode

forward voltage (VD).

Themaximumdutycycle(DMAX)occurswhentheconverter

operates at the minimum input voltage:

DMAX =

VOUT + VD

VIN(MIN) + VOUT + VD

Conversely, the minimum duty cycle (DMIN) occurs when

the converter operates at the maximum input voltage:

DMIN =

VOUT + VD

VIN(MAX) + VOUT + VD

Be sure to check that DMAX and DMIN obey:

DMAX < 1 – Minimum Off-Time(MAX) • fOSC(MAX)

and

DMIN > Minimum On-Time(MAX) • fOSC(MAX)

where Minimum Off-Time, Minimum On-Time and fOSC

are specified in the Electrical Characteristics table.

SEPIC Converter: The Maximum Output Current

Capability and Inductor Selection

As shown in Figure 8, the SEPIC converter contains two

inductors:L1andL2.L1andL2canbeindependent,butcan

also be wound on the same core, since identical voltages

are applied to L1 and L2 throughout the switching cycle.

For the SEPIC topology, the current through L1 is the

converter input current. Based on the fact that, ideally, the

output power is equal to the input power, the maximum

average inductor currents of L1 and L2 are:

IL1(MAX)(AVE) =IIN(MAX)(AVE) =IO(MAX) •

DMAX

− DMAX

IL2(MAX)(AVE) = IO(MAX)

In a SEPIC converter, the switch current is equal to IL1 +

IL2 when the power switch is on, therefore, the maximum

average switch current is defined as:

ISW(MAX)(AVE) = IL1(MAX)(AVE) + IL2(MAX)(AVE)

= IO(MAX) •

− DMAX

and the peak switch current is:

ISW(PEAK) = 1 +











 • IO(MAX) •

− DMAX

The constant c in the preceding equations represents

the percentage peak-to-peak ripple current in the switch,

relative to ISW(MAX)(AVE), as shown in Figure 9. Then, the

switch ripple current ∆ISW can be calculated by:

∆ISW = c • ISW(MAX)(AVE)

The inductor ripple currents ∆IL1 and ∆IL2 are identical:

∆IL1 = ∆IL2 = 0.5 • ∆ISW

Figure 9. The Switch Current Waveform of the SEPIC Converter

8331 F09

∆ISW = χ•ISW(MAX)(AVE)

ISW

DTS

ISW(MAX)(AVE)

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