LT8331 datasheet(16/24 Pages) AD

16 / 24 page

LT8334

Rev. 0

For more information www.analog.com

SEPIC CONVERTER APPLICATIONS

The LT8334 can be configured as a SEPIC (single-ended

primary inductance converter), as shown in Figure 6. This

topology allows for the input to be higher, equal, or lower

than the desired output voltage. The conversion ratio as

a function of duty cycle is given by Equation 17.

VOUT + VD

VIN

− D

(17)

in continuous conduction mode (CCM)

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.

CIN

CDC

COUT

VOUT

INTVCC GND

LT8334

EN/UVLO

FBX

VIN

8334 F06

Figure 6. LT8334 Configured in a SEPIC Topology

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 for-

ward voltage (VD).

The maximum duty cycle (DMAX) occurs when the converter

operates at the minimum input voltage (Equation 18).

DMAX =

VOUT + VD

VIN(MIN) + VOUT + VD

(18)

Conversely, the minimum duty cycle (DMIN) occurs

when the converter operates at the maximum input volt-

age(Equation 19).

DMIN =

VOUT + VD

VIN(MAX) + VOUT + VD

(19)

Be sure to check that DMAX and DMIN obey Equation 20.

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

and

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

(20)

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 6, the SEPIC converter contains two

inductors: L1 and L2. L1 and L2 can be independent,

but can also be wound on the same core, since iden-

tical voltages are applied to L1 and L2 throughout the

switching cycle.

For the SEPIC topology, the current through L1 is the con-

verter input current. Because, ideally, the output power is

equal to the input power, the maximum average inductor

currents of L1 and L2 are given by Equation 21.

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

DMAX

− DMAX

IL2(MAX)(AVG) = IO(MAX)

(21)

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 defined

with Equation 22.

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

= IO(MAX) •

− DMAX

(22)

APPLICATIONS INFORMATION

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