LT8331 datasheet(17/30 Pages) AD

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LT8365

Rev. A

For more information www.analog.com

APPLICATIONS INFORMATION

BOOST CONVERTER: LAYOUT HINTS

The high speed operation of the LT8365 demands careful

attention to board layout. Careless layout will result in per-

formance degradation. Figure 5 shows the recommended

component placement for a boost converter. Note the vias

under the exposed pad. These should connect to a local

ground plane for better thermal performance.

SEPIC CONVERTER APPLICATIONS

The LT8365 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:

VOUT + VD

VIN

1 − D

in continuous conduction mode (CCM).

Figure 6. LT8365 Configured in a SEPIC Topology

VOUT

VIN

FBX

GND

EN/UVLO

LT8365

VIN

VCC

INT

CIN

COUT

CDC

8365 F06

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 6, 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)(AVG) =IIN(MAX)(AVG) =IO(MAX) •

DMAX

1 − DMAX

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

= IO(MAX) •

1 − DMAX

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).

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