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LT8362

8362fa

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APPLICATIONS INFORMATION

APPLICATION CIRCUITS

The LT8362 can be configured for different topologies.

The first topology to be analyzed will be the boost con-

verter, followed by the SEPIC and inverting converters.

Boost Converter: Switch Duty Cycle

The LT8362 can be configured as a boost converter for

the applications where the converter output voltage is

higher than the input voltage. Remember that boost con-

verters are not short-circuit protected. Under a shorted

output condition, the inductor current is limited only by the

input supply capability. For applications requiring a step-

up converter that is short-circuit protected, please refer

to the Applications Information section covering SEPIC

converters.

The conversion ratio as a function of duty cycle is:

VOUT

VIN

1 − D

in continuous conduction mode (CCM).

For a boost converter operating in CCM, the duty cycle

of the main switch can be calculated based on the output

voltage (VOUT) and the input voltage (VIN). The maximum

duty cycle (DMAX) occurs when the converter has the

minimum input voltage:

DMAX =

VOUT − VIN(MIN)

VOUT

Discontinuous conduction mode (DCM) provides higher

conversion ratios at a given frequency at the cost of

reduced efficiencies, higher switching currents, and lower

available output power.

Boost Converter: Maximum Output Current Capability

and Inductor Selection

For the boost topology, the maximum average inductor

current is:

IL(MAX)(AVE)= IO(MAX) •

1 − DMAX

• 1

where

η (< 1.0) is the converter efficiency.

Due to the current limit of its internal power switch, the

LT8362 should be used in a boost converter whose maxi-

mum output current (IO(MAX)) is:

IO(MAX) ≤

VIN(MIN)

VOUT

• 2A − 0.5 • ΔISW

(

) • η

Minimum possible inductor value and switching frequency

should also be considered since they will increase inductor

ripple current ∆ISW.

The inductor ripple current ∆ISW has a direct effect on the

choice of the inductor value and the converter’s maximum

output current capability. Choosing smaller values of

∆ISW increases output current capability, but requires

large inductances and reduces the current loop gain

(the converter will approach voltage mode). Accepting

larger values of ∆ISW provides fast transient response and

allows the use of low inductances, but results in higher

input current ripple and greater core losses, and reduces

output current capability. It is recommended to choose a

∆ISW of approximately 0.75A.

Given an operating input voltage range, and having cho-

sen the operating frequency and ripple current in the

inductor, the inductor value of the boost converter can

be determined using the following equation:

L =

VIN(MIN)

ΔISW • fOSC

• DMAX

The peak inductor current is the switch current limit (max-

imum 3.1A), and the RMS inductor current is approxi-

mately equal to IL(MAX)(AVE).

Choose an inductor that can handle at least 3.1A without sat-

urating, and ensure that the inductor has a low DCR (copper-

wire resistance) to minimize I2R power losses. Note that in

some applications, the current handling requirements of the

inductor can be lower, such as in the SEPIC topology where

each inductor only carries one-half of the total switch cur-

rent. For better efficiency, use similar valued inductors with a

larger volume. Many different sizes and shapes are available

from various manufacturers (see Table 2). Choose a core

material that has low losses at the programmed switching

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