LT8331 datasheet(14/30 Pages) AD

14 / 30 page

LT8365

Rev. A

For more information www.analog.com

APPLICATIONS INFORMATION

not short-circuit protected. Under a shorted output condi-

tion, 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 Applica-

tions 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 re-

duced 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)(AVG)= IO(MAX) •

1 − DMAX

• 1

where η (< 1.0) is the converter efficiency.

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

LT8365 should be used in a boost converter whose maxi-

mum output current (IO(MAX)) is:

IO(MAX) ≤

VIN(MIN)

VOUT

• 1.5A − 0.5 • ΔISW

(

) • η

Minimumpossibleinductorvalueandswitchingfrequency

shouldalsobeconsideredsincetheywillincreaseinductor

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 0.60A.

Givenanoperatinginputvoltagerange,andhavingchosen

the operating frequency and ripple current in the inductor,

theinductorvalueoftheboostconvertercanbedetermined

using the following equation:

L =

VIN(MIN)

ΔISW • fOSC

• DMAX

The peak inductor current is the switch current limit

(maximum 2.7A), and the RMS inductor current is ap-

proximately equal to IL(MAX)(AVG).

Choose an inductor that can handle at least 2.7A without

saturating, and ensure that the inductor has a low DCR

(copper-wireresistance)tominimizeI2Rpowerlosses.Note

thatinsomeapplications,thecurrenthandlingrequirements

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

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

current. For better efficiency, use similar valued inductors

with a larger volume. Many different sizes and shapes are

availablefromvariousmanufacturers(seeTable2).Choosea

corematerialthathaslowlossesattheprogrammedswitch-

ing frequency, such as a ferrite core. The final value chosen

for the inductor should not allow peak inductor currents to

exceed 1.5A in steady state at maximum load. Due to toler-

ances, be sure to account for minimum possible inductance

value, switching frequency and converter efficiency.

For inductor current operation in CCM and duty cycles

above 50%, the LT8365's internal slope compensa-

tion prevents sub-harmonic oscillations provided the

inductor value exceeds a minimum value given by:

L >

VIN

–5 •D2 +10 •D – 1

(

)• fOSC

( )

•

2 •D – 1

(

)

1– D

(

)

Lower L values are allowed if the inductor current operates

in DCM or duty cycle operation is below 50%.

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