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LT8331

8331fa

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APPLICATIONS INFORMATION

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)(AVE)= IO(MAX) •

− DMAX

•

where

η (< 1.0) is the converter efficiency.

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

LT8331 should be used in a boost converter whose maxi-

mum output current (IO(MAX)) is:

IO(MAX) ≤

VIN(MIN)

VOUT

• 0.5A

− 0.5 • ∆ISW

(

) • 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(theconverterwillapproachvoltagemode).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 0.2A to 0.3A.

Givenanoperatinginputvoltagerange,andhavingchosen

the operating frequency and ripple current in the inductor,

theinductorvalueoftheboostconvertercanbedetermined

using the following equation:

VIN(MIN)

∆ISW • fOSC

• DMAX

The peak inductor current is the switch current limit

(maximum 0.7A), and the RMS inductor current is ap-

proximately equal to IL(MAX)(AVE).

Choose an inductor that can handle at least 0.7A without

saturating, 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 re-

quirements 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

valuedinductorswithalargervolume.Manydifferentsizes

and shapes are available from various manufacturers (see

Table 2). Choose a core material that has low losses at the

programmed switching frequency, such as a ferrite core.

The final value chosen for the inductor should not allow

peak inductor currents to exceed 0.5A in steady state at

maximum load. Due to tolerances, be sure to account for

minimum possible inductance value, switching frequency

and converter efficiency.

Table 2. Inductor Manufacturers

Sumida

(847) 956-0666

www.sumida.com

TDK

(847) 803-6100

www.tdk.com

Murata

(714) 852-2001

www.murata.com

Coilcraft

(847) 639-6400

www.coilcraft.com

Wurth

(605) 886-4385

www.we-online.com

BOOST CONVERTER: INPUT CAPACITOR SELECTION

Bypass the input of the LT8331 circuit with a ceramic ca-

pacitor of X7R or X5R type placed as close as possible to

the VIN and GND pins. Y5V types have poor performance

over temperature and applied voltage, and should not be

used. A 4.7µF to 10µF ceramic capacitor is adequate to

bypass the LT8331 and will easily handle the ripple cur-

rent. If the input power source has high impedance, or

there is significant inductance due to long wires or cables,

additional bulk capacitance may be necessary. This can

be provided with a low performance electrolytic capacitor.

A precaution regarding the ceramic input capacitor con-

cerns the maximum input voltage rating of the LT8331.

A ceramic input capacitor combined with trace or cable

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