LT3154AVPBF datasheet(19/32 Pages) AD

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LT3154

Rev. 0

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buck mode is maximum (highest VIN) and in boost mode

when the duty cycle is 50% (VOUT = 2VIN).

Recommended values based on switching frequency are

given in Table 4.

Table 4. Recommended Values

Frequency

Value

400kHz < fSW < 600kHz

2.2μH

600kHz < fSW < 900kHz

1.5μH

900kHz < fSW < 1.5MHz

1μH

1.5MHz < fSW < 2.5MHz

0.68μH

2.5MHz < fSW < 4.0MHz

0.47μH

In addition to its influence on power conversion efficiency,

the inductor DC resistance can also impact the maximum

output current capability of the buck‑boost converter par‑

ticularly at low input voltages. In buck mode, the output

current of the buck‑boost converter is primarily limited

by the inductor current reaching the average current limit

threshold defined by VC. However, in boost mode, espe‑

cially at large step‑up ratios, the output current capability

can also be limited by the total resistive losses in the

power stage. These losses include, switch resistances,

inductor DC resistance and PCB trace resistance. Avoid

inductors with a high DC resistance (DCR) as they can

degrade the maximum output current capability from

what is shown in the Typical Performance Characteristics

section. As a guideline, the inductor DCR should be simi‑

lar to the typical power switch resistance of 20mΩ. The

only exceptions are applications that have a maximum

output current much less than what the LT3154 is capable

of delivering.

Differentinductorcorematerialsandstyleshaveanimpact

on the size and price of an inductor at any given current

rating. Shielded construction is generally preferred as it

minimizes the chances of interference with other circuitry.

Thechoiceofinductorstyledependsupontheprice,sizing,

and EMI requirements of a particular application. Table 5

provides a small sampling of inductors that are well suited

to many LT3154 applications with L × W dimensions

around 3mm to 5mm.

APPLICATIONS INFORMATION

Table 5. Representative Surface Mount Inductors

SERIES

VALUE

(µH)

DCR

(mΩ)

MAX DC

CURRENT (A)

Bourns

www.bourns.com

SRP

0.47–1.5

5–15

Coilcraft

www.coilcraft.com

XAL, XEL

0.5–1.5

10–20

Cooper Bussmann/Eaton

www.eaton.com

HCM0703

0.68–1.5

6–15

Sumida

www.sumida.com

0420CDM

0.5–1.5

8–22

Taiyo Yuden

www.t‑yuden.com

MDW, NRS

0.5–1.5

10–30

TDK

www.tdk.com

SPM

0.6–1.5

15–40

Toko − Murata

www.murata.com

FDSD, DEM

0.68–1.5

10–22

Wurth

www.we‑online.com

WE‑MAPI, WE‑LHMI

0.68–2.2

6–15

Output Capacitor Selection

A low effective series resistance (ESR) output capacitor

should be connected at the output of the buck‑boost con‑

verterinordertominimizeoutputvoltageripple.Multilayer

ceramic capacitors are an excellent option as they have

low ESR and are available in small foot prints. The capaci‑

tor value should be chosen large enough to reduce the

output voltage ripple to acceptable levels. Neglecting the

capacitor’s ESR and ESL (effect series inductance), the

peak‑to‑peak output voltage ripple can be calculated by

the following formula, where fSW is the frequency in MHz

and COUT is the capacitance in µF. A formula for calculat‑

ing ∆IL, in buck mode is given in the Operation section.

ΔVP-P(BUCK) =

ΔIL

8fSW COUT

Volts

ΔVP-P(BOOST) =

ILOAD

fSW COUT

VOUT – VIN

VOUT

⎛

⎝⎜

⎞

⎠⎟

Volts

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