LTC3534 datasheet(21/34 Pages) AD

21 / 34 page

LTC3114-1

Rev. D

For more information www.analog.com

APPLICATIONS INFORMATION

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 and from the Typical Application circuits. As a

guideline, the inductor DCR should be significantly less

than the typical power switch resistance of 250mΩ. The

only exceptions are applications that have a maximum

output current much less than what the LTC3114-1 is

capable of delivering.

Different inductor core materials and styles have an impact

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.

The choice of inductor style depends upon the price, siz-

ing, and EMI requirements of a particular application.

Table 1 provides a small sampling of inductors that are

well suited to many LTC3114-1 applications.

Output Capacitor Selection

A low effective series resistance (ESR) output capacitor

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

verter in order to minimize output voltage ripple. Multilayer

ceramic capacitors are an excellent option as they have low

ESR and are available in small footprints. The capacitor 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 f is the frequency in MHz (1.2MHz for the LTC3114

-1), COUT is the capacitance in µF, tLOW is the switch pin min-

imum low time in us (0.1µs for the LTC3114-1) and ILOAD is

the output current in Amps.

ΔVP-P(BUCK) =

ILOADtLOW

COUT

Volts

ΔVP-P(BOOST) =

ILOAD

fCOUT

VOUT – VIN + tLOWfVIN

VOUT

⎛

⎝⎜

⎞

⎠⎟

Volts

Examining the previous equations reveal that the output

voltage ripple increases with load current and is gener-

ally higher in boost mode than in buck mode. Note that

these equations only take into account the voltage ripple

that occurs from the inductor current to the output being

discontinuous. They provide a good approximation to the

ripple at any significant load current but underestimate the

output voltage ripple at very light loads where the output

voltage ripple is dominated by the inductor current ripple.

In addition to the output voltage ripple generated across

the output capacitance, there is also output voltage rip-

ple produced across the internal resistance of the output

capacitor. The ESR-generated output voltage ripple is pro-

portional to the series resistance of the output capacitor

and is given by the following expressions where RESR is

Table 1. Representative Surface Mount Inductors

PART NUMBER

VALUE

(µH)

DCR

(mΩ)

MAX DC

CURRENT (A)

SIZE (mm)

W × L × H

Coilcraft

LPS6225

LPS6235

MSS1038

D03316P

4.7

6.8

3.2

2.8

3.3

3.0

6.2 × 6.2 × 2.5

6.2 × 6.2 × 3.5

10.2 × 10.5 × 3.8

12.9 × 9.4 × 5.2

Cooper-Bussmann

CD1-150-R

DR1030-100-R

FP3-8R2-R

DR1040-220-R

8.2

3.6

3.18

3.4

2.9

10.5 × 10.4 × 4.0

10.3 × 10.5 × 3.0

7.3 × 6.7 × 3.0

10.3 × 10.5 × 4.0

Panasonic

ELLCTV180M

ELLATV100M

3.0

3.3

12 × 12 × 4.2

10 × 10 × 4.2

Sumida

CDRH8D28/HP

CDR10D48MNNP

CDRH8D28NP

4.7

105

24.7

3.0

3.4

8.3 × 8.3 × 3

10.3 × 10.3 × 5

8.3 × 8.3 × 3

Taiyo-Yuden

NR10050T150M

3.6

9.8 × 9.8 × 5

TOKO

B1047AS-6R8N

B1179BS-150M

892NAS-180M

6.8

2.9

3.3

3.0

7.6 × 7.6 × 5

10.3 × 10.3 × 4

12.3 × 12.3 × 4.5

Wurth

7447789004

7440650068

744771133

744066150

4.7

6.8

2.9

3.6

2.7

3.2

7.3 × 7.3 × 3.2

10 × 10 × 3

12 × 12 × 6

10 × 10 × 3.8

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