LTC3549

9

3549f

be defined by the combination of the current needed to

charge the output capacitance and the current provided

to the load as the output voltage ramps up. The start-up

APPLICATIO S I FOR ATIO

The basic LTC3549 application circuit is shown on the first

page of this data sheet. External component selection is

driven by the load requirement and begins with the selection

Inductor Selection

For most applications, the value of the inductor will fall

in the range of 1µH to 10µH. Its value is chosen based

on the desired ripple current. Large value inductors

lower ripple current and small value inductors result in

the ripple current as shown in Equation 1. A reasonable

(40% of 250mA).

∆=

⎛

⎝⎜

⎞

⎠⎟

I

V

fL

V

V

L

OUT

OUT

IN

•

–

1

(1)

The DC current rating of the inductor should be at least

equal to the maximum load current plus half the ripple

current to prevent core saturation. Thus, a 300mA rated

inductor should be enough for most applications (250mA

+ 50mA). For better efficiency, choose a low DC resistance

inductor. The inductor value also has an effect on Burst

Mode operation. The transition to low current operation be-

gins when the inductor current peaks fall to approximately

to occur at lower load currents, which can cause a dip in

efficiency in the upper range of low current operation. In

Burst Mode operation, lower inductance values will cause

the burst frequency to increase.

Inductor Core Selection

Different core materials and shapes will change the

size/current and price/current relationship of an induc-

tor. Toroid or shielded pot cores in ferrite or permalloy

materials are small and don’t radiate much energy, but

generally cost more than powdered iron core inductors

with similar electrical characteristics. The choice of which

style inductor to use often depends more on the price vs

size requirements and any radiated field/EMI requirements

than on what the LTC3549 requires to operate. Table 1

shows some typical surface mount inductors that work

well in LTC3549 applications.

waveform, shown in the Typical Performance Character-

istics, shows the output voltage start-up from 0V to 1.2V

OPERATION

Table 1. Representative Surface Mount Inductors

MANU-

FACTURER

PART NUMBER

VALUE

(µH)

MAX DC

CURRENT

(A)

DCR

HEIGHT

(mm)

Taiyo

Yuden

LB2016T2R2M

LB2012T2R2M

LB2016T3R3M

LB2016T4R7M

2.2

2.2

3.3

4.7

315

240

280

210

0.13

0.23

0.2

0.25

1.6

1.25

1.6

1.6

Panasonic

ELT5KT4R7M

4.7

950

0.2

1.2

Murata

LQH32CN4R7M34

4.7

450

0.2

2

TDK

VLF3012AT2R2M1R0

VLF3012AT3R3MR87

VLF3012AT4R7MR74

VLF3010AT2R2M1R0

VLF3010AT3R3MR87

VLF3010AT4R7MR70

2.2

3.3

4.7

2.2

3.3

4.7

1

0.87

0.74

1

0.87

0.74

0.088

0.11

0.16

0.10

0.15

0.24

1.2

1.2

1.2

1.0

1.0

1.0

In continuous mode, the source current of the top MOSFET

voltage transients, a low ESR input capacitor sized for the

maximum RMS current must be used. The maximum RMS

capacitor current is given by:

C

quired I

I

VV

V

V

IN

RMS

OUT MAX

OUT

IN

OUT

IN

Re

–

()

/

≅

()

monly used for design because even significant devia-