LTC3577 datasheet(33/52 Pages) LINER

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LTC3577/LTC3577-1

3577f

OPERATION

process much simpler. The step-down switching regula-

tors are designed to work with inductors in the range of

2.2μH to 10μH. For most applications a 4.7μH inductor is

suggested for step-down switching regulators providing

up to 500mA of output current while a 3.3μH inductor is

suggested for step-down switching regulators providing

up to 800mA. Larger value inductors reduce ripple current,

which improves output ripple voltage. Lower value induc-

tors result in higher ripple current and improved transient

response time, but will reduce the available output current.

To maximize efﬁciency, choose an inductor with a low DC

resistance. For a 1.2V output, efﬁciency is reduced about

2% for 100mΩ series resistance at 400mA load current,

and about 2% for 300mΩ series resistance at 100mA load

current. Choose an inductor with a DC current rating at

least 1.5 times larger than the maximum load current to

ensure that the inductor does not saturate during normal

operation. If output short-circuit is a possible condition,

the inductor should be rated to handle the maximum peak

current speciﬁed for the step-down converters. Different

core materials and shapes will change the size/current

and price/current relationship of an inductor. 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. Inductors that are very thin or

have a very small volume typically have much higher core

and DCR losses, and will not give the best efﬁciency. The

choice of which style inductor to use often depends more

on the price versus size, performance, and any radiated

EMI requirements than on what the step-down switching

regulators requires to operate. The inductor value also has

an effect on Burst Mode operation. Lower inductor values

will cause Burst Mode switching frequency to increase.

Table 3 shows several inductors that work well with the

step-down switching regulators. These inductors offer a

good compromise in current rating, DCR and physical

size. Consult each manufacturer for detailed information

on their entire selection of inductors.

Input/Output Capacitor Selection

Low ESR (equivalent series resistance) ceramic capacitors

should be used at both step-down switching regulator

outputs as well as at each step-down switching regulator

input supply. Only X5R or X7R ceramic capacitors should

be used because they retain their capacitance over wider

voltage and temperature ranges than other ceramic types.

A 10μF output capacitor is sufﬁcient for the step-down

switching regulator outputs. For good transient response

Table 3. Recommended Inductors for Step-Down Switching Regulators

INDUCTOR TYPE

L (μH)

MAX IDC (A)

MAX DCR (Ω)

SIZE in mm (L

× W × H)

MANUFACTURER

DB318C

D312C

DE2812C

4.7

3.3

4.7

3.3

4.7

3.3

1.07

1.20

0.79

0.90

1.15

1.37

0.1

0.07

0.24

0.20

0.13*

0.105*

3.8

× 3.8 × 1.8

3.8

× 3.8 × 1.8

3.6

× 3.6 × 1.2

3.6

× 3.6 × 1.2

3.0

× 2.8 × 1.2

3.0

× 2.8 × 1.2

Toko

www.toko.com

CDRH3D16

CDRH2D11

CLS4D09

4.7

3.3

4.7

3.3

4.7

0.9

1.1

0.5

0.6

0.75

0.11

0.085

0.17

0.123

0.19

× 4 × 1.8

3.2

× 3.2 × 1.2

3.2

× 3.2 × 1.2

4.9

× 4.9 × 1

Sumida

www.sumida.com

SD3118

SD3112

SD12

SD10

4.7

3.3

4.7

3.3

4.7

3.3

4.7

3.3

1.3

1.59

0.8

0.97

1.29

1.42

1.08

1.31

0.162

0.113

0.246

0.165

0.117*

0.104*

0.153*

0.108*

3.1

× 3.1 × 1.8

3.1

× 3.1 × 1.8

3.1

× 3.1 × 1.2

3.1

× 3.1 × 1.2

5.2

× 5.2 × 1.2

5.2

× 5.2 × 1.2

5.2

× 5.2 × 1.0

5.2

× 5.2 × 1.0

Cooper

www.cooperet.com

LPS3015

4.7

3.3

1.1

1.3

0.2

0.13

3.0

× 3.0 × 1.5

3.0

× 3.0 × 1.5

Coil Craft

www.coilcraft.com

*Typical DCR

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