MP2355 – 3A, 23V, 380KHz STEP-DOWN CONVERTER

MP2355 Rev. 1.4

www.MonolithicPower.com

7

1/6/2006

MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.

© 2006 MPS. All Rights Reserved.

TM

Table 1—Inductor Selection Guide

Package

Dimensions

(mm)

Vendor/

Model

Core

Type

Core

Material

W

L

H

Sumida

CR75

Open

Ferrite

7.0

7.8

5.5

CDH74

Open

Ferrite

7.3

8.0

5.2

CDRH5D28

Shielded

Ferrite

5.5

5.7

5.5

CDRH5D28

Shielded

Ferrite

5.5

5.7

5.5

Sumida (continued)

CDRH6D28

Shielded

Ferrite

6.7

6.7

3.0

CDRH104R

Shielded

Ferrite

10.1 10.0

3.0

Toko

D53LC

Type A

Shielded

Ferrite

5.0

5.0

3.0

D75C

Shielded

Ferrite

7.6

7.6

5.1

D104C

Shielded

Ferrite

10.0 10.0

4.3

D10FL

Open

Ferrite

9.7

1.5

4.0

Coilcraft

DO3308

Open

Ferrite

9.4

13.0

3.0

DO3316

Open

Ferrite

9.4

13.0

5.1

Output Rectifier Diode

The output rectifier diode supplies the current to

the inductor when the high-side switch is off. To

reduce losses due to the diode forward voltage

and recovery times, use a Schottky diode.

Choose a diode which has a maximum reverse

voltage rating is greater than the maximum

input voltage, and who’s current rating is

greater than the maximum load current. Table 2

lists

example

Schottky

diodes

and

manufacturers.

Table 2—Diode Selection Guide

Diode

Voltage/Current

Rating

Manufacture

SK33

30V, 3A

Diodes Inc.

SK34

40V, 3A

Diodes Inc.

B330

30V, 3A

Diodes Inc.

B340

40V, 3A

Diodes Inc.

MBRS330

30V, 3A

On Semiconductor

MBRS340

40V, 3A

On Semiconductor

Input Capacitor

The input current to the step-down converter is

discontinuous, therefore a capacitor is required

to supply the AC current to the step-down

converter while maintaining the DC input

voltage. Use low ESR capacitors for the best

performance. Ceramic capacitors are preferred,

but tantalum or low-ESR electrolytic capacitors

may also suffice.

Since the input capacitor (C1) absorbs the input

switching current it requires an adequate ripple

current rating. The RMS current in the input

capacitor can be estimated by:

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

×

−

×

=

IN

OUT

IN

OUT

LOAD

1

C

V

V

1

V

V

I

I

where:

2

I

I

LOAD

1

C

=

For simplification, choose the input capacitor

whose RMS current rating greater than half of the

maximum load current.

The input capacitor can be electrolytic, tantalum

or ceramic. When using electrolytic or tantalum

capacitors, a

small, high

quality ceramic

capacitor, i.e. 0.1µF, should be placed as close to

the IC as possible. When using ceramic

capacitors, make sure that they have enough

capacitance to provide sufficient charge to

prevent excessive voltage ripple at input. The

input voltage ripple caused by capacitance can be

estimated by:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

×

×

=

∆

IN

OUT

IN

OUT

s

LOAD

IN

V

V

1

V

V

1

C

f

I

V