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MBRS130

3A, 23V, 340KHz Synchronous Rectified Step-Down Converter

Monolithic Power Systems 

TM

MP1570 – 3A, 23V, 340KHz SYNCHRONOUS RECTIFIED, STEP-DOWN CONVERTER

peak inductor current is below the maximum

switch current limit. The inductance value can

be calculated by:

L

=

VOUT

fS × ∆I

× ⎜⎜⎝⎛1 −

VOUT

VIN

⎟⎟⎠⎞

Where VIN is the input voltage, fS is the 340KHz

switching frequency, and ∆IL is the peak-to-

peak inductor ripple current.

Choose an inductor that will not saturate under

the maximum inductor peak current. The peak

inductor current can be calculated by:

ILP

= ILOAD

+

VOUT

2 × fS × L

× ⎜⎜⎝⎛1−

VOUT

VIN

⎟⎟⎠⎞

Where ILOAD is the load current.

Table 1 lists a number of suitable inductors

from various manufacturers. The choice of

which style inductor to use mainly depends on

the price vs. size requirements and any EMI

requirement.

Table 1—Inductor Selection Guide

Vendor/

Model

Core

Type

Sumida

CR75

Open

CDH74 Open

CDRH5D28 Shielded

CDRH5D28 Shielded

CDRH6D28 Shielded

CDRH104R Shielded

Toko

D53LC

Type A

Shielded

D75C Shielded

D104C

D10FL

Coilcraft

DO3308

Shielded

Open

Open

DO3316 Open

Core

Material

Ferrite

Ferrite

Ferrite

Ferrite

Ferrite

Ferrite

Ferrite

Ferrite

Ferrite

Ferrite

Ferrite

Ferrite

Package

Dimensions

(mm)

WL H

7.0 7.8 5.5

7.3 8.0 5.2

5.5 5.7 5.5

5.5 5.7 5.5

6.7 6.7 3.0

10.1 10.0 3.0

5.0 5.0 3.0

7.6 7.6 5.1

10.0 10.0 4.3

9.7 1.5 4.0

9.4 13.0 3.0

9.4 13.0 5.1

Optional Schottky Diode

During the transition between high-side switch

and low-side switch, the body diode of the low-

side power MOSFET conducts the inductor

current. The forward voltage of this body diode

is high. An optional Schottky diode may be

paralleled between the SW pin and GND pin to

improve overall efficiency. Table 2 lists example

Schottky diodes and their Manufacturers.

Table 2—Diode Selection Guide

Part Number

B130

SK13

MBRS130

Voltage/Current

Rating

30V, 1A

30V, 1A

30V, 1A

Vendor

Diodes, Inc.

Diodes, Inc.

International

Rectifier

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. Choose X5R or X7R

dielectrics when using ceramic capacitors.

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:

IC1 = ILOAD ×

VOUT

VIN

×⎜⎜⎛1−

⎝

VOUT

VIN

⎟⎞

⎟

⎠

The worst-case condition occurs at VIN = 2VOUT,

where:

IC1

=

ILOAD

2

For simplification, choose the input capacitor

whose RMS current rating greater than half of

the maximum load current.

MP1570 Rev. 1.5

www.MonolithicPower.com

6

1/31/2006

MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.

© 2006 MPS. All Rights Reserved.

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