MP2354 – 2A, 23V, 380KHz STEP-DOWN CONVERTER

MP2354 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 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

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

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

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:

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

×

−

×

=

IN

OUT

IN

OUT

LOAD

1

C

V

V

1

V

V

I

I

The worst-case condition occurs where:

2

I

I

LOAD

1

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

Output Capacitor

The output capacitor is required to maintain the

DC output voltage. Ceramic, tantalum, or low

ESR electrolytic capacitors are recommended.

Low ESR capacitors are preferred to keep the

output voltage ripple low. The output voltage

ripple can be estimated by:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

×

×

+

×

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

×

=

∆

2

C

f

8

1

R

V

V

1

L

f

V

V

S

ESR

IN

OUT

S

OUT

OUT

Where L is the inductor value, C2 is the output

series resistance (ESR) value of the output

capacitor.

In the case of ceramic capacitors, the

impedance at the switching frequency is

dominated by the capacitance. The output

voltage ripple is mainly caused by the

capacitance. For simplification, the output

voltage ripple can be estimated by:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

×

×

×

=

IN

OUT

2

S

OUT

OUT

V

V

1

2

C

L

f

8

V

∆V

In the case of tantalum or electrolytic

capacitors, the ESR dominates the impedance

at the switching frequency. For simplification,

the output ripple can be approximated to:

ESR

IN

OUT

S

OUT

OUT

R

V

V

1

L

f

V

∆V

×

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

×

=

The characteristics of the output capacitor also

affect the stability of the regulation system. The