MP1583 – 3A, 23V, 385KHz STEP-DOWN CONVERTER

MP1583 Rev. 3.1

www.MonolithicPower.com

6

6/20/2011

MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.

© 2011 MPS. All Rights Reserved.

APPLICATION INFORMATION

COMPONENT SELECTION

Setting the Output Voltage

The output voltage is set using a resistive

voltage divider from the output voltage to the

FB pin. The voltage divider divides the output

voltage down to the feedback voltage by the

ratio:

2

1

2

R

R

R

V

V

OUT

FB

+

=

the output voltage.

Thus the output voltage is:

2

2

1

22

.

1

R

R

R

V

V

OUT

+

×

=

A typical value for R2 can be as high as 100kΩ,

but a typical value is 10kΩ. Using that value, R1

is determined by:

)

)(

22

.

1

(

18

.

8

1

Ω

−

×

=

k

V

V

R

OUT

For example, for a 3.3V output voltage, R2 is

10kΩ, and R1 is 17kΩ.

Inductor

The inductor is required to supply constant

current to the output load while being driven by

the switched input voltage. A larger value

inductor will result in less ripple current and

lower output ripple voltage. However, larger

value inductors have a larger physical size,

higher

series

resistance,

and/or

lower

saturation current. A good rule for determining

the inductance to use is to allow the inductor

peak-to-peak ripple current to be approximately

30% of the maximum switch current limit. Also,

make sure that the peak inductor current is

below the maximum switch current limit. The

inductance value can be calculated by:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

×

=

IN

OUT

L

S

OUT

V

V

1

ΔI

f

V

L

inductor ripple current.

Choose an inductor that will not saturate under

the maximum inductor peak current.

The peak inductor current can be calculated by:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

×

×

+

=

IN

OUT

S

OUT

LOAD

LP

V

V

1

L

f

2

V

I

I

Table 1 lists a number of suitable inductors

from various manufacturers. The choice of

which inductor to use mainly depends on the

price vs. size requirements and any EMI

requirements.

Table 1—Inductor Selection Guide

Package

Dimensions

(mm)

Vendor/

Model

Core

Type

Core

Material

WL

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