MP1593 – 3A, 28V, 385KHz STEP-DOWN CONVERTER

MP1593 Rev. 1.9

www.MonolithicPower.com

6

9/14/2006

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© 2006 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

R

1

R

2

R

V

V

OUT

FB

+

=

the output voltage.

Thus the output voltage is:

2

R

2

R

1

R

22

.

1

+

×

=

R2 can be as high as 100kΩ, but a typical value

is 10kΩ. Using that value, R1 is determined by:

)

k

)(

22

.

1

V

(

18

.

8

1

R

OUT

Ω

−

×

=

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 that will

result in lower output ripple voltage. However,

larger value inductors will have larger physical

size, higher series resistance and/or lower

saturation current. A good standard 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

∆I

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

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