MP2372 – 3A, 28V, 925KHZ STEP-DOWN CONVERTER

MP2372 Rev. 0.92

www.MonolithicPower.com

7

12/22/2009

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APPLICATION INFORMATION

COMPONENT SELECTION

(Refer to Figure 5)

Setting the Output Voltage

The output voltage is set using a resistive

voltage divider from the output voltage to 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

92

.

0

+

×

=

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

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

is determined by:

)

k

)(

92

.

0

V

(

87

.

10

1

R

OUT

Ω

−

×

=

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,

the larger value inductor will 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 peak-to-peak ripple current in the inductor

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

1

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 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

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

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 whose maximum reverse

voltage rating is greater than the maximum

input voltage, and whose current rating is

greater than the maximum load current. Table 2

lists

example

Schottky

diodes

and

manufacturers.

Input Capacitor

The input current to the step-down converter is

discontinuous, therefore a capacitor is required