MP1423DP データシートの表示（PDF） - Monolithic Power Systems

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MP1423DP

3A, 23V, 385KHz Step-Down Converter

Monolithic Power Systems 

TM

MP1423 – 3A, 23V, 385KHz STEP-DOWN CONVERTER

APPLICATION INFORMATION

COMPONENT SELECTION

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:

VFB

=

VOUT

R2

R1 + R2

Where VFB is the feedback voltage and VOUT is

the output voltage.

Thus the output voltage is:

VOUT

= 1.22 × R1+ R2

R2

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

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

is determined by:

R1 = 8.18 × (VOUT − 1.22)(kΩ)

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 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:

L

=

VOUT

fS × ∆IL

× ⎜⎜⎝⎛1−

VOUT

VIN

⎟⎟⎠⎞

Where VIN is the input voltage, fS is the 385KHz

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

Core

Material

Package

Dimensions

(mm)

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

MP1423 Rev. 1.1

www.MonolithicPower.com

5

1/6/2006

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© 2006 MPS. All Rights Reserved.

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