MP9141ES-LF-Z データシートの表示（PDF） - Monolithic Power Systems

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MP9141ES-LF-Z

1.8A Step-Down DC to DC Converter

Monolithic Power Systems 

MP9141 – 1.8A STEP-DOWN DC TO DC CONVERTER

MPS CONFIDENTIAL AND PROPRIETARY INFORMATION– INTERNAL USE ONLY

APPLICATION INFORMATION

COMPONENT SELECTION

Setting the Output Voltage

The output voltage is set using a resistive

voltage divider from the output voltage to FB

(see Typical Application circuit on page 1). The

voltage divider divides the output voltage down

by the equation:

VFB

=

VOUT

× R3

(R2 + R3)

Thus the output voltage is:

VOUT

= 1.222 × (R2 + R3)

R3

R3 can be as high as 100kΩ, but a typical value

is 10kΩ. Using that value, R2 is determined by:

R2 ≅ 8.18 × (VOUT − 1.222)

For example, for a 3.3V output voltage, R3 is

10kΩ, and R2 is 16.9kΩ.

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 results in less ripple current that in turn

results in lower output ripple voltage. However,

the larger value inductor has a larger physical

size, higher series resistance and/or lower

saturation current. Choose an inductor that

does not saturate under the worst-case load

conditions. A good rule for determining the

inductance is to allow the inductor peak-to-peak

ripple current to be approximately 30% of the

maximum load current. Also, make sure that the

peak inductor current (the load current plus half

the peak-to-peak inductor ripple current) is

below the 2.4A minimum current limit.

The inductance value can be calculated by the

equation:

L

=

VOUT

×

(VIN

(VIN

−

×

VOUT )

f × ∆I)

Where VOUT is the output voltage, VIN is the

input voltage, f is the switching frequency, and

∆I is the peak-to-peak inductor ripple current.

Table 1 lists a number of suitable inductors

from various manufacturers.

Table 1—Inductor Selection Guide

Vendor/Model Core Type

Core

Material

Sumida

CR25

CDH74

CDRH5D28

CDRH5D28

CDRH6D28

CDRH104R

Open

Open

Shielded

Shielded

Shielded

Shielded

Toko

D53LC Type A Shielded

D75C

Shielded

D104C

Shielded

D10FL

Open

Coilcraft

DO3308

DO3316

Open

Open

Ferrite

Ferrite

Ferrite

Ferrite

Ferrite

Ferrite

Ferrite

Ferrite

Ferrite

Ferrite

Ferrite

Ferrite

Package

Dimensions (mm)

W

L

H

7.0 7.8 5.5

7.3 8.0 5.2

5.5 5.7 5.5

5.5 5.7 5.5

6.7 6.7 3.0

10.1 10.0 3.0

5.0 5.0 3.0

7.6 7.6 5.1

10.0 10.0 4.3

9.7 11.5 4.0

9.4 13.0 3.0

9.4 13.0 5.1

Input Capacitor

The input current to the step-down converter is

discontinuous, and therefore an input capacitor

C1 is required to supply the AC current to the

step-down converter while maintaining the DC

input voltage. A low ESR capacitor is required

to keep the noise at the IC to a minimum.

Ceramic capacitors are preferred, but tantalum

or low-ESR electrolytic capacitors may also

suffice.

The input capacitor value should be greater

than 10µF. The capacitor can be electrolytic,

tantalum or ceramic. However, since it absorbs

the input switching current it requires an

adequate ripple current rating. Its RMS current

rating should be greater than approximately 1/2

of the DC load current.

For insuring stable operation C2 should be

placed as close to the IC as possible.

Alternately a smaller high quality ceramic 0.1µF

capacitor may be placed closer to the IC and a

larger capacitor placed further away. If using

this technique, it is recommended that the

larger capacitor be a tantalum or electrolytic

type. All ceramic capacitors should be placed

close to the MP9141.

MP9141 Rev. 1.11

www.MonolithicPower.com

6

4/18/2011

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

© 2011 MPS. All Rights Reserved.

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