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21DQ04

2A, 32V, 330KHz Step-Down Converter

Monolithic Power Systems 

MP1591 – 2A, 32V, 330KHz 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.

The voltage divider divides the output voltage

down 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

= 1.230 × (R1+ R2)

R2

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

but 10kΩ is recommended. Using that value, R1

is determined by:

R1 ≅ 8.18 × (VOUT − 1.230)

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

10kΩ, and R1 is 17kΩ.

Inductor (L1)

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 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 to use for determining

the inductance is to allow the peak-to-peak

ripple current in the inductor to be

approximately 30% of the maximum load

current that the IC can provide. Also, make sure

that the peak inductor current (the load current

plus half the peak-to-peak inductor ripple

current) is below the 2.3A minimum current

limit.

The inductance value can be calculated by the

equation:

L1

=

VOUT

×

(VIN

(VIN

−

×

VOUT )

f × ∆I)

Where 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

Package

Dimensions

(mm)

WLH

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

Input Capacitor (C1)

The input current to the step-down converter is

discontinuous, and so a capacitor 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.

MP1591 Rev. 2.3

www.MonolithicPower.com

5

9/27/2006

MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.

© 2006 MPS. All Rights Reserved.

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