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MP1482

2A, 18V Synchronous Rectified Step-Down Converter

Monolithic Power Systems 

MP1482 – 2A, 18V SYNCHRONOUS RECTIFIED, STEP-DOWN CONVERTER

APPLICATIONS 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



0.923



R1  R2

R2

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

is 10kΩ. Using the typical value for R2, R1 is

determined by:

R1  10.83  (VOUT  0.923) (kΩ)

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

10kΩ, and R1 is 26.1kΩ.

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 VOUT is the output voltage, VIN is the

input voltage, fS is the switching frequency, and

ΔIL 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.

The choice of which style inductor to use mainly

depends on the price vs. size requirements and

any EMI requirements.

Optional Schottky Diode

During the transition between high-side switch

and low-side switch, the body diode of the low-

side power MOSFET conducts the inductor

current. The forward voltage of this body diode

is high. An optional Schottky diode may be

paralleled between the SW pin and GND pin to

improve overall efficiency. Table 1 lists example

Schottky diodes and their Manufacturers.

Table 1—Diode Selection Guide

Part Number

B130

SK13

Voltage/Current

Rating

30V, 1A

30V, 1A

MBRS130

30V, 1A

Vendor

Diodes, Inc.

Diodes, Inc.

International

Rectifier

Input Capacitor

The input current to the step-down converter is

discontinuous, therefore a capacitor is required

to supply the AC current to the step-down

converter while maintaining the DC input

voltage. Use low ESR capacitors for the best

performance. Ceramic capacitors are preferred,

but tantalum or low-ESR electrolytic capacitors

may also suffice. Choose X5R or X7R

dielectrics when using ceramic capacitors.

Since the input capacitor (C1) absorbs the input

switching current it requires an adequate ripple

current rating. The RMS current in the input

capacitor can be estimated by:

IC1  ILOAD 

VOUT

VIN

1



VOUT

VIN







MP1482 Rev. 1.31

www.MonolithicPower.com

7

7/9/2012

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© 2012 MPS. All Rights Reserved.

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