MP2403 – 3A, 32V, 250kHz INTEGRATED SYNCHRONOUS STEP-DOWN CONVERTER

MP2403 Rev.1.0

www.MonolithicPower.com

8

4/20/2010

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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:

2

R

1

R

2

R

V

V

OUT

FB

+

=

Thus the output voltage is:

2

R

2

R

1

R

80

.

0

+

×

=

output voltage.

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

10kΩ. Using that value, R1 is determined by:

)

k

)(

80

.

0

V

(

5

.

12

1

R

OUT

Ω

−

×

=

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

10kΩ, and R1 is 31.25kΩ.

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 value

is to allow the peak-to-peak ripple current in the

inductor to be approximately 30% of the maximum

switching 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

S

OUT

V

V

1

ΔI

f

V

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

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 2 lists example

Schottky diodes and their Manufacturers

.

Table 2—Diode Selection Guide

Part Number

Voltage/Current

Rating

Vendor

B340

40V, 3A

Diodes, Inc.

SK34

40V, 3A

Diodes, Inc.

MBRS340

40V, 3A

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 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:

)

V

V

1

(

V

V

I

I

IN

OUT

IN

OUT

LOAD

1

C

−

×

×

=

where:

2

I

I

LOAD

1

For simplification, choose an input capacitor with

an RMS current rating greater than half of the

maximum load current.