MP2303A – 3A, 28V, 360kHz SYNCHRONOUS RECTIFIED, STEP-DOWN CONVERTER

MP2303A Rev.1.1

www.MonolithicPower.com

9

7/25/2012

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© 2012 MPS. All Rights Reserved.

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:

OUT

R1 R2

V0.80

R2





the output voltage.

A typical value for 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









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

10kΩ, and R1 is 31.25kΩ.

Configuring the EN control

EN high to turn on the regulator and EN low to

turn it off. Do not float the pin.

For automatic start-up the EN pin can be pulled

up to input voltage through a resistive voltage

divider. Choose the values of the pull-up

determine the automatic start-up voltage:

UP

DOWN

IN START

DOWN

(R

R

)

V1.3

(V)

R







Note the EN voltage should be no greater than

6V. If the resistive voltage divider will make it

run over, please use a zener (below 6V) to

clamp it.

To avoid noise, a 10nF ceramic capacitor from

EN to GND is recommended.

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:





















IN

OUT

S

OUT

V

V

1

ΔI

f

V

L

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:

























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