MP2307 – 3A, 23V, 340KHz SYNCHRONOUS RECTIFIED STEP-DOWN CONVERTER

MP2307 Rev. 1.7

www.MonolithicPower.com

7

3/14/2006

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© 2006 MPS. All Rights Reserved.

TM

APPLICATIONS INFORMATION

COMPONENT SELECTION

Setting the Output Voltage

The output voltage is set using a resistive

voltage divider connected from the output

voltage to FB. 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

925

.

0

+

×

=

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

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

determined by:

)

925

.

0

V

(

81

.

10

1

R

×

=

(kΩ)

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

10kΩ, and R1 is 26.1kΩ. Table 1 lists

recommended resistance values of R1 and R2

for standard output voltages.

Table 1—Recommended Resistance Values

VOUT

R1

R2

1.8V

9.53kΩ

10kΩ

2.5V

16.9kΩ

10kΩ

3.3V

26.1kΩ

10kΩ

5V

44.2kΩ

10kΩ

12V

121kΩ

10kΩ

Inductor

The inductor is required to supply constant

current to the load while being driven by the

switched input voltage. A larger value inductor

will result in less ripple current that will in turn

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 inductance is to allow the peak-to-

peak ripple current 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

L

S

OUT

V

V

1

I

f

V

L

Choose an inductor that will not saturate under

the maximum inductor peak current, calculated

by:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

×

×

+

=

IN

OUT

S

OUT

LOAD

LP

V

V

1

L

f

2

V

I

I

The choice of which style inductor to use mainly

depends on the price vs. size requirements and

any EMI constraints.

Optional Schottky Diode

During the transition between the 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

B130

30V, 1A

Diodes, Inc.

SK13

30V, 1A

Diodes, Inc.

MBRS130

30V, 1A

International

Rectifier

Input Capacitor

The input current to the step-down converter is

discontinuous, therefore a capacitor is required

to supply the AC current 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 will also suffice. Choose X5R or

X7R dielectrics when using ceramic capacitors.