MP2309 – 1A, 23V, 340KHz SYNCHRONOUS RECTIFIED STEP-DOWN CONVERTER

MP2309 Rev. 1.4

www.MonolithicPower.com

6

3/13/2006

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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 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

923

.

0

+

×

=

the feedback voltage.

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

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

R1 is determined by:

)

923

.

0

V

(

87

.

10

1

R

×

=

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

10kΩ, and R1 is 26.1kΩ.

Table 1 lists recommended resistor values.

Table1—Recommended Resistors

VOUT

R1

R2

1.8V

9.53kΩ

10kΩ

2.5V

16.9kΩ

10kΩ

3.3V

26.1kΩ

10kΩ

5V

44.2Ω

10kΩ

12V

121kΩ

10kΩ

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

L

f

V

L

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

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 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 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.