MP2305 – 2A, 23V SYNCHRONOUS RECTIFIED, STEP-DOWN CONVERTER

MP2305 Rev. 1.7

www.MonolithicPower.com

6

4/17/2006

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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

+

=

the output voltage.

Thus the output voltage is:

2

R

2

R

1

R

923

.

0

+

×

=

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

(

83

.

10

1

R

×

=

(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:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

∆

×

=

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. 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 1 lists example

Schottky diodes and their Manufacturers.

Table 1—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.

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:

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

×

−

×

=

IN

OUT

IN

OUT

LOAD

1

C

V

V

1

V

V

I

I