MP2358 – 2A, 23V, 370KHz STEP-DOWN CONVERTER

MP2358 Rev. 1.3

www.MonolithicPower.com

6

3/23/2006

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© 2006 MPS. All Rights Reserved.

TM

APPLICATION 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

92

.

0

+

×

=

A typical value for R2 can be as high as 100kΩ,

but a typical value is 10kΩ. Using that value, R1

is determined by:

)

92

.

0

V

(

87

.

10

1

R

×

=

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

10kΩ, and R1 is 25.8kΩ.

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

the input voltage.

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

Output Rectifier Diode

The output rectifier diode supplies the current to

the inductor when the high-side switch is off. To

reduce losses due to the diode forward voltage

and recovery times, use a Schottky diode.

Choose a diode whose maximum reverse

voltage rating is greater than the maximum

input voltage, and whose current rating is

greater than the maximum load current.

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.

Since the input capacitor 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

2

C

V

V

1

V

V

I

I

where:

2

I

I

LOAD

2

C

=

For simplification, choose the input capacitor

whose RMS current rating greater than half of

the maximum load current.