MP4558

– 1A, 2MHz, 55V STEP-DOWN CONVERTER

MP4558 Rev. 1.01

www.MonolithicPower.com

12

10/28/2013

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APPLICATION INFORMATION

COMPONENT SELECTION

Setting the Output Voltage

Set the output voltage with a resistor divider

between the output voltage and the FB pin. The

voltage divider drops the output voltage down to

the feedback voltage by the ratio:

FB

OUT

R2

V =V

R1+R2



Thus the output voltage is:

OUT

FB

R1+R2

V

=V

R2



For example, for R2 =

10kΩ, R1 can be

determined by:

)

)(

8

.

0









k

(V

12.5

R1

OUT

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

10kΩ, and R1 is 31.6kΩ.

Inductor

The inductor supplies 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 lower the output ripple

voltage. However, a larger-valued inductor is

physically larger, has a higher series resistance,

or lower saturation current.

Generally, determine an appropriate inductance

value by selecting the peak-to-peak inductor

ripple current equal to approximately 30% of the

maximum switch current limit. Also, make sure

that the peak inductor current is below the

maximum switch current limit. Calculate the

inductance value with:

OUT

OUT

s

L

IN

VV

L1=

(1-

)

f

ΔI

V





Where:

 V

 V

 f

 ∆I

Choose an inductor that will not saturate under

the maximum inductor peak current. Calculate

the peak inductor current with:

























IN

OUT

S

OUT

LOAD

LP

V

V

1

1

L

f

2

V

I

I

Table 1 lists a number of suitable inductors from

various manufacturers. The choice the inductor

style mainly depends on the price vs. size

requirements and any EMI requirement.