MP4560 – 2A, 2MHz, 55V STEP-DOWN CONVERTER

MP4560 Rev. 1.0

www.MonolithicPower.com

10

11/5/2012

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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:

FB

OUT

R2

V=V

R1+R2

×

Thus the output voltage is:

OUT

FB

R1+R2

V=V

R2

×

For example, the value for R2 can be 10kΩ. With

this value, R1 can be determined by:

OUT

R1=12.5 (V

-0.8)(KΩ)

×

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

10kΩ, and R1 is 31.6kΩ.

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:

OUT

OUT

sL

IN

VV

L1=

(1-

)

f

ΔIV

×

×

the peak-to-peak inductor 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

1

L

f

2

V

I

I

Table 1 lists a number of suitable inductors from

various manufacturers. The choice of which style

inductor to use mainly depends on the price vs.

size requirements and any EMI requirement.