MP4561 – 1.5A, 2MHz, 55V STEP-DOWN CONVERTER

MP4561 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.

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. Table 2 lists example

Schottky diodes and manufacturers.