MP4350 – 2.5A, 4MHz, 20V STEP-DOWN CONVERTER

MP4350 Rev. 1.0

www.MonolithicPower.com

8

5/26/2008

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© 2008 MPS. All Rights Reserved.

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

+

=

Thus the output voltage is:

2

R

)

2

R

1

R

(

V

V

FB

OUT

+

=

About 20µA current from high side BS circuitry

can be seen at the output when the MP4350 is

at no load. In order to absorb this small amount

of current, keep R2 under 40KΩ. A typical value

for R2 can be 40.2kΩ. With this value, R1 can

be determined by:

)

k

)(

8

.

0

V

(

25

.

50

1

R

OUT

Ω

−

×

=

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

40.2kΩ, and R1 is 127kΩ.

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

1

L

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.