MP28313 – 2A, 16V, 340KHz SYNCHRONOUS RECTIFIED, STEP-DOWN CONVERTER

MP28313 Rev. 1.5

www.MonolithicPower.com

7

9/27/2010

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© 2010 MPS. All Rights Reserved.

APPLICATIONS INFORMATION

COMPONENT SELECTION

INPUT

5V to 16V

OUTPUT

3.3V

2A

MP28313

BS

IN

FB

SW

SS

GND

COMP

EN

1

2

3

5

6

4

8

7

Figure 2—Application Circuit

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

923

.

0

+

×

=

R2 can be as high as 100kΩ, but a typical value

is 10kΩ. Using the typical value for R2, R1 is

determined by:

)

923

.

0

V

(

83

.

10

1

R

×

=

(kΩ)

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

10kΩ, and R1 is 26.1kΩ.

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

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

The choice of which style inductor to use mainly

depends on the price vs. size requirements and

any EMI requirements.