MP2355 – 3A, 23V, 380KHz STEP-DOWN CONVERTER

MP2355 Rev. 1.4

www.MonolithicPower.com

6

1/6/2006

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© 2006 MPS. All Rights Reserved.

TM

APPLICATIONS INFORMATION

MP2355_TAC_F02

MP2355

LX

VIN

BST

32

4

6

7

5

1

8

FB

SS

COMP

GND

RUN

OUTPUT

3.3V / 3A

INPUT

4.75V to 23V

OPEN

AUTOMATIC

STARTUP

C6

OPEN

C4

10nF

C3

4.7nF

C5

10nF

D1

B330A

Figure 2—MP2355 with Murata 22µF, 10V Ceramic Output Capacitor

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

22

.

1

+

×

=

the output voltage.

A typical value for R2 can be as high as 100kΩ,

but a typical value is 10kΩ. Using that value, R1

is determined by:

)

k

)(

22

.

1

V

(

18

.

8

1

R

OUT

Ω

−

×

=

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

10kΩ, and R1 is 17kΩ.

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

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

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.