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Fabricante

EV2355DN-00A

3A, 23V, 380KHz Step-Down Converter

Monolithic Power Systems 

TM

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

APPLICATIONS INFORMATION

INPUT

4.75V to 23V

C5

10nF

3

2

OPEN

AUTOMATIC

STARTUP

VIN

8

RUN

BST

LX 4

MP2355

1 SS

6

FB

GND

COMP

5

7

C4

10nF

C6

OPEN

C3

4.7nF

D1

B330A

OUTPUT

3.3V / 3A

MP2355_TAC_F02

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:

VFB

=

VOUT

R2

R1+ R2

Thus the output voltage is:

VOUT

= 1.22 × R1+ R2

R2

Where VFB is the feedback voltage and VOUT is

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:

R1 = 8.18 × (VOUT − 1.22)(kΩ)

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

10kΩ, and R1 is 17kΩ.

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:

L

=

VOUT

fS × ∆IL

× ⎜⎜⎝⎛1−

VOUT

VIN

⎟⎟⎠⎞

Where VIN is the input voltage, fS is the 380KHz

switching frequency, and ∆IL is 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:

ILP

= ILOAD

+

VOUT

2 × fS × L

× ⎜⎜⎝⎛1−

VOUT

VIN

⎟⎟⎠⎞

Where ILOAD is the load current.

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.

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.

MP2355 Rev. 1.5

www.MonolithicPower.com

6

5/1/2006

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© 2006 MPS. All Rights Reserved.

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