MP1472 – 2A, 18V SYNCHRONOUS RECTIFIED, STEP-DOWN CONVERTER

MP1472 Rev. 1.0

www.MonolithicPower.com

7

9/2/2011

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APPLICATIONS 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





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

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

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.

Table 1—Inductor Selection Guide

Part Number

Inductance (µH)

Max DCR (Ω)

Current Rating (A)

Dimensions

L x W x H (mm

Wurth Electronics

7440650068

6.8

0.033

3.6

10x10x2.8

744066100

10

0.035

3.6

10x10x3.8

744066150

15

0.050

3.2

10x10x3.8

TDK

SLF10165T-6R8N4R33PF

6.8

0.014

4.3

10x10x4.5

SLF10165T-100M3R83PF

10

0.0185

3.8

10x10x4.5

SLF10165T-150M3R13PF

15

0.027

3.1

10x10x4.5

Toko

#B952AS-6R8N

6.8

0.035

3.1

10.4x10.4x4.8

#B892NAS-100M

10

0.0225

4.2

12.3x12.3x4.5

#B892NAS-150M

15

0.0355

3.2

12.3x12.3x4.5