MP9403 – 3A, 7V, 250kHz INTEGRATED SYNCHRONOUS STEP-DOWN CONVERTER

MP9403 Rev.0.9

www.MonolithicPower.com

7

2/9/2010

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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

+

=

Thus the output voltage is:

2

R

2

R

1

R

80

.

0

+

×

=

the output voltage.

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

is 10kΩ. Using that value, R1 is determined by:

)

k

)(

80

.

0

V

(

5

.

12

1

R

OUT

Ω

−

×

=

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

10kΩ, and R1 is 31.6kΩ.

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 value is to allow the peak-to-peak

ripple current in the inductor to be approximately

30% of the maximum switching 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

S

OUT

V

V

1

ΔI

f

V

L

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

744777004

4.7

0.040

4.0

7.3x7.3x4.5

7440650068

6.8

0.033

3.6

10x10x2.8

744066100

10

0.035

3.6

10x10x3.8

TDK

VLF10040T-4R7N5R4

4.7

0.015

5.4

10x9.7x4

VLF10040T-6R8N4R5

6.8

0.023

4.5

10x9.7x4

VLF10040T-100M3R8

10

0.033

3.8

10x9.7x4

Toko

1010ASW-4R7M

4.7

0.015

4.4

12.3x12.3x4.5

B992AS-6R8N

6.8

0.028

3.8

8.3x8.3x6.8

1010ASW-100M

10

0.022

3.6

12.3x12.3x4.5