MP2452―1A, 36V, 1MHz, STEP-DOWN CONVERTER

MP2452 Rev. 1.0

www.MonolithicPower.com

11

9/27/2010

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APPLICATION 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

(

V

V

FB

OUT

+

=

The feedback resistor R1 also sets the

feedback loop bandwidth with the internal

compensation capacitor.

Choose R1 around 124kΩ for optimal transient

response. R2 is then given by:

1

0.794V

V

R1

R2

OUT

−

=

Table 1―Resistor Selection vs. Output

Voltage Setting

R1

R2

0.8V

124kΩ (1%)

NS

1.2V

124kΩ (1%)

243kΩ (1%)

3.3V

124kΩ (1%)

39.2kΩ (1%)

5V

124kΩ (1%)

23.7kΩ (1%)

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.

Generally, 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 load

current. 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

1

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

1

L

f

2

V

I

I

Table 2 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.