MPQ2451―INDUSTRIAL-GRADE 36V, 2MHz, 0.6A, STEP-DOWN CONVERTER, AEC-Q100 QUALIFIED

MPQ2451 Rev 1.02

www.MonolithicPower.com

11

5/3/2013

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© 2013 MPS. All Rights Reserved.

APPLICATION INFORMATION

COMPONENT SELECTION

Setting the Output Voltage

The output voltage is set using a resistive

voltage divider from the output voltage to the

using the following equation:

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:

OUT

R1

R2

V

1

0.8V

=

−

Table 1: Resistor Selection vs. Output Voltage

Setting

R1

R2

0.8V

124kΩ (1%)

NS

1.2V

124kΩ (1%)

249kΩ (1%)

3.3V

124kΩ (1%)

40.2kΩ (1%)

5V

124kΩ (1%)

23.7kΩ (1%)

Inductor

The inductor supplies constant current to the

output load while being driven by the switched

input voltage. A larger-value inductor will result

in less ripple current and lower output ripple

voltage. However, the larger-value inductor is

typically physically larger, has a higher series

resistance, or has a lower saturation current.

To determine the inductance, allow the peak-to-

peak ripple current in the inductor to be

approximately 30% of the maximum load

current. Also, chose a peak inductor current

below the maximum switch current limit. The

inductance value can be calculated by:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

×

=

IN

OUT

L

S

OUT

V

V

1

ΔI

f

V

1

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

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.