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EV2351DQ-00A

1.5A, 23V, 1.4MHz Step-Down Converter

Monolithic Power Systems 

TM

MP2351 – 1.5A, 23V, 1.4MHz STEP-DOWN CONVERTER

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:

VFB

=

VOUT

R2

R1 + R2

Where VFB is the feedback voltage and VOUT is

the output voltage.

Thus the output voltage is:

VOUT

=

1.23

×

R1 + R2

R2

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

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

R1 = 8.18 × (VOUT − 1.23)

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

10kΩ, and R1 is 17kΩ.

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:

L

=

VOUT

fS × ∆IL

× ⎜⎜⎝⎛1−

VOUT

VIN

⎟⎟⎠⎞

Where VIN is the input voltage, fS is the

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.

Output Rectifier Diode

The output rectifier diode supplies the current to

the inductor when the high-side switch is off. To

reduce losses due to the diode forward voltage

and recovery times, use a Schottky diode.

Choose a diode whose maximum reverse

voltage rating is greater than the maximum

input voltage, and whose current rating is

greater than the maximum load current.

Input Capacitor

The input current to the step-down converter is

discontinuous, therefore a capacitor is required

to supply the AC current to the step-down

converter while maintaining the DC input

voltage. Use low ESR capacitors for the best

performance. Ceramic capacitors are preferred,

but tantalum or low-ESR electrolytic capacitors

may also suffice.

Since the input capacitor (C1) absorbs the input

switching current it requires an adequate ripple

current rating. The RMS current in the input

capacitor can be estimated by:

IC1 = ILOAD ×

VOUT

VIN

×⎜⎜⎛1−

⎝

VOUT

VIN

⎟⎞

⎟

⎠

The worst-case condition occurs at VIN = 2VOUT,

where:

IC1

=

ILOAD

2

For simplification, choose the input capacitor

whose RMS current rating greater than half of

the maximum load current.

MP2351 Rev. 1.5

www.MonolithicPower.com

5

1/6/2006

MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.

© 2006 MPS. All Rights Reserved.

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