MP1584 データシートの表示（PDF） - Monolithic Power Systems

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MP1584

3A, 1.5MHz, 28V Step-Down Converter

Monolithic Power Systems 

MP1584 – 3A, 1.5MHz, 28V 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

Thus the output voltage is:

VOUT



VFB

(R1  R2)

R2

About 20µA current from high side BS circuitry

can be seen at the output when the MP1584 is

at no load. In order to absorb this small amount

of current, keep R2 under 40KΩ. A typical

value for R2 can be 40.2kΩ. With this value, R1

can be determined by:

R1  50.25  (VOUT  0.8)(k)

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

40.2kΩ, and R1 is 127kΩ.

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:

L1 

VOUT

fS  ΔIL

 1 

VOUT

VIN



Where VOUT is the output voltage, VIN is the input

voltage, fS is the switching frequency, and ΔIL 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



2

VOUT

 fS 

L1



1 

VOUT

VIN



Where ILOAD is the load current.

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.

MP1584 Rev. 1.0

www.MonolithicPower.com

10

8/8/2011

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