MP1586 – 3A, 28V, 50kHz TO 400kHz STEP-DOWN CONVERTER

MP1586 Rev. 0.91

www.MonolithicPower.com

11

1/8/2010

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Table 1—Inductor Selection Guide

Part Number

Inductance (µH)

Max DCR (Ω) Current Rating (A)

Dimensions

L x W x H (mm

Wurth Electronics

744066100

10

0.035

3.6

10x10x3.8

744770115

15

0.027

5

12x12x8

744770122

22

0.043

4.1

12x12x8

TDK

SLF12565T-100M4R8

10

0.0202

4.8

12.5x12.5x6.5

SLF12565T-150M4R2

15

0.0237

4.2

12.5x12.5x6.5

SLF12565T-220M3R5

22

0.0316

3.5

12.5x12.5x6.5

Toko

919AS-100M

10

0.0265

4.3

10.3x10.3x4.5

931BS-150M

15

0.0158

4.6

12.3x12.3x8

931BS-220M

22

0.0283

3.9

12.3x12.3x8

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. Table 2

lists

example

Schottky

diodes

and

manufacturers.

Table 2—Diode Selection Guide

Diodes

Voltage/

Current

Rating

Manufacturer

B340A-13-F

40V, 3A

Diodes Inc.

CMSH3-40MA

40V, 3A

Central Semi

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.

For simplification, choose the input capacitor

with RMS current rating greater than half of the

maximum load current.

The input capacitor (C1) can be electrolytic,

tantalum or ceramic. When using electrolytic or

tantalum capacitors, a small, high quality

ceramic capacitor, i.e. 0.1μF, should be placed

as close to the IC as possible. When using

ceramic capacitors, make sure that they have

enough capacitance to provide sufficient charge

to prevent excessive voltage ripple at input. The

input voltage ripple caused by capacitance can

be estimated by:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

×

×

=

Δ

IN

OUT

IN

OUT

S

LOAD

IN

V

V

1

V

V

1

C

f

I

V