MP8761 — 8A, 18V, SYNCHRONOUS STEP-DOWN CONVERTER

MP8761 Rev. 1.1

www.MonolithicPower.com

19

11/4/2013

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below

the

maximum

switch-current

limit.

Calculate the inductance value as:

)

V

V

(

I

f

V

L

IN

OUT

L

SW

OUT

−

×

Δ

×

=

1

(27)

current.

Choose an inductor that will not saturate under

the maximum inductor peak current. The peak

inductor current can be calculated as:

)

V

V

(

L

f

V

I

I

IN

OUT

SW

OUT

OUT

LP

−

×

×

×

+

=

1

2

(28)

Table 1 lists a few highly-recommended high-

efficiency inductors.

Table 1: Inductor Selection Guide

Part Number

Manufacturer

Inductance

(µH)

DCR

(mΩ)

Current

Rating (A)

Dimensions

L × W × H (mm

Switching

Frequency

(kHz)

744325072

Wurth

0.72

1.35

35

10.2 × 10.5 × 4.7

500

744325180

Wurth

1.8

3.5

18

10.2 × 10.5 × 4.7

500

FDA1055-2R2M

TOKO

2.2

3.94

20.6

11.6 × 10.8 × 5.5

500

Typical Design Parameters

Tables 2 and 3 list recommended component

values for typical output voltages (1V, 2.5V, 3.3V)

and switching frequency (500kHz). Refer to

Table 2 for design cases without external ramp

compensation and Table 3 for design cases with

external ramp compensation. An external ramp is

not needed when using high-ESR capacitors,

such as electrolytics or POSCAPs. Use an

external ramp when using low-ESR capacitors,

such as ceramic capacitors. For cases not listed

in this datasheet, conctact a local sales

representative for an Excel spreadsheet to assist

with the calculation.

(V)

L

(μH)

R1

(kΩ)

R2

(kΩ)

(kΩ)

1

0.72

13.3

20

357

2.5

1.8

63.4

20

887

3.3

2.2

91

20

1200

(V)

L

(μH)

R1

(kΩ)

R2

(kΩ)

R4

(kΩ)

C4

(pF)

(kΩ)

1

0.72

13.7

20

750

220

357

2.5

1.8

66.5

20

1000

220

887

3.3

2.2

95.3

20

1200

220

1200