MP8763D ― 12A, 18V, SYNCHRONOUS STEP-DOWN CONVERTER

MP8763D Rev. 1.1

www.MonolithicPower.com

21

1/15/2015

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© 2015 MPS. All Rights Reserved.

The ESR dominates the switching-frequency

impedance for POSCAPs,. The ESR ramp

voltage is high enough to stabilize the system.

thus eliminating the need for an external ramp.

Select a minimum ESR value around 12mΩ to

ensure stable operation. For simplification, the

output ripple can be approximated as:

ESR

IN

OUT

SW

OUT

OUT

R

)

V

V

1

(

L

F

V

V

×

−

×

×

=

Δ

(26)

Inductor

The inductor supplies constant current to the

output load while being driven by the switching

input voltage. A larger value inductor results in

less ripple current and lower output ripple voltage,

but is larger physical size, has a higher series

resistance, and/or lower saturation current.

Generally, select an inductor value that allows

the inductor peak-to-peak ripple current to 30%

to 40% of the maximum switch current limit. Also,

design for a peak inductor current that is below

the maximum switch current limit. Calculate the

inductance value as:

)

V

V

1

(

I

F

V

L

IN

OUT

L

SW

OUT

−

×

Δ

×

=

(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

1

(

L

F

2

V

I

I

IN

OUT

SW

OUT

OUT

LP

−

×

×

×

+

=

(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 x W x H (mm

Switching

Frequency

(kHz)

744325072

Wurth

0.72

1.35

35

10.2 x 10.5 x 4.7

500

FDU1250C-1R0M

TOKO

1

1.72

31.3

13.3 x 12.1 x 5

500

FDA1055-1R5M

TOKO

1.5

2.8

24

11.6 x 10.8 x 5.5

500

744325180

Wurth

1.8

3.5

18

10.2 x 10.5 x 4.7

500

FDA1055-2R2M

TOKO

2.2

3.94

20.6

11.6 x 10.8 x 5.5

500

FDA1055-3R3M

TOKO

3.3

5.92

15.6

11.6 x 10.8 x 5.5

500

HC7-3R9-R

Cooper

3.9

7.9

10.6

13.8 x 13 x 5.5

500

Typical Design Parameter Tables

The following tables include recommended

component values for typical output voltages (1V,

2.5V, 3.3V) and switching frequency (500kHz).

Refer to Tables 2 for design cases without

external ramp compensation and Tables 3 for

design cases with external ramp compensation.

An external ramp is not needed when using high-

ESR

capacitors,

such

as

electrolytic

or

POSCAPs. Use an external ramp when using

low-ESR capacitors, such as ceramic capacitors.

For cases not listed in this datasheet, an excel

spreadsheet

provided

by

local

sales

representatives can assist with the calculations.

(V)

L

(μH)

R1

(kΩ)

R2

(kΩ)

R7

(8)

(kΩ)

1

0.72

12.7

20

357

2.5

1.5

61.9

20

825

3.3

1.8

88.7

20

1083

(V)

L

(μH)

R1

(kΩ)

R2

(kΩ)

R4

(kΩ)

C4

(pF)

R7

(8)

(kΩ)

1

0.72

12.7

20

750

220

357

2.5

1.5

64.9

20

1000

220

825

3.3

1.8

93.1

20

1200

220

1083

Note:

8) Frequency is about 500kHz at full load condition.