MP8764 ― 12A, 18V, SYNCHRONOUS STEP-DOWN CONVERTER WITH LATCH-OFF OCP

MP8764 Rev. 1.2

www.MonolithicPower.com

20

6/21/2016

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© 2016 MPS. All Rights Reserved.

In the case of POSCAP capacitors, the ESR

dominates the impedance at the switching

frequency. The ramp voltage generated from the

ESR is high enough to stabilize the system.

Therefore, an external ramp is not needed. A

minimum ESR value around 12m

is required to

ensure stable operation of the converter. For

simplification,

the

output

ripple

can

be

approximated as:

ESR

IN

OUT

SW

OUT

OUT

R

)

V

V

1

(

L

F

V

V

×

−

×

×

=

∆

(26)

Inductor

The inductor is required to supply constant

current to the output load while being driven by

the switching input voltage. A larger value

inductor will result in less ripple current and lower

output ripple voltage. However, a larger value

inductor will have a larger physical size, higher

series resistance, and/or lower saturation current.

A good rule for determining the inductor value is

to allow the peak-to-peak ripple current in the

inductor to be approximately 30~40% 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 as:

)

V

V

1

(

I

F

V

L

IN

OUT

L

SW

OUT

−

×

∆

×

=

(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)

The inductors listed in Table 1 are highly

recommended for the high efficiency they can

provide.

Table 1—Inductor Selection Guide

Part Number

Manufacturer

Inductance

(µH)

DCR

(m )

Current

Rating (A)

Dimensions

L x W x H (mm

Switching

Frequency

(kHz)

PCMC-135T-R68MF

Cyntec

0.68

1.7

34

13.5 x 12.6 x 4.8

600

FDA1254-1R0M

TOKO

1

2

25.2

13.5 x 12.6 x 5.4

300~600

FDA1254-1R2M

TOKO

1.2

2.05

20.2

13.5 x 12.6 x 5.4

300~600

Typical Design Parameter Tables

The

following

tables

include

recommended

component values for typical output voltages (1V,

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

Refer to Tables 2 for design cases without

external ramp compensation and Tables 3 for

design cases with external ramp compensation.

External ramp is not needed when high-ESR

capacitors, such as electrolytic or POSCAPs are

used. External ramp is needed when low-ESR

capacitors, such as ceramic capacitors are used.

For cases not listed in this datasheet, a calculator

in excel spreadsheet can also be requested

through a local sales representative to assist with

the calculation.

(V)

L

(µH)

R1

(k )

R2

(k )

R7

(8)

(k )

1

1

12.7

20

340

2.5

1.5

61.9

20

825

3.3

2.2

88.7

20

1083

(V)

L

(µH)

R1

(k )

R2

(k )

R4

(k )

C4

(pF)

R7

(8)

(k )

1

1

12.7

20

750

220

340

2.5

1.5

64.9

20

1000

220

825

3.3

2.2

93.1

20

1200

220

1083

Notes:

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