MP8762GL datasheet(20/39 Pages) MPS

20 / 39 page

MP8762 ― 10A, 18V, SYNCHRONOUS STEP-DOWN CONVERTER

MP8762 Rev. 1.4

www.MonolithicPower.com

11/4/2013

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

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 12mΩ is required to

ensure stable operation of the converter. For

simplification,

the

output

ripple

can

approximated as:

ESR

OUT

)

(

−

(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:

)

(

OUT

−

(27)

Where 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 as:

)

(

OUT

−

(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

(mΩ)

Current

Rating (A)

Dimensions

L x W x H (mm

Switching

Frequency

(kHz)

PCMC-135T-R68MF

Cyntec

0.68

1.7

13.5 x 12.6 x 4.8

600

FDA1254-1R0M

TOKO

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 (300kHz,

500kHz, and 800kHz). Refer to Tables 2-4 for

design

cases

without

external

ramp

compensation and Tables 5-7 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.

Table 2—FSW=300kHz, VIN=12V

VOUT

(V)

(μH)

(kΩ)

2.2

17.4

26.1

604

2.5

2.2

45.3

14.3

1500

3.3

2.2

10.5

2000

Table 3—FSW=500kHz, VIN=12V

VOUT

(V)

(μH)

(kΩ)

17.4

26.1

357

2.5

45.3

14.3

887

3.3

10.5

1200

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