NX2415 datasheet(8/21 Pages) MICROSEMI

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NX2415

Rev.4.8

05/06/08

APPLICATION INFORMATION

Symbol Used In Application Information:

VIN

- Input voltage

VOUT

- Output voltage

IOUT

- Output current

DVRIPPLE - Output voltage ripple

- Operation frequency for each channel

DIRIPPLE - Inductor current ripple

Design Example

The following is typical application for NX2415.

VIN = 12V

VOUT=1.2V

IOUT=50A

IOUT_max=60A

DVRIPPLE <=12mV

DVDROOP<=120mV @30A step

FS=400kHz

Phase number N=2

Output Inductor Selection

The selection of inductor value is based on induc-

tor ripple current, power rating, working frequency and

efficiency. Larger inductor value normally means smaller

ripple current. However if the inductance is chosen too

large, it brings slow response and lower efficiency. Usu-

ally the ripple current ranges from 20% to 40% of the

output current. This is a design freedom which can be

decided by design engineer according to various appli-

cation requirements. The inductor value can be calcu-

lated by using the following equations:

OUT

RIPPLE

INS

OUTPUT

RIPPLE

V -VV

××

∆

∆×

...(1)

where k is between 0.2 to 0.4.

Select k=0.2, then

OUT

12V-1.2V

1.2V1

50A

12V

400kHz

0.2

××

OUT

=0.54uH

Choose inductor from Vishay IHLP_5050FD-01

with L=0.68uH DCR=1.4m

Ω.

Current Ripple is recalculated as

OUT

RIPPLE

OUT

INS

V -VV

12V-1.2V 1.2V1

3.97A

0.68uH

12V

400kHz

∆

××

×=

...(2)

Output Capacitor Selection

Output capacitor value is basically decided by the

output voltage ripple, capacitor RMS current rating and

load transient.

Based on Voltage Ripple

For electrolytic, POSCAP bulk capacitor, the ESR

(equivalent series resistance) and inductor current typi-

cally determines the output voltage ripple.

RIPPLE

desire

RIPPLE

12mV

ESR

3.022m

3.97A

∆

=Ω

∆

...(3)

If low ESR is required, for most applications, mul-

tiple capacitors in parallel are better than a big capaci-

tor. For example, for 12mV output ripple, SANYO OS-

CON capacitors 2R5SEPC1000MX(1000uF 7m

Ω) are

chosen.

R IPPLE

ES RI

×∆

∆

...(4)

Number of Capacitor is calculated as

3.97A

12mV

Ω×

N =2.3

For ceramic capacitor, the current ripple is deter-

mined by the number of capacitor instead of ESR

RIPPLE

OUT

RIPPLE

8 FV

∆

× ×∆

...(5)

Typically, the calculated capacitance is so small

that the output voltage droop during the transient can

not meet the spec although ripple is small.

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