NX2124ACSTR datasheet(6/17 Pages) MICROSEMI

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NX2124/2124A

Rev.1.8

02/28/08

APPLICATION INFORMATION

Symbol Used In Application Information:

VIN

- Input voltage

VOUT

- Output voltage

IOUT

- Output current

DVRIPPLE - Output voltage ripple

- Working frequency

DIRIPPLE - Inductor current ripple

Design Example

The following is typical application for NX2124, the

schematic is figure 1.

VIN = 5V

VOUT=1.8V

FS=300kHz

IOUT=9A

DVRIPPLE <=20mV

DVDROOP<=100mV @ 9A step

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

RIPPLE

OUTPUT

V -VV

=kI

××

∆

...(1)

where k is between 0.2 to 0.4.

Select k=0.3, then

××

OUT

5V-1.8V

1.8V1

0.3 9A

300kHz

=1.4uH

Choose inductor from COILCRAFT DO5010P-

152HC with L=1.5uH is a good choice.

Current Ripple is recalculated as

∆

××

×=

OUT

RIPPLE

OUT

INS

V -VV

5V-1.8V

1.8v1

2.56A

1.5uH

300kHz

...(2)

Output Capacitor Selection

Output capacitor is basically decided by the

amount of the output voltage ripple allowed during steady

state(DC) load condition as well as specification for the

load transient. The optimum design may require a couple

of iterations to satisfy both condition.

Based on DC Load Condition

The amount of voltage ripple during the DC load

condition is determined by equation(3).

∆

× ∆+

××

RIPPLE

OUT

ESRI

8 FC

...(3)

Where ESR is the output capacitors' equivalent

series resistance,C

OUT is the value of output capacitors.

Typically when large value capacitors are selected

such as Aluminum Electrolytic,POSCAP and OSCON

types are used, the amount of the output voltage ripple

is dominated by the first term in equation(3) and the

second term can be neglected.

For this example,electrolytic capacitors are cho-

sen as output capacitors, the ESR and inductor current

typically determines the output voltage ripple.

∆

=Ω

∆

RIPPLE

desire

RIPPLE

20mV

ESR

7.8m

2.56A

...(4)

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

tiple capacitors in parallel are better than a big capaci-

tor. For example, SANYO electrolytic capacitor

16ME1500WG is chosen.

R IPPLE

ES RI

×∆

∆

...(5)

Number of Capacitor is calculated as

13m

2.56A

20mV

Ω×

N =1.7

The number of capacitor has to be round up to a

integer. Choose N =2.

If ceramic capacitors are chosen as output ca

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