NX2142ACUTR datasheet(9/18 Pages) MICROSEMI

9 / 18 page

NX2142/2142A

Rev. 1.1

10/28/07

APPLICATION INFORMATION

Symbol Used In Application Information:

VIN

- Input voltage

VOUT

- Output voltage

IOUT

- Output current

DVRIPPLE - Output voltage ripple

- Switching frequency

DIRIPPLE

- Inductor current ripple

Design Example

Power stage design requirements:

VINMIN=8V

VINMAX=20V

VOUT=5V

IOUT =3A

DVRIPPLE <=50mV

DVTRAN<=150mV @ 1.5A step

FS=600kHz

Output Inductor Selection

The selection of inductor value is based on in-

ductor 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. Usually 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 application requirements. The inductor value

can be calculated by using the following equations:

INMAX

OUT

RIPPLE

INMAXS

RIPPLE

OUTPUT

-VV

=kI

××

...(1)

where k is between 0.2 to 0.4.

Select k=0.3, then

OUT

20V-5V

5V1

0.3 3A

20V

600kHz

=6.9uH

××

Choose LOUT=6.8uH, then coilcraft inductor

DO3316P-682HC is a good choice.

Current Ripple is calculated as

OUT

RIPPLE

OUT

INS

V -VV

20V-5V

5V1

0.919A

6.8uH

20V

600kHz

××

×=

...(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 specifica-

tion 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 capaci-

tors.

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, Aluminum Electrolytic is cho-

sen as output capacitor, the ESR and inductor current

typically determines the output voltage ripple.

RIPPLE

desire

RIPPLE

50mV

ESR

54m

0.919A

∆

=Ω

∆

...(4)

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

tiple capacitors in parallel are better than a big capaci-

tor. For example, for 50mV output ripple, Electrolytic

6ME1000WG with 30m

Ω are chosen.

R IPPLE

ES RI

×∆

∆

...(5)

Number of Capacitor is calculated as

30m

0.919A

50mV

Ω×

N =0.55

The number of capacitor has to be round up to a

integer. Choose N =1.

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