NX2142ACUTR datasheet(10/18 Pages) MICROSEMI

10 / 18 page

NX2142/2142A

Rev. 1.1

10/28/07

If ceramic capacitors are chosen as output ca-

pacitors, both terms in equation (3) need to be evalu-

ated to determine the overall ripple. Usually when this

type of capacitors are selected, the amount of capaci-

tance per single unit is not sufficient to meet the tran-

sient specification, which results in parallel configura-

tion of multiple capacitors.

For example, one 100uF, X5R ceramic capaci-

tor

with 2m

Ω ESR is used. The amount of output ripple

RIPPLE

0.919A

8 600kHz 100uF

1.838mV 1.9mV

3.738mV

∆

Ω×+

××

One ceramic capacitors are needed. Although

this can meet DC ripple spec, however it needs to be

studied for transient requirement.

Based On Transient Requirement

Typically, the output voltage droop during tran-

sient is specified as

∆V

droop

∆V

tran

@step load

STEP

During the transient, the voltage droop during

the transient is composed of two sections. One sec-

tion is dependent on the ESR of capacitor, the other

section is

a function of the inductor, output capacitance as well

as input, output voltage. For example, for the

overshoot when load from high load to light load

with a

STEP transient load, if assuming the band-

width of system is high enough, the overshoot can

be estimated as the following equation.

OUT

overshoot

step

OUT

ESRI

2LC

∆

× ∆

×τ

××

...(6)

where

τ is the a function of capacitor,etc.

crit

step

OUT

crit

OUT

ifLL

ESR C

ifLL

≤



 ×∆

τ= 

−

×≥





...(7)

where

OUT

crit

step

ESR C

ESRCV

××

∆∆

...(8)

where ESR

E and CE represents ESR and capaci-

tance of each capacitor if multiple capacitors are used

in parallel.

The above equation shows that if the selected

output inductor is smaller than the critical inductance,

the voltage droop or overshoot is only dependent on

the ESR of output capacitor.

For low frequency ca-

pacitor such as electrolytic capacitor, the product of

ESR and capacitance is high and

crit

≤

is true. In

that case, the transient spec is mostly like to depen-

dent on the ESR of capacitor.

Most case, the output capacitor is multiple ca-

pacitor in parallel. The number of capacitor can be cal-

culated by the following

step

OUT

tran

ESRI

2L CV

×∆

×τ

∆

× ×

×∆

...(9)

where

crit

step

crit

OUT

ifLL

ESR

ifLL

≤



 ×∆

τ= 

−

×≥





...(10)

For example, assume voltage droop during tran-

sient is 150mV for 1.5A load step.

If the Electrolytic 6ME1000WG(1000uF, 30mohm

ESR) is used, the crticial inductance is given as

OUT

crit

step

ESRCV

30m

1000 F 5V

100H

1.5A

××

∆

Ω×

µ×

=µ

The selected inductor is 6.8uH which is much

smaller than critical inductance. In that case, the out-

put voltage transient not only dependent on the ESR,

but also capacitance.

number of capacitor is

step

tran

ESRI

30m

1.5A

200mV

0.225

×∆

∆

Ω×

The number of capacitors has to satisfy both ripple

and transient requirement. Overall, we choose N=1.

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