NX2114CSTR datasheet(9/20 Pages) MICROSEMI

9 / 20 page

NX2114/2114A

Rev. 4.0

06/20/06

If ceramic capacitors are chosen as output ca-

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

to determine the overall ripple. Usually when this type of

capacitors are selected, the amount of capacitance per

single unit is not sufficient to meet the transient specifi-

cation, which results in parallel configuration of multiple

capacitors.

For example, one 100uF, X5R ceramic capacitor

with 2m

Ω ESR is used. The amount of output ripple is

∆

Ω×+

××

=+=

RIPPLE

2.3A

8 300kHz 100uF

4.6mV 9.6mV

13.2mV

Although this meets DC ripple spec, however it

needs to be studied for transient requirement.

Based On Transient Requirement

Typically, the output voltage droop during transient

is specified as:

DROOP

TRAN

∆∆

@ step load

STEP

During the transient, the voltage droop during the

transient is composed of two sections. One Section 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 tran-

sient load, if assuming the bandwidth 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 out-

put inductor is smaller than the critical inductance, the

voltage droop or overshoot is only dependent on the ESR

of output capacitor. For low frequency capacitor such

as electrolytic capacitor, the product of ESR and ca-

pacitance is high and

crit

≤

is true. In that case, the

transient spec is dependent on the ESR of capacitor.

In most cases, the output capacitors are multiple

capacitors in parallel. The number of capacitors can be

calculated 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 100mV for 6A load step.

If the POSCAP 2R5TPE220MC (220uF, 12m

Ω ) is

used, the critical inductance is given as

OUT

crit

step

ESRCV

15m

220 F 1.6V

0.88H

××

∆

Ω×

µ×

=µ

The selected inductor is 1.5uH which is bigger than

critical inductance. In that case, the output voltage tran-

sient not only dependent on the ESR, but also capaci-

tance.

number of capacitors is

step

OUT

ESRC

1.5 H 6A

15m

220 F

2.3us

1.6V

×∆

τ =

−×

µ×

−

Ω ×

µ=

step

OUT

tran

ESRI

2L CV

15m

60mV

1.6V

2.3us

2 1.5 H 220 F 60mV

1.7

×∆

×τ

∆

× ×

×∆

Ω×

× µ ×

µ×

The number of capacitors has to satisfied both ripple

and transient requirement. Overall, we can choose N=2.

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