NX2116/2116A/2116B/2117/2117A

7

Rev. 3.0

03/14/06

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 capacitor

with 2m

∆

=

Ω×+

××

=

RIPPLE

2.56A

V

2m

2.55A

8 600kHz 100uF

10.4mV

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

V

<V

∆∆

@ step load

DI

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

DI

STEP

transient load, if assuming the bandwidth of system is

high enough, the overshoot can be estimated as the fol-

lowing equation.

2

OUT

overshoot

step

OUT

V

V

ESRI

2LC

∆

=

× ∆

+

×τ

××

...(6)

where

crit

step

OUT

crit

OUT

0

ifLL

LI

ESR C

ifLL

V

≤



 ×∆

τ= 

−

×≥





...(7)

where

OUT

OUT

EE

OUT

crit

step

step

ESR C

V

ESRCV

L

II

×

×

××

==

∆∆

...(8)

where ESR

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

LL

≤

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

E

step

2

OUT

tran

E

tran

ESRI

V

N

V

2L CV

×∆

=

+

×τ

∆

× ×

×∆

...(9)

where

crit

step

E

E

crit

OUT

0

ifLL

LI

ESR

C

ifLL

V

≤



 ×∆

τ= 

−

×≥





...(10)

For example, assume voltage droop during tran-

sient is 100mV for 9A load step.

If the POSCAP 2R5TPE220MC(220uF, 12m

Ω ) is

used, the critical inductance is given as

EE

OUT

crit

step

ESRCV

L

I

12m

220 F 1.8V

0.56H

9A

××

==

∆

Ω×

µ×

=µ

The selected inductor is 1uH 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

EE

OUT

LI

ESRC

V

1 H

9A

12m

220 F

2.36us

1.8V

×∆

τ =

−×

µ×

=

−

Ω ×

µ=

E

step

2

OUT

tran

E

tran

2

ESRI

V

N

V

2L CV

12m

9A

100mV

1.8V

(2.36us)

2 1 H 220 F 100mV

1.3

×∆

=

+

×τ

∆

× ×

×∆

Ω×

=+

×

× µ ×

µ×

=

The number of capacitors has to satisfied both ripple

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