NX2139A datasheet(12/21 Pages) MICROSEMI

12 / 21 page

NX2139A

Rev. 2.3

03/19/09

and the second term can be neglected.

For this example, one POSCAP 2R5TPE330MC

is chosen as output capacitor, the ESR and inductor

current typically determines the output voltage ripple.

When VIN reach maximum voltage, the output volt-

age ripple is in the worst case.

RIPPLE

desire

RIPPLE

60mV

ESR

12m

∆

=Ω

∆

...(6)

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

tiple capacitors in parallel are needed. The number of

output capacitor can be calculate as the following:

R IPPLE

ES RI

×∆

∆

...(7)

12m

60mV

Ω×

N =1

The number of capacitor has to be round up to a

integer. Choose N =1.

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 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 over-

shoot when load from high load to light load with a

STEP transient load, if assuming the bandwidth of sys-

tem is high enough, the overshoot can be estimated

as the following equation.

OUT

overshoot

step

OUT

ESRI

2LC

∆

× ∆

×τ

××

...(8)

where

τ is the a function of capacitor,etc.

crit

step

OUT

crit

OUT

ifLL

ESR C

ifLL

≤



 ×∆

τ= 

−

×≥





...(9

where

OUT

crit

step

ESR C

ESRCV

××

∆∆

...(10)

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

∆

× ×

×∆

...(11)

where

crit

step

crit

OUT

ifLL

ESR

ifLL

≤



 ×∆

τ= 

−

×≥





...(12)

For example, assume voltage droop during tran-

sient is 60mV for 3A load step.

If one POSCAP 2R5TPE330MC(330uF, 12mohm

ESR) is used, the crticial inductance is given as

OUT

crit

step

ESRCV

12m

3300 F 1.8V

23.76H

××

∆

Ω×

µ×

=µ

The selected inductor is 1.5uH which is smaller

than critical inductance. In that case, the output volt-

age transient mainly dependent on the ESR.

number of capacitor is

step

tran

ESRI

12m

60mV

0.6

×∆

∆

Ω×

Choose N=1.

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