Feb2-07 Rev O

SP7663 Wide Input Voltage Range 6A, 600kHz Buck Regulator

© 2007 Sipex Corporation

APPLICATIONS INFORMATION

Output Capacitor Selection

The required ESR (Equivalent Series Re-

sistance) and capacitance drive the selec-

tion of the type and quantity of the output

capacitors. The ESR must be small enough

that both the resistive voltage deviation due

to a step change in the load current and

the output ripple voltage do not exceed

the tolerance limits expected on the output

voltage. During an output load transient,

the output capacitor must supply all the ad-

ditional current demanded by the load until

the SP7663 adjusts the inductor current to

the new value.

must be large enough so that the output

voltage is held up while the inductor cur-

rent ramps to the value corresponding

to the new load current. Additionally, the

ESR in the output capacitor causes a step

in the output voltage equal to the current.

Because of the fast transient response and

inherent 00% to 0% duty cycle capability

provided by the SP7663 when exposed to

output load transients, the output capacitor

is typically chosen for ESR, not for capaci-

tance value.

The ESR of the output capacitor, combined

with the inductor ripple current, is typically

the main contributor to output voltage ripple.

The maximum allowable ESR required to

maintain a specifiedoutputvoltageripplecan

be calculated by:

where:

The total output ripple is a combination of

the ESR and the output capacitance value

andcanbecalculatedasfollows:

D = Duty Cycle

Input Capacitor Selection

The input capacitor should be selected for

ripple current rating, capacitance and volt-

age rating. The input capacitor must meet

the ripple current requirement imposed

by the switching current. In continuous

conduction mode, the source current of

thehigh-sideMOSFETisapproximatelya

accurately, the current wave form is trap-

ezoidal,givenafiniteturn-onandturn-off,

switchtransitionslope.Mostofthiscurrent

is supplied by the input bypass capaci-

tors.TheRMScurrenthandlingcapability

of the input capacitors is determined at

maximum output current and under the

assumption that the peak-to-peak inductor

ripplecurrentislow,itisgivenby:

The worst case occurs when the duty cycle

Dis50%andgivesanRMScurrentvalue

adequate ripple current rating to ensure reli-

able operation.

The power dissipated in the input capaci-

toris:

2

CiN(rMS)

Thiscanbecomeasignificantpartofpower

losses in a converter and hurt the overall

energy transfer efficiency. The input volt-

age ripple primarily depends on the input