SP7655_07 datasheet(8/16 Pages) SIPEX

8 / 16 page

Rev E: 3/2/07

SP7655 Wide Input Voltage Range 8A, 300kHz, Buck Regulator

Output Capacitor Selection

The required ESR (Equivalent Series Resis-

tance) and capacitance drive the selection of the

type and quantity of the output capacitors. The

ESR must be small enough that both the resis-

tive 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 addi-

tional current demanded by the load until the

SP7655 adjusts the inductor current to the new

value.

In order to maintain VOUT ,the capacitance must

be large enough so that the output voltage is held

up while the inductor current ramps to the value

corresponding to the new load current. Addi-

tionally, the ESR in the output capacitor causes

a step in the output voltage equal to the current.

Because of the fast transient response and inher-

ent 0% to100% duty cycle capability provided

by the SP7655 when exposed to an output load

transient, the output capacitor is typically cho-

sen for ESR, not for capacitance value.

The ESR of the output capacitor, combined with

the inductor ripple current, is typically the main

contributor to output voltage ripple. The maxi-

mum allowable ESR required to maintain a

specified output voltage ripple can be calculated

by:

RESR

VOUT

IPK-PK

where:

VOUT = Peak-to-Peak Output Voltage Ripple

IPK-PK= Peak-to-Peak Inductor Ripple Current

The total output ripple is a combination of the

ESR and the output capacitance value and can

be calculated as follows:

VOUT =

(IPP(1–D))2+(IPPRESR)2

COUTFS

FS= Switching Frequency

D = Duty Cycle

COUT= Output Capacitance Value

Input Capacitor Selection

The input capacitor should be selected for ripple

current rating, capacitance and voltage rating.

The input capacitor must meet the ripple current

requirement imposed by the switching current.

In continuous conduction mode, the source cur-

rent of the high-side MOSFET is approximately

a square wave of duty cycle VOUT/VIN. More

accurately the current wave form is trapezoidal,

given a finite turn-on and turn-off, switch tran-

sition slope. Most of this current is supplied by

the input bypass capacitors. The RMS current

handling capability of the input capacitors is

determined at maximum output current and

under the assumption that the peak-to-peak in-

ductor ripple current is low, it is given by:

CIN(RMS)= I OUT(max) D(1 - D)

The worst case occurs when the duty cycle D is

50% and gives an RMS current value equal to

IOUT/2.

Select input capacitors with adequate ripple

current rating to ensure reliable operation.

The power dissipated in the input capacitor is:

)

(

)

(

CIN

ESR

rms

CIN

This can become a significant part of power

losses in a converter and hurt the overall energy

transfer efficiency. The input voltage ripple

primarily depends on the input capacitor ESR

and capacitance. Ignoring the inductor ripple

current, the input voltage ripple can be deter-

mined by:

APPLICATIONS INFORMATION

)

(

)

(

(max)

)

(

OUT

MAX

OUT

CIN

E SR

out

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