ISL81601 datasheet(48/54 Pages) RENESAS

48 / 54 page

FN9299 Rev.3.1

Page 48 of 53

May 27, 2021

ISL81601

8. Component Selection Guideline

8.2

Inductor Selection

The inductor is selected to meet the output voltage ripple requirements. The inductor value determines the

converter’s ripple current and the ripple voltage is a function of the ripple current and the output capacitor(s) ESR.

The ripple voltage expression is given in the capacitor selection section and the ripple current is approximated by

Equation 28 for Buck mode and Equation 29 for Boost mode.

The ripple current ratio is usually 30% to 70% of the inductor average current at the full output load condition.

8.3

Output Capacitor Selection

In general, select the output capacitors to meet the dynamic regulation requirements including ripple voltage and

load transients. Selection of output capacitors is also dependent on the inductor, so some inductor analysis is

required to select the output capacitors.

One of the parameters limiting the converter’s response to a load transient is the time required for the inductor

current to slew to its new level. The ISL81601 provides either 0% or maximum duty cycle in response to a load

transient.

The response time is the time interval required to slew the inductor current from an initial current value to the load

current level. During this interval, the difference between the inductor current and the transient current level must

be supplied by the output capacitor(s). The output capacitance can be minimized if faster loop compensation is

used. Also, if the load transient rise time is slower than the inductor response time, as in a hard drive or CD drive, it

reduces the requirement on the output capacitor.

The maximum capacitor value required to provide the full, rising step, transient load current during the response

time of the inductor is shown in Equation 30 for Buck mode and Equation 31 for Boost mode:

where COUT is the output capacitor(s) required, L is the inductor, ITRAN is the transient load current step, VIN is the

input voltage, VOUT is output voltage, and DVOUT is the drop in output voltage allowed during the load transient.

High frequency capacitors initially supply the transient current and slow the load rate of change seen by the bulk

capacitors. The bulk filter capacitor values are generally determined by the Equivalent Series Resistance (ESR) and

voltage rating requirements as well as actual capacitance requirements.

In Buck mode, the output voltage ripple is due to the inductor ripple current and the ESR of the output capacitors as

defined by Equation 32:

where

ILBuck is calculated in Equation 28.

I

LBuck

VIN VOUT

–

 V

OUT



fSW

 L

 V



----------------------------------------------------------

(EQ. 28)

I

LBoost

VOUT VIN

–

 V



fSW

 L

 V

OUT



----------------------------------------------------

(EQ. 29)

COUTBuck

 I

TRAN



2VIN VOUT

–

 DV

OUT



------------------------------------------------------------------

(EQ. 30)

COUTBoost

 V

OUT

 I

TRAN



2VIN

 DV

OUT



-------------------------------------------------------

(EQ. 31)

VRIPPLE

I

LBuck ESR



(EQ. 32)

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