ISL6558IBZ datasheet(13/17 Pages) RENESAS

13 / 17 page

ISL6558

FN9027 Rev 13.00

Page 13 of 17

August 28, 2015

Component Selection Guidelines

OUTPUT CAPACITOR SELECTION

Output capacitors are required to filter the output inductor

current ripple and supply the load transient current. The

filtering requirements are a function of the channel switching

frequency and the output ripple current. The load transient

requirements are a function of the slew rate (di/dt) and the

magnitude of the transient load current. These requirements

are generally met with a mix of capacitors and careful layout.

Some modern microprocessors can produce transient load

rates above 200A/

s. High frequency capacitors are used to

supply the initial transient current and slow the rate-of-change

seen by the bulk capacitors. Bulk filter capacitor values are

generally determined by the ESR and voltage rating

requirements rather than actual capacitance requirements.

High frequency decoupling capacitors should be placed as

close to the power pins of the load as physically possible. Be

careful not to add inductance in the circuit board wiring that

could cancel the usefulness of these low inductance

components. Consult with the manufacturer of the load

device for any specific decoupling requirements.

Specialized low-ESR capacitors intended for switching

regulator applications are recommended for the bulk

capacitors. The bulk capacitor’s ESR determines the output

ripple voltage and the initial voltage drop following a high

slew-rate transient edge. Aluminum electrolytic capacitor ESR

values are related to case size with lower ESR available in

larger case sizes. However, the ESL of these capacitors

increases with case size and can reduce the usefulness of the

capacitor to high slew-rate transient loading. Unfortunately,

ESL is not a specified parameter. Work with your capacitor

supplier and measure the capacitor’s impedance with

frequency to select a suitable component. In most cases,

multiple electrolytic capacitors of small case size perform

better than a single large case capacitor.

OUTPUT INDUCTOR SELECTION

The output inductor is selected to meet the voltage ripple

requirements and minimize the converter response time to a

load transient. In a multi-phase converter topology, the ripple

current of one active channel partially cancels with the other

active channels to reduce the overall ripple current. The

reduction in total output ripple current results in a lower

overall output voltage ripple.

The inductor selected for the power channels determines the

channel ripple current. Increasing the value of inductance

reduces the total output ripple current and total output

voltage ripple. However, increasing the inductance value will

slow the converter response time to a load transient.

One of the parameters limiting the converter’s response time

to a load transient is the time required to slew the inductor

current from its initial current level to the transient current

level. During this interval, the difference between the two

levels must be supplied by the output capacitance.

Minimizing the response time can minimize the output

capacitance required.

The channel ripple current is approximated by the following

equation:

VOUT

+12V

VIA CONNECTION TO GROUND PLANE

ISLAND ON POWER PLANE LAYER

ISLAND ON CIRCUIT PLANE LAYER

LO1

COUT

CIN

+5VIN

PHASE

VCC

USE INDIVIDUAL METAL RUNS

COMP

ISL6558

PWM

RFB

CBP

VSEN

ISEN

RISEN

HIP6601B

CBOOT

CBP

VCC

FS/EN

PVCC

ISOLATE OUTPUT STAGES

FOR EACH CHANNEL TO HELP

PWM

BOOT

ROS

FIGURE 9. PRINTED CIRCUIT BOARD POWER PLANES AND ISLANDS

ICH

VIN VOUT

–

FSWxL

----------------------------------x

VOUT

VIN

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

(EQ. 10)

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