ISL6553

FN4931 Rev 1.00

Page 13 of 15

August 2004

Component Selection Guidelines

Output Capacitor Selection

The output capacitor is selected to meet both the dynamic load

requirements and the voltage ripple requirements. The load

transient for the microprocessor CORE is characterized by

high slew rate (di/dt) current demands. In general, multiple

high quality capacitors of different size and dielectric are

paralleled to meet the design constraints.

Modern microprocessors produce severe transient load rates.

High frequency capacitors supply the initially transient current

and slow the load rate-of-change seen by the bulk capacitors.

The bulk filter capacitor values are generally determined by the

ESR (effective series resistance) 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 on

specific decoupling requirements.

Use only specialized low-ESR capacitors intended for

switching-regulator applications 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’s

edge. In most cases, multiple capacitors of small case size

perform better than a single large case capacitor.

Bulk capacitor choices include aluminum electrolytic, OS-Con,

Tantalum and even ceramic dielectrics. An aluminum

electrolytic capacitor’s ESR value is related to the case size

with lower ESR available in larger case sizes. However, the

equivalent series inductance (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. Consult the capacitor

manufacturer and measure the capacitor’s impedance with

frequency to select a suitable component.

Output Inductor Selection

One of the parameters limiting the converter’s response to a

load transient is the time required to change the inductor

current. Small inductors in a multi-phase converter reduces the

response time without significant increases in total ripple

current.

The output inductor of each power channel controls the ripple

current. The control IC is stable for channel ripple current

(peak-to-peak) up to twice the average current. A single

channel’s ripple current is approximately:

The current from multiple channels tend to cancel each other

and reduce the total ripple current. Figure 12 gives the total

ripple current as a function of duty cycle, normalized to the

parameter

at zero duty cycle. To determine the

total ripple current from the number of channels and the duty

cycle, multiply the y-axis value by

.

Small values of output inductance can cause excessive power

dissipation. The ISL6553 is designed for stable operation for

ripple currents up to twice the load current. However, for this

condition, the RMS current is 115% above the value shown in

the following MOSFET Selection and Considerations section.

With all else fixed, decreasing the inductance could increase the

power dissipated in the MOSFETs by 30%.

I

–

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

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



=

Vo







Vo





+12V

VIA CONNECTION TO GROUND PLANE

ISLAND ON POWER PLANE LAYER

ISLAND ON CIRCUIT PLANE LAYER

KEY

PHASE

VCC

USE INDIVIDUAL METAL RUNS

COMP

ISL6553

PWM

FB

VSEN

ISEN

HIP6601

VCC

FS/DIS

PVCC

LOCATE NEXT TO IC PIN

LOCATE NEXT

TO FB PIN

LOCATE NEXT TO IC PIN(S)

ISOLATE OUTPUT STAGES

FOR EACH CHANNEL TO HELP

LOCATE NEAR TRANSISTOR

FIGURE 11. PRINTED CIRCUIT BOARD POWER PLANES AND ISLANDS