ISL6521

FN9148 Rev 2.00

Page 10 of 14

Feb 8, 2005

Component Selection Guidelines

Output Capacitor Selection

The output capacitors for each output have unique

requirements. In general, the output capacitors should be

selected to meet the dynamic load regulation requirements.

Additionally, the PWM converters require an output capacitor to

filter the current ripple. The load transient for some embedded

processors requires high quality capacitors to supply the high

slew rate (di/dt) current demands.

PWM Output Capacitors

High performance embedded processors can produce

transient load rates above 1A/ns. 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 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. 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. 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.

Linear Output Capacitors

The output capacitors for the linear regulators provide dynamic

load current. The linear controllers use dominant pole

compensation integrated into the error amplifier and are

insensitive to output capacitor selection. Output capacitors

should be selected for transient load regulation.

PWM Output Inductor Selection

The PWM converter requires an output inductor. The output

inductor is selected to meet the output voltage ripple

requirements and sets the converter’s response time to a load

transient. The inductor value determines the converter’s ripple

current and the ripple voltage is a function of the ripple current.

The ripple voltage and current are approximated by the

following equations:

Increasing the value of inductance reduces the ripple current

and voltage. However, the large inductance values increase

the converter’s response time to a load transient.

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

load transient is the time required to change the inductor

current. Given a sufficiently fast control loop design, the

ISL6521 will provide either 0% or 100% 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 post-transient current level. During this interval the

difference between the inductor current and the transient

current level must be supplied by the output capacitor(s).

Minimizing the response time can minimize the output

capacitance required.

The response time to a transient is different for the application

of load and the removal of load. The following equations give

the approximate response time interval for application and

removal of a transient load:

FIGURE 7. PRINTED CIRCUIT BOARD POWER PLANES AND

ISLANDS

Q1

Q2

Q3

Q4

+12V

VIA CONNECTION TO GROUND PLANE

ISLAND ON POWER PLANE LAYER

ISLAND ON CIRCUIT OR POWER PLANE LAYER

CR1

ISL6521

PGND

LGATE

UGATE

PHASE

DRIVE3

KEY

GND

VCC

DRIVE2

OCSET

DRIVE4

Q5

+

+

+

+

+

I

–



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

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



=



I



ESR



=



–

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

=



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

=