ISL6554

FN9003 Rev 3.00

Page 13 of 16

February 11, 2005

copper filled polygons on the top and bottom circuit layers for

the phase nodes. Use the remaining printed circuit layers for

small signal wiring. The wiring traces from the driver IC to the

MOSFET gate and source should be sized to carry at least

one ampere of current.

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 ISL6554 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%.

50

100

10

20

200

500 1,000

5,000 10,000

2,000

1

2

5

10

20

50

100

200

500

1,000

I

–



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

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



=

Vo

 LxF

SW





Vo

 LxF

SW





1.0

0.8

0.6

0.4

0.2

0

0

0.1

0.2

0.3

0.4

0.5

SINGLE

CHANNEL

2 CHANNEL

3 CHANNEL

4 CHANNEL

FIGURE 11. RIPPLE CURRENT vs DUTY CYCLE