FN9092 Rev 2.00

Page 20 of 22

Dec 29, 2004

ISL6569A

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Layout Considerations

The following multi-layer printed circuit board layout

strategies minimize the impact of board parasitics on

converter performance. The following sections highlight

some important practices which should not be overlooked

during the layout process.

Component Placement

Within the allotted implementation area, orient the switching

components first. The switching components are the most

critical because they switch large amounts of energy and

tend to generate large amounts of noise. How the switching

components are placed should also take into account power

dissipation. Align the output inductors and MOSFETs such

that space between the components is minimized while

creating the PHASE plane. Place the Intersil HIP660X

drivers as close as possible to the MOSFETs they control to

reduce the parasitics due to trace length between critical

driver input and output signals. If possible, duplicate the

same placement of switching components for each phase.

Next, place the input and output capacitors. Position one

high-frequency ceramic input capacitor next to each upper

MOSFET drain. Place the bulk input capacitors as close to

the upper MOSFET drains as dictated by the component

size and dimensions. Long distances between input

capacitors and MOSFET drains results in too much trace

inductance and a reduction in capacitor performance. Locate

the output capacitors between the inductors and the load,

while keeping them in close proximity around the

microprocessor socket.

The ISL6569A can be placed off to one side or centered

relative to the individual phase switching components.

Routing of sense lines and PWM signals will guide final

placement. Critical small signal components to place close

feedback resistor, and compensation components.

Bypass capacitors for the ISL6569A and HIP660X driver

bias supplies must be placed next to their respective pins.

Stray trace parasitics will reduce their effectiveness.

Plane Allocation and Routing

Dedicate one solid layer, usually a middle layer, for a ground

plane. Make all critical component ground connections with

vias to this plane. Dedicate one additional layer for power

planes; breaking the plane up into smaller islands of

common voltage. Use the remaining layers for small signal

wiring.

Route PHASE planes of copper filled polygons on the top

and bottom once the switching component placement is set.

Size the trace width between the driver gate pins and the

MOSFET gates to carry 1A of current. When routing

components in the switching path, use short wide traces to

reduce the associated parasitics.

0.3

0.1

0

0.2

FIGURE 16. NORMALIZED INPUT-CAPACITOR RMS

CURRENT vs DUTY CYCLE FOR 2-PHASE

CONVERTER

00.4

1.0

0.2

0.6

0.8