9

FN9055.8

March 20, 2007

When the converter is sinking current, it is behaving as a

boost converter that is regulating its input voltage. This

means that the converter is boosting current into the input

rail of the regulator. If there is nowhere for this current to go,

such as to other distributed loads on the rail or through a

voltage limiting protection device, the capacitance on this rail

will absorb the current. This situation will allow the voltage

level of the input rail to increase. If the voltage level of the rail

is boosted to a level that exceeds the maximum voltage

rating of any components attached to the input rail, then

those components may experience an irreversible failure or

experience stress that may shorten their lifespan. Ensuring

that there is a path for the current to flow other than the

capacitance on the rail will prevent this failure mode.

Application Guidelines

Layout Considerations

Layout is very important in high frequency switching

converter design. With power devices switching efficiently at

300kHz or 600kHz, the resulting current transitions from one

device to another causing voltage spikes across the

interconnecting impedances and parasitic circuit elements.

These voltage spikes can degrade efficiency, radiate noise

into the circuit, and lead to device overvoltage stress.

Careful component layout and printed circuit board design

minimizes the voltage spikes in the converters.

As an example, consider the turn-off transition of the PWM

MOSFET. Prior to turn-off, the MOSFET is carrying the full load

current. During turn-off, current stops flowing in the MOSFET

and is picked up by the lower MOSFET. Any parasitic

inductance in the switched current path generates a large

voltage spike during the switching interval. Careful component

selection, tight layout of the critical components, and short, wide

traces minimizes the magnitude of voltage spikes.

There are two sets of critical components in a DC/DC

converter using the ISL6526, ISL6526A. The switching

components are the most critical because they switch large

amounts of energy, and therefore tend to generate large

amounts of noise. Next are the small signal components

which connect to sensitive nodes or supply critical bypass

current and signal coupling.

A multi-layer printed circuit board is recommended. Figure 4

shows the connections of the critical components in the

represent numerous physical capacitors. Dedicate one solid

layer, usually a middle layer of the PC board, for a ground

plane and make all critical component ground connections

with vias to this layer. Dedicate another solid layer as a

power plane and break this plane into smaller islands of

common voltage levels. Keep the metal runs from the

PHASE terminals to the output inductor short. The power

plane should support the input power and output power

nodes. Use 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 GATE pins to the MOSFET gates should be kept short

and wide enough to easily handle the 1A of drive current.

The switching components should be placed close to the

ISL6526, ISL6526A first. Minimize the length of the connections

placing them nearby. Position both the ceramic and bulk input

capacitors as close to the upper MOSFET drain as possible.

Position the output inductor and output capacitors between the

upper MOSFET and lower MOSFET and the load.

The critical small signal components include any bypass

capacitors, feedback components, and compensation

the VCC pin with a via directly to the ground plane. Place the

PWM converter compensation components close to the FB

and COMP pins. The feedback resistors for both regulators

should also be located as close as possible to the relevant

FB pin with vias tied straight to the ground plane as required.

ISLAND ON POWER PLANE LAYER

ISLAND ON CIRCUIT PLANE LAYER

KEY

COMP

ISL6526, ISL6526A

UGATE

R4

FB

GND

CPVOUT

FIGURE 4. PRINTED CIRCUIT BOARD POWER PLANES

AND ISLANDS

BOOT

VIA CONNECTION TO GROUND PLANE

Q1

PHASE

D1

Q2

LGATE

PHASE

VCC

ISL6526, ISL6526A