11

FN9188.1

February 9, 2005

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 2

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.

LDO, the ground pad, pin 29, should be connected to the

internal ground plane through at least four vias. This allows

the heat to move away from the IC and also ties the pad to

the ground plane through a low impedance path.

The switching components should be placed close to the

ISL6548 first. Minimize the length of the connections

by 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 and lower MOSFETs and the load.

The critical small signal components include any bypass

capacitors, feedback components, and compensation

components. Place the PWM converter compensation

components close to the FB and COMP pins. The feedback

resistors should be located as close as possible to the FB

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

Feedback Compensation - PWM Buck Converter

Figure 3 highlights the voltage-mode control loop for a

synchronous-rectified buck converter. The output voltage

triangular wave to provide a pulse-width modulated (PWM)

The modulator transfer function is the small-signal transfer

peak-to-peak oscillator voltage

5VSBY

ISLAND ON POWER PLANE LAYER

ISLAND ON CIRCUIT AND/OR POWER PLANE LAYER

5VDUAL

KEY

COMP

ISL6548

UGATE

FB

DRIVE3

5VSBY

FIGURE 2. PRINTED CIRCUIT BOARD POWER PLANES

AND ISLANDS

FB3

VIA CONNECTION TO GROUND PLANE

PHASE

GNDP

P12V

LGATE

VDDQ(2)

VTT(2)

GND PAD

DRIVE2_U

FB2

DRIVE4

FB4

REFADJ4

DRIVE2_L

ISL6548