ISL6532B

FN9120 Rev 3.00

Page 10 of 15

Jul 2004

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 the ISL6532B

switching converter. 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 ground pad, pin 21, 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

ISL6532B first. Minimize the length of the connections between

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 5 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

Modulator Break Frequency Equations

The compensation network consists of the error amplifier

closed loop transfer function with the highest 0dB crossing

degrees. The equations below relate the compensation

locating the poles and zeros of the compensation network:

5VSBY

ISLAND ON POWER PLANE LAYER

ISLAND ON CIRCUIT PLANE LAYER

KEY

COMP

ISL6532B

UGATE

FB

GND

5VSBY

FIGURE 4. PRINTED CIRCUIT BOARD POWER PLANES

AND ISLANDS

VIA CONNECTION TO GROUND PLANE

GND

P12V

LGATE

P5VSBY

VDDQ(2)

VTT(2)

NCH

GND PAD

FLC

1

2

 x LO x CO

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

=

FESR

1

2

 x ESR x CO

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

=