10

spikes can degrade efficiency, radiate noise into the circuit,

and lead to device over-voltage stress. Careful component

layout and printed circuit board design minimizes these

voltage spikes.

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

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 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

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 5 highlights the voltage-mode control loop for a

synchronous-rectified buck converter. The output voltage

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

The modulator transfer function is the small-signal transfer

peak-to-peak oscillator voltage

Modulator Break Frequency Equations

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

F

LC

1

2

π x L

O

x C

O

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

=

F

ESR

1

2

π x ESR x C

O

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

=

ISL6532B