7

 2004 Semtech Corp.

www.semtech.com

POWER MANAGEMENT

SC2618

Layout Guidelines

Careful attention to layout requirements are necessary

for successful implementation of the SC2618 controller.

High currents switching at up to 500kHz are present in

the application and their effect on ground plane voltage

differentials must be understood and minimized.

1). The high power parts of the circuit should be laid out

first. A ground plane should be used, the number and

position of ground plane interruptions should be such as

to not unnecessarily compromise ground plane integrity.

Isolated or semi-isolated areas of the ground plane may

be deliberately introduced to constrain ground currents

to particular areas, for example the input capacitor and

bottom FET ground.

2). The loop formed by the Input Capacitor(s) (Cin), the

Top FET (Q1) and the Bottom FET (Q2) must be kept as

small as possible. This loop contains all the high current,

fast transition switching. Connections should be as wide

and as short as possible to minimize loop inductance.

Minimizing this loop area will a) reduce EMI, b) lower

ground injection currents, resulting in electrically “cleaner”

grounds for the rest of the system and c) minimize source

ringing, resulting in more reliable gate switching signals.

Cin should consist of the bulk input capacitors and must

be supplemented with one or more ceramic decoupling

capacitors as close as possible to the FETs

3). The connection between the junction of Q1, Q2 and

the output inductor should be a wide trace or copper

region. It should be as short as practical. Since this con-

nection has fast voltage transitions, keeping this con-

nection short will minimize EMI. The connection between

the output inductor and the output capacitors should be

a wide trace or copper area, there are no fast voltage or

current transitions in this connection and length is not

so important, however adding unnecessary impedance

will reduce efficiency.

L

Cout

10uF

U1

SC2618

5

6

4

1

3

2

VCC

DH

BST

FB

DL

GND

Q1

10

VPWR IN

Vout

GND

0.1uF

Cin

Q2

VBST IN

VCC IN