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 2004 Semtech Corp.

www.semtech.com

POWER MANAGEMENT

SC2688

Careful attention to layout requirements are necessary for

successful implementation of the SC2688 PWM control-

ler. High currents switching at 140kHz are present in the

application and their effect on ground plane voltage differ-

entials 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 bot-

tom 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

Layout Guidelines

transition switching. Connections should be as wide and

as short as possible to minimize loop inductance. Mini-

mizing 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.

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

the output inductor should be a wide trace or copper re-

gion. It should be as short as practical. Since this connec-

tion has fast voltage transitions, keeping this connection

short will minimize EMI. The connection between the out-

put inductor and the sense resistor should be a wide trace

or copper area, there are no fast voltage or current transi-

tions in this connection and length is not so important,

however adding unnecessary impedance will reduce effi-

ciency.

Vout

12V IN

3.3V

Vo Lin1

5V

+

10

Cout Lin1

1.00k

Heavy lines indicate

high current paths.

Layout Diagram

SC2688

1

2

3

4

5

6

7

8

13

12

14

15

16

11

10

9

GATE1

LDOS1

LDOS2

VCC

LDOEN

CS-

CS+

DH

GATE2

LDOV

EN

BST

DL

AGND

FB

PGND

Q1

Q2

L

Cin

5mOhm

2.32k

Cout

Q3

Cin Lin

0.1uF

0.1uF

Vo Lin2

Cout Lin2

Q4

SC2688