PROGRAMMABLE SYNCHRONOUS DC/DC

CONVERTER, DUAL LOW DROPOUT

REGULATOR CONTROLLER

© 1999 SEMTECH CORP.

PRELIMINARY - January 6, 1999

652 MITCHELL ROAD NEWBURY PARK CA 91320

SC1185

SC1185A

8

LAYOUT GUIDELINES

Careful attention to layout requirements are necessary

for successful implementation of the SC1185 PWM

controller. High currents switching at 140kHz are pre-

sent in the application and their effect on ground plane

voltage differentials must be understood and mini-

mized.

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 in-

tegrity. Isolated or semi-isolated areas of the ground

plane may be deliberately introduced to constrain

ground currents to particular areas, for example the in-

put 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 cur-

rent, fast transition switching. Connections should be as

wide and as short as possible to minimize loop induc-

tance. 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) mini-

mize 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

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

connection has fast voltage transitions, keeping this

connection short will minimize EMI. The connection be-

tween the output inductor and the sense resistor should

be a wide trace or copper area, there are no fast volt-

age or current transitions in this connection and length

is not so important, however adding unnecessary

impedance will reduce efficiency.

Vout

12V IN

3.3V

Vo Lin1

Vo Lin2

5V

4uH

5mOhm

+

Cout

+

Cin

10

Q2

0.1uF

Q3

+

Cout Lin1

2.32k

Q4

+

Cout Lin2

1.00k

+

Cin Lin

SC1185

AGND

1

VCC

5

REF

6

PWRGOOD

7

CS-

8

CS+

9

PGNDH

10

DH

11

BSTH

15

EN

16

VOSENSE

17

VID4

18

VID3

19

VID2

20

VID1

21

VID0

22

DL

13

PGNDL

12

BSTL

14

GATE2

24

GATE1

2

LDVO

23

LDOS1

3

LDOS2

4

Q1

0.1uF

Heavy lines indicate

high current paths.

Layout diagram for the SC1185