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 2002 Semtech Corp.

www.semtech.com

SC2453

POWER MANAGEMENT

PRELIMINARY

Applications Information (Cont.)

DUAL LDO CONTROLLERS

The SC2453 provides positive and negative adjustable

linear regulator controllers. The positive linear regulator

uses a PNP transistor to regulate output voltage. This is

set by a voltage divider connected from the output to FB

to AGND. Referring to the front page Application Circuit,

select R8 in the 5K

Ω to 20KΩ range. Calculate R7 with

the following equation:













−

=

1

5

.

0

V

R

R

OUT

8

7

The negative linear regulator uses a NPN transistor to

regulate output voltage. This is set by a voltage divider

connected from the output to FB to a positive reference.

Referring to the front page Application Circuit, select R16

in the 5K

Ω to 20KΩ range. Calculate R12 with the fol-

lowing equation:









=

REF

OUT

16

12

V

V

R

R

where V

LAYOUT GUIDELINES

Careful attention to layout requirements are necessary

for successful implementation of the SC2453 PWM con-

troller. High switching current is present in the applica-

tion and their effect on ground plane voltage differen-

tials 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, such as the input capacitor or the

bottom FET ground.

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

Top FET (Q

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.

3). The connection between the junction of Q

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. Also keep the Phase con-

nection to the IC short. The top FET gate charge currents

flow in this trace.

4) The Output Capacitor(s) (Cout) should be located as

close to the load as possible. Fast transient load cur-

rents are supplied by Cout only and connections between

Cout and the load must be short, wide copper areas to

minimize inductance and resistance.

5) The SC2453 is best placed over a quiet ground plane

area. Avoid pulse currents in the Cin, Q

in this area. GND should be returned to the ground plane

close to the package and close to the ground side of

(one of) the output capacitor(s). If this is not possible,

the GND pin may be connected to the ground path be-

tween the Output Capacitor(s) and the Cin, Q1, Q2 loop.

Under no circumstances should GND be returned to a

ground inside the Cin, Q1, Q2 loop.

6) A separate analog ground plane connects to the

SC2453 AGND pin. All analog grounding paths including

decoupling capacitors, feedback resistors, compensation

components, and current-limit setting resistors should

be connected to this plane.

7) Ideally, the grounds for the two LDO sections should

be returned to the ground side of (one of) the output

capacitor(s).