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SC2603A

POWER MANAGEMENT

Applications Information (Cont.)

where Ipeak is the input peak current through the power

MOSFET.

Figure 3. Current Waveforms

Layout Guidelines

Careful attentions to layout requirements are necessary

for successful implementation of the SC2603A PWM con-

troller. High currents switching at 200kHz 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 bot-

tom Schottky ground.

2) The loop formed by the output Capacitor(s) (Cout), the

FET (Q1), the current sensing resistor, and the Schottky (D1)

must be kept as small as possible, as shown on the layout

diagram in Fig.4. This loop contains all the high current, fast

transition switching. Connections should be as wide and

as short as possible to minimize loop inductance. Minimiz-

ing this loop area will reduce EMI, lower ground injection

currents, resulting in electrically “cleaner” grounds for the

rest of the system and minimize source ringing, resulting

t0

t1

t2

t3

t4

OCP

Supply Current Limiting

SS

Normal

Loading

in more reliable gate switching signals.

3) The connection between the junction of Q1, D1 and the

output capacitor 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.

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

close to the load as possible, fast transient load currents

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 SC2603A is best placed over an isolated ground

plane area. The soft-start capacitor, the Vin decoupling ca-

pacitor, and the current sensing filter capacitor should also

connected to this ground pad area. This isolated ground

area should be connected to the main ground by a trace

that runs from the GND pin to the ground side of the output

capacitor. If this is not possible, the GND pin may be con-

nected to the ground path between the Output Capacitor

and the Cin, Q1, D1 loop. Under no circumstances should

GND be returned to a ground inside the Cin, Q1, D1 loop.

6) Input voltage of the SC2603A should be supplied from

the power rail through a 2.2~10Ω resistor, the Vin pin

should be decoupled directly to GND by a 0.1µF~1µF

ceramic capacitor, trace lengths should be as short as

possible.

7) An RC low pass filter is required to filter out leading

edge current spikes on the current sensing resistor. The

filter capacitor needs to be very close to the CS and GND

to reduce noise pickup.