SC1103 Semtech Corporation, SC1103 Datasheet - Page 5

• Manufacturer Part Number: SC1103
• Description: VOLTAGE MODE PWM CONTROLLER
• Manufacturer: Semtech Corporation

© 2000 SEMTECH CORP.
PRELIMINARY - February 29, 2000
LAYOUT GUIDELINES
Careful attention to layout requirements are necessary
for successful implementation of the SC1103 PWM
controller. High currents switching at 200kHz 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 integrity. Isolated or semi-isolated areas of the
ground plane may be deliberately introduced to con-
strain ground currents to particular areas, for example
the input capacitor and bottom Schottky ground.
2). The loop formed by the Input Capacitor(s) (Cin),
the Top FET (Q1) and the Schottky (D1) 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 reduce EMI,
lower ground injection currents, resulting in electrically
“cleaner” grounds for the rest of the system and mini-
mize source ringing, resulting in more reliable gate
switching signals.
3). The connection between the junction of Q1, D1
Fig. 7 Layout diagram for the SC1103
10
Heavy lines indicate
high current paths.
0.1uF
1
3
2
4
SC1103CS
VCC
CS(-)
CS(+)
PGND
0.1uF
GND
BST
DH
FB
8
7
6
5
24V IN
VOLTAGE MODE PWM CONTROLLER
Rb
Ra
Q1
D1
Cin
and the output inductor should be a wide trace or cop-
per region. It should be as short as practical. Since
this connection has fast voltage transitions, keeping
this connection short will minimize EMI. The connec-
tion between the output inductor and the sense resis-
tor 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 unnec-
essary impedance will reduce efficiency.
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 SC1103 is best placed over an isolated ground
plane area. GND and PGND should be returned to this
isolated ground. 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 (one of) the
output capacitor(s). If this is not possible, the GND pin
may be connected to the ground path between the
Output Capacitor(s) and the Cin, Q1, D1 loop. Under
no circumstances should GND be returned to a ground
inside the Cin, Q1, D1 loop.
6) Vcc for the SC1103 should be supplied from the
VIN supply through a 10 resistor, the Vcc pin should
12V
+
4uH
1.00k
652 MITCHELL ROAD NEWBURY PARK CA
5mOhm
2.32k
Cout
+
Vout
SC1103
5