SC418
26
Applications Information (continued)
tor. For a conventional PWM constant-frequency con-
verter, as load increases the duty cycle also increases
slightly to compensate for IR and switching losses in the
MOSFETs and inductor. A adaptive on-time converter
must also compensate for the same losses by increasing
the effective duty cycle (more time is spent drawing
energy from V
IN
as losses increase). Because the on-time is
essentially constant for a given V
OUT
/V
IN
combination, to
offset the losses the off-time will reduce slightly as load
increases. The net effect is that switching frequency
increases slightly with increasing load.
PCB Layout Guidelines
As with any switch-mode converter, good PCB layout is
essential to achieving high performance. The following
guidelines will provide an optimum PCB layout.
The device layout can be considered in four parts.
Grounding for PGND and AGND
Power components
Low-noise analog circuits
Bypass capacitors
Grounding for PGND and AGND
A ground plane layer for PGND is recommended
to minimize the effects of switching noise, resis-
tive losses, and to maximize heat removal from
the power components.
A separate ground plane or island should be used
for AGND and all associated components. The
AGND island should avoid overlapping switching
signals on other layers (DH/DL/BST/LX).
Connect PGND and AGND together with a zero
ohm resistor or copper trace. Make the connec-
tion near the AGND and PGND pins of the IC.
Power Components
Use short, wide connections between the power
components.
Input capacitors and high-side MOSFETs
High-side and Low-side MOSFETs and inductor
(LX connection). Use wide copper traces to
provide high current carrying capacity and for
heat dissipation.
Inductor and output capacitors.
"
"
"
"
"
"
"
"
"
"
All PGND connections the input capacitors,
low-side MOSFETs, output capacitors, and the
PGND pin of the SC48.
An inner layer ground plane is recom-
mended.
Each power component requires a short,
low impedance connection to the PGND
plane.
Place vias to the PGND plane directly near
the component pins.
Use short wide traces for the pin connections
from the SC48 (LX, DH, DL and BST). Do not
route these traces near the sensitive low-noise
analog signals (FB, FBL, TON, V
OUT
).
Low-noise Analog Circuits
Low-noise analog circuits are sensitive circuits that are
referenced to AGND. Due to their high impedance and
sensitivity to noise, it is important that these circuits be
kept away from the switching signals.
Use a plane or solid area for AGND. Place all
components connected to AGND above this
area.
Use short direct traces for the AGND con-
nections to all components.
Place vias to the AGND plane directly near
the component pins.
Proper routing of the V
OUT
sense trace is essential
since it feeds into the FB resistor divider. Noise
on the FB waveform will cause instability and
multiple pulsing.
Connect the V
OUT
sense trace directly to
the output capacitor or a ceramic bypass
capacitor.
Route this trace over to the VOUT pin,
carefully avoiding all switching signals
and power components.
Route this trace in a quiet layer if possible.
Route this trace away from the switch-
ing traces and components, even if the
trace is longer. Avoid shorter trace routing
through the power switching area.
If a bypass capacitor is used at the IC side
of the V
OUT
sense trace, it should be placed
near the FB resistor divider.
"
?/DIV>
?/DIV>
?/DIV>
"
"
?/DIV>
?/DIV>
"
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
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