AOZ125
Page 10 of 10 Rev. 1.1 March 2012
www.aosmd.com
PCB Layout Consideration
As with all high current switching buck regulators, circuit
board layout is the key in preventing inductive
overshoots and undershoots transient spikes. There are
essentially two high pulsing current loops in a
synchronous buck converter. The first switching loop is
formed when the high-side MOSFET is switched on.
The circulating current flows from the input capacitors,
to the filter inductor, to the output capacitors (load),
then returns to the input capacitor through power
ground. It is imperative that a low inductive ceramic
capacitor be placed directly across VIN (PIN2) and
PGND (PIN1) to reduce the switching loop during the
high side turn on transition.
The second loop is formed when the low-side MOSFET
is switched on during synchronous operation. This loop
is formed from the inductor, to the output capacitors
(load), then returns through the low-side MOSFET. The
PCB layout for the AOZ1252 is shown below. It can be
seen that the input bypassing capacitors are placed in
close proximity to VIN and PGND. Analog ground and
power grounds are separated to filter out noise from the
main power switching loops. The sensitive nodes that
require connection through analog ground (AGND) are
COMP, FB, and VSS.
PCB guideline that can be helpful:
1. The exposed (LX) node serves as the interconnect
of both high-side and low-side MOSFETs. It is
suggested that a large copper plane added
underneath the IC to improve thermal dissipation.
2. To improve high-side thermal dissipation, maximize
the copper area connected to the VIN pin.
3. The input capacitor should be connected in close
proximity to the VIN pin and the PGND pins.
4. A ground plane is strongly recommended. Separate
PGND from AGND and connect them at a single
point to avoid noise coupling.
5. Form a short wide trace from LX to the output
inductor and capacitors.
6. The LX switching node is the noisiest of all pins;
therefore, all sensitive nodes must be far away from
this pin.
Not Recommended For New Designs
