21
2004 Semtech Corp.
www.semtech.com
SC1175
POWER MANAGEMENT
Power and signal traces must be kept separated for
noise considerations. Feedback, current sense traces
and analog ground should not cross any traces or
planes carrying high switching currents, such as the
input loop or the phase node.
The input loop, consisting of the input capacitors and
both MOSFETs must be kept as small as possible. All of
the high switching currents occur in this loop. The
enclosed loop area must be kept small to minimize
inductance and radiated and conducted emissions.
Designing for minimum trace length is not always the
best approach, often a more optimum layout can be
achieved by keeping loop area constraints in mind.
It is important to keep gate lengths short, the IC must
be close to the power switches. This is more difficult in
a dual channel device than a single and requires that
the two power paths run on either side of a centrally
located controller.
Grounding requirements are always conflicting in a
buck converter, especially at high power, and the trick
is to achieve the best compromise. Power ground
(PGND) should be returned to the bottom MOSFET
source to provide the best gate current return path.
Analog ground (GND) should be returned to the ground
side of the output capacitors so that the analog
circuitry in the controller has an electrically quiet
reference and to provide the greatest feedback
accuracy. The problem is that the differential voltage
capability of the two IC grounds is limited to about 1V
for proper operation and so the physical separation
between the two grounds must also be minimized. If
the grounds are too far apart, fast current transitions
in the connection can generate voltage spikes exceed-
ing the 1V capability, resulting in unstable and erratic
behavior.
The feedback divider must be close to the IC and be
returned to analog ground. Current sense traces must
be run parallel and close to each other and to analog
ground.
The IC must have a ceramic decoupling capacitor
across its supply pins, mounted as close to the device
as possible. The small ceramic, noise-filtering capaci-
tors on the current sense lines should also be placed
as close to the IC as possible.
