REV. A
ADP3158/ADP3178
–13–
LAYOUT AND COMPONENT PLACEMENT GUIDELINES
The following guidelines are recommended for optimal perfor-
mance of a switching regulator in a PC system:
General Recommendations
1. For best results, a four-layer PCB is recommended. This
should allow the needed versatility for control circuitry
interconnections with optimal placement, a signal ground
plane, power planes for both power ground and the input
power (e.g., 5 V), and wide interconnection traces in the
rest of the power delivery current paths.
2. Whenever high currents must be routed between PCB
layers, vias should be used liberally to create several parallel
current paths so that the resistance and inductance intro-
duced by these current paths is minimized and the via
current rating is not exceeded.
3. If critical signal lines (including the voltage and current
sense lines of the controller IC) must cross through power
circuitry, it is best if a ground plane can be interposed
between those signal lines and the traces of the power
circuitry. This serves as a shield to minimize noise injec-
tion into the signals at the cost of making signal ground a
bit noisier.
4. The GND pin should connect first to a ceramic bypass
capacitor (on the VCC pin) and then into the power ground
plane. However, the ground plane should not extend under
other signal components, including the controller IC itself.
5. The output capacitors should also be connected as closely
as possible to the load (or connector) that receives the
power (e.g., a microprocessor core). If the load is distributed,
the capacitors should also be distributed, and generally in
proportion to where the load tends to be more dynamic. It
is also advised to keep the planar interconnection path short
(i.e., have input and output capacitors close together).
6. Absolutely avoid crossing any signal lines over the switching
power path loop, described below.
Power Circuitry
7. The switching power path should be routed on the PCB to
encompass the smallest possible area in order to minimize
radiated switching noise energy (i.e., EMI). Failure to take
proper precaution often results in EMI problems for the
entire PC system as well as noise-related operational prob-
lems in the power converter control circuitry. The switching
power path is the loop formed by the current path through
the input capacitors, the two FETs, and the power Schottky
diode, if used, including all interconnecting PCB traces and
planes. The use of short and wide interconnection traces is
especially critical in this path for two reasons: it minimizes
the inductance in the switching loop, which can cause high-
energy ringing, and it accommodates the high current demand
with minimal voltage loss.
8. A power Schottky diode (1 ~ 2 A dc rating) placed from the
lower MOSFET’s source (anode) to drain (cathode) will
help to minimize switching power dissipation in the upper
MOSFET. In the bñôÖÑx of an effective Schottky diode,
this dissipation occurs through the following sequence of
switching events. The lower MOSFET turns off in advance
of the upper MOSFET turning on (necessary to prevent
cross-conduction). The circulating current in the power
converter, no longer finding a path for current through the
channel of the lower MOSFET, draws current through the
inherent body-drain diode of the MOSFET. The upper
MOSFET turns on, and the reverse recovery characteristic
of the lower MOSFET’s body-drain diode prevents the drain
voltage from being pulled high quickly. The upper MOSFET
then conducts very large current while it momentarily has a
high voltage forced across it, which translates into added
power dissipation in the upper MOSFET. The Schottky diode
minimizes this problem by carrying a majority of the circu-
lating current when the lower MOSFET is turned off, and
by virtue of its essentially nonexistent reverse recovery time.
9. Whenever a power-dissipating component (e.g., a power
MOSFET) is soldered to a PCB, the liberal use of vias,
both directly on the mounting pad and immediately sur-
rounding it, is recommended. Two important reasons for
this are: improved current rating through the vias (if it is a
current path), and improved thermal performance—espe-
cially if the vias extend to the opposite side of the PCB where
a plane can more readily transfer the heat to the air.
10. The output power path, though not as critical as the switch-
ing power path, should also be routed to encompass a small
area. The output power path is formed by the current path
through the inductor, the current sensing resistor, the out-
put capacitors, and back to the input capacitors.
11. For best EMI containment, the ground plane should extend
fully under all the power components. These are: the input
capacitors, the power MOSFETs and Schottky diode, the
inductor, the current sense resistor, any snubbing elements
that might be added to dampen ringing, and the output
capacitors.
