ADP3162 Analog Devices, ADP3162 Datasheet - Page 11

• Manufacturer Part Number: ADP3162
• Description: 5-Bit Programmable 2-Phase Synchronous Buck Controller
• Manufacturer: Analog Devices

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.
2.
3.
4.
5.
6.
7.
8.
Power Circuitry
9.
For good results, at least 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. Keep in mind that
each square unit of 1 ounce copper trace has a resistance
of ~ 0.53 mΩ at room temperature.
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.
If critical signal lines (including the voltage and current
sense lines of the ADP3162) must cross through power
circuitry, it is best if a signal ground plane can be inter-
posed between those signal lines and the traces of the
power circuitry. This serves as a shield to minimize noise
injection into the signals at the expense of making signal
ground a bit noisier.
The power ground plane should not extend under signal
components, including the ADP3162 itself. If necessary,
follow the preceding guideline to use the signal ground
plane as a shield between the power ground plane and the
signal circuitry.
The GND pin of the ADP3162 should be connected first
to the timing capacitor (on the CT pin), and then into the
signal ground plane. In cases where no signal ground plane
can be used, short interconnections to other signal ground
circuitry in the power converter should be used.
The output capacitors of the power converter should be
connected to the signal ground plane even though power
current flows in the ground of these capacitors. For this
reason, it is advised to avoid critical ground connections
(e.g., the signal circuitry of the power converter) in the
signal ground plane between the input and output capaci-
tors. It is also advised to keep the planar interconnection
path short (i.e., have input and output capacitors close
together).
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 also should be distributed, and generally in pro-
portion to where the load tends to be more dynamic.
Absolutely avoid crossing any signal lines over the switching
power path loop, described below.
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
10. An optional power Schottky diode (3 A–5 A dc rating) from
11. A small ferrite bead inductor placed in series with the drain
12. Whenever a power dissipating component (e.g., a power
13. The output power path, though not as critical as the switch-
14. For best EMI containment, the power ground plane should
PC system as well as noise-related operational problems in
the power converter control circuitry. The switching power
path is the loop formed by the current path through the
input capacitors, the power MOSFETs, and the power
Schottky diode, if used (see next), including all intercon-
necting 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.
each lower MOSFET’s source (anode) to drain (cathode)
will help to minimize switching power dissipation in the
upper MOSFETs. In the absence 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 diode of the MOSFET. The upper MOSFET
turns on, and the reverse recovery characteristic of the
lower MOSFET’s body 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.
The Schottky diode has to be connected with very short
copper traces to the MOSFET to be effective.
of the lower MOSFET can also help to reduce this previ-
ously described source of switching power loss.
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, and
improved thermal performance from vias extended to the
opposite side of the PCB where a plane can more readily
transfer the heat to the air.
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.
extend fully under all the power components except the
output capacitors. These components are: the input capaci-
tors, the power MOSFETs and Schottky diodes, the
inductors, the current sense resistor, and any snubbing
element that might be added to dampen ringing. Avoid
extending the power ground under any other circuitry or
signal lines, including the voltage and current sense lines.
ADP3162