ADP3160 Analog Devices, ADP3160 Datasheet - Page 14

• Manufacturer Part Number: ADP3160
• Description: 5-Bit Programmable 2-Phase Synchronous Buck Controller
• Manufacturer: Analog Devices

ADP3160/ADP3167
Power Circuitry
10. An optional power Schottky diode (3 A–5 A dc rating) from
6. The output capacitors of the power converter should be
7. The output capacitors should also be connected as closely
8. Absolutely avoid crossing any signal lines over the switching
9. The switching power path should be routed on the PCB to
connected to the signal ground plane even though power
current flows in the ground of these capacitors. For this
reason, it is advisable to avoid critical ground connections
(e.g., the signal circuitry of the power converter) in the
signal ground plane between the input and output capacitors.
It is also advisable to keep the planar interconnection path
short (i.e., have input and output capacitors close together).
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.
power path loop, described next.
encompass the smallest possible area to minimize radiated
switching noise energy (i.e., EMI). Failure to take proper
precautions often results in EMI problems for the entire 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 interconnecting PCB
traces and planes. The use of short and wide interconnec-
tion 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
–14–
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
Signal Circuitry
15. The output voltage is sensed and regulated between the FB
16. The CS+ and CS– traces should be Kelvin-connected to the
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 circulating 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 previously
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 surrounding
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 capacitors,
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.
pin and the GND pin (which connects to the signal ground
plane). The output current is sensed (as a voltage) by the
CS+ and CS– pins. In order to avoid differential mode noise
pickup in the sensed signal, the loop area should be small.
Thus the FB trace should be routed atop the signal ground
plane and the CS+ and CS– pins (the CS+ pin should be
over the signal ground plane as well).
current sense resistor, so that the additional voltage drop
due to current flow on the PCB at the current sense resistor
connections does not affect the sensed voltage.
REV. B