REV. A

ADP3162

–11–

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 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 m

Ω at room temperature.

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 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.

4.

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.

5.

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.

6.

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).

7.

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.

8.

Absolutely avoid crossing any signal lines over the switching

power path loop, described below.

Power Circuitry

9.

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 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.

10. An optional power Schottky diode (3 A–5 A dc rating) from

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.

11. A small ferrite bead inductor placed in series with the drain

of the lower MOSFET can also help to reduce this previ-

ously described source of switching power loss.

12. 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, 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.

13. 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.

14. For best EMI containment, the power ground plane should

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.