ADP3158JR datasheet(13/16 Pages) AD

13 / 16 page

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

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.

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 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.

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.

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).

Absolutely avoid crossing any signal lines over the switching

power path loop, described below.

Power Circuitry

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.

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.

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.

ADP3158JR Folha de dados(PDF) 13 Page - Analog Devices

ADP3158JR Folha de dados(HTML) 13 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Folha de dados Download

Ligação URL