LM2706TL datasheet(14/15 Pages) NSC

14 / 15 page

Application Information (Continued)

and act as antennas. Following this rule reduces radiated

noise. Place the capacitors and inductor within 0.2 in

(5 mm) of the LM2706.

3. Arrange the components so that the switching current

loops curl in the same direction. During the first half of

each cycle, current flows from the input filter capacitor,

through the LM2706 and inductor to the output filter ca-

pacitor and back through ground, forming a current loop.

In the second half of each cycle, current is pulled up from

ground, through the LM2706 by the inductor, to the output

filter capacitor and then back through ground, forming a

second current loop. Routing these loops so the current

curls in the same direction prevents magnetic field rever-

sal between the two half-cycles and reduces radiated

noise.

4. Connect the ground pins of the LM2706, and filter capaci-

tors together using generous component-side copper fill

as a pseudo-ground plane. Then, connect this to the

ground-plane (if one is used) with several vias. This

reduces ground-plane noise by preventing the switching

currents from circulating through the ground plane. It also

reduces ground bounce at the LM2706 by giving it a

low-impedance ground connection.

5. Use wide traces between the power components and for

power connections to the DC-DC converter circuit. This

reduces voltage errors caused by resistive losses across

the traces. Because the FB trace of the LM2706 carries

current from the bypass switch, this trace must also be

wide.

6. Route noise sensitive traces, such as the voltage feed-

back path, away from noisy traces between the power

components. The voltage feedback trace must remain

close to the LM2706 circuit and should be routed directly

from FB to V

OUT at the output capacitor and should be

routed opposite to noisy components. This reduces EMI

radiated onto the DC-DC converter’s own voltage feed-

back trace.

LM2706TL Datasheet(PDF) 14 Page - National Semiconductor (TI)