LM26 datasheet(14/15 Pages) NSC

14 / 15 page

Application Information (Continued)

Arrange the components so that the switching current

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

each cycle, current flows from the input filter capacitor,

through the LM2618 and inductor to the output filter

capacitor and back through ground, forming a current

loop. In the second part of each cycle, current is pulled

up from ground, through the LM2618 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 mag-

netic field reversal between the two part-cycles and

reduces radiated noise.

Connect the ground pins of the LM2618 and filter ca-

pacitors together using generous component-side cop-

per 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 LM2618 by giving it

a low-impedance ground connection.

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.

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 LM2618 circuit and should be direct and

routed away from noisy components. This reduces EMI

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

back trace.

Place noise sensitive circuitry, such as radio IF blocks,

away from the DC-DC converter, CMOS digital blocks

and

other

noisy

circuitry.

Interference

with

noise-sensitive circuitry in the system can be reduced

through distance.

In mobile phones, for example, a common practice is to

place the DC-DC converter on one corner of the board,

arrange the CMOS digital circuitry around it (since this also

generates noise), and then place sensitive preamplifiers and

IF stages on the diagonally opposing corner. Often, the

sensitive circuitry is shielded with a metal pan and power to

it is post-regulated to reduce conducted noise, using

low-dropout linear regulators, such as the LP2966.

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