LM3674 datasheet(14/15 Pages) NSC

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Application Information (Continued)

Good layout for the LM3674 can be implemented by follow-

ing a few simple design rules, as illustrated in .

Place the LM3674, inductor and filter capacitors close

together and make the traces short. The traces between

these components carry relatively high switching cur-

rents and act as antennas. Following this rule reduces

radiated noise. Special care must by given to place the

input filter capacitor very close to the V

IN and GND pin.

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 LM3674 and inductor to the output filter

capacitor and back through ground, forming a current

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

up from ground, through the LM3674 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 half-cycles and re-

duces radiated noise.

Connect the ground pins of the LM3674, 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 LM3674 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 LM3674 circuit and should be direct but

should be routed opposite to noisy components. This

reduces EMI radiated onto the DC-DC converter’s own

voltage feedback trace. A good approach is to route the

feedback trace on another layer and to have a ground

plane between the top layer and layer on which the

feedback trace is routed. In the same manner for the

adjustable part it is desired to have the feedback divid-

ers on the bottom layer.

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.

20167231

FIGURE 5. Board Layout Design Rules for the LM3674

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