LM26480 datasheet(27/30 Pages) NSC

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Board Layout Considerations

PC board layout is an important part of DC-DC converter de-

sign. Poor board layout can disrupt the performance of a DC-

DC converter and surrounding circuitry by contributing to EMI,

ground bounce, and resistive voltage loss ii the traces. These

can send erroneous signals to the DC-DC converter IC, re-

sulting in poor regulation or instability. Poor layout can also

result in re-flow problems leading to poor solder joints, which

can result in erratic or degraded performance.

Good layout for the LM26480 bucks can be implemented by

following a few simple design rules, as illustrated in Figure

30040468

FIGURE 3. Board Layout Design Rules for the LM26480

Place the buck inductor and filter capacitors close

together and make the trace short. The traces between

these components carry relatively high switching

currents and act as antennas. Following this rule reduces

radiated noise. Place the capacitors and inductor close

to the buck.

Arrange the components so that the switching current

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

each cycle, current flows from the input filter capacitor,

through the buck 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 buck 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 reversal between the two half-cycles and reduces

radiated noise.

Connect the ground pins of the buck, and filter capacitors

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 buck 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

Rout noise sensitive traces, such as the voltage

feedback path, away from noisy traces between the

power components. The voltage feedback trace must

remain close to the buck circuit and should be routed

directly from FB to VOUT at the output capacitor and

should be routed opposite to noise components. This

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

voltage feedback trace.

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 cir-

cuitry is shielded with a metal pan and power to it is post-

regulated to reduce conducted noise, using low-dropout

linear regulators.

LM26480 Datasheet(PDF) 27 Page - National Semiconductor (TI)

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