LM3670 datasheet(15/17 Pages) NSC

15 / 17 page

Application Information (Continued)

BOARD LAYOUT CONSIDERATIONS

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

design. 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 in the traces.

These can send erroneous signals to the DC-DC converter

IC, resulting in poor regulation or instability.

Good layout for the LM3670 can be implemented by follow-

ing a few simple design rules, as illustrated in .

Place the LM3670 on TBDmil (TBD/1000 in.) pads.

Place the LM3670, 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. Place the capacitors and inductor within

0.2 in. (5 mm) of the LM3670.

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 LM3670 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 LM3670 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 LM3670, 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 LM3670 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 LM3670 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.

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.

20075831

FIGURE 6. Board Layout Design Rules for the LM3670

LM3670 Datasheet(PDF) 15 Page - National Semiconductor (TI)

LM3670 Datasheet(HTML) 15 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Datasheet Download

Link URL