LM3679

SNVS510A – SEPTEMBER 2007 – REVISED FEBRUARY 2008

www.ti.com

Figure 26. Board Layout Design Rules for the LM3679

Good layout for the LM3679 can be implemented by following a few simple design rules, as illustrated in Figure.

1. Place the LM3679 on 10.82 mil pads. As a thermal relief, connect to each pad with a 7 mil wide,

approximately 7 mil long trace, and then incrementally increase each trace to its optimal width. The important

criterion is symmetry to ensure the solder bumps on the re-flow evenly (DSBGA PACKAGE ASSEMBLY

AND USE).

2. Place the LM3679, inductor and filter capacitors close together and make the traces short. The traces

between these components carry relatively high switching currents and act as antennas. Following this rule

and GND pin.

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 LM3679 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 LM3679 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.

4. Connect the ground pins of the LM3679, 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 LM3679 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

6. Route 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 LM3679 circuit and should be routed

reduces EMI radiated onto the DC-DC converter’s own voltage feedback trace.

7. 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.

16

Submit Documentation Feedback

Copyright © 2007–2008, Texas Instruments Incorporated

Product Folder Links: LM3679