LM3241
www.ti.com
SNOSB38B –JANUARY 2009–REVISED APRIL 2013
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. Poor layout can also result in re-flow problems leading to poor solder joints between the DSBGA
package and board pads — poor solder joints can result in erratic or degraded performance. Good layout for the
LM3241 can be implemented by following a few simple design rules, as illustrated in Figure 31.
Figure 31. LM3241 Board Layout
1. Place the LM3241 on 10.82 mil pads. As a thermal relief, connect each pad with a 7mil wide, approximately
7mil long trace, and then incrementally increase each trace to its optimal width. VIN and GND traces are
especially recommended to be as wide as possible. The important criterion is symmetry to ensure the solder
bumps re-flow evenly. (See AN-1112, Surface Mount Technology (SMD) Assembly Considerations..)
2. Place the LM3241, inductor, and filter capacitors close together and make the traces short. The traces
between these components carry relatively high switching current and act as antennae. Following this rule
reduces radiated noise. Special care must be given to place the input filter capacitor very close to the
VIN and GND pads.
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 LM3241 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 LM3241 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 pads of the LM3241 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 LM3241 by giving it a low impedance ground connection.
5. Use side 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 output voltage feedback point should be taken approximately 1.5 nH away from the output
capacitor. The feedback trace also should be routed opposite to noise components. The voltage feedback
trace must remain close to the LM3241 circuit and should be routed directly from FB to VOUT at the
inductor and should be routed opposite to noise components. This allows fast feedback and reduces
EMI radiated onto the DC-DC converter’s own voltage feedback trace.
Copyright © 2009–2013, Texas Instruments Incorporated
Submit Documentation Feedback
17
Product Folder Links: LM3241
TI [ TEXAS INSTRUMENTS ]
