10.1.1.1 Energy Efficiency

Minimize resistive losses by using wide traces between the power components and doubling up traces on multiple layers when possible

10.1.1.2 EMI

By its very nature, any switching converter generates electrical noise. The circuit board designer’s challenge is to minimize, contain, or attenuate such switcher-generated noise. A high-frequency switching converter, such as the LM3243, switches Ampere level currents within nanoseconds, and the traces interconnecting the associated components can act as radiating antennas. The following guidelines are offered to help to ensure that EMI is maintained within tolerable levels.

To help minimize radiated noise:

• Place the LM3243 switcher, its input capacitor, and output filter inductor and capacitor close together, and make the interconnecting traces as short as possible.
• 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 internal PFET of the LM3243 and the inductor, to the output filter capacitor, then back through ground, forming a current loop. In the second half of each cycle, current is pulled up from ground, through the internal synchronous NFET of the LM3243 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.
• Make the current loop area(s) as small as possible. Interleave doubled traces with ground planes or return paths, where possible, to further minimize trace inductances.

To help minimize conducted noise in the ground-plane:

• Reduce the amount of switching current that circulates through the ground plane: Connect the ground bumps of the LM3243 and its input filter capacitor together using generous component-side copper fill as a pseudo-ground plane. Then connect this copper fill to the system ground-plane (if one is used) by multiple vias located at the input filter capacitor ground terminal. The multiple vias help to minimize ground bounce at the LM3243 by giving it a low-impedance ground connection.

To help minimize coupling to the DC-DC converter voltage feedback trace:

• Route noise sensitive traces, such as the voltage feedback path (FB), as directly as possible from the switcher FB pad to the VOUT pad of the output capacitor, but keep it away from noisy traces between the power components.

To help minimize noise coupled back into power supplies:

• Use a star connection to route from the VBATT power input to Switcher PVIN and to VBATT_PA.
• Route traces for minimum inductance between supply pins and bypass capacitor(s).
• Route traces to minimize inductance between bypass capacitors and the ground plane.
• Maximize power supply trace inductance(s) to reduce coupling among function blocks.
• Inserting a ferrite bead in-line with power supply traces can offer a favorable tradeoff in terms of board area, by attenuating noise that might otherwise propagate through the supply connections, allowing the use of fewer bypass capacitors.

VBATT Star Connection: It is critically important to use a ‘Star’ connection from VBATT supply to LM3243 PVIN and from VBATT to PA modules as implementing a ‘daisy chain’ supply connection may add noise to the PA output.

Figure 26. VBATT Star Connection on VIN And VBATT_PA