10.1.1 C_OUT-to-C_LOAD Inductance

Minimize inductance in the path between LM3281 C_OUT capacitor and the load bypass capacitors C_LOAD1 and C_LOAD2 for best performance. Total power path inductance from the LM3281 output to the load (including vias and traces) should target < 1 nH and must not exceed 2 nH.

10.1.2 LM3281-to-C_IN Inductance

Minimize inductance between LM3281 pins (VIN, GND) and the LM3281 input bypass capacitor C_IN for best performance. The LM3281 device and C_IN capacitor should be placed to permit shortest possible top-metal routing for these connections.

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 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 which can result in erratic or degraded performance of the converter. By its very nature, any switching converter generates electrical noise, and the circuit board designer’s challenge is to minimize, contain, or attenuate such switcher-generated noise. A high-frequency switching converter, such as the LM3281, switches Ampere level currents within nanoseconds, and the traces interconnecting the associated components can act as radiating antennas. The following general guidelines are offered to help mitigate EMI and facilitate good layout design.

• Place the LM3281 switcher, its input capacitor, and output filter inductor and capacitor close together, and make the Inter-connecting 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 LM3281 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 LM3281 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.
• Reduce the amount of switching current that circulates through the ground plane: Connect the ground bump of the LM3281 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) with multiple vias. These multiple vias help to minimize ground bounce at the LM3281 by giving it a low-impedance ground connection.
• Minimize resistive losses by using wide traces between the power components and doubling up traces on multiple layers when needed.
• Route noise sensitive traces, such as the voltage feedback path, 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.
• Take advantage of the inherent inductance of circuit traces to reduce coupling among various function blocks on the board, by way of the power supply traces.