11.1 Layout Guidelines

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 device, resulting in poor regulation or instability. Implement good layout for the LM3686 by following a few simple design rules:

1. Place the LM3686, inductor,and filter capacitor close together and make the traces short. The traces between these components carry relatively high switching currents and act as antennas. Following this rule reduces radiated noise. Special care must be given to place the input filter capacitor very close to the V_BATT and PGND pin. Place the output capacitor of the linear regulator close to the output pin.
2. 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 LM3686 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 LM3686 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.
3. Connect the ground pins of the LM3686 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 LM3686 by giving it a low impedance ground connection. Route SGND to the ground-plane by a separate trace.
4. 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.
5. Route noise sensitive traces, such as the voltage feedback path (FB_DCDC), away from noisy traces between the power components. The voltage feedback trace must remain close to the LM3686 circuit, must be direct, and must be routed opposite to noisy components. This reduces EMI radiated onto the DC-DC converter voltage feedback trace. A good approach is to route the feedback trace on another layer and to have a ground plane between the top layer and layer on which the feedback trace is routed.
6. 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.

In mobile phones, for example, a common practice is to place the DC-DC converter on one corner of the board, arrange the CMOS digital circuitry around it (since this also generates noise), and then place sensitive pre-amplifiers and IF stages on the diagonally opposing corner. Often, the sensitive circuitry is shielded with a metal plane; power to it is post-regulated to reduce conducted noise, a good field of application for the on-chip low-dropout linear regulator.

11.2 Layout Example

Figure 24. LM3686 Layout

11.3 DSBGA Package Assembly and Use

Use of the DSBGA package requires specialized board layout, precision mounting and careful re-flow techniques, as detailed in AN-1112 DSBGA Wafer Level Chip Scale Package. Refer to the section Surface Mount Technology (SMD) Assembly Considerations. For best results in assembly, alignment ordinals on the PC board must be used to facilitate placement of the device. The pad style used with DSBGA package must be the non-solder mask defined (NSMD) type. This means that the solder-mask opening is larger than the pad size. This prevents a lip that otherwise forms if the solder mask and pad overlap, from holding the device off the surface of the board and interfering with mounting. See AN-1112 DSBGA Wafer Level Chip Scale Package for specific instructions how to do this. The 12-pin package used for LM3686 has 300 micron solder balls and requires 275 micron pads for mounting on the circuit board. The trace to each pad must enter the pad with a 90° entry angle to prevent debris from being caught in deep corners. Initially, the trace to each pad must not exceed 183 micron, for a section approximately 183 micron long or longer, as a thermal relief —then each trace must neck up or down to its optimal width. The important criteria is symmetry. This ensures the solder bumps on the LM3686 re-flow evenly and that the device solders level to the board. In particular, special attention must be paid to the pads for bumps A1 and B1 because PGND and VBATT are typically connected to large copper planes, inadequate thermal relief can result in late or inadequate re-flow of these bumps. The DSBGA package is optimized for the smallest possible size in applications with red or infrared opaque cases. Because the DSBGA package lacks the plastic encapsulation characteristic of larger devices, it is vulnerable to light. Backside metallization and/or epoxy coating, along with frontside shading by the printed circuit board, reduce this sensitivity. However, the package has exposed die edges. In particular, DSBGA devices are sensitive to light, in the red and infrared range, shining on the exposed die edges of the package.