10.1 Layout Guidelines

When planning layout, there are a few things to consider when trying to achieve a clean, regulated output. The most important consideration when completing a boost converter layout is the close coupling of the GND connections of the C_OUT capacitor and the LM2735-Q1 PGND pin. The GND ends should be close to one another and be connected to the GND plane with at least two through-holes. There should be a continuous ground plane on the bottom layer of a two-layer board except under the switching node island. The FB pin is a high impedance node and care should be taken to make the FB trace short to avoid noise pickup and inaccurate regulation. Place the feedback resistors as close as possible to the IC, with the AGND of R1 placed as close as possible to the GND (pin 5 for the WSON) of the IC. The V_OUT trace to R2 should be routed away from the inductor and any other traces that are switching. High AC currents flow through the V_IN, SW and V_OUT traces, so they should be as short and wide as possible. However, making the traces wide increases radiated noise, so the designer must make this trade-off. Radiated noise can be decreased by choosing a shielded inductor. The remaining components should also be placed as close as possible to the IC. See Application Note AN-1229 SIMPLE SWITCHER® PCB Layout Guidelinesfor further considerations and the LM2735-Q1 demo board as an example of a 4-layer layout.

Below is an example of a good thermal and electrical PCB design. This is very similar to TI's LM2735-Q1 demonstration boards that are obtainable through the TI website. The demonstration board consists of a 2-layer PCB with a common input and output voltage application. Most of the routing is on the top layer, with the bottom layer consisting of a large ground plane. The placement of the external components satisfies the electrical considerations, and the thermal performance has been improved by adding thermal vias and a top layer Dog-Bone.