10.1 Layout Guidelines

The LM5008A regulation and overvoltage comparators are very fast, and as such responds to short-duration noise pulses. Layout considerations are therefore critical for optimum performance:

1. Minimize the area of the high di/dt switching current loop consisting of the VIN pin, input ceramic capacitor, SW node and freewheeling power diode. Keep the input capacitor as close as possible to the VIN pin and route a short, direct connection to the RTN pin using polygon copper pours.
2. Minimize SW copper area to reduce radiated noise related to high dv/dt.
3. Locate all components as physically close as possible to their respective pins, thereby minimizing noise pickup in the printed-circuit tracks.
4. The FB trace should be away from noise sources and inductors. The lower feedback resistor should connect to ground close to the IC RTN.

If the internal dissipation of the LM5008A converter produces excessive junction temperatures during normal operation, optimal use of the PCB ground plane can help considerably to dissipate heat. The exposed pad on the bottom of the WSON-8 package can be soldered to a ground plane on the PCB, and that plane should extend out from beneath the IC to help dissipate the heat. Additionally, the use of wide PCB traces for power connection can also help conduct heat away from the IC. Judicious positioning of the LM5008A converter within the end product, along with use of any available air flow (forced or natural convection), can help reduce the operating junction temperature.