11.1 Layout Guidelines
Layout is a critical portion of PMIC designs.
- Place VOUT, and LX on the top layer and an inner power plane for VIN.
- Fit also on the top layer connections for the remaining pins of the PMIC and a large top side area filled with ground.
- The top layer ground area sould be connected to the internal ground layer(s) using vias at the input bypass capacitor, the output filter cpacitor and directly under the TPS65251 device to provide a thermal path from the Powerpad land to ground.
- The AGND pin should be tied directly to the power pad under the IC and the power pad.
- For operation at full rated load, the top side ground area together with the internal ground plane, must provide adequate heat dissipating area.
- There are several signals paths that conduct fast changing currents or voltages that can interact with stray inductance or parasitic capacitance to generate noise or degrade the power supplies performance. To help eliminate these problems, the VIN pin should be bypassed to ground with a low ESR ceramic bypass capacitor with X5R or X7R dielectric. Care should be taken to minimize the loop area formed by the bypass capacitor connections, the VIN pins, and the ground connections. Since the LX connection is the switching node, the output inductor should be located close to the LX pins, and the area of the PCB conductor minimized to prevent excessive capacitive coupling.
- The output filter capacitor ground should use the same power ground trace as the VIN input bypass capacitor. Try to minimize this conductor length while maintaining adequate width.
- The compensation should be as close as possible to the COMP pins. The COMP and OSC pins are sensitive to noise so the components associated to these pins should be located as close as possible to the IC and routed with minimal lengths of trace.