A proper layout is critical for the operation of a switched mode power supply, even more at high switching frequencies. Therefore, the PCB layout of the TPS62851x demands careful attention to ensure operation and to get the performance specified. A poor layout can lead to issues like the following:
See Figure 12-1 for the recommended layout of the TPS62851x, which is designed for common external ground connections. The input capacitor must be placed as close as possible between the VIN and GND pin.
Provide low inductive and resistive paths for loops with high di/dt. Therefore, paths conducting the switched load current must be as short and wide as possible. Provide low capacitive paths (with respect to all other nodes) for wires with high dv/dt. Therefore, the input and output capacitance must be placed as close as possible to the IC pins and parallel wiring over long distances and narrow traces must be avoided. Loops which conduct an alternating current should outline an area as small as possible, as this area is proportional to the energy radiated.
Sensitive nodes like FB need to be connected with short wires and not nearby high dv/dt signals (for example, SW). As they carry information about the output voltage, they must be connected as close as possible to the actual output voltage (at the output capacitor). The capacitor on the SS/TR pin as well as the FB resistors, R1 and R2, must be kept close to the IC and be connected directly to the pin and the system ground plane.
The package uses the pins for power dissipation. Thermal vias on the VIN and GND pins help to spread the heat into the PCB.
The recommended layout is implemented on the EVM and shown in the TPS62851xEVM-139 Evaluation Module User's Guide.