TI recommends to use one of the middle layers as a solid ground plane. Ground plane provides shielding for sensitive circuits and traces. Ground plane also provides a quiet reference potential for the control circuitry. The AGND and PGND pins must be connected to the ground plane using vias right next to the bypass capacitors. The PGND pin is connected to the source of the internal low-side MOSFET switch, and also connected directly to the grounds of the input and output capacitors. The PGND net contains noise at the switching frequency and can bounce due to load variations. The PGND trace, as well as VIN and SW traces, must be constrained to one side of the ground plane. The other side of the ground plane contains much less noise and must be used for sensitive routes.

TI recommends to provide adequate device heat sinking by using the PAD of the IC as the primary thermal path. Use a minimum 4 × 2 array of 12-mil thermal vias to connect the PAD to the system ground plane heat sink. The vias must be evenly distributed under the PAD. Use as much copper as possible, for system ground plane, on the top and bottom layers for the best heat dissipation. Use a four-layer board with the copper thickness for the four layers, starting from the top of 2 oz / 1 oz / 1 oz / 2 oz. Four layer boards with enough copper thickness provide low current conduction impedance, proper shielding, and lower thermal resistance.

The thermal characteristics of the TPS2585x-Q1 are specified using the parameter θ_JA, which characterizes the junction temperature of silicon to the ambient temperature in a specific system. Although the value of θ_JA is dependent on many variables, it still can be used to approximate the operating junction temperature of the device. To obtain an estimate of the device junction temperature, one can use the following relationship:

where

• T_J = Junction temperature in °C
• P_D = V_IN × I_IN × (1 – Efficiency) – 1.1 × I_OUT² × DCR in Watt
• DCR = Inductor DC parasitic resistance in Ω
• θ_JA = Junction-to-ambient thermal resistance of the device in °C/W
• T_A = Ambient temperature in °C

The maximum operating junction temperature of the TPS2585x-Q1 is 150°C. θ_JA is highly related to PCB size and layout, as well as environmental factors such as heat sinking and air flow.