ZHCSK92E September 2019 – March 2022 TPS25840-Q1 , TPS25842-Q1
PRODUCTION DATA
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 PGND pins must be connected to the ground plane using vias right next to the bypass capacitors. PGND pin is connected to the source of the internal LS switch. The PGND net contains noise at switching frequency and can bounce due to load variations. 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. AGND and PGND must be connected under the QFN package PAD.
TI recommends to provide adequate device heat sinking by using the PAD of the IC as the primary thermal path. Use a minimum 2 row, 2 column "+" 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 TPS2584x-Q1 are specified using the parameter, θJA, which characterize the junction temperature of silicon to the ambient temperature in a specific system. Although the value of θJA 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
TJ = Junction temperature is in °C
PD = VIN × IIN × (1 – Efficiency) – 1.1 × IOUT2 × DCR in Watt
DCR = Inductor DC parasitic resistance in Ω
θJA = Junction to ambient thermal resistance of the device in °C/W
TA = Ambient temperature in °C
θJA is highly related to PCB size and layout, as well as environmental factors such as heat sinking and air flow.