12.1.2 Power Dissipation and Junction Temperature

The total power dissipation inside TPS65283, TPS65283-1 should not to exceed the maximum allowable junction temperature of 125°C. The maximum allowable power dissipation is a function of the thermal resistance of the package (R_θJA) and ambient temperature.

The following analysis gives an approximation in calculating junction temperature based on the power dissipation in the package. However, note that thermal analysis is strongly dependent on additional system-level factors. Such factors include air flow, board layout, copper thickness and surface area, and proximity to other devices dissipating power. Good thermal design practice must include all system-level factors in addition to individual component analysis.

To calculate the temperature inside the device under continuous load, use the following procedure.

1. Define the total continuous current through buck converter (including the load current through power switches). Make sure the continuous current does not exceed maximum load current requirement.
2. From the graphs in this section, determine the expected losses (y-axis) in watts for buck converter inside the device. The loss P_{D_BUCK} depends on the input supply and the selected switching frequency.
3. Determine the load current I_OUT through the power switches. Read R_DS(on) of power switch from the typical characteristics graph.
4. Calculate the power loss through power switches with P_{D_PW} = R_DS(on) × I_OUT.
5. The Thermal Information table provides the thermal resistance R_θJA for specific packages and board layouts.
6. To calculate the maximum temperature inside the IC, use Equation 21.
Equation 21. T_J = (P_{D_BUCK} + P_{D_PW} ) × R_θJA + T_A
where
• T_A = Ambient temperature (°C)
• R_θJA = Thermal resistance (°C/W)
• P_{D_BUCK} = Total power dissipation in buck converter (W)
• P_{D_PW} = Total power dissipation in power switches (W)

Figure 56. Power Dissipation of TPS65283

A. VIN = 12 V, Vout1 = 1.2 V / 3 A, Vout2 = 5 V / 2 A, VSW_in = 5 V, ISW_OUT = 1.2 A

B. EVM board: 4-layer PCB, 1.6-mm thickness, 35-µm copper thickness, 68-mm × 68-mm size, 9 vias at thermal pad

Figure 57. Thermal Signature of TPS65283EVM