This section demonstrates how to analyze the power dissipation and junction temperature of the TPD3S716-Q1 to validate that the application requirements of an I_VBUS operating current level of 1.5 A and a maximum operating ambient temperature of 105 °C can be met.

It is good design practice to estimate power dissipation and maximum expected junction temperature of TPD3S716-Q1. This is important to insure the device does not go into thermal shutdown in normal operation and that the long term reliability of the device is maintained. Using Equation 4 to Equation 6, the system designer can control choices of the device's proximity to other power dissipating devices and the design of the printed circuit board (PCB). These have a direct influence on maximum junction temperature. Other factors, such as airflow and maximum ambient temperature, are often determined by system considerations. It is important to remember that these calculations do not include the effects of adjacent heat sources, and enhanced or restricted air flow. Addition of extra PCB copper area around these devices is recommended to reduce the thermal impedance and maintain the junction temperature as low as practical.

For TPD3S716-Q1, the operating junction temperature must be kept below 150°C in order to prevent the device from going into thermal shutdown. Equation 4 is used to calculate the junction temperature of the device:

where

• I_OUT = Rated OUT pin current (A)
• R_ON = Power path on-resistance at an assumed T_J (Ω)
• T_A = Maximum ambient temperature (°C)
• T_J = Maximum junction temperature (°C)
• R_θJA = Thermal resistance (°C/W)

This application example requires an I_VBUS operating current level of 1.5 A. TPD3S716-Q1 has maximum junction temperature derating requirements depending on the maximum operating current of the device according to Equation 5:

where

• T_J(MAX) = Maximum allowed junction temperature (°C)
• I_{VBUS(MAX OPERATING)} = Maximum I_VBUS operating current (A)

See Figure 9-7 for a plot of the reliability curve equation. Using this equation, 138.1°C is the maximum allowed junction temperature in this application.

This example requires a maximum operating ambient temperature of 105°C. To determine if this can be supported using Equation 4, the maximum V_BUS path R_ON must be determined. Equation 6 calculates the maximum V_BUS path R_ON possible for TPD3S716-Q1 for a given junction temperature:

where

• R_ON(MAX) = Maximum V_BUS R_ON at a given junction temperature (Ω)
• T_J = Device junction temperature (°C)

See Figure 9-8 for a plot of the maximum V_BUS path R_ON vs. Junction Temperature curve. Using the above equation, the maximum V_BUS R_ON possible for TPD3S716-Q1 at 138.1°C is R_ON(MAX) = 0.118 Ω.

Using the calculated parameters for this example and the standard datasheet R_θJA for TPD3S716-Q1, the maximum operating ambient temperature possible in this example is T_A = 111°C. Because this is greater than the application requirement of 105°C, TPD3S716-Q1 can safely be operated at 1.5 A with R_θJA = 98.8 (°C/W). If the resulting ambient temperature in the above calculations resulted in a T_A < 105 °C, methods for improving R_θJA would need to be taken. See the Layout Optimized for Thermal Performance section for guidelines on improving R_θJA for TPD3S716-Q1. The example given in the Layout Optimized for Thermal Performance yields an R_θJA = 57 (°C/W). Excellent thermal performance of TPD3S716-Q1 can be achieved with the proper PCB layout.