The low on-resistance of the N-channel MOSFETs allows the small surface-mount packages to pass large currents. It is good design practice to check power dissipation to ensure that the junction temperature of the device is within the recommended operating conditions. The below analysis gives an approximation for calculating junction temperature based on the power dissipation in the package. However, it is important to 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.

The following procedure shows how to approximate the junction temperature rise due to power dissipation in a single channel. The TPS2062A/66A devices contain two channels, so the total device power must sum the power in each power switch.

Begin by determining the r_DS(on) of the N-channel MOSFET relative to the input voltage and operating temperature. Use the highest operating ambient temperature of interest and read r_DS(on) from the typical characteristics graph as an initial estimate. Power dissipation is calculated by:

P_D = r_DS(on)× I_OUT²

P_T = 2 x P_D

Where:

P_D = Power dissipation/channel (W)

P_T = Total power dissipation for both channels (W)

r_DS(on) = Power switch on-resistance (Ω)

I_OUT = Maximum current-limit threshold (A)

Finally, calculate the junction temperature:

T_J = P_T x R_ΘJA + T_A

Where:

T_A= Ambient temperature °C

R_ΘJA = Thermal resistance (°C/W)

P_T = Total power dissipation (W)

Compare the calculated junction temperature with the initial estimate. If they are not within a few degrees, repeat the calculation using the "refined" r_DS(on) from the previous calculation as the new estimate. Two or three iterations are generally sufficient to achieve the desired result. The final junction temperature is highly dependent on thermal resistance R_θJA, and thermal resistance is highly dependent on the individual package and board layout. The "Dissipation Rating Table" at the beginning of this document provides example thermal resistances for specific packages and board layouts.