SLVS931B November 2009 – December 2016 TPS2556 , TPS2557
PRODUCTION DATA.
The low on-resistance of the N-channel MOSFET allows small surface-mount packages to pass large currents. It is good design practice to estimate power dissipation and junction temperature. This analysis gives an approximation for calculating junction temperature based on the power dissipation in the package. However, 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.
Begin by determining the rDS(ON) of the N-channel MOSFET relative to the input voltage and operating temperature. As an initial estimate, use the highest operating ambient temperature of interest and read rDS(ON) from the typical characteristics graph. Using this value, the power dissipation can be calculated by Equation 8.
where
Finally, calculate the junction temperature with Equation 9.
where
Compare the calculated junction temperature with the initial estimate. If they are not within a few degrees, repeat the calculation using the refined rDS(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, and thermal resistance is highly dependent on the individual package and board layout.