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TPS25221

SLVSDT3D – JANUARY 2018 – REVISED DECEMBER 2019

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Power Dissipation and Junction Temperature (continued)

PD = rDS(on) × IOUT

2

where

•

PD = Total power dissipation (W)

•

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

•

IOUT = Maximum current-limit threshold (A)

•

This step calculates the total power dissipation of the N-channel MOSFET.

(7)

Finally, calculate the junction temperature:

TJ = PD × θJA + TA

where

•

TA = Ambient temperature (°C)

•

θJA = Thermal resistance (°C/W)

•

PD = Total power dissipation (W)

(8)

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 θJA, and thermal resistance is highly dependent on the individual package and board layout.

The table provides example thermal resistances for specific packages and board layouts.