LDO thermal & dropout calculator
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Operating point
Thermal & dropout check
Dissipated power
(Vin − Vout)·Iout + Vin·Iq
Junction temperature
Ta + P·Rθ_JA
Dropout margin
Vin − (Vout + Vdo)
Junction reaches 153.8 °C — above the ~125 °C limit. Reduce Iout, lower Vin, or switch to a bigger package / a DC-DC converter.
About the model
An LDO burns the difference between input and output as heat: P = (Vin − Vout)·Iout + Vin·Iq. At fixed load the efficiency is roughly Vout/Vin, so a 5 V → 3.3 V rail at 300 mA dumps about 0.5 W — a lot for a SOT-23, which only sheds a fraction of that before hitting its ~125 °C junction limit. Junction temperature is Ta + P·Rθ_JA, where Rθ_JA is the package's junction-to-ambient thermal resistance from the datasheet. Dropout is the smallest Vin − Vout the regulator can hold; below it the pass device saturates and output follows input minus a diode drop.
For high drop or high current, a DC-DC converter dissipates far less: switching losses replace the linear drop, which is why the verdict pushes big, hot LDO rails that way.
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