LDO thermal & dropout calculator
Check whether an LDO runs too hot: enter input/output voltage, load current and the thermal resistance, and read the power, junction temperature and dropout.
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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Frequently asked questions
How is the power dissipated by an LDO calculated?
An LDO burns the input-minus-output voltage as heat: P = (Vin − Vout)·Iout + Vin·Iq, with currents in mA. With the defaults of 5 V in, 3.3 V out and 300 mA load the drop wastes roughly (5 − 3.3) × 0.3 ≈ 0.5 W — which is why a large Vin−Vout gap at high current runs hot.
How is junction temperature estimated?
Junction temperature follows Tj = Ta + P·Rθ_JA, where Rθ_JA is the package junction-to-ambient thermal resistance. The tool ships typical values — SOT-23 ≈ 250 °C/W, SOT-223 ≈ 55 °C/W, DPAK ≈ 62 °C/W — and flags a junction above the ~125 °C limit, suggesting a lower drop, reduced current or a DC-DC converter.
What does the dropout check tell me?
Dropout is the smallest Vin − Vout the regulator can hold before the pass device saturates and regulation is lost. The tool compares Vin against Vout + Vdo (default 0.3 V) and reports the margin; if Vin falls below that sum it warns that the LDO has dropped out of regulation.
LDOs burn the input-minus-output voltage as heat — the bigger the drop, the hotter the part.ICBOMS provides this tool for reference only.