Linear Regulator Thermal Calculator
Estimate linear regulator or LDO junction temperature, thermal margin, and maximum current from electrical loss and thermal path assumptions.
Inputs and tolerances
Estimate steady-state junction temperature, thermal margin, and maximum current for a linear regulator or LDO.
θJA is strongly board dependent. Datasheet values often assume a specific test PCB, copper area, airflow, and mounting condition that may not match the final product.
Thermal result
This is a steady-state lumped thermal estimate. It does not model transient thermal impedance, pulsed loads, copper spreading in detail, airflow variation, package hot spots, thermal shutdown, or regulator safe operating area.
Convert linear-regulator loss into junction-temperature margin
Electrical dissipation tells you how many watts the regulator must lose. Thermal resistance, ambient temperature, and junction-temperature limit tell you whether that operating point is realistic.
θJA board estimate
Use a single junction-to-ambient value for quick package and PCB feasibility checks.
Case and heatsink path
Sum junction-to-case, interface, and sink-to-ambient resistances when that path dominates heat flow.
Maximum current estimate
Work backwards from allowable dissipation to estimate the thermal current limit for the entered conditions.
Worked example
For 12 V to 5 V at 200 mA, with 5 mA quiescent current, the regulator dissipates 1.46 W. With θJA = 50 °C/W and 25 °C ambient, the estimated junction temperature is 98 °C.
Power loss
Load-related loss = (12 V - 5 V) x 0.2 A = 1.4 W.
Quiescent loss = 12 V x 0.005 A = 0.06 W.
Total dissipation = 1.46 W.
Temperature rise
Temperature rise = 1.46 W × 50 °C/W = 73 °C.
Estimated junction temperature = 25 °C + 73 °C = 98 °C.
Margin to 125 °C = 27 °C.
Common mistakes and limits
Treating θJA as universal
θJA depends on package, copper area, vias, airflow, board stack-up, enclosure, and the datasheet test board.
Ignoring quiescent current
High-input-voltage LDOs can add meaningful heat from Vin x Iq, especially in always-on rails.
Using steady-state for pulses
Transient loads need transient thermal impedance or measured temperature, not only a steady-state lumped model.
Related calculators and next checks
Linear regulator power calculator
Start with the simpler electrical dissipation check before detailed thermal margin review.
Buck converter calculator
Compare against a switching regulator when linear dissipation creates too much heat.
Battery life calculator
Fold quiescent current and regulator losses into a battery-powered load profile.
Regulator thermal guide
Review thermal resistance, copper area, airflow, and temperature-rise assumptions.
Engineering reference
Equations, assumptions, and design guidance
Estimates steady-state linear regulator junction temperature, thermal margin, and maximum output current from electrical dissipation and a lumped thermal path.
Equations and variables
Pd = (Vin - Vout) * Iout + Vin * IqTj = Ta + Pd × θJATj = Ta + Pd × (θJC + θCS + θSA)Pdmax = (Tjmax - Ta) / thetaTotal- Vin
- Regulator input voltage (V)
- Vout
- Regulator output voltage (V)
- Iout
- Output load current (A)
- Iq
- Regulator quiescent current (A)
- Ta
- Ambient temperature (°C)
- Tjmax
- Maximum junction temperature (°C)
- θ
- Thermal resistance (°C/W)
Assumptions and limitations
Assumptions
- The regulator is a linear regulator or LDO, not a switching regulator.
- The thermal model is steady-state and lumped into one total thermal resistance.
Limitations
- Transient thermal impedance, copper spreading detail, airflow variation, package hot spots, thermal shutdown, current limit, dropout, safe operating area, and manufacturer-specific derating are not modelled.
Worked example and design use
12 V to 5 V at 200 mA with θJA = 50 °C/W
Inputs: Vin = 12 V, Vout = 5 V, Iout = 0.2 A, Iq = 5 mA, Ta = 25 °C
Outputs: Pd = 1.46 W, temperature rise = 73 °C, Tj = 98 °C, margin to 125 °C = 27 °C
Design guidance
- Use this estimate before selecting a package or deciding whether a buck converter is required.
- Check the datasheet thermal test conditions and final PCB copper before treating θJA as reliable.