LDO Efficiency, Power Dissipation and Dropout: A Worked Thermal Check

An LDO’s efficiency is limited by the ratio of output voltage to input voltage. A larger package can help remove heat, but cannot recover the power lost across the pass element. Calculate efficiency and dissipation before deciding whether a linear regulator is suitable for the rail. ^{[1, 2]}

Include the ground current

In steady regulation, the source supplies both the load and the regulator’s ground-pin current. If I_G is the ground current at the actual load, input power is V_IN(I_OUT + I_G). Subtract the load’s output power to find what the regulator dissipates:

Only when ground current is small relative to load current does η ≈ V_OUT/V_IN. At a light load, that approximation can become optimistic. Use the datasheet’s ground-current figure for the operating point; unloaded quiescent current and loaded ground current need not be identical. ^[1]

Worked example: 5 V to 3.3 V at 500 mA

This illustrative operating point assumes 1 mA ground current, 0.3 V dropout, 40 °C ambient and an effective θ_JA of 60 °C/W. These are inputs for the calculation, not specifications of a selected component.

Reproduce the operating point in the LDO calculator →

How much load can the thermal budget support?

For a 125 °C junction limit, the assumed thermal path allows (125 − 40)/60 = 1.4167 W. After subtracting 5 mW for ground current, the model allows 1.4117/1.7 = 830.4 mA. That is a thermal estimate only: the IC’s current rating, dropout at that current and protection behaviour still have to permit it.

Changing only the input to 12 V makes dissipation 4.362 W and efficiency 27.44%. The linear temperature estimate becomes 301.7 °C, far beyond the entered limit. That calculation identifies an infeasible operating point; it does not predict stable operation at that temperature.

See why the direct 12 V linear conversion exceeds the thermal budget →

Dropout is a separate limit

The nominal headroom at 5 V is 1.7 V, comfortably above the illustrative 0.3 V dropout. Reducing the input improves efficiency, but sufficient headroom must remain at minimum input, maximum output, peak load and the relevant temperature. Once the pass element cannot maintain regulation, the efficiency formula no longer describes the requested regulated output. ^[4]

Use the right thermal and supply assumptions

θ_JA depends on the test board and environment. Datasheet values help compare conditions, but your copper area, vias, airflow and nearby heat sources change the result. θ_JC is a different metric and cannot simply replace θ_JA in the ambient-temperature calculation. ^[3]

A buck feeding an LDO can reduce the voltage dropped in the linear stage. For example, an assumed 90%-efficient buck feeding the 5 V example gives a combined efficiency of 0.90 × 0.6587 ≈ 59.28%, with losses distributed between the stages. Choose the intermediate rail with dropout margin and check the LDO’s PSRR at the buck’s switching frequency; the cascade does not guarantee a quiet output. ^{[2, 4]}

Compare the currents and losses of a buck-plus-LDO supply tree →

Tools used in this guide

• LDO Regulator Power Dissipation – Linear regulator dissipation, junction temperature, efficiency and dropout headroom, with the maximum load for a Tj limit.
• Power Tree Planner – Budget source and regulator currents across cascaded rails and loads, estimate losses, check ratings and record power-up dependencies.
• Copper Area for Heat Spreading – How much copper a hot part needs: copper-pour thermal resistance by convection and radiation, junction temperature and the smallest pour for a Tj limit.
• Junction Temperature – Junction temperature from power and the thermal-resistance chain (θJA or θJC + θCS + θSA), and the maximum power for a Tj limit.

Sources

1. Analog Devices, Understand Low-Dropout Regulator Concepts to Achieve Optimal Designs: efficiency, ground current, headroom and dropout.
2. Analog Devices AN-140, Basic Concepts of Linear Regulator and Switching Mode Power Supplies: linear versus switching power conversion.
3. Texas Instruments SPRA953D, Semiconductor and IC Package Thermal Metrics: limitations of θJA and correct uses of thermal metrics.
4. Analog Devices, How to Successfully Apply Low-Dropout Regulators: operating conditions, dropout and noise-sensitive supplies.

Related guides

• How Much Copper Does a Regulator Need? PCB Heat Spreading in Numbers
• Thermal Via Arrays: Barrel Geometry, Filling and Temperature Drop
• Buck Converter Formulas: Duty Cycle, Inductor and Capacitors, Worked Through

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