Thermal Design and Junction Temperature in LDOs
An LDO turns the voltage difference between its input and output into heat inside the package. That heat raises the die temperature, and if the die gets too hot the device either shuts down to protect itself or, in the worst case, suffers accelerated aging. Thermal design is therefore not an afterthought for a linear regulator; it is the calculation that determines whether the part can deliver the rated current at all. This article explains how power dissipation becomes heat, how to read thermal resistance, how to estimate junction temperature, and how the PCB and package spread the heat.
- Where the Heat Comes From
The power dissipated by an LDO is, to a first approximation, the input-to-output voltage difference multiplied by the load current, plus a small term from its own quiescent current. For example, dropping 5 V to 3.3 V at 500 mA dissipates about 0.85 W, and dropping 12 V to 3.3 V at 500 mA dissipates over 4 W. Unlike a switching converter, which stores and transfers energy efficiently, a linear regulator burns this power continuously as long as the load is on. This is why LDOs are chosen for small voltage differences: the smaller Vin minus Vout, the less heat. The quiescent term is usually negligible at moderate loads, but at light load it becomes part of the total dissipation.
- Thermal Resistance: The Path From Die to Ambient
Heat flows from the silicon junction, through the die attach, the package, and the board, to the surrounding air. This path has an effective thermal resistance, expressed in degrees Celsius per watt. The two parameters on a datasheet are theta-JA, the resistance from junction to ambient, and theta-JC, the resistance from junction to the outside of the package case. Theta-JA is the number a board designer usually uses, because the part sits on a PCB rather than on a perfect heatsink. Theta-JC is relevant only when a real heatsink or a well-coupled metal surface is attached. Small SOT-23 or DFN packages have relatively high theta-JA, on the order of 100 to 200 degrees C per watt, while larger packages or those with an exposed pad can be several times lower.
- Calculating Junction Temperature
The calculation is a direct analogy to Ohm's law. The junction temperature equals the ambient temperature plus the power dissipation multiplied by the thermal resistance: Tj equals Ta plus P times theta-JA. For example, a part dissipating 0.8 W with a theta-JA of 120 degrees C per watt, in a 50 degrees C ambient, reaches about 146 degrees C. Most silicon is rated for a maximum junction temperature of 125 or 150 degrees C, so this example is already above the safe limit and would need either a lower dissipation, a better package, or more copper on the board. The designer should always perform this calculation at worst case: maximum input voltage, maximum load current, and maximum ambient temperature, including internal enclosure heating.
- Why the PCB Matters
The theta-JA quoted on a datasheet is almost always measured on a standard test board with a defined amount of copper. In a real product, the copper area connected to the thermal pad, the ground plane underneath, and the number of vias all change the actual resistance. A package with an exposed thermal pad relies entirely on the PCB copper to drain heat; with no copper it behaves like a tiny package. Practically, enlarging the copper area around the device, tying it to a ground plane, and adding multiple vias can halve the effective thermal resistance. This is why package choice and layout go together: a small DFN with a good board can outperform a larger package on a poor one.
- Heatsinking and Derating
When the dissipation exceeds what the PCB can handle, the options are to reduce the dissipation or to add thermal help. Reducing the voltage difference, for example by using a lower input rail or a switching pre-regulator, is usually the most effective step. If the difference cannot be reduced, a larger package with lower theta-JA, an exposed pad, or a bolted heatsink may be needed. At high ambient temperatures, the allowable dissipation shrinks linearly: as the ambient approaches the maximum allowed junction temperature, the part must be derated. Datasheets provide a derating curve that plots maximum allowable dissipation against ambient temperature, and that curve is the practical limit.
- Thermal Protection Behavior
Nearly all LDOs include internal thermal shutdown: if the junction exceeds a threshold, typically around 150 to 170 degrees C, the part turns off to protect itself, and it restarts when the die cools by some hysteresis, often tens of degrees. This prevents destructive failure during a fault, but repeated thermal cycling is not healthy for the system. A design that relies on thermal shutdown during normal operation is a design that is too hot, and it should be reworked. Thermal shutdown is a fault backstop, not an operating mode.
Summary
- An LDO dissipates roughly the input-to-output voltage difference times the load current as heat.
- Junction temperature is estimated as ambient temperature plus dissipation times the thermal resistance theta-JA.
- The PCB copper, ground plane, and vias are major parts of the heat path, especially for exposed-pad packages.
- Design worst case: maximum Vin, maximum load, and maximum ambient, and respect the derating curve.
- Thermal shutdown protects the device but signals an over-heated design; reducing Vin minus Vout is usually the best cure.
