LDO Packages and Thermal Management: Keeping the Pass Element Cool
An LDO is, by design, a device that converts unwanted voltage difference into heat. At low current and small headroom the heat is negligible, but as the load current or the input-to-output difference grows, the package and the printed circuit board become active parts of the power design. A regulator rated for 1 A on paper can deliver far less if its package cannot shed the heat, and the difference between parts with similar silicon often comes down to how well they are packaged. This article reviews the common LDO packages, the thermal resistance parameters, and the board-level design choices that actually limit the heat.
- Where the Heat Comes From
The power dissipated by an LDO is approximately the input-to-output voltage difference multiplied by the load current, plus the small quiescent power. At 5 V input to 3.3 V output at 500 mA, the dissipation is about 0.85 W. At 12 V to 3.3 V at the same current, it is nearly 4.5 W, which is impossible to handle in a tiny SOT-23. The designer must first choose whether the LDO is even a viable option for the expected dissipation, and then choose a package and board layout that keep the junction temperature within the rated limit, typically in the range of 125 to 150 degrees Celsius.
- Common Package Types
Small signal LDOs come in packages such as SOT-23, SC-70, and XDFN-style wafer-level packages, which are ideal for low-current applications where heat is not a concern. Higher-current parts move to SOT-223, DPAK, TO-252, and SOIC with an exposed thermal pad. The through-hole TO-220 family is still used in industrial and bench supplies where a heatsink can be attached. The package size is set by thermal needs more than by silicon needs: the die itself is small, but the package must spread the heat into the board.
- Theta-JA and What It Really Means
The junction-to-ambient thermal resistance, written theta-JA, is the number most often quoted on datasheets, but it is also the most misleading. It is measured on a standardized test board with a specific amount of copper, and a part quoted at 100 degrees Celsius per watt on the test board may run at 200 degrees Celsius per watt on a real two-layer board. Theta-JC, junction-to-case, is more useful when a heatsink is attached, and theta-JP, junction-to-pad, is relevant for exposed-pad packages. The practical lesson is to treat the datasheet thermal numbers as a starting point, not a guarantee, and to design in enough copper that the real thermal resistance comes out well below the quoted maximum.
- PCB Copper and Thermal Pads
The board is the main heatsink for most surface-mount LDOs. Several square centimeters of copper on the top layer, connected to the thermal pad with multiple vias to an inner plane or a bottom-layer copper area, can drop the effective thermal resistance by a factor of two or three compared with a small pad. The vias should be small, plentiful, and capped or filled to prevent solder wicking into them. For high-power parts, a dedicated ground plane under the package is the single most effective layout choice. The thermal pad must be soldered, not just tented, because the solder joint is the primary thermal path.
- Heatsinks and Forced Air
When the dissipation exceeds what the board can handle, a surface-mount heatsink can be attached to the package, or the part can be moved to a through-hole package with a bolt-on heatsink. Forced air further improves the cooling by lowering the effective ambient temperature. In sealed enclosures, the internal ambient can be much higher than the room, and the designer must derate the thermal resistance accordingly. A useful rule is to assume an internal ambient 20 to 40 degrees above the outside air in a sealed box, and to size the heatsink for that worst case.
- Protecting Against Thermal Overstress
Most modern LDOs include thermal shutdown, which disables the pass element when the junction exceeds a threshold, typically around 150 to 170 degrees Celsius, and re-enables it when the part cools. This prevents destruction but is not a substitute for good design: a part that cycles through thermal shutdown is unstable, noisy, and unpleasant for the user. The correct approach is to keep the normal operating junction temperature well below the shutdown threshold, with margin for hot days and blocked vents.
- Practical Heat Budget
A quick thermal check should be done before the part is chosen. Estimate the worst-case dissipation, estimate the effective theta-JA of the chosen package on the planned board, multiply to get the temperature rise above ambient, and add the worst-case internal ambient. If the result exceeds the rated maximum junction temperature, either reduce the headroom by lowering the upstream voltage, spread the dissipation across multiple regulators, move to a switching converter, or upgrade the package and copper. Heat budgets that look fine on paper often fail in the field because the real board has less copper than the datasheet test board.
Summary
- An LDO dissipates the product of input-to-output differential and load current as heat.
- Package size is mostly about heat spreading, not silicon size; SOT-23 parts are for low current, DPAK and exposed-pad SOIC for higher current.
- Theta-JA is measured on an idealized board, so real values are usually worse; design extra copper accordingly.
- A ground plane under the thermal pad, with multiple vias, is the most effective board-level cooling technique.
- Thermal shutdown protects the part but is not a design strategy; the normal operating temperature must stay well below the limit.
Top comments (0)