LDO or Switching Regulator: Choosing for Noise and Efficiency
The choice between a linear regulator and a switching regulator is usually presented as efficiency against noise. The real decision involves the drop out, the thermal path and where in the chain the conversion happens.
The Fundamental Difference
A linear regulator dissipates the difference between the input and the output as heat. Its efficiency is the ratio of the output voltage to the input voltage, so a large difference is a large loss.
A switching regulator converts with an efficiency that is nearly independent of the ratio, at the cost of a switching node that radiates and a ripple on the output.
The linear regulator’s output noise is very low, which makes it the natural choice for an analog or radio supply. The switching regulator’s noise can be filtered, and the filter adds cost and board area.
Drop Out and Headroom
A linear regulator requires a minimum difference between input and output to regulate, which is the drop out voltage. A low drop out regulator works with a small difference, which allows a lower input voltage and therefore less loss.
The drop out matters at the end of a battery discharge, where the input approaches the output. A regulator with insufficient headroom will let the output fall, which appears as a device that resets before the battery is empty.
The headroom also matters for a switching regulator, though in a different way, since it needs a minimum input to start and to drive its own circuitry. Our power integrity notes describe how the supply chain is arranged.
Thermal Considerations of a Linear Regulator
The power dissipated is the voltage difference times the current. That heat must leave the package, and the thermal resistance from the junction to the ambient determines the temperature rise.
A surface mount package with a thermal pad conducts heat into the board, which becomes the heatsink. The copper area around the pad and the thermal vias beneath it are part of the thermal design, and the calculation must include them.
Where the dissipation is large, a linear regulator requires a copper area that may be larger than a switching regulator’s entire circuit. The comparison is therefore not only efficiency but board area. Our switching regulator notes describe the layout requirements of the alternative.
A linear regulator is right where the voltage difference is small, the current is modest and the noise requirement is tight. The efficiency loss is acceptable and the simplicity is valuable.
A switching regulator is right where the voltage difference is large, the current is high or the input varies over a wide range. The filtering required for a sensitive load is part of its cost.
A combination is often best: a switching regulator for the conversion and a linear regulator after it to clean the supply for the sensitive part. The linear regulator then sees a small difference and dissipates little. Our component reliability notes describe the tolerance budget for the chain.
A switching regulator’s output ripple can be reduced by the choice of the switching frequency, the inductor and the output capacitor. A higher frequency allows a smaller inductor and produces a ripple that is easier to filter.
The noise that matters is often not the output ripple but the radiated field from the switching loop and the conducted noise on the input. Both are controlled by the layout rather than by the component values.
The feedback node is the most sensitive point in the regulator, because it carries a small signal and runs from the output back to the controller. It should be routed away from the switch node and the inductor.
The verification is a measurement of the output ripple and the efficiency at the load extremes, and a measurement of the conducted and radiated emission.
For a sensitive load, the noise should be measured at the load rather than at the regulator output, since the trace and the decoupling change what the load sees.
The temperature of the regulator should be measured at the maximum load and the maximum ambient, and compared against the junction limit with the thermal path accounted for. Our design release checklist notes where the results are recorded.
Process Control and Verification
On a design of this kind, switching regulator is the item that decides how the rest of the board is arranged. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.
Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end.
Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.
Process Control and Verification
On a design of this kind, switching regulator is the item that decides how the rest of the board is arranged. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.
Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch.
Process Control and Verification
On a design of this kind, switching regulator is the item that decides how the rest of the board is arranged. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.
Is a linear regulator always quieter? Its output noise is low and it also has a high power supply rejection ratio, which means it removes the input noise. That combination is why it is used after a switching stage.
Can a switching regulator be used for a radio supply? It can, with careful layout and filtering. The risk is the radiated field rather than the output ripple.
What does gopcb provide for regulator selection? We provide a comparison of efficiency, drop out, thermal path and noise for the actual load, layout for the chosen topology, thermal measurement at the load extremes, and noise measurements at the load rather than at the regulator.
