Regulators — LDO vs Switching#

Every embedded system needs at least one voltage regulator to convert a supply rail into the clean, stable voltage the MCU and its peripherals require. The two fundamental topologies — linear (LDO) and switching — differ in efficiency, noise, size, and cost. Choosing the wrong one wastes power, introduces noise into sensitive circuits, or generates more heat than the board can dissipate.

LDO Fundamentals#

A low-dropout regulator (LDO) works by burning excess voltage as heat. The power dissipated is straightforward: P = (Vin - Vout) * Iload. Converting 5V to 3.3V at 200mA dissipates (5 - 3.3) * 0.2 = 340mW — manageable in a SOT-223 package. Converting 12V to 3.3V at the same current dissipates 1.74W, which exceeds most small-package thermal limits without a copper pour or heatsink.

To determine whether a given package can survive that dissipation, combine the power calculation with the package’s thermal resistance. For example, a SOT-23-5 package with Rth_ja of 250°C/W dissipating 340mW raises the junction temperature 85°C above ambient. At a 40°C ambient, the junction reaches 125°C — right at the maximum for most regulators. Switching to a SOT-223 (Rth_ja ~60°C/W) brings the junction rise down to ~20°C, a much safer margin.

The “dropout voltage” is the minimum Vin - Vout the regulator can sustain while keeping output in regulation. The AMS1117-3.3, a ubiquitous cheap LDO, has a 1.0V dropout — it needs at least 4.3V in to produce a stable 3.3V. The AP2112-3.3 drops out at just 250mV and draws only 55µA quiescent current, making it far better suited for battery-powered designs where every milliamp matters.

Output Capacitor Stability#

Not all ceramic capacitors are suitable for every LDO. Many older LDO designs rely on a minimum amount of equivalent series resistance (ESR) in the output capacitor for loop stability. A very low-ESR ceramic capacitor (often below 10 milliohms) can push the regulator into oscillation, producing ripple at hundreds of kilohertz on what should be a clean rail. Tantalum capacitors, with ESR typically in the 100–500 milliohm range, naturally satisfy this requirement. Newer LDO families — including the AP2112 and MCP1700 — are explicitly designed for ceramic capacitor stability, specified down to 1µF X5R output capacitance with no minimum ESR. Always verify the LDO datasheet’s capacitor requirements before selecting output components.

Switching Regulator Basics#

Switching regulators use an inductor and high-frequency switching (typically 500kHz–2MHz) to convert voltages with 85–95% efficiency. A buck converter steps voltage down; a boost converter steps it up; a buck-boost handles both directions. The TPS563200 is a widely used synchronous buck converter that delivers up to 3A at 3.3V from inputs up to 17V, achieving roughly 90% efficiency at moderate loads — dissipating only ~440mW versus the 10.4W an LDO would waste at the same operating point. Its integrated high-side and low-side FETs simplify layout, making it a common choice for cost-sensitive designs.

The TPS63001 is a buck-boost converter commonly used with single-cell LiPo batteries. It maintains a 3.3V output whether the battery is at 4.2V (freshly charged) or 2.5V (near cutoff), seamlessly transitioning between buck and boost modes as the cell discharges.

Efficiency Curve Shape#

Switching regulator efficiency is not a single number — it varies with load current. At moderate-to-high loads (roughly 10–80% of rated output), efficiency peaks in the 85–95% range. At very light loads (below a few milliamps), switching losses and quiescent draw dominate, and efficiency can drop to 30–50%. Some converters offer pulse-frequency modulation (PFM) or pulse-skip mode at light loads to reduce switching losses, maintaining 70–80% efficiency down into the microamp range. By contrast, an LDO converting 3.7V to 3.3V achieves a fixed efficiency of Vout/Vin = 89%, regardless of load current — making LDOs the more efficient choice when the dropout is small and the load is light.

Noise Characteristics#

LDOs produce inherently clean output — their output noise is typically 10–50µV RMS, making them ideal for powering ADCs, audio codecs, and RF front-ends. Switching regulators generate ripple at their switching frequency, typically 10–50mV peak-to-peak before filtering. In mixed-signal designs, a common architecture uses a switching regulator for bulk power conversion, followed by an LDO on the analog supply rail to reject switching noise.

When to Use Which#

For low current draw (under ~300mA) with a small Vin-Vout differential, an LDO is simpler, cheaper, and quieter. For higher currents, large input-output differentials, or battery-powered systems where efficiency directly determines runtime, a switching regulator is the right choice. Many production boards use both — a buck converter for the main 3.3V rail and an LDO for noise-sensitive analog power.

Tips#

Calculate LDO power dissipation before committing to a package — (Vin - Vout) * Imax gives worst-case thermal load, and exceeding the package rating causes thermal shutdown or reduced lifespan
For battery-powered designs, select LDOs with quiescent current under 100µA — parts like the AP2112 (55µA) or MCP1700 (1.6µA) extend standby battery life significantly
Place the switching regulator’s inductor and input capacitor as close to the IC as possible — long traces add parasitic inductance that degrades efficiency and increases EMI
Use the regulator’s datasheet-recommended capacitor values and types — ceramic X5R or X7R for switching regulators, as Y5V capacitors lose too much capacitance at DC bias
When in doubt about noise sensitivity, add a ferrite bead and 10µF capacitor between a switching regulator output and an analog supply pin

Caveats#

LDO thermal limits sneak up on higher input voltages — A regulator that runs cool at 5V-to-3.3V may overheat at 9V-to-3.3V with the same load, because power dissipation scales linearly with the voltage differential
Switching regulator efficiency curves are not flat — Datasheets show peak efficiency at a specific load point; at very light loads (under 1mA), many switching regulators drop to 50–60% efficiency, worse than an LDO
The AMS1117 is not actually low-dropout — Despite appearing on countless breakout boards, its 1.0V dropout makes it a poor choice for 3.3V output from a 3.7V LiPo cell that sags to 3.5V under load
Switching regulators require careful PCB layout — A poorly routed buck converter can emit enough electromagnetic interference to corrupt nearby SPI or I2C signals, even if the schematic is correct

In Practice#

A board that works on USB power but resets intermittently on a 3.7V LiPo often has an LDO with too much dropout — as the battery voltage sags under load, the regulator falls out of regulation and the MCU browns out
An ADC reading that shows periodic noise at a fixed frequency (e.g., 500kHz ripple) commonly traces back to a switching regulator on the same board — adding an LC filter or replacing the analog supply with an LDO resolves it
A regulator that runs warm under light load but becomes too hot to touch at full system current is dissipating more than the package can handle — switching to a buck converter or adding thermal relief copper is the corrective step
A circuit that draws 200mA continuously from a CR2032 coin cell through an LDO and dies in hours instead of days is dominated by the LDO’s operating current plus the wasted heat — a micropower buck converter dramatically extends runtime
An LDO output that oscillates at 200–400kHz when loaded, visible on a scope as clean ripple rather than random noise, typically indicates that the output capacitor ESR is too low for the regulator’s compensation loop — replacing the ceramic output cap with a tantalum, or switching to a ceramic-stable LDO, resolves the oscillation