Power Supply Basics
Linear regulators versus switching converters, why decoupling capacitors are everywhere, and how to choose the right power architecture for an embedded board.
Every embedded board needs to turn whatever voltage it's given — a battery, a USB port, a wall adapter — into the clean, regulated rails its ICs expect. The capacitors and inductors from the previous sub-lesson are the building blocks of how that's actually done.
Linear regulators
A linear regulator (including the common LDO, "low-dropout" variant) works like a variable resistor that continuously adjusts itself to hold the output voltage constant. It's simple, cheap, and produces very little output noise — which is why it's the default choice for powering sensitive analog circuitry.
The cost is efficiency: a linear regulator dissipates the voltage difference as heat.
efficiency ≈ V_out / V_in
power dissipated = (V_in − V_out) × I_loadStepping 5 V down to 3.3 V at 500 mA dissipates (5 − 3.3) × 0.5 = 0.85 W as heat — fine with a small
heatsink, but a large 12 V-to-3.3 V drop at any real current would need active cooling.
Switching regulators
A switching regulator (buck, boost, or buck-boost) rapidly switches a transistor on and off, using an inductor and capacitor to smooth the result into a steady output voltage. Because energy is transferred rather than dissipated, efficiency is typically 85–95% regardless of the voltage difference — but switching at hundreds of kHz to MHz introduces its own high-frequency noise that has to be filtered.
| Linear (LDO) | Switching (buck/boost) | |
|---|---|---|
| Efficiency | Low when ΔV is large | High, largely independent of ΔV |
| Output noise | Very low | Higher, at the switching frequency |
| Component count | Minimal | More (inductor, diode/sync switch, caps) |
| Best for | Sensitive analog rails, small ΔV | Battery-powered systems, large ΔV |
Decoupling, again
Every IC power pin needs a local decoupling capacitor — typically 100 nF placed as close to the pin as the layout allows, sometimes backed by a larger 1–10 µF bulk capacitor per board section. This isn't optional polish: without it, the fast current spikes a digital IC draws on every clock edge cause local voltage sag that can corrupt logic levels or trigger resets.
Why this matters in practice
Choosing a power architecture is a real design decision, not a default: a battery-powered sensor node wants switching regulators for efficiency, while the analog front end of a precision measurement circuit often wants a clean linear rail even at the cost of some wasted power. Most real boards use both — switching regulation for the bulk power budget, with linear regulators downstream for noise- sensitive subsystems.
With the electrical foundations in place, the next lesson moves into firmware itself: Embedded Firmware Basics.