What Is an LDO (Linear Regulator)?
An LDO (Low Dropout Regulator) is a linear voltage regulator that can work even when the voltage difference between input and output is very small. It's known for its quiet, clean output.
How Does It Work?
An LDO drives an internal pass transistor like a variable resistor. It drops the excess voltage across itself to hold the output constant; this difference turns into heat equal to voltage × current.
Dropout Voltage
The smallest input-output difference an LDO needs to regulate the output is called the dropout voltage. For example, an LDO with a 0.2 V dropout needs at least ~3.5 V input for a 3.3 V output.
| Pros | Cons |
|---|---|
| Very low noise | Low efficiency (heat) |
| Fast response | Only steps down |
| Simple, few parts | Weak at a large difference |
Where Is It Used?
Common in RF modules, ADC/sensor references, audio circuits and as a "cleanup" stage after a switching converter.
The Quiet-Operation Advantage
Since there's no switching in an LDO, it produces no high-frequency noise. So it's connected in series after a switching converter's output to "clean" the voltage. The higher its PSRR (power supply rejection) value, the better it suppresses noise on the supply.
Common Mistakes
- Large voltage difference: stepping from 12 V to 3.3 V with an LDO produces a lot of heat; there a buck converter is a better fit.
- Wrong capacitor: some LDOs need an output capacitor with a specific ESR.
- Ignoring dropout: if the input is too low the regulation breaks down.
Frequently Asked Questions
What's the difference between an LDO and a normal linear regulator?+
How is LDO efficiency calculated?+
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