Not every rail on a laptop board is made by a switching converter. Small, low-current rails — for audio, clocks, references, some always-on logic and parts of the USB-C circuitry — are often made by linear regulators called LDOs. They are tiny, quiet and simple, which makes them easy to overlook. But when an LDO's rail is missing, the chip it feeds stops working, and when an LDO runs hot, it is usually warning you about something else. This guide explains how LDOs work, where to find them and how to diagnose them.
How a linear regulator works
An LDO places a transistor — the pass element — between its input and output. A control loop adjusts the transistor so that the output stays at the set voltage, whatever the input (above a minimum) and the load. The voltage difference between input and output is dropped across the transistor, and the power that represents becomes heat. The device is described under ldo.
"LDO" stands for low dropout: such regulators keep working with only a small difference between input and output.
Why boards use LDOs
| Advantage | Where it matters |
|---|---|
| Low noise | Audio codecs, clock generators, analog references |
| Simplicity | Small rails that need only a few parts |
| Small size | Tight areas where an inductor will not fit |
| Fast start | Early, always-on supplies |
Their disadvantage is efficiency: an LDO wastes the difference between input and output as heat. They suit small currents or small voltage drops.
Where to find LDOs
- Always-on low-power rails — a tiny LDO often makes the first supply for the EC before the main always-on converters start, as described in Always-On Rails (3VALW, 5VALW): What They Do.
- Audio — the codec's analog supply.
- Clocks and PLLs — clean supplies for timing circuits.
- USB-C — the PD controller's supply from VBUS on a flat battery.
- Charger ICs — many have an internal LDO output for their own drivers, brought out to a pin with a capacitor.
- Small device rails — card readers, sensors, some panel logic.
On the schematic, LDOs are usually small ICs with an input, output, ground, enable and sometimes a power-good pin, with capacitors on input and output and occasionally a feedback divider. Reading those pages is covered in Reading Voltage Regulator Pages in Laptop Schematics.
Dropout
An LDO needs its input a little above its output — the dropout voltage. If the input falls below output plus dropout, the output follows the input down. On a laptop board, a "low" LDO output can therefore mean a low input rather than a faulty LDO. Check the input first.
Stability and the output capacitor
The LDO's control loop relies on its output capacitor to be stable. The datasheet specifies the minimum capacitance and an acceptable range of equivalent series resistance (ESR). A missing, wrong or degraded output capacitor can make the output oscillate — appearing as a voltage that looks slightly wrong on a meter, or noise on a scope. ESR is described under esr. When replacing an LDO's output capacitor, use the value and type in the schematic.
Heat
An LDO's dissipation is the voltage it drops times the current it supplies. A small package can handle only so much before it overheats and shuts down. A hot LDO usually means one of:
- the load draws more than it should — often a partial short downstream;
- the input is higher than designed, increasing the drop;
- the design is simply working the part hard.
LDO Power Dissipation Calculator estimates the dissipation and junction temperature for a given input, output and load.
Diagnosing an LDO rail
With the board powered:
- Input — present and above the output plus dropout?
- Enable — at its active level?
- Output — at the expected voltage?
- Power-good (if present) — high?
With power removed:
- Output to ground — a short on the output stops the LDO or makes it current-limit and heat up.
- Input to ground — a shorted LDO can pull its input rail down too.
| Finding | Likely cause |
|---|---|
| Output missing, input and enable present, output not shorted | Failed LDO |
| Output missing, output shorted | Load or output capacitor |
| Output low, input low | Input rail problem, not the LDO |
| Output correct, LDO very hot | Excess load downstream, or input too high |
| Output slightly off and noisy | Output capacitor missing or wrong |
Enable and power-good on LDOs
Many LDOs have an enable input, driven by the EC, a sleep-state signal or another rail's power-good, and some have a power-good output of their own. They follow the same rules as larger regulators: the enable must be at its active level for the output to appear, and the power-good must be pulled up to show a high level. An LDO whose enable is tied to its input starts as soon as the input rail exists — common for always-on helpers.
Internal LDOs
Several larger chips contain LDOs inside and bring their outputs to a pin that needs only a capacitor: charger ICs, PD controllers, switching controllers and the EC itself often do this. The voltage on such a pin is a quick health check for the chip: if its input is present and the internal LDO output is missing, the chip is disabled or damaged. If the pin is shorted to ground through a failed capacitor, the chip cannot run at all — a common and easily fixed fault.
Failure modes
- Open — no output despite correct inputs; common after overheating.
- Shorted input to output — the output rises to the input voltage, which can damage chips on the rail.
- Shorted to ground — pulls the input rail down; the LDO gets hot under injection.
Replacing an LDO
Use the same part, or one with the same pinout, fixed output voltage (or adjustable with the same feedback arrangement), current rating, dropout, enable polarity and stability requirements. Tiny LDO packages often carry short top marks; read the part number from the schematic.
Example: no audio and a hot little chip
A laptop has no audio device in the operating system. The codec's digital supply is present, but its analog supply measures well below its nominal value. That analog supply comes from a small LDO, which is hot to the touch. Its input is correct. With power removed, the LDO's output reads only a few ohms to ground, and injecting a low voltage warms a capacitor next to the codec, not the LDO. Replacing the capacitor brings the analog supply to its proper value, the LDO cools down and the codec appears. The hot LDO was the symptom, not the cause.
FAQ
What does LDO mean?
Low dropout: a linear regulator that works with only a small difference between input and output.
Why do boards still use LDOs?
They are quiet, simple and small — ideal for low-current, noise-sensitive rails.
Why is my LDO hot?
Usually too much load downstream or an input higher than designed. Check for a partial short on the output.
Can a wrong capacitor affect an LDO?
Yes. LDOs need specific output capacitance and ESR for stability.
How do I tell an LDO from a switching regulator on the board?
An LDO has no inductor; a switching regulator always does.
Bottom line
LDOs make small, quiet rails by dropping the voltage difference as heat. Check input, enable and output, remember dropout and output-capacitor requirements, and treat a hot LDO as a pointer to excess load. Replace with parts that match voltage, current, dropout and stability needs.