◈ Glossary · Concept

LDO
Low-Dropout linear regulator

Concept Board-level reference April 2026

What it is

An LDO (Low-Dropout Regulator) is a linear voltage regulator IC that converts a higher input voltage to a lower, precisely controlled output voltage with minimal voltage drop across the regulator itself. The defining characteristic is a very small differential voltage (dropout voltage) between input and output—typically 100–500 mV at rated load—versus the 1–2 V dropout of older regulators.

LDOs use a high-gain error amplifier and series pass transistor to maintain constant output. The pass transistor is sized and biased such that it operates in a nearly saturated state, dissipating less power and running cooler than traditional linear regulators. They accept a wide input range and deliver a stable reference voltage, making them essential for generating secondary rails from primary bus voltages on board-level systems.

Common applications on logic boards: generating 3.3 V, 1.8 V, 1.2 V, and specialty voltages from a single higher input rail (usually PPBUS or 5 V). Unlike switching buck regulators, LDOs produce low output noise—critical for sensitive analog and RF circuits—but dissipate more heat under high load.

In practice

When troubleshooting a board, you encounter LDOs at every secondary rail distribution point. A failed or missing LDO causes an entire secondary rail to collapse, killing processors, memory, or power sequencing logic downstream.

Typical fault scenario: PP3V3_S5 rail is missing. You trace back from the load and find an AMS1117 (common 1A LDO) fed by PPBUS. Probe its input (VIN) and output (VOUT) pins with your meter. If VIN is present at 5.0 V but VOUT reads 0 V, the LDO is shorted, open, or disabled. If both are 0 V, check the enable pin (EN)—it may be pulled low by a failed control IC or held in shutdown state.

Measurement approach: Always measure from the LDO output pin directly to ground, not across a load resistor. LDOs exhibit output impedance; a voltmeter across a discharge resistor may read artificially low. If the IC is LQFP or BGA, use the via pads or test points marked on the schematic. Check capacitor values: undersized output caps can cause oscillation and unstable voltage.

Heat dissipation failure: An LDO under sustained high load (e.g., sourcing 500 mA from 5 V to 3.3 V) dissipates 0.85 W. If thermal pad is not soldered, IC temperature climbs and thermal shutdown engages, cutting output. You'll see intermittent dropout, then recovery as it cools. Reflow the thermal pad or replace the IC with proper thermal interface material (TIM).

Common LDO IC Max Input Typical Output Max Current Dropout @ max load
AMS1117 15 V 1.2–5.0 V 1000 mA 1.3 V @ 1A
TPS71533 5.5 V 3.3 V, 1.8 V 500 mA 220 mV @ 500 mA
LP8550 28 V 3.3 V, 5.0 V 1500 mA 400 mV @ 1.5A
MCP1700 6.5 V 3.3 V, 2.5 V 250 mA 178 mV @ 250 mA
Always verify the enable pin state. Many LDOs have active-low SHUTDOWN or EN pins tied to power sequencing logic. A disabled LDO supplies zero current regardless of input. Check the control IC driving that pin—it may be dead or held in reset.
LDO output noise is crucial for sensitive analog circuits. If a precision ADC, audio codec, or RF chip fed by an LDO behaves erratically (glitching, skewed readings), suspect a failing LDO capacitor or a switching regulator too close by. Measure VOUT with an oscilloscope on AC coupling—expect ripple under 50 mVpk.

See also

Related terms in this glossary:

  • VRM — Voltage Regulator Module; buck converters for CPU core rails.
  • Decoupling Capacitor — Output filtering; critical for LDO stability.
  • PWM — Pulse-Width Modulation; switching regulators (higher efficiency alternative).
  • MOSFET — Pass transistor inside LDO die; subject to damage and thermal failure.