An LDO regulator (low-dropout regulator) is a type of voltage regulator that takes a higher DC voltage and converts it to a stable, lower DC voltage, even when the difference between input and output is very small. Where older linear regulators need the input to be at least 2 volts above the output, an LDO can work with a gap of less than 300 millivolts, and some modern designs manage below 100 millivolts. That small required gap is the “low dropout” the name refers to.
How an LDO Works
An LDO has three core parts working together in a feedback loop: a voltage reference, an error amplifier, and a pass element (a transistor that controls how much current flows to the output). The voltage reference sets the target. The error amplifier constantly compares a sample of the actual output voltage against that reference. If the output dips too low, the amplifier drives the pass transistor harder, allowing more current through. If the output rises too high, it backs off. This adjustment happens continuously, keeping the output voltage locked to the target.
The output voltage itself is set by a pair of resistors that divide the output and feed a fraction of it back to the error amplifier. By changing the ratio of those resistors, a designer can select different output voltages from the same LDO circuit.
What Makes It “Low Dropout”
The dropout voltage is the minimum difference between input and output that the regulator needs to keep working properly. A standard linear regulator using an older transistor design might need the input to sit 2 volts above the output. That’s fine when you’re making 2.5 V from a 5 V supply, but useless when you need 3.3 V from a 3.6 V lithium-ion battery.
LDOs achieve their lower dropout by using a different transistor arrangement. Traditional regulators use what’s called an emitter follower topology, which inherently wastes more voltage. LDOs instead use an open-collector or open-drain topology, where the transistor can be driven much closer to full saturation with the voltages available. The result is that the transistor itself wastes very little voltage, letting the regulator operate with input and output nearly equal.
Efficiency and Heat
Every LDO converts the voltage difference between input and output into heat. The power it wastes is straightforward to calculate: multiply the voltage drop (input minus output) by the current flowing through. If you’re dropping 1 volt at 500 milliamps, the LDO dissipates half a watt as heat.
This means LDOs are most efficient when the input voltage is only slightly above the output. A 5 V to 3.3 V conversion at 1 amp wastes 1.7 watts, which is significant and requires attention to thermal management. But a 3.6 V to 3.3 V conversion at the same current wastes just 0.3 watts. The closer the input is to the output, the less heat you deal with and the higher the efficiency. For large voltage drops or high currents, a switching regulator is usually a better choice.
Why Noise Performance Matters
The biggest advantage LDOs hold over switching regulators is clean output. Switching regulators chop the input voltage on and off thousands of times per second, which inherently creates electrical noise, spikes, and ripple. An LDO has no switching action at all, so its output is far quieter.
LDOs also actively reject noise already present on their input. This ability is measured as power supply rejection ratio (PSRR), expressed in decibels. A well-designed LDO might suppress input noise by 60 dB or more at moderate frequencies, meaning the noise that reaches the output is roughly a thousand times smaller than what came in. Performance drops at higher frequencies, which is why some advanced designs push usable rejection up to 10 MHz to cover the range where switching regulators create the most interference.
The LDO’s own internal voltage reference is its primary source of self-generated noise, typically specified in microvolts. For most digital circuits, this is negligible. For sensitive analog loads, manufacturers offer “ultralow noise” versions specifically optimized for this spec.
Common Applications
LDOs show up wherever clean, stable power matters more than raw efficiency. A very common arrangement places an LDO after a switching regulator: the switcher handles the heavy lifting of a large voltage conversion efficiently, and the LDO cleans up the output ripple and noise before it reaches a sensitive load.
Specific examples of noise-sensitive loads that rely on LDOs include image sensors in DSLR cameras, infrared sensors in thermal cameras, radio frequency amplifiers, precision analog circuits in medical equipment, and clock and timing circuits in data centers. In battery-powered devices, LDOs are popular because the battery voltage is often only slightly above the voltage the circuit needs, making the efficiency penalty small while gaining the benefit of a simple, compact solution with no switching noise.
Quiescent and Shutdown Current
Two current specs on an LDO datasheet describe what the regulator itself consumes. Quiescent current is the current the LDO draws just to keep its internal circuitry running (the reference and error amplifier) when no load is connected. For low-power LDOs, this is typically in the microamp range. Ground current is often used interchangeably with quiescent current in datasheets, referring to the same idle consumption.
Shutdown current is different. It’s the tiny current the LDO draws when it’s disabled but still connected to the power source. For battery-powered designs where the device spends long periods in sleep mode, both quiescent and shutdown current directly affect battery life.
Output Capacitor and Stability
An LDO’s feedback loop can oscillate if the output capacitor isn’t chosen correctly. The capacitor’s internal resistance, called equivalent series resistance (ESR), plays a critical role. It introduces a stabilizing effect in the feedback loop that counteracts the phase shifts from other parts of the circuit. With the right ESR, a design can achieve a phase margin of 70 degrees or more, which is extremely stable.
If the ESR is too high (generally above 10 ohms), the stabilizing effect overshoots and the loop bandwidth extends into a range where other parasitic effects cause instability. If the ESR is too low, as is common with ceramic capacitors, the stabilizing effect shifts to a frequency too high to help, and the regulator oscillates. Most cases of LDO oscillation in real circuits trace back to the output capacitor’s ESR being outside the acceptable range. Every LDO datasheet specifies a range of acceptable capacitor values and ESR, and following those recommendations is essential.
Some newer LDO designs include internal compensation that makes them stable with very low ESR capacitors, including standard ceramics. These are sometimes marketed as “ceramic stable” and simplify the design process considerably.