LDO
An LDO (Low-Dropout regulator) takes a slightly-higher input and produces a cleaner, lower-noise output, with very low headroom (often ≤ 200 mV). It's the right choice for noise-sensitive analog circuits.
An error amplifier compares the output to a band-gap reference, and gates a P-MOSFET (or PNP) pass transistor to deliver exactly the right amount of current. The pass element burns the difference between input and output as heat.
In plain terms
Like a faucet that drops a higher pressure down to a usable lower pressure — but converts the difference to heat. Good for clean rails, bad for big drops.
Why designers use it
- Drop a noisy 5 V rail to a clean 3.3 V rail for sensitive ADCs and RF chips.
- Provide quiet bias rails to op-amp and audio stages.
- Replace a switcher when efficiency matters less than EMC and noise.
Best for
- Clean analog rail
- RF chip supply
- Low-noise bias
Key specifications
- Vout range: 0.6 V – 36 V (Fixed or adjustable)
- Iout: 50 mA – 5 A
- Dropout: 30 mV – 1 V (At rated current)
- PSRR @ 1 kHz: 60 – 90 dB
- Quiescent current: 1 µA – 5 mA
When not to use it
- Where the input is much higher than the output — most of the energy becomes heat. Use a buck.
- Where battery life is critical and load currents are large.
Common mistakes
- Skipping the input/output ceramic caps — many LDOs oscillate without specified ESR caps.
- Picking an LDO with low PSRR for a noisy input — it'll pass the noise straight through at HF.
Where you will find it
- A pulse-oximeter uses an LDO to power its photodiode amplifier from a noisy lithium cell: the switching buck that runs the display would couple ripple into the nanoamp-level photodiode signal, so a 20 µA quiescent-current LDO post-regulates to a clean rail that keeps SpO₂ measurement accurate.
- A GPS receiver's RF front-end draws its supply from a 50 dB PSRR LDO: the antenna amplifier needs the supply quiet to below −150 dBm noise floor, and a switcher would inject switching harmonics directly into the 1575 MHz receive band.
- A wearable heart-rate monitor uses an ultra-low-dropout regulator (150 mV dropout) to squeeze every last milliamp-hour from a coin cell: the LDO keeps the sensor alive right down to 2.0 V as the cell dies, extending battery life by 30 % compared to a 300 mV LDO.
A short history
The Low-Dropout (LDO) linear regulator is a refinement of the classical series-pass linear regulator, using a P-channel MOSFET or PNP pass transistor that allows the regulator to operate with the input only millivolts above the output. Robert Dobkin at National Semiconductor formalised the LDO concept in a 1977 Electronic Design article; National's earlier LM330 / LM2930 (1976) were the first commercial LDO regulators, and the LM2940 family that followed in the 1980s became one of the most widely deployed automotive 1 A LDOs, targeting equipment where every volt of headroom mattered. Linear Technology, Maxim, and Texas Instruments expanded the LDO category through the 1990s with low-noise, high-PSRR variants suited for clocking, RF, and ADC reference applications. Today LDOs are used everywhere a clean, low-noise rail is needed downstream of a switching regulator—almost universally for analogue, RF, and clocking domains.
Good to know
- An LDO's headroom efficiency = Vout / Vin — drop 5 V to 3.3 V and you only get 66 % no matter how good the part is. The rest is heat.
- PSRR is what makes LDOs the favourite for clean analog rails downstream of a noisy switcher; 80 dB of supply rejection at 1 kHz is typical.
- The classic LM317 (1976) is still produced; its three-pin 1.2 V – 37 V adjustable output is the textbook 'first regulator' for hobbyists.