Linear Reg.
A series linear regulator holds its output near a target below its input. The pass element dissipates power, so thermal design and input-output headroom are central constraints.
An error amplifier compares feedback with a reference and adjusts the pass element. A useful load-related heat estimate is (Vin − Vout) × Iload, with regulator supply current adding further loss. The exact device determines dropout, stability, transient response and permitted output capacitors.
In plain terms
A controlled valve that adjusts how much voltage is dropped. It can regulate only while the source and load stay within its limits.
Why designers use it
- Making a suitable lower-voltage rail with a relatively simple circuit.
- Providing low-noise regulation where the selected device's noise and rejection meet the requirement.
- Post-regulating another supply when sufficient headroom is available.
- Meeting transient requirements with an appropriately selected and tested regulator.
Best for
- Analog rails (ADC, op-amp)
- Low-noise post-regulation after a switcher
- Tiny current loads
Key specifications
- Dropout: At the intended load and temperature
- Thermal limits: Input-output differential, load and board cooling
- Stability: Required capacitance, ESR and operating conditions
- Noise and rejection: Check frequency, bandwidth and test conditions
- Transient response: Check the actual load-step and capacitor conditions
When not to use it
- When the output must be higher than the input.
- When calculated power dissipation or temperature exceeds the design limits.
- When input headroom, output-current, stability or transient requirements cannot be met.
Common mistakes
- Treating an LDO as zero-dropout.
- Ignoring the exact output-capacitor and stability requirements.
- Checking nominal current but not worst-case dissipation and junction temperature.
- Assuming all linear regulators have the same noise, PSRR or transient response.
Where you will find it
- ADI AN-140 describes generating 3.3 V from a 12 V input and explains why the pass element's heat loss matters.
- A post-regulator can reduce supply disturbances only within its documented rejection, frequency and headroom limits.
A short history
A series linear regulator adjusts a transistor's conduction to hold its output near a reference voltage. Feedback corrects for changes in input voltage or load within the regulator's limits. The voltage dropped across the pass transistor produces heat, so efficiency and thermal design matter when the input is much higher than the output.
Good to know
- Ignoring supply current, dropping 12 V to 5 V at 1 A produces 7 W of regulator heat.
- An LDO is a type of linear regulator, not a separate loss-free topology.