Linear Regulators

A linear regulator produces a steady DC output by continuously adjusting a pass device between the input supply and the load. It is simple, quiet, and predictable, but the unused voltage is converted directly into heat.

Learning Objectives

By the end of this lesson, you should be able to calculate linear-regulator power loss, check dropout and thermal limits, choose input and output capacitors, and decide when a linear regulator is better than a switching regulator.

Basic Operation

A linear regulator compares its output with an internal reference and changes the pass transistor drive until the output returns to the target voltage.

flowchart LR VIN[Input voltage] --> PASS[Pass transistor] PASS --> VOUT[Regulated output] REF[Reference] --> ERR[Error amplifier] FB[Feedback divider] --> ERR ERR --> PASS VOUT --> FB

If the input rises or the load current falls, the regulator reduces pass-device conduction. If the load current rises, it increases conduction. This feedback action keeps VOUT nearly constant.

Dropout Voltage

A regulator needs enough headroom between input and output:

$$
V_{IN(min)} \ge V_{OUT} + V_{DROP}
$$

A 7805 may need about 2 V of dropout, so a 5 V output usually needs at least about 7 V at the input. An LDO has lower dropout, sometimes a few hundred millivolts, but only under specified load current and temperature.

Power Loss and Efficiency

The main cost of a linear regulator is heat:

$$
P_{LOSS}=(V_{IN}-V_{OUT})I_{LOAD}
$$

Ideal efficiency is approximately:

$$
\eta \approx \frac{V_{OUT}}{V_{IN}}\times100%
$$

This assumes regulator quiescent current is small compared with the load.

Worked Example: 12 V to 5 V at 1 A

$$
P_{OUT}=5V\times1A=5W
$$

$$
P_{LOSS}=(12V-5V)\times1A=7W
$$

$$
\eta\approx\frac{5}{12}\times100%=41.7%
$$

Seven watts in a small package is usually too much without serious heat sinking. This design should probably use a buck regulator first.

Thermal Check

Datasheets specify thermal resistance from junction to ambient, often written as thetaJA in degC/W.

$$
T_J=T_A+P_{LOSS}\theta_{JA}
$$

If a regulator dissipates 0.6 W in a package with thetaJA = 60 degC/W at TA = 50 degC:

$$
T_J=50+0.6\times60=86degC
$$

That may be acceptable if the maximum junction temperature is 125 degC, but margin must include enclosure temperature, copper area, and nearby heat sources.

Capacitors and Stability

Most regulators need input and output capacitors close to the pins. They reduce source impedance, improve transient response, and keep the control loop stable.

Put the input capacitor between VIN and ground near the regulator.
Put the output capacitor between VOUT and ground near the regulator.
Check datasheet limits for capacitance and ESR, especially for older LDOs.
Use a bulk capacitor when the load changes quickly or wiring is long.

Fixed and Adjustable Regulators

Common fixed regulators include 7805, 7812, and many modern LDOs. Adjustable regulators such as LM317 use a resistor divider:

$$
V_{OUT}=V_{REF}\left(1+\frac{R_2}{R_1}\right)+I_{ADJ}R_2
$$

For LM317, VREF is typically about 1.25 V. The adjust-pin current term is often small but should be checked in precision designs.

When Linear Regulators Are a Good Choice

Low current loads.
Small difference between input and output voltage.
Noise-sensitive analog rails, ADC references, RF biasing, and audio circuits.
Post-regulation after a switching converter.
Simple prototypes and educational circuits.

When to Avoid Them

Battery products where efficiency matters.
High current with large voltage drop.
Enclosures with poor airflow or high ambient temperature.
Designs where the input may fall below dropout.
Products that must survive short circuits without overheating.

Common Mistakes

Checking output voltage but not power dissipation.
Ignoring dropout during battery discharge.
Using capacitors that violate ESR stability requirements.
Forgetting reverse current paths when output is externally powered.
Assuming thermal shutdown is normal operation.

Summary

Linear regulators are quiet and easy to use, but every volt dropped at load current becomes heat. A good design checks dropout, load current, capacitor requirements, thermal rise, and fault behavior before choosing a package.

Mind Map

mindmap root((Linear Regulator)) Core concept Feedback controls pass device Extra voltage becomes heat Quiet DC rail Applications Analog supply MCU prototype Post regulator Reference rail Formulas Ploss equals Vin minus Vout times Iload Efficiency approx Vout over Vin Vin min equals Vout plus Vdrop Tj equals Ta plus Ploss thetaJA Design rules Check dropout Place capacitors close Verify ESR Size thermal path Practical checks Load step Battery low input Package temperature Startup behavior Common mistakes No heat calculation Wrong capacitor ESR Too little headroom Thermal shutdown reliance