Linear Voltage Regulators

Linear voltage regulators are fundamental building blocks of power supply design, providing stable, low-noise DC output voltages from higher and potentially variable input sources. Unlike switching regulators that use pulse-width modulation and energy storage elements, linear regulators operate by continuously dissipating excess power as heat through a series pass element, trading efficiency for simplicity, low noise, and excellent transient response.

From discrete series pass transistor circuits to highly integrated three-terminal regulators, linear regulation technology spans a wide range of complexity and capability. This article explores the principles, topologies, and practical considerations essential for designing and applying linear voltage regulators in electronic systems where clean, stable power is paramount.

Fundamentals of Linear Regulation

Linear voltage regulators maintain a constant output voltage by varying the resistance of a control element in series or parallel with the load. The regulator continuously compares the output voltage against a reference and adjusts the control element to compensate for changes in input voltage, load current, or temperature.

Basic Operating Principle

A linear regulator functions as a variable resistor controlled by a feedback loop. When the output voltage tends to rise (due to decreased load or increased input), the control element increases its resistance, dropping more voltage and maintaining the output. Conversely, when output tends to fall, the control element decreases its resistance. This continuous analog adjustment provides inherently clean output with no switching noise.

Key Performance Parameters

Several parameters characterize linear regulator performance:

Line Regulation: The change in output voltage for a given change in input voltage, typically expressed as millivolts per volt or as a percentage. Good regulators achieve line regulation below 0.01%/V
Load Regulation: The change in output voltage from no load to full load, expressed as millivolts or percentage. Values below 0.1% indicate excellent load regulation
Dropout Voltage: The minimum difference between input and output voltage required to maintain regulation. Standard regulators require 2-3V dropout; LDO types operate with less than 0.5V
Quiescent Current: The current consumed by the regulator itself, exclusive of load current. Critical for battery-powered applications
Output Noise: The AC component superimposed on the DC output, typically specified as microvolts RMS over a bandwidth. Linear regulators excel here compared to switching types
Ripple Rejection: The ability to attenuate AC components on the input, measured in decibels at a specified frequency

Efficiency Considerations

Linear regulator efficiency is fundamentally limited by the input-output voltage differential:

Efficiency = Vout / Vin (ideally, neglecting quiescent current)

For a 5V output from a 12V input, maximum efficiency is approximately 42%. The remaining 58% of input power dissipates as heat in the pass element. This thermal dissipation limits linear regulators to applications where the voltage differential is small or the load current is modest. However, the simplicity, low noise, and fast transient response often justify the efficiency penalty in appropriate applications.

Series Pass Regulator Design

Series pass regulators place the control element in series between the input supply and load, modulating the voltage drop to maintain constant output. This topology handles the majority of linear regulator applications and forms the basis for integrated regulator ICs.

Basic Series Pass Circuit

The fundamental series pass regulator comprises four essential blocks:

Reference Voltage: A stable voltage source, typically a Zener diode, bandgap reference, or precision reference IC, against which the output is compared
Error Amplifier: Compares a fraction of the output voltage to the reference and generates a control signal proportional to the error
Pass Element: A transistor (BJT or MOSFET) that acts as a variable resistance, controlled by the error amplifier output
Feedback Network: Usually a resistive voltage divider that scales the output voltage for comparison with the reference

NPN Pass Transistor Configurations

Traditional series regulators use an NPN transistor as the pass element, typically in common-collector (emitter follower) configuration. The error amplifier drives the base, and the emitter delivers current to the load. This arrangement offers several advantages:

Natural current gain: The transistor beta multiplies the error amplifier drive current, allowing a modest amplifier to control substantial load currents
Inherent stability: The emitter follower provides unity voltage gain with current gain, simplifying feedback compensation
Wide bandwidth: NPN emitter followers exhibit excellent high-frequency response

The primary limitation is dropout voltage. The NPN pass transistor requires its collector to be at least 1-2V above the emitter (output) to remain in the active region, plus any voltage dropped in current sensing resistors. Total dropout typically reaches 2-3V in practical implementations.

Darlington Pass Configurations

For higher current applications where the error amplifier cannot supply sufficient base current, a Darlington pair provides increased current gain. The two-transistor combination squares the effective beta, reducing drive requirements dramatically. However, the Darlington configuration adds approximately 0.6V to the dropout voltage and can exhibit thermal runaway concerns if not properly compensated.

PNP and P-Channel Pass Elements

Using a PNP transistor or P-channel MOSFET as the pass element allows the input voltage to appear directly at the pass device collector or drain, with the emitter or source connected to the output. This configuration achieves much lower dropout voltages, forming the basis for LDO (Low-Dropout) regulators discussed in a later section.

Shunt Regulator Configurations

Shunt regulators place the control element in parallel with the load rather than in series. The pass element diverts excess current away from the load to maintain constant voltage. While less common than series regulators for primary power supplies, shunt configurations excel in specific applications.

Basic Shunt Regulator Operation

In a shunt regulator, a series resistor limits current from the input supply. The shunt element (typically a transistor or Zener diode) maintains the output voltage by conducting whatever current the load does not require. When load current increases, shunt current decreases proportionally. When load current decreases, shunt current increases to absorb the excess.

Zener Diode Shunt Regulators

The simplest shunt regulator uses a Zener diode as both the reference and control element. The Zener maintains approximately constant voltage across its terminals as current varies within its operating range. Design considerations include:

Series resistor selection: Must provide adequate Zener current at minimum load while not exceeding maximum Zener current at no load
Power dissipation: The Zener dissipates maximum power at minimum load current
Dynamic impedance: Lower Zener impedance provides better regulation but requires higher current
Temperature coefficient: Zener voltage varies with temperature; 5.6V Zeners exhibit minimal tempco

Active Shunt Regulators

Replacing the Zener with an active circuit improves regulation dramatically. The TL431 adjustable precision shunt regulator exemplifies this approach, providing programmable output voltage with excellent accuracy and stability. Active shunt regulators find extensive use in:

Reference voltage generation: Creating precision voltages for ADCs, DACs, and measurement circuits
Feedback networks: Providing the reference and error amplifier functions in switching power supply feedback loops
Voltage clamping: Protecting circuits from overvoltage conditions
LED drivers: Maintaining constant current through LED strings

Shunt Regulator Limitations

Shunt regulators suffer from poor efficiency, especially at light loads where most input power dissipates in the shunt element. They also require careful matching between the series resistance and load current range. These limitations restrict shunt regulators to low-power applications or situations where their specific characteristics, such as simplicity, bidirectional operation, or overvoltage clamping, provide unique benefits.

Three-Terminal Regulator Applications

Three-terminal fixed-voltage regulators revolutionized power supply design by integrating reference, error amplifier, pass transistor, and protection circuits into a single package. The classic 78xx (positive) and 79xx (negative) series, introduced in the 1970s, remain ubiquitous despite their age, while modern variants offer improved performance.

Standard 78xx/79xx Regulators

The 78xx series provides fixed positive output voltages (5V, 6V, 8V, 9V, 10V, 12V, 15V, 18V, 24V) with current capability of 1A or more depending on the package. Key characteristics include:

Input/Output/Ground terminals: Simple connection requires only input and output capacitors
Dropout voltage: Typically 2V minimum, requiring input at least 2V above the rated output
Line regulation: Typically 0.01-0.1% for input variations
Load regulation: Typically 0.1-0.5% from no load to full load
Built-in protection: Thermal shutdown, current limiting, and safe operating area protection

The 79xx series provides corresponding negative voltages with similar specifications, though internal topological differences result in slightly different performance characteristics.

Capacitor Requirements

Proper capacitor selection ensures stability and performance:

Input capacitor: 0.33uF minimum recommended at the regulator input, placed physically close to the device. Larger values improve ripple rejection
Output capacitor: 0.1uF minimum for stability, though 10uF or more improves transient response. ESR requirements depend on the regulator type
Capacitor type: Ceramic capacitors work well for smaller values; aluminum electrolytics suit larger values but require attention to ESR

Enhanced Performance Configurations

Simple circuit additions extend three-terminal regulator capabilities:

Output voltage adjustment: Adding a resistor between the ground pin and ground raises the output voltage by lifting the reference point. The adjustment resistor forms a divider with the regulator's quiescent current path
Current boosting: An external pass transistor increases current capability beyond the regulator's rating while the IC provides regulation and protection
Foldback current limiting: External resistors can modify the current limit characteristic to reduce pass element dissipation during overload
Tracking regulators: Summing positive and negative outputs creates tracking dual supplies for operational amplifier circuits

Adjustable Voltage Regulators

Adjustable regulators offer flexibility to generate any output voltage within a specified range using external resistors to set the output. The LM317 (positive) and LM337 (negative) represent the most widely used adjustable linear regulators, though numerous alternatives exist with varying current capabilities and features.

LM317/LM337 Operating Principles

The LM317 maintains a constant 1.25V reference voltage between its output and adjustment (ADJ) terminals. A resistive divider between output and ground, with its midpoint connected to ADJ, sets the output voltage according to:

Vout = Vref * (1 + R2/R1) + Iadj * R2

where Vref is 1.25V, R1 connects from output to ADJ, R2 connects from ADJ to ground, and Iadj is the adjustment pin current (typically 50uA). For low values of R2, the Iadj term becomes negligible, simplifying the expression to:

Vout = 1.25V * (1 + R2/R1)

Practical designs use R1 values of 120-240 ohms to ensure adequate minimum load current through the regulator for stable operation.

Output Voltage Range and Accuracy

The LM317 output voltage adjusts from 1.25V (with R2=0) to 37V (limited by maximum input-output differential). The reference voltage tolerance of +-4% represents the dominant source of output voltage uncertainty. Precision applications may require individual calibration or selection of tighter-tolerance variants.

Protection and Bypassing

Recommended protection elements include:

Input protection diode: A diode from output to input prevents reverse voltage damage if the input collapses while output capacitors remain charged
ADJ pin bypass capacitor: A 10uF capacitor from ADJ to ground improves ripple rejection by 15-20dB and enhances transient response
Output diode: A diode from ground to output prevents the adjustment network from back-driving the regulator when output capacitors discharge

Current Regulator Applications

The LM317 also functions as an adjustable current regulator by placing a sense resistor between the output and ADJ terminals with no external load path for the ADJ pin. The output current becomes:

Iout = 1.25V / Rsense

This configuration suits LED drivers, battery chargers, and constant-current loads. For example, a 12-ohm resistor provides approximately 100mA of regulated current regardless of load voltage (within dropout constraints).

Current Limiting and Foldback Protection

Current limiting protects both the regulator pass element and the load from damage during fault conditions. Various current limiting approaches offer different trade-offs between simplicity, protection effectiveness, and power dissipation.

Simple Current Limiting

Basic current limiting uses a sense resistor in series with the load to monitor current flow. When the voltage across this resistor reaches approximately 0.6V, it turns on a transistor that diverts drive from the pass element, preventing further current increase. The current limit threshold is:

Ilimit = 0.6V / Rsense

For a 0.6-ohm sense resistor, the limit is approximately 1A. While simple, this approach causes maximum power dissipation in the pass element during short-circuit conditions when the pass element must sustain full input voltage at the limited current.

Foldback Current Limiting

Foldback limiting reduces the current limit as output voltage decreases, dramatically reducing pass element dissipation during shorts. As the output approaches zero volts, the current limit folds back to a fraction (typically 20-30%) of the normal limit. This characteristic creates a safe operating envelope that keeps the pass element within its power rating even during sustained faults.

Foldback limiting uses a resistive divider from the output voltage to modify the sense transistor bias point. As output voltage drops, the transistor turns on at progressively lower load currents. While excellent for protection, foldback limiting can prevent startup into capacitive loads that draw high initial charging current.

Constant Power Limiting

Some sophisticated regulators implement constant power limiting, which reduces current inversely with the input-output differential to maintain constant pass element dissipation regardless of operating point. This provides optimal utilization of the pass transistor's safe operating area across all conditions.

Safe Operating Area Considerations

Pass transistor destruction can result from:

Excessive junction temperature: From sustained high power dissipation
Secondary breakdown: A bipolar transistor failure mode occurring at high voltage and moderate current where localized heating creates current-hogging hot spots
Excessive current: Beyond the maximum current rating regardless of voltage

Effective protection must address all three failure mechanisms by limiting current, implementing thermal shutdown, and potentially incorporating secondary breakdown protection for high-voltage applications.

Thermal Protection Circuits

Thermal protection prevents regulator destruction from overheating, whether caused by excessive load current, inadequate heat sinking, high ambient temperature, or sustained fault conditions. All modern integrated regulators include thermal shutdown, and discrete designs should incorporate similar protection.

Thermal Shutdown Operation

Thermal shutdown circuits typically use a temperature-sensing element (thermistor, diode, or integrated temperature sensor) located near or on the pass transistor die. When the junction temperature exceeds a threshold (typically 150-175 degrees Celsius), the shutdown circuit reduces or eliminates drive to the pass element, allowing the device to cool.

Most thermal shutdown implementations include hysteresis to prevent rapid cycling between on and off states. The regulator shuts down at an upper threshold and does not resume operation until temperature falls to a lower threshold, typically 10-20 degrees below the shutdown point.

Discrete Thermal Protection

For discrete regulator designs, thermal protection can be implemented using:

Thermistors: NTC thermistors mounted on the heat sink or pass transistor case sense temperature. A voltage divider with the thermistor drives a comparator that controls regulator operation
Thermal cutoff devices: One-shot thermal fuses or resettable thermal switches provide backup protection
Temperature-sensing ICs: Devices like the LM35 or DS18B20 provide accurate temperature measurement for sophisticated protection schemes
Integrated thermal shutdown ICs: Purpose-built protection ICs combine temperature sensing with latching or non-latching shutdown logic

Heat Sink Design Considerations

Proper thermal management prevents thermal protection from activating during normal operation:

Power dissipation calculation: Pd = (Vin - Vout) * Iload + Vin * Iquiescent
Thermal resistance budget: Junction-to-ambient thermal resistance must keep junction temperature below maximum at worst-case ambient
Heat sink selection: Thermal resistance required = (Tjmax - Tambient) / Pd - theta-jc - theta-cs, where theta-jc is junction-to-case and theta-cs is case-to-sink thermal resistance
Airflow effects: Forced airflow dramatically reduces heat sink thermal resistance; natural convection requires larger heat sinks

Low-Dropout (LDO) Regulators

Low-dropout regulators minimize the voltage difference required between input and output to maintain regulation, enabling efficient operation from low-headroom sources such as batteries approaching discharge or post-regulator applications where minimizing power loss is critical.

LDO Architecture

LDO regulators achieve low dropout by using a PNP bipolar transistor or P-channel MOSFET as the pass element, with the input connected to the emitter/source and output at the collector/drain. This topology eliminates the saturation voltage that limits NPN/N-channel designs:

PNP pass element: Dropout equals the collector-emitter saturation voltage plus any sense resistor drop, typically 300-700mV at rated current
P-channel MOSFET: Dropout equals Id * Rds(on), potentially below 100mV for properly sized devices at moderate currents

Stability Considerations

LDO regulators present challenging stability requirements due to the high-impedance output node and the PNP/PMOS pass element's current-source characteristic. Key stability factors include:

Output capacitor ESR: Many LDOs require output capacitor ESR within a specific range. Too low ESR removes a zero needed for phase margin; too high degrades transient response
Capacitor type: Tantalum and aluminum electrolytic capacitors provide beneficial ESR; low-ESR ceramics may require ESR compensation networks or LDOs specifically designed for ceramic capacitors
Minimum load current: Some LDOs require minimum load current for stability; a bleed resistor may be necessary in light-load applications
Internal compensation: Modern LDOs increasingly incorporate internal compensation allowing stable operation with any capacitor type

LDO Performance Tradeoffs

LDO design involves balancing competing requirements:

Quiescent current vs. transient response: Lower quiescent current reduces efficiency loss at light loads but slows the error amplifier, degrading transient response
Dropout vs. ground current: Larger pass devices reduce dropout but require more gate/base drive current
Noise vs. complexity: Achieving very low output noise requires sophisticated error amplifier design and careful reference implementation
PSRR vs. bandwidth: Power supply rejection ratio typically degrades at higher frequencies as the control loop gain decreases

LDO Applications

LDO regulators excel in applications including:

Battery-powered devices: Maximizing usable battery life by operating to lower cell voltages
Post-switching regulator filtering: Creating clean supply rails from switching converter outputs with minimal additional voltage drop
RF and analog circuits: Providing ultra-low-noise supplies for sensitive amplifiers, PLLs, and data converters
Automotive applications: Operating from cold-cranking voltage dips while maintaining regulation

Negative Voltage Regulators

Negative voltage regulators provide regulated negative output voltages referenced to ground, essential for bipolar analog circuits, operational amplifiers, and other applications requiring both positive and negative supplies.

Negative Regulator Topologies

Negative regulators use circuit topologies complementary to their positive counterparts:

Series pass with NPN transistor: The NPN transistor connects with its collector to ground and emitter to the negative output, providing the same functionality as a PNP in a positive regulator. This topology enables low dropout operation
PNP-based designs: Mirror the NPN positive regulators, with higher dropout voltage but simpler drive requirements
Integrated solutions: The 79xx series and LM337 provide negative equivalents to positive three-terminal and adjustable regulators

79xx Series Characteristics

The 79xx series provides fixed negative voltages with specifications similar to, but not identical to, their positive counterparts:

Pinout differences: Pin assignments differ from 78xx; ground and input are swapped. Care must be taken when replacing positive with negative regulators in a circuit
Minimum load requirements: Many 79xx regulators require minimum load current (5-10mA) for stability; a load resistor may be needed
Different transient response: Internal topology differences result in different dynamic behavior compared to positive regulators
Same capacitor requirements: Similar input and output capacitor values maintain stability

Design Considerations for Negative Rails

Designing negative supply circuits requires attention to:

Ground reference: All voltages are negative with respect to ground; measurement equipment must be properly configured
Capacitor polarity: Electrolytic capacitor polarity reverses from positive rail designs; the positive terminal connects to ground
Protection diodes: Diode orientation reverses compared to positive regulator circuits
Current direction: Current flows from ground toward the negative supply; meters may show negative current readings

Dual-Tracking Supplies

Dual-tracking power supplies maintain matched positive and negative output voltages that track together during startup, shutdown, and load variations. This tracking prevents potentially destructive latch-up conditions in CMOS circuits and ensures proper biasing of operational amplifiers and other symmetric circuits.

Tracking Requirements

Different applications have varying tracking requirements:

Startup/shutdown tracking: The positive and negative rails should rise and fall together to prevent either rail from being present alone, which can forward-bias protection diodes or cause latch-up
Static matching: The magnitude of positive and negative voltages should be equal under steady-state conditions
Dynamic tracking: Transient load variations on one rail should not excessively affect the other
Tracking accuracy: Specifications range from 5% for general analog circuits to 0.1% for precision applications

Implementation Approaches

Several techniques achieve tracking between positive and negative regulators:

Master-slave configuration: One regulator serves as master with a fixed reference. The other regulator (slave) uses the master output as its reference, tracking any variations in the master
Common reference: Both regulators derive their references from a common source, with matched gain in each feedback path
Integrated dual regulators: Purpose-built ICs incorporate matched reference and error amplifiers for inherent tracking
Active tracking circuits: Separate tracking circuitry monitors both outputs and adjusts one to match the other

Practical Tracking Circuit

A practical master-slave tracker uses the positive regulator as the master with standard resistor-set output voltage. The negative regulator receives a reference voltage derived from the positive output through an inverting amplifier with unity gain magnitude. This configuration ensures the negative rail matches the positive rail magnitude.

Startup tracking requires additional circuitry to ensure neither rail leads or lags significantly. Common approaches include:

Soft-start capacitors: Matched RC time constants on both regulators' reference inputs provide similar ramp rates
Sequential enable: Logic circuitry enables the second regulator only after the first reaches regulation
Active sequencing ICs: Purpose-built sequencing controllers manage supply rail ordering

Noise and PSRR Optimization

Linear regulators' primary advantage over switching types lies in their inherently low output noise and high power supply rejection ratio (PSRR). Understanding and optimizing these characteristics is essential for noise-sensitive applications.

Noise Sources in Linear Regulators

Output noise originates from several sources within the regulator:

Reference noise: The voltage reference contributes wideband noise that appears at the output multiplied by the regulator's gain. Bandgap references typically exhibit lower noise than Zener-based references
Error amplifier noise: Input-referred noise from the error amplifier is amplified by the closed-loop gain
Pass element noise: The pass transistor contributes 1/f noise and thermal noise, though these are typically attenuated by the feedback loop
Resistor noise: Feedback network resistors generate thermal noise proportional to their resistance and temperature

Techniques for Low-Noise Operation

Minimizing output noise involves:

Reference filtering: Bypassing the reference or adjustment pin with a capacitor filters reference noise
Low-noise reference selection: Buried Zener and precision bandgap references offer lower noise than standard references
Low feedback resistor values: Reducing feedback network resistance decreases thermal noise contribution
Output filtering: Additional LC or RC filtering on the output attenuates remaining noise
Selecting low-noise LDOs: Some LDOs are specifically designed for low noise, with optimized internal circuitry

Power Supply Rejection

PSRR quantifies the regulator's ability to attenuate input voltage variations, including ripple from rectified AC, switching converter noise, and other supply disturbances. PSRR is expressed in decibels:

PSRR (dB) = 20 * log10(delta-Vin / delta-Vout)

PSRR varies with frequency, typically exceeding 60dB at DC and low frequencies but degrading to 20-30dB or less at higher frequencies as the error amplifier's gain-bandwidth product limits correction capability.

Improving PSRR

Strategies for enhancing power supply rejection include:

Pre-regulation: An RC filter or pre-regulator stage ahead of the main regulator provides additional input filtering
Cascaded regulation: Two regulators in series multiply their individual PSRR values (in dB, the values add)
Feed-forward compensation: Some regulators inject a compensating signal that cancels input variations, improving high-frequency PSRR
Higher bandwidth error amplifiers: Extending the control loop bandwidth maintains PSRR to higher frequencies

Practical Design Example

A complete design example illustrates the application of linear regulator principles. Consider a 5V, 500mA regulated supply from a 9-12V input for powering sensitive analog circuitry.

Design Requirements

Output voltage: 5.0V +/-2%
Output current: 0-500mA
Input voltage: 9-12V DC
Output noise: less than 50uV RMS (10Hz-100kHz)
Load regulation: better than 0.5%
Thermal protection required

Component Selection

For this application, an LDO regulator offers unnecessarily low dropout given the available headroom, so a standard LM7805 or equivalent provides a cost-effective solution with proven reliability:

Regulator: LM7805CT in TO-220 package provides the required current with internal thermal protection
Input capacitor: 10uF aluminum electrolytic provides bulk energy storage; 100nF ceramic close to the input pin handles high-frequency bypassing
Output capacitor: 10uF low-ESR tantalum ensures stability and good transient response; 100nF ceramic for high-frequency performance

Thermal Design

Maximum power dissipation occurs at maximum input voltage and full load:

Pd = (12V - 5V) * 0.5A = 3.5W

The LM7805 in TO-220 has junction-to-case thermal resistance of approximately 4 degrees C/W. With a maximum junction temperature of 125 degrees C and 40 degrees C ambient, the maximum allowable thermal resistance from case to ambient is:

(125 - 40) / 3.5 - 4 = 20.3 degrees C/W

A small heat sink with thermal resistance below 20 degrees C/W, along with thermal compound or an insulating pad, provides adequate cooling. In an enclosed space with limited airflow, a larger heat sink may be necessary.

Noise Reduction

To meet the stringent noise requirement, additional filtering is beneficial:

LC filter: A 10uH inductor followed by 100uF capacitor creates a low-pass filter with corner frequency around 5kHz, attenuating high-frequency noise
Ferrite bead: A ferrite bead in series with the output provides additional high-frequency impedance
Attention to layout: Keep high-current paths away from sensitive signal traces; use ground planes and appropriate decoupling at load points

Conclusion

Linear voltage regulators remain indispensable in modern electronics despite the proliferation of switching converters. Their simplicity, low noise, excellent transient response, and freedom from electromagnetic interference make them ideal for powering noise-sensitive analog circuits, serving as post-regulators after switching converters, and providing clean power in applications where efficiency is less critical than power quality.

Understanding the fundamentals of series and shunt regulation, three-terminal and adjustable regulators, current limiting and thermal protection, LDO operation, and noise optimization enables engineers to select appropriate devices and design effective linear power supplies. While switching regulators dominate applications where efficiency is paramount, linear regulators continue to excel in their niche, providing the clean, stable power that sensitive circuits demand.

As integrated circuit technology advances, linear regulators continue to evolve with lower dropout voltages, reduced quiescent currents, improved PSRR, and sophisticated protection features, ensuring their continued relevance in the ever-expanding landscape of electronic power conversion.