Linear Power Supplies
Linear power supplies convert AC or higher DC voltage to regulated DC output using continuously conducting pass elements that dissipate excess energy as heat. Unlike switching power supplies that achieve efficiency through rapid on-off transitions, linear supplies maintain their regulating element in the active region, providing inherently smooth, low-noise output. This fundamental approach makes linear supplies the preferred choice for noise-sensitive applications despite their lower efficiency.
The simplicity of linear regulation belies the sophisticated engineering required for high-performance designs. From the voltage reference establishing accuracy to the pass transistor handling power dissipation, every component contributes to overall performance. Modern linear supplies achieve excellent regulation, fast transient response, and exceptionally low output noise, making them essential for precision instrumentation, audio equipment, radio frequency systems, and medical devices.
Operating Principles
Series Pass Regulation
The series pass regulator forms the heart of most linear power supplies. A variable resistance element, typically a bipolar transistor or MOSFET, connects in series between the unregulated input and regulated output. A feedback loop continuously monitors the output voltage, comparing it to a precision reference and adjusting the pass element's conductance to maintain constant output. When input voltage rises or load current decreases, the pass element increases its voltage drop; when conditions reverse, the drop decreases accordingly.
The pass transistor operates in its linear region between saturation and cutoff, hence the name "linear" regulator. The voltage dropped across the pass element multiplied by the load current equals the power dissipated as heat, representing the fundamental efficiency limitation. A regulator converting 12V input to 5V output at 1A dissipates 7W in the pass transistor while delivering only 5W to the load, achieving just 42% efficiency.
Feedback Loop Operation
The feedback loop comprises the voltage reference, error amplifier, and pass transistor driver. A resistor divider samples the output voltage and feeds it to the error amplifier's inverting input. The reference voltage connects to the non-inverting input. Any difference between these voltages produces an error signal that drives the pass transistor to correct the output. High loop gain ensures tight regulation; gains of 80-100 dB produce regulation measured in millivolts per ampere or per volt of input change.
Loop stability requires careful frequency compensation. The output capacitor, load characteristics, and pass transistor parameters create poles and zeros that shape the loop response. Inadequate phase margin leads to oscillation or ringing on transients. The compensation network, typically integrated within the regulator IC, ensures stable operation across the specified range of output capacitance and load conditions.
Dropout Voltage
Dropout voltage is the minimum input-to-output voltage differential required for proper regulation. Standard linear regulators require 2-3V dropout, while low-dropout (LDO) regulators operate with dropouts below 1V, some achieving as low as 100mV at moderate currents. The pass transistor type determines dropout performance: PNP bipolar transistors and P-channel MOSFETs enable LDO operation because they require only that the gate or base voltage be below the input voltage.
Operating near dropout affects several parameters. Loop gain decreases as the pass transistor approaches saturation, potentially degrading regulation and transient response. Noise rejection often worsens at low dropout. Some LDO regulators specify performance parameters at both nominal and minimum dropout conditions, allowing designers to understand the trade-offs involved in operating close to dropout limits.
Voltage Reference Circuits
Bandgap References
The bandgap reference provides the stable voltage that defines output accuracy. This circuit exploits the complementary temperature coefficients of two voltage sources within a bipolar transistor. The base-emitter voltage decreases approximately 2mV per degree Celsius, while the voltage difference between transistors operating at different current densities increases with temperature. Proper scaling and summation produces a temperature-stable output near 1.25V, corresponding to silicon's bandgap voltage extrapolated to absolute zero.
Modern bandgap references incorporate curvature correction to compensate for nonlinear temperature effects, achieving temperature coefficients of 10-50 parts per million per degree Celsius. Premium references use higher-order correction and production trimming to achieve single-digit ppm/C performance. These references establish the fundamental accuracy limit for linear power supplies.
Buried Zener References
Buried Zener references offer superior long-term stability compared to bandgap references. The Zener diode forms beneath the silicon surface, eliminating surface effects that cause noise and drift in conventional Zeners. Long-term drift rates below 10 parts per million per year make buried Zeners the choice for precision instrumentation and metrology applications. However, their higher cost and power consumption limit them to demanding applications.
Temperature compensation circuits accompanying the buried Zener maintain overall temperature stability. The Zener current must remain stable, as breakdown voltage varies with current. Complete buried Zener reference modules integrate heating elements and temperature control for the most demanding applications, achieving stability measured in parts per million over extended periods.
Reference Noise Considerations
Reference noise directly impacts output noise since the error amplifier multiplies reference noise by the regulation ratio. Zener diodes produce shot noise proportional to operating current; buried Zeners generate lower noise due to their subsurface construction. Bandgap references produce noise from internal transistors, with performance varying significantly by design and manufacturer.
External filtering can reduce reference noise contribution. Many regulators provide bypass pins where external capacitors filter the internal reference, rolling off noise above a corner frequency determined by internal resistance and external capacitance. This simple technique reduces output noise by 10-20 dB in the audio frequency range, significantly improving performance for noise-sensitive applications.
Pass Transistor Design
Bipolar Pass Transistors
Bipolar transistors remain common in linear regulators due to their high current gain and mature manufacturing. NPN transistors offer best performance but require drive voltage above the output, complicating positive regulator design. PNP transistors enable simple positive regulator topologies with direct base drive from the error amplifier. The choice affects dropout voltage, frequency response, and thermal characteristics.
Darlington configurations cascade two transistors to achieve current gain products of several thousand, reducing drive current requirements to microamperes. However, Darlingtons exhibit higher saturation voltages than single transistors, increasing minimum dropout. The Sziklai pair, combining NPN and PNP transistors, provides Darlington-like gain with improved saturation characteristics, making it popular in integrated regulators.
MOSFET Pass Elements
MOSFETs eliminate base current, improving efficiency and simplifying drive circuits. P-channel MOSFETs are natural choices for positive LDO regulators since their gate voltage can be below the source (input) voltage. N-channel devices offer lower on-resistance for equivalent die size but require gate voltage above the drain (output), necessitating charge pumps or bootstrap circuits in positive regulators.
MOSFET on-resistance determines dropout voltage and efficiency. As current increases, the voltage drop across the pass MOSFET increases proportionally. Selecting devices with appropriately low RDS(on) minimizes dropout while keeping die size and cost reasonable. Temperature effects must be considered, as on-resistance increases approximately 0.5% per degree Celsius in typical MOSFETs.
Thermal Considerations
The pass transistor dissipates most of the power in a linear regulator, making thermal design critical. Power dissipation equals the product of voltage drop and output current: a regulator converting 12V to 3.3V at 500mA dissipates 4.35W continuously. This heat must be removed to maintain junction temperature below the maximum rating, typically 125-150 degrees Celsius for silicon devices.
Thermal resistance from junction to ambient determines the temperature rise for a given power dissipation. Package selection, heat sink design, and PCB layout all affect thermal resistance. Surface-mount devices rely on copper planes for heat spreading; through-hole packages may use external heat sinks. Proper thermal design maintains adequate margin below maximum junction temperature under worst-case conditions of maximum ambient temperature, minimum airflow, and maximum power dissipation.
Protection Circuits
Current Limiting
Current limiting protects both the regulator and load from excessive current during fault conditions. Simple current limiting monitors load current through a sense resistor and reduces pass transistor drive when current exceeds the limit threshold. The current limit point is typically set 20-50% above maximum rated load current to provide margin for transients while protecting against sustained overloads.
Constant current limiting maintains fixed maximum current regardless of output voltage, creating maximum power dissipation under short-circuit conditions where the full input voltage drops across the pass transistor. Foldback current limiting reduces both voltage and current during overloads, minimizing pass transistor dissipation. The trade-off is potential startup difficulty with loads presenting low initial impedance.
Thermal Protection
Integrated thermal shutdown disables the regulator output when junction temperature exceeds a safe threshold, typically 150-170 degrees Celsius. This provides last-resort protection when heat sinking proves inadequate for the operating conditions. Once the device cools below the hysteresis threshold, operation resumes automatically. While essential for safety, thermal shutdown should not be relied upon for normal operation, as repeated thermal cycling stresses the device.
External thermal protection can supplement or replace integrated protection, particularly in discrete designs. Temperature sensors mounted to heat sinks trigger shutdown through external logic or analog circuits. Proportional thermal derating gradually reduces output current as temperature rises, maintaining some output capability while preventing overtemperature rather than abruptly shutting down at a threshold.
Safe Operating Area
The safe operating area (SOA) defines the combinations of voltage and current a transistor can sustain without damage. Linear regulators operate continuously within the SOA, unlike switching applications where transistors traverse the SOA only during brief transitions. DC SOA boundaries are more restrictive than pulsed SOA, and thermal derating at elevated temperatures further constrains the operating region.
Secondary breakdown in bipolar transistors creates additional SOA limitations at high voltages. This phenomenon, where localized heating causes current concentration and thermal runaway within the device, occurs before the device reaches its rated power or current. Proper design ensures operation well within SOA boundaries under all anticipated conditions including fault scenarios.
Reverse Polarity Protection
Reverse polarity protection prevents damage when input or output connections are inadvertently reversed. Input reverse protection typically uses a series diode or MOSFET to block reverse current, though the diode's forward drop increases effective dropout voltage. Output reverse protection uses an output diode to clamp reverse voltage, protecting against conditions where the output is externally driven above the input.
More sophisticated protection integrates these functions without the voltage penalties of discrete diodes. Internal body diodes in MOSFET pass devices may provide inherent reverse protection. Circuit topology choices can also provide protection; some designs are inherently immune to certain fault conditions without explicit protection components.
Noise and Ripple Performance
Power Supply Rejection
Power supply rejection ratio (PSRR) quantifies how well the regulator attenuates input voltage variations. Expressed in decibels, PSRR equals 20 log(input ripple / output ripple). A regulator with 60 dB PSRR reduces 1V input ripple to 1mV at the output. PSRR is frequency-dependent, typically degrading at higher frequencies as loop gain decreases. At DC and low frequencies, PSRR may exceed 80 dB; above the loop bandwidth, PSRR decreases at 20 dB per decade or more.
The output capacitor provides additional filtering above the loop bandwidth, making capacitor selection important for high-frequency noise rejection. Low-impedance capacitors, particularly ceramics, offer excellent high-frequency bypass. However, ceramic capacitance varies with voltage and temperature, and some regulators require minimum ESR for stability, constraining capacitor choices.
Output Noise Sources
Linear regulator output noise originates primarily from the voltage reference and error amplifier. Bandgap references produce characteristic noise with a 1/f corner typically between 100 Hz and 10 kHz, above which white noise dominates. The noise contribution appears at the output multiplied by the feedback network gain. Low-noise designs use optimized references and quiet amplifier topologies to minimize these contributions.
Thermal noise from feedback resistors adds to output noise, becoming significant in high-gain configurations. Lower resistance values reduce thermal noise but increase quiescent current. Proper filtering and shielding prevent external noise pickup through the feedback network. Layout practices that minimize loop areas and provide adequate ground planes improve noise performance in the final design.
Achieving Ultra-Low Noise
Applications requiring the lowest possible noise use specialized low-noise regulators with optimized references and amplifiers. Output noise densities below 10 nanovolts per root-hertz are achievable in premium devices. Additional post-filtering with LC networks can further reduce noise at the cost of transient response. Active noise reduction techniques using feedforward compensation can cancel reference noise, achieving exceptional results.
System-level noise reduction combines regulator selection with careful layout, grounding, and filtering strategies. Star grounding prevents ground loop currents from modulating supply voltage. Shield and guard techniques prevent capacitive coupling of noise into sensitive circuits. The lowest noise performance requires attention to every aspect of the power delivery path from regulator output to load input.
Transient Response
Load Transient Fundamentals
When load current changes suddenly, output voltage deviates until the feedback loop corrects the error. The initial deviation depends on output capacitor impedance and current step magnitude. The recovery involves the regulator driving the output back toward regulation at a rate determined by loop bandwidth and slew rate limitations. Total transient response includes both the initial deviation and recovery time to return within specification.
Fast transients stress both regulator and load. Digital circuits with rapidly switching current demands produce severe transients that can cause noise, timing errors, or malfunction if supply voltage deviates excessively. Characterizing transient response under realistic load conditions ensures the power supply meets system requirements.
Output Capacitor Effects
The output capacitor dominates initial transient response by supplying current to the load before the regulator can react. Capacitor impedance, primarily ESR at high frequencies, determines the instantaneous voltage drop for a current step. The voltage then recovers at a rate set by capacitor size and regulator response time. Minimizing transient deviation requires low-impedance capacitors and fast loop response.
Ceramic capacitors offer the lowest ESR and ESL, providing excellent high-frequency transient performance. However, their capacitance varies significantly with applied voltage in Class 2 dielectrics, potentially reducing effective capacitance at operating voltage. Combining ceramics with bulk electrolytic or polymer capacitors often yields optimal performance, with ceramics handling fast transients and electrolytics providing energy storage.
Loop Bandwidth Optimization
Higher control loop bandwidth enables faster transient response but requires careful stability analysis. Bandwidth is ultimately limited by the need to maintain adequate phase margin; aggressive bandwidth expansion can cause oscillation or ringing. The output capacitor characteristics interact with the loop compensation, requiring design iteration to optimize both transient response and stability.
LDO regulators face particular bandwidth challenges near dropout, where pass transistor transconductance changes and the dominant pole frequency shifts. Performance at low dropout may differ significantly from performance at higher input-output differentials. Datasheets often specify transient response under multiple operating conditions to characterize this variation.
Application Considerations
Input and Output Capacitor Selection
Input capacitors provide local energy storage and reduce source impedance seen by the regulator. They must handle ripple current without excessive heating or voltage droop. Low-ESR capacitors improve high-frequency performance but may interact with source inductance to create resonances requiring damping. Input capacitance requirements increase with load current and source impedance.
Output capacitor selection balances transient response, noise filtering, and stability requirements. The regulator datasheet specifies acceptable capacitance and ESR ranges; deviation risks oscillation or degraded performance. Some LDOs require minimum ESR for stability, precluding all-ceramic solutions. Others are designed for ceramic capacitors but may require higher capacitance than specified for optimal transient response.
Power Dissipation Management
Efficient thermal design keeps junction temperature within limits under worst-case conditions. Power dissipation calculations must account for maximum input voltage, minimum output voltage tolerance, and maximum load current. Selecting a regulator with appropriate package and thermal characteristics avoids the need for external heat sinks in many applications.
PCB thermal design significantly affects maximum power handling in surface-mount devices. Thermal vias beneath the thermal pad transfer heat to inner layers or the board's opposite side. Copper area devoted to heat spreading determines thermal resistance to ambient. Thermal simulation or empirical measurement validates that the design maintains adequate temperature margin.
Parallel and Series Configurations
Paralleling linear regulators increases current capacity beyond single-device limits. However, simple output paralleling causes unequal current sharing due to reference voltage and sense point differences. Forced current sharing uses active circuits or droop compensation to balance currents among paralleled devices. Each device must be protected against reverse current from others during fault conditions.
Series configurations increase output voltage or achieve cascaded regulation. Two regulators in series provide additional noise rejection and improved load isolation. Pre-regulation before a precision regulator reduces its dissipation and input voltage variation, improving both efficiency and regulation performance. The pre-regulator can be a switching supply, combining switching efficiency with linear output quality.
Remote Sensing
Remote sensing compensates for voltage drops between the regulator output and load. Separate sense wires connect directly to the load, and the regulator adjusts its output to maintain correct voltage at the sense point regardless of distribution resistance. This technique is essential when cable or trace resistance would otherwise cause unacceptable load voltage error.
Sense lines require protection against open-circuit conditions that could cause the regulator to drive maximum output voltage. Resistors from output to sense terminals provide default sensing if sense lines disconnect. Filtering on sense lines prevents noise pickup from corrupting the feedback signal, particularly in electrically noisy environments.
Linear Regulator Types
Fixed Voltage Regulators
Fixed voltage regulators provide specific output voltages determined by internal resistor ratios. Standard voltage families include 3.3V, 5V, and 12V for digital logic, plus various voltages for specific applications. Fixed regulators offer simplicity and inherent protection against feedback network faults that could cause adjustable regulators to produce incorrect voltage.
The 78xx and 79xx series established standard pinouts and specifications still used today. Modern equivalents offer improved performance including lower dropout, better accuracy, and enhanced protection features while maintaining pin compatibility. Multiple manufacturers produce interchangeable devices, ensuring supply security and competitive pricing.
Adjustable Regulators
Adjustable regulators accept external resistor programming to set output voltage. A reference voltage appears between the output and adjust pins; the output voltage equals this reference multiplied by the resistor divider ratio plus one. Standard parts like the LM317 and LM1117 provide wide adjustment ranges from about 1.25V to the device's maximum voltage rating.
Precision resistors in the feedback divider determine output accuracy. Temperature coefficients of both resistors should match to maintain accuracy over temperature. The adjust pin typically sources a small bias current that flows through the lower divider resistor, introducing a small voltage offset requiring compensation in precision applications.
Low-Dropout Regulators
LDO regulators operate with input-output differentials below 1V, enabling efficient regulation from batteries and other sources with limited voltage headroom. Modern LDOs achieve dropout voltages of 100-300mV at rated current, minimizing power loss in the pass element. PMOS and PNP pass devices inherently support low-dropout operation.
LDO performance parameters often degrade near dropout. PSRR, transient response, and output noise may all worsen when operating with minimum headroom. Designers must evaluate performance at actual operating conditions, not just nominal specifications, to ensure adequate margin for all requirements.
Specialty Regulators
Numerous specialty linear regulators address specific application requirements. Low-noise regulators optimize for minimum output noise with enhanced references and quiet amplifiers. High-PSRR regulators provide exceptional supply rejection for sensitive analog circuits. Fast-transient regulators use wide bandwidth loops for digital load applications.
Application-specific regulators integrate additional functions relevant to their target applications. Camera power regulators include load switches and sequencing. Audio regulators optimize for low noise in the audio band. RF regulators provide excellent high-frequency PSRR and noise performance. These integrated solutions simplify designs while ensuring optimized performance for their intended applications.
Comparison with Switching Regulators
Efficiency Trade-offs
Switching regulators achieve efficiencies of 85-95% by using transistors as switches rather than variable resistors. Linear regulators' maximum theoretical efficiency equals the output-to-input voltage ratio, typically 30-70% in practice after accounting for quiescent current. This efficiency difference translates directly to heat dissipation and thermal management requirements.
The efficiency penalty of linear regulators decreases when the input-output differential is small. An LDO converting 3.6V to 3.3V achieves 92% efficiency at the voltage ratio alone, potentially exceeding switching regulator efficiency at low power levels where switching losses dominate. Optimal solutions often combine switching pre-regulation with linear post-regulation.
Noise Characteristics
Linear regulators inherently produce quieter outputs than switching regulators, which generate noise at the switching frequency and its harmonics. Switch-mode ripple typically measures tens of millivolts; linear regulator noise measures microvolts. This orders-of-magnitude difference makes linear regulation essential for the most noise-sensitive applications.
Post-regulation with a linear regulator following a switching supply captures the advantages of both technologies: switching efficiency for bulk power conversion and linear regulation for clean output. The linear regulator rejects the switching ripple while adding minimal additional noise, achieving low overall noise with good system efficiency.
Application Selection Guidelines
Linear regulators are preferred when noise sensitivity dominates requirements, when power dissipation is acceptable, when design simplicity is valued, or when cost and board space for inductors and complex control are prohibitive. They excel in precision analog, audio, RF, and medical applications where output quality trumps efficiency.
Switching regulators are preferred when efficiency is paramount, when large input-output differentials exist, when high current is required, or when thermal constraints limit power dissipation. Battery-powered, high-power, and thermally constrained applications typically favor switching solutions despite their complexity and noise characteristics.
Design Best Practices
Component Selection
Selecting regulators with adequate current capability and appropriate thermal packages prevents overheating and ensures reliability. Margin between maximum rated current and actual operating current provides headroom for transients and accounts for efficiency degradation at elevated temperatures. Thermal resistance specifications must support actual thermal design, not just ideal conditions.
Capacitor selection follows manufacturer recommendations for stability while optimizing for application requirements. Temperature effects on ceramic capacitors can dramatically reduce effective capacitance; specify capacitors for actual operating voltage and temperature. Multiple parallel capacitors may be necessary to meet both transient response and stability requirements.
PCB Layout Guidelines
Minimize trace length between input capacitor and regulator input pin to reduce parasitic inductance. Similarly, the output capacitor should connect as close as possible to the output pin with short, wide traces. Ground connections should converge at a single point near the regulator to prevent ground loops from degrading regulation or introducing noise.
Thermal considerations influence layout significantly. Thermal pads require via connections to ground planes or dedicated thermal copper. Keep heat-generating components away from temperature-sensitive circuits. Adequate copper area for heat spreading is essential for surface-mount packages that rely on the PCB for thermal management.
Testing and Verification
Verify regulation performance across input voltage range and load current from minimum to maximum. Measure transient response using realistic load step magnitudes and slew rates. Confirm thermal performance under worst-case conditions of maximum ambient temperature and maximum power dissipation. These tests validate that the design meets requirements throughout its operating envelope.
Noise measurements require appropriate bandwidth limiting and proper probe techniques. Measure at the actual load connection point, not at the regulator output, to capture any noise added by distribution traces. Compare measured noise against application requirements to ensure adequate margin. Environmental testing validates performance across temperature, humidity, and other relevant conditions.
Conclusion
Linear power supplies remain indispensable despite the efficiency advantages of switching alternatives. Their inherently quiet operation, fast transient response, and design simplicity make them optimal for noise-sensitive applications ranging from precision instrumentation to audio equipment to radio frequency systems. Understanding the principles of series pass regulation, voltage references, protection circuits, and thermal management enables engineers to create linear power supplies meeting the most demanding specifications.
The key to successful linear power supply design lies in understanding and managing the inherent trade-offs: efficiency versus noise, transient response versus stability, protection completeness versus cost and complexity. Proper component selection, careful layout, and thorough verification transform these principles into practical, reliable power solutions that enable sensitive electronic systems to achieve their full performance potential.