Electronics Guide

Basic Series Regulator Operation

The series pass regulator operates by placing a variable resistance element in series between the unregulated input and the regulated output. A feedback loop continuously adjusts this resistance to maintain constant output voltage regardless of input variations or load changes. When input voltage rises or load current decreases, the pass element increases its voltage drop; when input falls or load increases, the drop decreases. This continuous adjustment maintains output regulation.

The pass element, typically a bipolar transistor or MOSFET, operates in its linear region between saturation and cutoff. The control circuit compares a sample of the output voltage to a precision reference and drives the pass transistor to minimize the difference. The gain around this feedback loop determines regulation accuracy; higher loop gain produces tighter regulation but requires careful compensation to maintain stability.

Pass Transistor Selection

The pass transistor must handle the full load current while dissipating power equal to the voltage drop times the current. Power dissipation requirements often dominate pass transistor selection, as the device must safely handle worst-case dissipation with adequate margin. For a regulator with 15V input, 5V output, and 1A load, the pass transistor dissipates 10 watts continuously, requiring substantial heat sinking.

Bipolar transistors offer high current gain and excellent saturation characteristics but require base current that reduces efficiency and complicates drive circuits. PNP transistors enable simple positive regulator designs where the pass device connects between unregulated input and output. NPN transistors provide higher performance but require floating drive circuits or compound configurations.

MOSFETs eliminate gate current requirements and offer very low dropout voltage when fully enhanced, making them attractive for low-dropout (LDO) designs. N-channel MOSFETs provide the best performance but require gate voltage above the drain (output), necessitating charge pumps or bootstrap circuits in positive regulators. P-channel devices accept simple drive but have higher on-resistance for equivalent die size.

Darlington and Compound Configurations

Single transistors often provide insufficient current gain to drive directly from the error amplifier, particularly at high output currents where bipolar transistor beta decreases. Darlington configurations cascade two transistors, with the first driving the base of the second, providing current gain equal to the product of individual betas. This easily achieves gains of several thousand, reducing drive requirements to microamperes.

The compound configuration, also called a Sziklai pair, combines NPN and PNP transistors to achieve Darlington-like gain with improved saturation characteristics. The driver transistor operates as an emitter follower, driving the pass transistor's base. This arrangement provides lower minimum dropout voltage than a Darlington while maintaining high current gain. Many integrated regulators use compound output stages internally.

Error Amplifier Design

The error amplifier compares a divided-down sample of the output voltage to the reference voltage and amplifies the difference to drive the pass transistor. High DC gain ensures tight regulation; gains of 80-100 dB are common in precision regulators. The amplifier must also provide adequate bandwidth for good transient response while maintaining stability with the reactive load presented by the output capacitor and pass transistor.

Operational amplifiers serve well as error amplifiers in discrete regulator designs, providing high gain, flexible compensation, and predictable characteristics. Single-supply op-amps simplify circuit design when the regulator operates from unipolar supplies. Rail-to-rail output capability maximizes the available control range, ensuring the pass transistor can be driven to full conduction or cutoff as needed.

Discrete error amplifier designs using individual transistors offer maximum flexibility for optimization but require more careful design. Common approaches include differential pairs with current source loads, cascoded stages for high gain, and class AB output stages for symmetric drive capability. Discrete designs can achieve noise performance superior to general-purpose op-amps when optimized for the application.

Shunt Regulator Principles

Shunt regulators maintain constant voltage by diverting excess current around the load rather than blocking it with a series element. A resistor or current source supplies more current than the load requires at the regulated voltage, and the shunt regulator absorbs the excess. When load current increases, the shunt current decreases proportionally; when load decreases, shunt current increases to maintain the total current through the supply impedance.

The shunt approach offers inherent short-circuit protection, as the maximum current is limited by the supply impedance. The output cannot exceed the programmed regulation voltage even with zero load. These characteristics make shunt regulators attractive for reference applications and as protection elements. However, efficiency is poor because power dissipation equals the supply voltage times the total current, regardless of load.

Zener Diode Regulators

The simplest shunt regulator uses a Zener diode to establish voltage. When reverse-biased beyond its breakdown voltage, the Zener conducts and clamps the voltage across it to the breakdown value. A series resistor limits the current, sized to supply the maximum load current plus adequate Zener current for regulation. Zener regulation improves at higher currents, so a minimum Zener current of several milliamperes is typically maintained.

Standard Zener diodes exhibit significant temperature coefficients, positive for breakdown voltages above about 5.5V and negative below. The temperature coefficient passes through zero near 5.6V, making this voltage optimal for temperature-stable references. Special temperature-compensated Zeners combine series elements with opposing temperature coefficients to achieve stability of a few parts per million per degree Celsius.

Adjustable Shunt Regulators

Adjustable shunt regulators such as the TL431 and its many derivatives combine a precision voltage reference with an internal error amplifier. An external resistor divider sets the regulation voltage, with the divider tap connected to the reference input. When the cathode voltage rises above the programmed threshold, the device increases conduction; when voltage falls, conduction decreases. These devices provide reference accuracy comparable to precision references with the flexibility of adjustable output.

Applications extend beyond simple voltage regulation to include overvoltage protection, voltage monitoring, and feedback elements in switching power supplies. The TL431 family has become ubiquitous in isolated power supply feedback circuits, where it provides both voltage reference and error amplification functions across the isolation boundary through an optocoupler.

High-Current Shunt Regulators

High-current shunt regulators use a transistor or MOSFET to shunt current, with a precision reference establishing the voltage threshold. The reference drives the transistor base or gate through an error amplifier that compares output voltage to the reference. This arrangement achieves regulation accuracy determined by the reference and error amplifier rather than the imprecise breakdown characteristics of high-power Zener diodes.

The shunt transistor must dissipate substantial power, especially at light loads when it carries nearly the full supply current. Thermal design is critical; the transistor temperature directly affects regulation accuracy through temperature-dependent parameters. Heat sinking and thermal protection become primary design concerns at high power levels.

Bandgap Reference Fundamentals

The bandgap reference, invented by Robert Widlar and refined by Paul Brokaw, achieves excellent temperature stability by combining two voltage sources with opposite temperature coefficients. The base-emitter voltage of a bipolar transistor decreases with temperature at approximately -2 mV per degree Celsius. The difference in base-emitter voltage between two transistors operating at different current densities increases with temperature. Proper scaling and summing of these voltages produces a temperature-independent output near 1.25V, corresponding to the bandgap voltage of silicon extrapolated to absolute zero.

Modern bandgap references include curvature correction to compensate for the slightly nonlinear temperature dependence of the base-emitter voltage. First-order correction reduces the temperature coefficient to tens of parts per million per degree Celsius. Higher-order correction, requiring more complex circuitry, achieves single-digit ppm/C performance suitable for precision instrumentation. Production trimming optimizes individual devices for minimum temperature coefficient.

Buried Zener References

Buried Zener references use subsurface Zener diodes formed beneath the silicon surface, eliminating surface effects that cause noise and instability in conventional Zeners. These references achieve long-term stability measured in parts per million per year, superior to bandgap references. They find application in precision voltmeters, calibrators, and other instruments where absolute accuracy matters.

The buried Zener operates at a fixed current, with temperature compensation provided by additional circuitry. Initial accuracy depends on production calibration, with laser trimming of thin-film resistors achieving accuracies of 0.01% or better. These references require stable operating conditions; temperature control further improves long-term stability in the most demanding applications.

Reference Selection Criteria

Selecting a voltage reference requires balancing initial accuracy, temperature coefficient, long-term stability, noise, and cost. Initial accuracy ranges from several percent for simple Zeners to 0.01% for precision buried Zener references. Temperature coefficients span from hundreds of ppm/C for uncorrected designs to below 1 ppm/C for the best temperature-compensated references.

Noise performance varies dramatically among reference types. Zener diodes produce shot noise proportional to current; buried Zeners generate less noise due to their subsurface operation. Bandgap references produce noise from the internal transistors, with performance varying by design and bandwidth. Low-frequency noise (1/f noise) affects DC accuracy; wideband noise matters for applications sampling the reference at high rates.

Application requirements determine the appropriate reference technology. Simple Zener diodes suffice for coarse regulation and protection functions. Bandgap references serve most precision applications cost-effectively. Buried Zener references address the most demanding requirements in metrology and precision measurement, where their cost is justified by superior performance.

Reference Application Circuits

Voltage references rarely connect directly to loads; buffer amplifiers provide current capability and reverse the output polarity when needed. Low-offset operational amplifiers in follower or inverting configurations maintain the reference accuracy while providing milliampere output currents. The buffer output impedance must be low enough to supply the load without significant voltage drop.

References operating from noisy power supplies benefit from filtering on both supply and output. RC or LC filters on the supply input reduce high-frequency noise that might modulate the reference output. Output filtering addresses the reference's own noise, with time constants selected to match the application bandwidth requirements.

Simple Current Limiting

Basic current limiting adds a current-sensing element and limiting transistor to prevent excessive output current. A small resistor in series with the output develops a voltage proportional to load current. When this voltage exceeds approximately 0.6V (one base-emitter drop), a limiting transistor conducts and reduces the pass transistor drive, limiting further current increase.

The sense resistor value determines the current limit: for a 0.6V threshold, a 0.6 ohm resistor limits current to 1A, a 0.06 ohm resistor limits to 10A. Lower values minimize power loss and voltage drop but require more precise sensing. The current limit point varies with temperature due to the temperature coefficient of the base-emitter voltage, decreasing by approximately 2 mV per degree Celsius.

Constant Current Limiting

Constant current limiting maintains a fixed maximum output current regardless of output voltage. When the load attempts to draw more than the limit, the regulator reduces output voltage to maintain the programmed current. This creates a characteristic where output voltage remains regulated until current reaches the limit, then output voltage drops while current remains constant. A short circuit produces maximum power dissipation in the pass transistor at the current limit times the input voltage.

Constant current limiting is straightforward to implement and provides predictable behavior for most overloads. However, the high power dissipation during sustained overcurrent or short circuit conditions requires careful thermal design. The pass transistor must safely dissipate the product of input voltage and current limit continuously, often necessitating substantial heat sinking or active thermal protection.

Foldback Current Limiting

Foldback current limiting reduces both output voltage and current during overload, minimizing pass transistor power dissipation. As output voltage drops below the regulation point, the current limit progressively decreases. Under short circuit conditions, current may reduce to 20-30% of the full-load rating, dramatically reducing power dissipation compared to constant current limiting.

Foldback implementation adds a voltage-dependent component to the current limit threshold. A resistor divider from the output biases the limiting transistor's base, so that lower output voltage increases the base voltage and triggers limiting at lower currents. The foldback ratio and knee voltage are set by resistor values, with sharper foldback providing better short-circuit protection but potentially complicating startup with capacitive or low-resistance loads.

Foldback limiting can cause startup problems with loads that present low initial resistance, such as discharged capacitors or incandescent lamps. The regulator may current-limit before the load reaches its normal operating point, preventing startup. Techniques to address this include delayed foldback activation, soft-start circuits that slowly increase the current limit, or constant current limiting during startup with foldback enabled after a timeout.

Current Sensing Methods

Low-side current sensing places the sense resistor between the load and ground, simplifying the sensing circuit but interrupting the ground path. The sense voltage is ground-referenced, allowing simple comparison circuits. However, the load no longer connects directly to ground, which may affect noise and ground integrity in sensitive applications.

High-side current sensing places the sense resistor between the regulator and load, maintaining the load's ground connection but requiring differential sensing at near-output voltage levels. High-side current sense amplifiers address this challenge, using precision resistor networks to reject common-mode voltage while amplifying the differential sense signal. This approach is essential for positive-ground systems and preserves ground integrity for the load.

Lossless current sensing techniques detect current without a dedicated sense resistor, using the voltage drop across existing circuit elements. The pass transistor on-resistance, inductor DCR in switching regulators, or trace resistance can provide current information. These methods require accurate knowledge of the sensing element's resistance and its temperature coefficient for accurate limiting.

Thermal Runaway in Linear Regulators

Linear regulators face the risk of thermal runaway when power dissipation increases with temperature. If component parameters shift with temperature in a direction that increases dissipation, rising temperature leads to higher dissipation, further temperature rise, and potential destruction. The primary concern is the pass transistor, where current gain typically decreases with temperature in bipolar devices, potentially increasing base drive power.

Thermal runaway is prevented by ensuring adequate heat sinking, implementing thermal protection, and designing the circuit so that temperature rise does not increase power dissipation. Proper thermal design maintains a negative feedback relationship: higher temperature leads to reduced current or increased resistance, limiting further power increase.

Integrated Thermal Shutdown

Integrated voltage regulators include on-chip thermal shutdown that disables the output when junction temperature exceeds a safe threshold, typically 150-170 degrees Celsius. The protection circuit monitors a temperature-sensitive element adjacent to the power transistor and triggers shutdown when the threshold is reached. Once the device cools, operation resumes automatically.

Thermal shutdown provides last-resort protection against conditions that overwhelm normal heat sinking. It should not be relied upon for normal operation, as repeated thermal cycling stresses the device and reduces reliability. Proper thermal design ensures the device operates well below the shutdown threshold under all anticipated conditions.

External Thermal Protection

Discrete regulator designs require external thermal protection circuits. Temperature sensors mounted to the heat sink or pass transistor case monitor temperature and trigger protection when thresholds are exceeded. Simple implementations use temperature switches or thermistors; more sophisticated designs use analog temperature sensors for proportional response.

Proportional thermal limiting gradually reduces output current as temperature rises, rather than abruptly shutting down at a threshold. This approach maintains some output capability while preventing overtemperature, often preferable to complete shutdown in systems where total loss of power has serious consequences. The current derating curve matches the safe operating area of the pass transistor at elevated temperatures.

Safe Operating Area Considerations

Transistor safe operating area (SOA) defines the combinations of voltage and current the device can safely sustain. Linear regulator pass transistors operate within the SOA continuously, unlike switching applications where transistors traverse the SOA only during brief transitions. The DC SOA is typically more restrictive than the 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 creates current hogging and thermal runaway within the device, occurs before the device reaches its power or current ratings. The forward-biased secondary breakdown boundary slopes downward on the SOA curve, limiting high-voltage operation more severely than low-voltage operation.

Power Supply Rejection Ratio

Power supply rejection ratio (PSRR) quantifies how well a regulator attenuates input voltage variations. Expressed in decibels, PSRR equals 20 times the log of input variation divided by output variation. A regulator with 60 dB PSRR reduces a 1V input ripple to 1mV at the output. PSRR typically degrades at higher frequencies as the control loop runs out of gain, making it important to specify PSRR at the frequency of concern.

Linear regulators achieve high PSRR at low frequencies due to their high-gain feedback loops. At DC and low frequencies, PSRR may exceed 80 dB. As frequency increases, PSRR degrades at rates determined by the loop dynamics, typically falling to 40-50 dB at the bandwidth limit and continuing to decrease at higher frequencies. The output capacitor provides additional filtering above the loop bandwidth.

Input Filtering

Prefiltering the input to a linear regulator reduces the ripple the regulator must reject, improving output quality. Simple RC filters attenuate ripple at rates of 20 dB per decade; LC filters achieve 40 dB per decade. The filter must handle the full input current without excessive voltage drop, limiting resistance values and requiring inductors rated for DC bias current.

The input capacitor to a linear regulator provides local energy storage that reduces the effective source impedance seen by the regulator. Large input capacitance improves both PSRR and transient response by presenting a stiffer input. Low-ESR capacitors are essential, as the series resistance limits high-frequency filtering effectiveness and contributes to ripple.

Output Filtering

The output capacitor is critical to linear regulator performance, providing energy storage for transient response and filtering for noise and ripple. Capacitor selection balances capacitance value, equivalent series resistance (ESR), equivalent series inductance (ESL), and voltage rating. The regulator stability requirements may constrain capacitor choice, particularly ESR range in low-dropout designs.

Additional LC filtering after the regulator output provides extremely low ripple for the most sensitive applications. The filter attenuates both the regulator's own output noise and any residual ripple that passes through. Ferrite beads provide simple high-frequency filtering; full LC networks achieve greater attenuation but must be designed to avoid resonances that could amplify noise at certain frequencies.

Noise Sources and Mitigation

Linear regulator 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 error amplifier adds its own noise contribution, which appears at the output multiplied by the regulator's closed-loop gain. Selecting low-noise references and amplifiers directly improves output noise performance.

External filtering of the reference input or bypass pins on integrated regulators reduces reference noise contribution. Many regulators provide a bypass pin where an external capacitor filters the internal reference, rolling off noise above a corner frequency set by internal resistance and external capacitance. This technique can reduce output noise by 10-20 dB in the audio band.

Load Transient Fundamentals

When load current changes suddenly, the regulator's output voltage deviates from the regulated value until the feedback loop corrects the error. The initial voltage change depends on output capacitor impedance and the magnitude of the current step. The regulator then drives the output back toward regulation at a rate determined by loop bandwidth and slew rate limitations. The total transient response includes both the initial deviation and the recovery time.

Load transient specifications typically include the current step magnitude, slew rate, initial voltage deviation, and recovery time to return within a specified voltage band. Fast transients stress both the regulator and the load; the regulator must respond quickly to minimize deviation, while the load must tolerate whatever deviation occurs during the response time.

Output Capacitor Selection

The output capacitor dominates initial transient response. When load current steps, the capacitor supplies the current change before the regulator can respond. The voltage droop equals the current step times the capacitor impedance (primarily ESR at high frequencies, capacitive reactance at lower frequencies) times the 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 with voltage and temperature, and some regulators require minimum ESR for stability. Polymer and tantalum capacitors provide good performance with more stable characteristics. Combining multiple capacitor types often yields the best overall performance, with ceramics handling fast transients and bulk capacitors providing energy storage.

Loop Bandwidth Considerations

Higher control loop bandwidth enables faster transient response, reducing both the magnitude and duration of voltage deviations. However, bandwidth is limited by stability requirements; the loop must maintain adequate phase margin to prevent oscillation. The output capacitor ESR and the pass transistor characteristics create poles and zeros that shape the loop response and constrain achievable bandwidth.

Low-dropout regulators face particular bandwidth challenges due to the high output impedance of the pass transistor near dropout. The dominant pole frequency decreases as dropout is approached, reducing bandwidth and degrading transient response. Designs optimized for transient response may compromise dropout performance or require specific output capacitor characteristics to maintain stability.

Feedforward and Fast Response Techniques

Feedforward techniques improve transient response by anticipating load changes rather than waiting for the feedback loop to detect and correct errors. Sensing load current directly and adjusting pass transistor drive proportionally provides immediate response to current changes, reducing the deviation the feedback loop must correct. This technique is particularly effective for predictable load transients.

Some regulators include fast transient response modes that temporarily override normal regulation to drive the output quickly toward the target. When a large deviation is detected, the regulator may increase loop gain, disable compensation networks, or switch to a different operating mode optimized for rapid correction. After the transient settles, normal regulation resumes.

Sense Line Fundamentals

Remote sensing connects the regulator's feedback input directly to the load rather than to the regulator output terminals. This compensates for voltage drops in the power distribution path, maintaining accurate voltage at the point of load regardless of cable or trace resistance. Without remote sensing, a 100-milliohm distribution impedance with 1A load current produces a 100mV error at the load, potentially exceeding the regulation specification.

Remote sensing requires separate sense wires from the load back to the regulator. These wires carry negligible current, so their resistance does not affect regulation accuracy. The regulator adjusts its output voltage to compensate for distribution drops, maintaining the sense point at the programmed voltage. The output terminal voltage exceeds the load voltage by the distribution drop.

Sense Line Protection

Sense lines must be protected against open and short circuit conditions that could cause regulator malfunction. An open sense line causes the regulator to drive its output to maximum voltage attempting to raise the apparently low sense voltage. Resistors from the output to each sense terminal provide default sensing if the sense lines open, limiting the overvoltage to a safe level.

Filtering on the sense lines prevents noise pickup from corrupting the feedback signal. The sense lines run alongside power cables in electrically noisy environments and can couple switching noise, motor brush noise, or other interference. RC filtering with corner frequencies below the regulator bandwidth attenuates this noise without affecting regulation.

Four-Terminal Sensing

Complete remote sensing systems use four-terminal (Kelvin) connections at the load, with separate force and sense terminals for both positive and negative rails. The force terminals connect to the power distribution; the sense terminals connect directly to the load circuit at the point of regulation. This arrangement provides the most accurate load voltage regulation possible, limited only by the sense amplifier accuracy rather than distribution resistance.

Proper sense point location requires attention to the load's current distribution. Sensing at a single point provides accurate regulation only at that point; other locations see voltage differences due to PCB trace resistance. For loads with multiple components drawing current from different locations, the optimal sense point minimizes the maximum voltage error across all load elements.

Differential Sensing

Differential remote sensing uses a high-common-mode-rejection amplifier to sense the voltage between positive and negative load terminals, rejecting ground potential differences between the regulator and load. This technique is essential when ground noise or ground loops would corrupt single-ended sensing, common in systems with long cable runs or multiple ground references.

The differential sense amplifier must provide high common-mode rejection (typically 60 dB or more) at frequencies where ground noise is significant. Input protection prevents damage from voltage transients on the sense lines. Matched sense line impedances preserve the common-mode rejection by ensuring equal coupling of any interference to both inputs.

Current Sharing Fundamentals

Parallel operation of multiple power supplies or regulator stages increases current capacity beyond the capability of a single unit. However, simply connecting outputs in parallel does not guarantee equal current sharing; minor differences in reference voltage, sense point location, or output impedance cause one unit to supply most or all of the current while others contribute little. Achieving balanced current sharing requires active techniques to force equalization.

The importance of current sharing depends on the application. If the goal is simply increased current capacity and any unit can supply the full load, unbalanced sharing is acceptable as long as no unit is overloaded. If the goal includes redundancy, thermal balancing, or efficient utilization of all units, forced current sharing becomes essential.

Droop Current Sharing

Droop current sharing, also called passive current sharing, uses the output impedance of each regulator to naturally balance currents. Each regulator's output voltage decreases slightly with increasing output current, creating a droop characteristic. When paralleled, the regulator producing higher voltage supplies more current, which reduces its output voltage; this continues until currents equalize at a common output voltage.

The droop magnitude, typically 1-5% of nominal output voltage from no load to full load, represents a trade-off between regulation accuracy and current sharing precision. Larger droop provides better sharing but worse regulation; smaller droop maintains tighter output voltage but allows greater current imbalance. Matching droop characteristics among paralleled units is essential for good sharing.

Active Current Sharing

Active current sharing uses dedicated control circuitry to force equal current distribution among paralleled units. Each unit measures its output current and compares it to either a reference value or the average of all units. The control loop adjusts each unit's output to equalize currents while a master reference determines the output voltage.

Master-slave configurations designate one unit as the master, which regulates voltage normally and broadcasts a current reference to the slave units. Slaves regulate their current to match the master's current command, inherently sharing the load. This approach provides excellent sharing but creates a single point of failure if the master fails.

Democratic current sharing compares each unit's current to the average of all units, with each unit adjusting to match the average. No single unit is critical; any unit can fail and the others continue sharing the remaining load. This approach requires communication among all units, typically through an analog current-sharing bus or digital communication link.

Hot-Swap and Redundancy

Hot-swap capability enables removing and replacing power supply units without shutting down the system. This requires redundant capacity so the remaining units can supply the full load while one unit is removed. Hot-swap mechanics include physical interlocks preventing insertion damage, soft-start circuits limiting inrush current, and ORing devices isolating failed units without affecting the output.

ORing can be implemented with diodes, which provide simple and reliable isolation but introduce forward voltage drop and power loss. Active ORing uses MOSFETs controlled to block reverse current when their unit's voltage falls, providing much lower insertion loss. Proper ORing ensures that a failed unit does not drag down the output bus or backfeed from healthy units.

Supply Sequencing Requirements

Many electronic systems require multiple supply voltages that must power up and down in specific sequences to prevent damage or malfunction. FPGAs and complex processors often require core voltage before I/O voltage to prevent latchup. Memory devices may have specific sequencing requirements for proper initialization. Power-down sequences are equally important to prevent data loss or component stress.

Sequencing specifications define the required order of supplies, the delay between sequential supplies, and the allowable variation in these parameters. Some sequences are flexible, requiring only that certain supplies precede others by any amount. Others are strict, specifying precise delays to coordinate with initialization timing requirements.

Hardware Sequencing Methods

Simple sequencing uses the power-good output of one regulator to enable the next in sequence. When the first supply reaches regulation, its power-good signal enables the second supply, which in turn enables the third when it reaches regulation. This creates a deterministic sequence with timing set by individual regulator soft-start periods.

Delay circuits between stages provide controlled delays when the inherent regulator timing is insufficient. RC networks with comparators create adjustable delays; precision timing requires dedicated delay ICs or digital timers. The delay accuracy must account for component tolerances and temperature variations to ensure reliable operation across all conditions.

Supply Tracking

Supply tracking maintains a defined relationship between two or more supply voltages during power-up and power-down. Ratio tracking maintains supplies in constant proportion; coincident tracking brings all supplies up together. Rail tracking is a special case where one supply follows a reference supply, maintaining a fixed offset or ratio through all operating conditions.

Tracking is implemented by using one supply's output as the reference for others, scaled by resistor dividers to set the relationship. The tracking supply follows the reference with speed limited by its control loop bandwidth. During power-up and power-down, all tracked supplies move together, preventing the voltage differentials that could damage loads sensitive to supply relationships.

Sequencing Controllers

Dedicated sequencing controllers consolidate power management functions including sequencing, tracking, voltage monitoring, and fault response. These devices monitor multiple supply voltages and control regulator enable pins according to programmed sequences. Programmable threshold voltages, delays, and fault responses accommodate diverse system requirements.

Digital sequencing controllers offer the greatest flexibility, with sequences defined through configuration registers rather than external components. They can implement complex conditional logic, such as aborting power-up if a supply fails to reach regulation within a timeout period. Fault logging and communication interfaces provide visibility into power system status and history.

Negative Rail Requirements

Many analog circuits require negative supply voltages in addition to positive. Operational amplifiers processing signals referenced to ground need bipolar supplies to handle both positive and negative signal excursions. Some devices, particularly depletion-mode transistors, require negative gate bias. Legacy interfaces and certain specialty devices may specify negative supply rails.

The negative rail current requirements are often much lower than positive, as negative supplies typically bias high-impedance inputs or provide small bias currents. This asymmetry allows efficient negative generation from the positive rail using charge pumps or inverting converters, avoiding the need for separate transformer windings or high-power negative regulators.

Charge Pump Inverters

Charge pumps generate negative voltage by sequentially connecting capacitors to different potentials using semiconductor switches. A flying capacitor charges from the positive rail, then its connections reverse to transfer charge to an output capacitor, producing negative voltage. Successive switching cycles maintain the output voltage. Charge pumps provide simple, low-noise negative generation suitable for low-current applications.

Voltage doubler charge pumps produce output magnitude equal to the input (minus diode drops and losses), so a 5V input produces approximately -5V output. Regulated charge pumps include feedback control to maintain precise output voltage regardless of input or load variations. Unregulated charge pumps produce output that varies with input and load, suitable only where regulation is not required.

Inverting Linear Regulators

Linear regulators can produce negative output from positive input using the appropriate topology. Three-terminal regulators designed for positive output can generate negative output by connecting the common terminal to the output and the output terminal to ground. The input current equals the load current plus the regulator quiescent current; the ground current flows through the load.

This configuration requires the input supply to float above the negative output rather than being ground-referenced. The regulator dissipates power based on the voltage from its input to output terminals, not from input to ground. Output current flows from ground through the load to the regulator output, which is the most negative point in the circuit.

Negative Linear Regulator ICs

Dedicated negative linear regulator ICs, such as the 79xx series, provide regulated negative output from negative input. These complement their positive counterparts (78xx series) with similar specifications and pinouts adapted for negative voltages. The pass transistor is a PNP type with collector connected to the negative input and emitter to the output, maintaining current flow from input to output despite the reversed polarity.

Low-dropout negative regulators (negative LDOs) use P-channel MOSFETs to minimize dropout voltage, enabling efficient regulation with small input-output differentials. Selection criteria parallel those for positive LDOs: output current, dropout voltage, noise, PSRR, and stability requirements with various output capacitors.

Floating Supply Applications

Floating power supplies have no inherent connection to system ground, allowing them to reference any potential as their common terminal. Applications include high-side gate drivers that must supply voltage above the switching node, current sensing amplifiers operating at elevated common-mode voltages, and isolated measurement systems requiring independent ground references.

The floating supply output terminals define a local voltage reference that can move with respect to the main system ground. Load circuits connected to the floating supply operate normally within their local reference frame while potentially operating at high voltage with respect to system ground.

Bootstrap Power Supplies

Bootstrap supplies use a capacitor charged through a diode to generate a floating supply that follows a switching node. When the switch is low, the bootstrap capacitor charges from a fixed supply through the diode. When the switch goes high, the capacitor's positive terminal rises with the switch, maintaining voltage above the switch to power the high-side gate driver. The bootstrap capacitor provides energy for one switching cycle; subsequent low intervals recharge it.

Bootstrap supplies are inherently limited to applications with minimum off-time to refresh the capacitor charge. Continuous high-side operation eventually depletes the bootstrap capacitor, losing gate drive. Charge pumps or isolated supplies address applications requiring extended high-side operation without low-side intervals.

Isolated DC-DC Converters

Isolated DC-DC converters provide completely floating output with no galvanic connection to the input. Transformer coupling transfers energy across the isolation boundary while maintaining DC isolation. The output can be connected to any reference potential within the converter's isolation voltage rating, typically hundreds or thousands of volts.

Small isolated converters for gate drive and measurement applications use push-pull or flyback topologies to achieve high isolation in compact packages. The output regulation may be fixed or adjustable, with feedback either through optocouplers across the isolation boundary or using primary-side sensing for lower-cost unregulated outputs.

Floating Supply Considerations

Floating supplies must withstand the full common-mode voltage between their output and system ground. Isolation voltage ratings specify the maximum allowable potential difference. Creepage and clearance distances on PCBs must accommodate this voltage to prevent breakdown. Some applications require reinforced isolation meeting safety standards for human protection.

Parasitic capacitance between floating and grounded circuits creates paths for common-mode current to flow, potentially disrupting operation or violating safety requirements. Low-capacitance transformers and careful layout minimize coupling. Common-mode transients on the floating node can inject currents that upset sensitive circuits; filtering and balanced design mitigate these effects.

Constant Voltage and Constant Current Modes

Laboratory power supplies operate in either constant voltage (CV) mode or constant current (CC) mode depending on load conditions. In CV mode, the supply maintains programmed voltage while current varies with load demand. When load current reaches the programmed current limit, the supply transitions to CC mode, maintaining constant current while voltage adjusts to accommodate the load. Front panel indicators show the active mode.

The transition between modes should be smooth and stable, without oscillation or overshoot. The crossover region where both voltage and current are near their limits requires careful control design to prevent instability. High-quality laboratory supplies achieve seamless transitions that are transparent to the load.

Output Programming and Display

Precision voltage and current adjustment enable setting outputs to exact values. Multi-turn potentiometers or digital encoders provide fine adjustment capability. Digital displays show actual output values with resolution matching or exceeding the adjustment precision. Some supplies include separate coarse and fine adjustment controls for rapid setup followed by precise trimming.

Programmable supplies accept voltage and current setpoints through communication interfaces, enabling automated operation. Standard interfaces include GPIB (IEEE-488), USB, RS-232, and Ethernet with SCPI command protocols. Remote programming enables integration with automated test systems and data acquisition applications.

Output Enable and Soft Start

The output enable function disconnects the supply output without changing programmed values, allowing setup before connecting to the load. This prevents exposing loads to full voltage during setup and enables rapid switching between energized and de-energized states during testing. Front panel buttons and remote commands control the output state.

Soft-start circuits gradually increase output voltage when enabled, limiting inrush current to capacitive loads and preventing overshoot. Adjustable soft-start times accommodate different load requirements. Some supplies include pre-programmed startup sequences that can set voltage, current limit, and timing parameters for automated test procedures.

Protection Features

Laboratory supplies incorporate comprehensive protection against overvoltage, overcurrent, and overtemperature conditions. Overvoltage protection (OVP) shuts down the output if voltage exceeds a programmed threshold, protecting loads sensitive to voltage stress. The OVP threshold is set above the normal operating voltage but below the load's damage threshold.

Overpower protection limits total output power to prevent overheating in sustained high-dissipation conditions. This complements current limiting by accounting for the voltage-current product rather than current alone. Intelligent supplies reduce capability smoothly as thermal limits approach rather than abruptly shutting down.

Multiple Output Configurations

Many laboratory supplies provide multiple independent outputs from a single chassis. Independent outputs can be set to different voltages and current limits, powering diverse loads from one instrument. Series connection of outputs produces higher voltage; parallel connection increases current capacity. Tracking modes synchronize outputs for applications requiring matched or ratioed supplies.

Master-slave configurations link multiple supplies for coordinated operation. The master provides voltage reference and control signals; slaves follow the master's output while contributing to current capacity. This enables creating high-power supplies from multiple smaller units or achieving tracking and sequencing across separate instruments.

Accuracy vs. Precision

Voltage regulation accuracy refers to how closely the actual output matches the intended setpoint. Precision refers to the repeatability and stability of the output over time and varying conditions. A power supply can be precise without being accurate (consistently offset from the setpoint) or accurate without being precise (varying randomly but averaging to the correct value). High-performance applications require both accuracy and precision.

Initial accuracy depends on reference voltage calibration, resistor divider matching, and amplifier offset. These parameters can be trimmed during production to achieve specified accuracy. Long-term stability depends on component aging, particularly of the voltage reference. Environmental stability requires low temperature coefficients throughout the signal chain.

Temperature Compensation

Output voltage variation with temperature arises from temperature coefficients of references, resistors, and active devices. Minimizing temperature effects requires selecting low-temperature-coefficient components and matching temperature coefficients in ratiometric circuits. Temperature-compensated references are essential for precision applications.

Beyond component selection, temperature compensation techniques can further improve stability. Measurement of actual temperature enables software compensation in digital systems. Oven-controlled references maintain the reference at constant temperature regardless of ambient conditions. These techniques achieve stability measured in parts per million per degree Celsius in the most demanding applications.

Long-Term Stability

Component aging causes gradual changes in output voltage over months and years. Voltage references drift as internal stresses relax and materials evolve. Resistors change value due to moisture absorption, oxidation, and crystalline restructuring. Even apparently stable circuits exhibit measurable drift over extended periods.

Specifying and achieving long-term stability requires understanding the aging mechanisms and selecting appropriate components. Buried Zener references offer the best long-term stability, with drift rates below 10 ppm per year. Thin-film and wire-wound resistors provide better stability than thick-film or carbon types. Burn-in accelerates initial aging, reducing subsequent drift during normal operation.

Calibration and Traceability

Precision power supplies require periodic calibration to verify and adjust output accuracy. Calibration compares the supply's output to a traceable reference standard and adjusts or documents any deviation. The calibration interval depends on required accuracy and observed drift rate, typically ranging from three months to two years for precision equipment.

Traceability links the calibration to national or international standards through an unbroken chain of comparisons. Each measurement in the chain adds uncertainty; minimizing the number of links and using high-accuracy transfer standards preserves accuracy. Calibration certificates document the traceability chain and measured uncertainties for each calibrated parameter.

Noise Considerations in Precision Regulators

Noise establishes the fundamental limit on achievable precision, as output voltage fluctuations prevent discrimination below the noise level. Voltage reference noise, amplifier noise, and resistor thermal noise all contribute to total output noise. Minimizing noise requires selecting low-noise components and optimizing circuit topology to minimize noise gain.

Measurement bandwidth affects observed noise; wider bandwidth captures more noise energy, increasing measured noise voltage. Specifying noise requires stating the measurement bandwidth, typically 10 Hz to 100 kHz for power supply applications. Some specifications distinguish between low-frequency noise (DC to 10 Hz) and wideband noise, as different applications have different sensitivities to these components.

Component Selection Guidelines

Proper component selection is fundamental to achieving reliable, high-performance linear regulator designs. Pass transistors must be rated for worst-case voltage, current, and power dissipation with adequate margin. Voltage references should be selected for the required accuracy, temperature coefficient, and noise performance. Resistors in the feedback divider require precision values with low temperature coefficients.

Capacitor selection affects stability, noise, and transient response. Output capacitor ESR must fall within the stable range for the regulator topology. Low-ESR ceramic capacitors offer excellent performance but may destabilize some regulators designed for tantalum characteristics. Input capacitors must handle ripple current without excessive heating or voltage droop.

Layout Considerations

PCB layout significantly impacts linear regulator performance. The feedback sense point should connect as close as possible to the load, minimizing the trace resistance included within the regulation loop. Power and ground traces should be sized for full load current with minimal voltage drop. Sense traces should avoid noisy areas and run as direct routes to minimize pickup.

Thermal design integrates with electrical layout. Pass transistors and other dissipative components require heat sinking or thermal connection to copper planes. Component placement should facilitate heat flow to ambient. Thermal vias beneath power components improve heat transfer to inner layers or the opposite side of the board.

Stability Analysis

Linear regulator stability requires adequate phase margin in the feedback loop. The combination of output capacitor ESR, capacitance, and load determines the loop poles and zeros. Simulation and analysis verify stability across the full range of operating conditions including load current, input voltage, and temperature variations.

Low-dropout regulators present particular stability challenges due to the high output impedance of the pass transistor near dropout. As dropout is approached, the dominant pole frequency decreases, potentially causing instability with capacitors that are stable under normal operation. Manufacturer guidelines specify acceptable capacitor ranges; deviating from these ranges risks oscillation.

Testing and Verification

Thorough testing verifies that the design meets specifications under all operating conditions. Regulation measurements confirm line and load regulation performance. Transient response testing with controlled load steps characterizes dynamic performance. Noise measurements verify output noise levels across the relevant frequency range.

Environmental testing evaluates performance across temperature, humidity, and altitude ranges. Margin testing operates the design beyond specified limits to identify weaknesses and verify design margins. Long-term testing accelerates aging to predict field reliability and identify infant mortality failures before deployment.