Negative Voltage Regulators

1. Definition and Purpose of Negative Voltage Regulators

Definition and Purpose of Negative Voltage Regulators

A negative voltage regulator is an electronic circuit or integrated device designed to maintain a stable, predefined negative output voltage despite variations in input voltage, load current, or temperature. Unlike positive voltage regulators, which maintain a positive output, negative regulators ensure a consistent negative potential relative to ground or a reference point.

Fundamental Operating Principle

The core function of a negative voltage regulator is governed by feedback control, where the output voltage is compared against a reference voltage. Any deviation triggers corrective action via an error amplifier and pass element (typically a transistor or MOSFET). The regulator adjusts conduction to maintain:

$$ V_{out} = -V_{ref} \left(1 + \frac{R_1}{R_2}\right) $$

where Vref is the internal reference voltage, and R1, R2 form an external feedback network. Stability is achieved through compensation networks that mitigate phase shifts.

Key Design Parameters

$$ T_j = T_a + P_d \cdot \theta_{JA} $$

where Tj is junction temperature, Ta ambient temperature, and Pd power dissipation.

Practical Applications

Negative voltage regulators are indispensable in systems requiring symmetrical power rails, such as:

Historical context reveals early implementations using discrete zener diodes and bipolar transistors, later supplanted by monolithic ICs like the 79xx series (e.g., 7905 for -5V). Modern designs employ low-dropout (LDO) topologies with PNP or PMOS pass elements to minimize power loss.

Comparative Analysis with Positive Regulators

While operational principles mirror positive regulators, key distinctions include:

Negative Voltage Regulator Block Diagram Vin Vout
Negative Voltage Regulator Functional Block Diagram Block diagram showing the functional components of a negative voltage regulator, including input, error amplifier, pass element, feedback network, and output. V_in Error Amplifier Pass Element V_out R1 R2 V_ref GND
Diagram Description: The diagram would physically show the block-level flow of a negative voltage regulator, including input/output paths and feedback network connections.

1.2 Key Characteristics and Specifications

Output Voltage and Line Regulation

The primary function of a negative voltage regulator is to maintain a stable output voltage (VOUT) despite variations in input voltage (VIN) or load current (ILOAD). Line regulation quantifies this stability and is defined as:

$$ \text{Line Regulation} = \frac{\Delta V_{OUT}}{\Delta V_{IN}} \times 100\% $$

For precision applications, high-performance regulators like the LM337 achieve line regulation below 0.01%/V. The output voltage is typically adjustable via external resistors, with the relationship given by:

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

where VREF is the reference voltage (typically -1.25V) and IADJ is the adjust pin current (~50µA).

Load Regulation and Dropout Voltage

Load regulation measures the output voltage variation under changing load conditions:

$$ \text{Load Regulation} = \frac{V_{NL} - V_{FL}}{I_{FL} - I_{NL}} $$

where VNL and VFL are no-load and full-load voltages, respectively. Modern regulators maintain load regulation within 1-5mV. The dropout voltage—the minimum required VIN-to-VOUT differential—is critical for low-voltage applications. For example, the LT3015 features a 300mV dropout at 1.5A.

Thermal Performance

Power dissipation (PDISS) is calculated as:

$$ P_{DISS} = (V_{IN} - V_{OUT}) \times I_{LOAD} + V_{IN} \times I_{Q} $$

where IQ is the quiescent current. Thermal resistance (θJA) determines junction temperature rise:

$$ T_J = T_A + P_{DISS} \times \theta_{JA} $$

For TO-220 packages, θJA typically ranges from 50-65°C/W. Proper heatsinking is essential for high-current applications.

Noise and Ripple Rejection

Negative regulators suppress input noise through power supply rejection ratio (PSRR):

$$ \text{PSRR} = 20 \log \left(\frac{\Delta V_{IN}}{\Delta V_{OUT}}\right) $$

High-performance designs like the ADP7182 achieve >70dB PSRR at 1kHz. External bypass capacitors further enhance noise rejection, with the optimal value often determined by:

$$ C_{BY} = \frac{1}{2\pi f_{CROSS} \times ESR} $$

where fCROSS is the regulator's crossover frequency.

Protection Features

Advanced negative regulators incorporate:

For example, the MIC29712 implements a ±50V input transient rating with internal ESD protection up to 4kV.

1.3 Comparison with Positive Voltage Regulators

Negative and positive voltage regulators serve complementary roles in power supply design, but their operational principles and applications differ significantly. While both stabilize output voltage, their topologies, grounding schemes, and performance characteristics diverge in ways that influence circuit design.

Polarity and Ground Reference

Positive regulators, such as the LM78xx series, maintain a positive output relative to ground. The ground pin serves as the reference, and the load connects between the output and ground. In contrast, negative regulators like the LM79xx series produce an output that is negative relative to ground, requiring the load to be connected between ground and the negative output terminal.

$$ V_{out} = -|V_{ref}| \left(1 + \frac{R_2}{R_1}\right) $$

This polarity inversion in negative regulators necessitates careful attention to grounding in mixed-signal systems where both positive and negative supplies coexist.

Stability Considerations

Negative regulators exhibit different stability characteristics compared to their positive counterparts. The feedback loop in a negative regulator must account for the inverted phase response, which can affect compensation requirements. Empirical data shows that negative regulators typically require larger output capacitors (often 2-3× the value) for equivalent stability margins.

Thermal Management

The power dissipation equation remains identical in form for both regulator types:

$$ P_{diss} = (V_{in} - V_{out}) \times I_{load} $$

However, negative regulators often demonstrate slightly higher junction-to-case thermal resistance (θJC) in practice due to package design constraints. For example, the LM7905 typically exhibits 5°C/W θJC compared to 3°C/W for an LM7805 in identical packages.

Noise Performance

Spectrum analysis reveals that negative regulators generate approximately 20-30% higher high-frequency noise (10kHz-1MHz range) than equivalent positive regulators. This difference stems from the inverted control loop architecture and can be mitigated through proper bypassing:

Application-Specific Tradeoffs

In operational amplifier power systems, the pairing of positive and negative regulators introduces unique considerations:

Parameter Positive Regulator Negative Regulator
Line Regulation 0.01%/V typical 0.015%/V typical
Load Regulation 10mV (100mA step) 15mV (100mA step)
Dropout Voltage 2V @ 1A 2.5V @ 1A

The increased dropout voltage in negative regulators becomes particularly relevant in low-voltage battery-powered systems where every millivolt of headroom affects efficiency.

Transient Response Characteristics

Negative regulators demonstrate 15-20% slower transient response to load current steps compared to positive regulators with equivalent output capacitance. This behavior originates from the minority carrier dynamics in the PNP pass transistors commonly used in negative regulator designs, as opposed to the NPN transistors in positive regulators.

$$ t_{response} = \frac{C_{out} \Delta V}{I_{step}} $$

where ΔV represents the allowable voltage deviation during the transient event.

2. Linear Negative Voltage Regulators

2.1 Linear Negative Voltage Regulators

Operating Principle

Linear negative voltage regulators function by dissipating excess power as heat to maintain a stable negative output voltage. Unlike positive regulators, they require a negative input voltage and a common ground reference. The fundamental operation relies on a pass transistor (typically a PNP or NPN in a Darlington configuration) controlled by an error amplifier that compares a fraction of the output voltage to a reference.

The output voltage VOUT is determined by:

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

where VREF is the internal reference voltage, and R1, R2 form the feedback network.

Key Components

Performance Metrics

The efficiency η of a linear negative regulator is given by:

$$ \eta = \frac{|V_{OUT}| \cdot I_{OUT}}{|V_{IN}| \cdot I_{IN}} \times 100\% $$

Due to the inherent power dissipation, efficiency is often low, especially for large input-output differentials. The dropout voltage, defined as the minimum required |VIN| - |VOUT| for regulation, is a critical parameter:

$$ V_{DROP} = |V_{IN}| - |V_{OUT}| $$

Common Topologies

Three-terminal regulators like the LM79xx series (e.g., LM7905 for -5V) are widely used due to their simplicity. More advanced designs, such as the LT1033, offer adjustable output voltages and lower dropout characteristics.

Fixed Negative Regulators

These provide a preset output voltage (e.g., -5V, -12V) with minimal external components. The stability of the output depends heavily on input capacitance to suppress oscillations.

Adjustable Negative Regulators

Devices like the LM337 allow the output voltage to be set via external resistors. The governing equation is:

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

where IADJ is the small adjustment pin current (typically ~50µA).

Thermal Considerations

Power dissipation PD is a critical design constraint:

$$ P_D = (|V_{IN}| - |V_{OUT}|) \cdot I_{OUT} $$

Exceeding the maximum junction temperature (typically 125°C–150°C) necessitates a heatsink. The required thermal resistance θSA is calculated as:

$$ \theta_{SA} \leq \frac{T_{J(MAX)} - T_A}{P_D} - \theta_{JC} - \theta_{CS} $$

where TA is ambient temperature, θJC is junction-to-case resistance, and θCS is case-to-sink resistance.

Practical Applications

Internal Block Diagram of a Linear Negative Voltage Regulator A functional block diagram showing the internal structure of a linear negative voltage regulator, including pass transistor, error amplifier, reference voltage, and feedback resistors. V_IN Pass Transistor V_OUT R1 R2 Error Amplifier V_REF
Diagram Description: A diagram would visually demonstrate the internal block structure of a linear negative voltage regulator, showing the relationship between the pass transistor, error amplifier, and feedback network.

2.2 Switching Negative Voltage Regulators

Switching negative voltage regulators employ high-frequency switching elements (typically MOSFETs or BJTs) to efficiently convert a positive input voltage to a regulated negative output. Unlike linear regulators, which dissipate excess power as heat, switching regulators achieve higher efficiency through pulse-width modulation (PWM) or pulse-frequency modulation (PFM).

Operating Principle

The core mechanism involves an inductor-based energy transfer cycle:

$$ V_{out} = -D \cdot V_{in} \quad \text{(Buck-Boost Topology)} $$

where D is the duty cycle (0 < D < 1).

Key Topologies

Inverting Buck-Boost

Most common for negative output generation. The output voltage polarity is inverted relative to the input:

$$ \frac{V_{out}}{V_{in}} = -\frac{D}{1-D} $$

Cuk Converter

Uses capacitive energy transfer for lower output ripple. Its transfer function is:

$$ \frac{V_{out}}{V_{in}} = -\frac{D}{1-D} $$

Notably, the Cuk converter provides continuous input and output currents.

Control Methods

Modern IC implementations use:

Design Considerations

Critical parameters include:

Practical Applications

Switching negative regulators are indispensable in:

Inverting Buck-Boost Regulator
Inverting Buck-Boost Regulator Operation Schematic diagram of an inverting buck-boost regulator showing energy transfer cycle with MOSFET switch, inductor, diode, and input/output capacitors. Includes time-domain waveforms for PWM signal and inductor current. V_in MOSFET L V_out I_L (charge) I_L (discharge) PWM Signal Inductor Current Switch ON Switch OFF
Diagram Description: The diagram would physically show the energy transfer cycle (charge/discharge phases) and component interactions in an inverting buck-boost regulator.

2.3 Adjustable vs. Fixed Output Regulators

Fundamental Differences

Negative voltage regulators are categorized into fixed and adjustable types based on their output voltage configurability. Fixed regulators, such as the LM7905 (-5V) or LM7912 (-12V), provide a predefined output voltage determined by internal bandgap references and resistive dividers. In contrast, adjustable regulators like the LM337 allow the output voltage to be set externally using a resistor network.

Mathematical Basis for Adjustable Regulators

The output voltage of an adjustable negative regulator is governed by the feedback network between its output (VOUT) and adjust (ADJ) pins. For the LM337, the relationship is derived from Kirchhoff's voltage law:

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

where:

Practical Trade-offs

Fixed Regulators

Adjustable Regulators

Stability Considerations

Adjustable regulators demand careful compensation to avoid oscillations. The feedback network introduces a pole at:

$$ f_p = \frac{1}{2\pi R_2 C_{ADJ}} $$

where CADJ is the bypass capacitor (typically 10µF). A minimum load current (e.g., 10mA for LM337) is often required to maintain regulation.

Application-Specific Selection

In precision instrumentation, adjustable regulators dominate for their ability to fine-tune voltages. Fixed regulators are preferred in high-volume consumer electronics for cost and reliability. For example, telecom systems use adjustable regulators to accommodate varying backplane requirements, while fixed regulators power legacy analog circuits.

Historical Context

The LM337 (1976) emerged as a counterpart to the LM317, addressing the need for programmable negative supplies in op-amp biasing and audio equipment. Fixed regulators like the 79xx series became industry standards due to their robustness in early digital systems.

LM337 Adjustable Regulator Circuit Schematic diagram of an LM337 adjustable negative voltage regulator circuit, showing resistor network configuration and external components. LM337 VIN VOUT ADJ R1 R2 CADJ CIN COUT VIN VOUT
Diagram Description: The diagram would show the resistor network configuration for an adjustable negative voltage regulator, illustrating the physical connections between the LM337's pins and external components.

3. Basic Circuit Configuration

3.1 Basic Circuit Configuration

Negative voltage regulators, such as the LM79xx series, follow a topology distinct from their positive counterparts. The fundamental circuit consists of three terminals: input (VIN), ground (GND), and output (VOUT), where the output voltage is regulated to a negative value relative to ground. The input must be more negative than the output to maintain regulation.

Core Components

The minimal configuration requires:

Mathematical Analysis

The dropout voltage (VDO) for a negative regulator is defined as:

$$ V_{DO} = |V_{IN}| - |V_{OUT}| $$

where VIN and VOUT are negative. For stable operation, VIN must satisfy:

$$ |V_{IN}| \geq |V_{OUT}| + V_{DO} $$

The power dissipation (PD) is calculated as:

$$ P_D = (|V_{IN}| - |V_{OUT}|) \cdot I_{LOAD} $$

Practical Implementation

A typical LM7905 circuit (-5V output) with a -9V input includes:

LM7905 VOUT (-5V) VIN (-9V) GND

Stability Considerations

Negative regulators exhibit higher sensitivity to capacitive load ESR. For stability:

$$ ESR_{COUT} \leq \frac{1}{2\pi \cdot f_c \cdot C_{OUT}} $$

where fc is the regulator's crossover frequency (typically 0.5–1 MHz for monolithic designs). Low-ESR ceramic capacitors may require a small series resistor (0.1–1 Ω) to avoid oscillation.

3.2 Input and Output Capacitor Selection

Stability and Transient Response Considerations

The selection of input and output capacitors in a negative voltage regulator circuit is critical for ensuring stability, minimizing output ripple, and improving transient response. Unlike positive regulators, negative regulators often exhibit different stability characteristics due to the inversion of feedback polarity. The output capacitor (COUT) must compensate for the regulator's internal pole while maintaining phase margin.

The minimum output capacitance required to ensure stability can be derived from the regulator's open-loop gain and phase response. For a typical negative linear regulator (e.g., LM337), the output capacitance must satisfy:

$$ C_{OUT} \geq \frac{1}{2 \pi \cdot f_{UGF} \cdot R_{ESR}} $$

where fUGF is the unity-gain frequency of the regulator's error amplifier, and RESR is the equivalent series resistance of the capacitor. Excessive RESR can introduce a zero that degrades phase margin, while insufficient capacitance leads to poor load transient response.

Input Capacitor Requirements

The input capacitor (CIN) serves two primary purposes: reducing input voltage ripple and providing instantaneous current during load transients. For a negative regulator, the input capacitor must handle the reverse current flow during switching or transient conditions. The minimum input capacitance is determined by:

$$ C_{IN} \geq \frac{I_{PEAK} \cdot \Delta t}{\Delta V_{IN}} $$

where IPEAK is the peak current demand, Δt is the transient duration, and ΔVIN is the allowable input voltage droop. Low-ESR ceramic capacitors (X5R/X7R) are preferred for high-frequency decoupling, while bulk electrolytic or tantalum capacitors handle slower transients.

Output Ripple and Noise Suppression

Output ripple voltage (VRIPPLE) is a function of the capacitor's impedance at the switching frequency. For a negative regulator, the ripple can be approximated by:

$$ V_{RIPPLE} = I_{LOAD} \cdot \sqrt{R_{ESR}^2 + \left( \frac{1}{2 \pi f_{SW} C_{OUT}} \right)^2 } $$

where fSW is the ripple frequency (or switching frequency in LDOs with bypass switching). Parallel combinations of ceramic and tantalum capacitors are often used to achieve low RESR and high capacitance density.

Practical Design Guidelines

In high-noise environments, additional filtering with an RC network at the output may be necessary to suppress high-frequency artifacts. The time constant of this network should not interfere with the regulator's control loop bandwidth.

Negative Regulator Capacitor Effects Waveform diagram illustrating input/output ripple and capacitor impedance effects in a negative voltage regulator. V_IN V_OUT V_RIPPLE f_SW Input/Output Ripple Waveforms 10Hz 1kHz 100kHz 10MHz Frequency (log scale) 1mΩ 100mΩ 10Ω Impedance (log scale) ESR C_OUT C_IN phase margin Capacitor Impedance vs Frequency Transient Response
Diagram Description: The section discusses stability, ripple, and transient response, which are best visualized with waveforms and impedance relationships.

3.3 Heat Dissipation and Thermal Considerations

Negative voltage regulators, like their positive counterparts, dissipate power as heat when operating under load. The power dissipation Pdiss is primarily governed by the voltage drop across the regulator and the output current. For a negative regulator with an input voltage Vin and output voltage Vout, the power dissipated is:

$$ P_{diss} = (V_{in} - V_{out}) \cdot I_{load} + V_{in} \cdot I_{q} $$

where Iload is the load current and Iq is the quiescent current of the regulator. The first term dominates in most practical applications, making thermal management critical for high-current or high-dropout scenarios.

Thermal Resistance and Junction Temperature

The junction temperature Tj of the regulator must be kept within safe limits to prevent thermal shutdown or degradation. The relationship between power dissipation, thermal resistance, and ambient temperature is:

$$ T_j = T_a + ( heta_{jc} + heta_{cs} + heta_{sa}) \cdot P_{diss} $$

where:

For TO-220 packages, typical θjc values range from 1–5°C/W, while θja (junction-to-ambient without a heatsink) can exceed 50°C/W. Exceeding the maximum junction temperature (often 125–150°C) risks device failure.

Heatsink Selection and Design

Heatsinks reduce θsa to maintain Tj within limits. The required thermal resistance of the heatsink is derived by rearranging the junction temperature equation:

$$ heta_{sa} \leq \frac{T_{j,max} - T_a}{P_{diss}} - ( heta_{jc} + heta_{cs}) $$

Forced air cooling can further reduce θsa by 30–50%. Thermal interface materials (TIMs) like silicone pads or thermal grease minimize θcs, typically achieving 0.1–1.0°C/W.

Practical Considerations

TO-220 Package Heatsink Thermal Resistance Network

3.4 Protection Mechanisms (Overcurrent, Overvoltage)

Overcurrent Protection

Negative voltage regulators, like their positive counterparts, must incorporate overcurrent protection to prevent catastrophic failure under excessive load conditions. The most common method employs a current-sensing resistor in series with the output, feeding a comparator that triggers a shutdown or current-limiting mechanism when the threshold is exceeded. For a regulator with a maximum output current Imax, the power dissipation in the pass transistor during a short-circuit event is given by:

$$ P_{diss} = (V_{in} - V_{out}) \cdot I_{max} $$

where Vin is the input voltage and Vout is the regulated output voltage. Foldback current limiting is often implemented in high-power designs to reduce Pdiss under fault conditions, where the current limit decreases as Vout approaches zero.

Overvoltage Protection

Overvoltage events can occur due to input transients, inductive load kickback, or regulator failure. A crowbar circuit, typically using an SCR or zener-triggered MOSFET, is employed to clamp the output voltage to a safe level. The triggering voltage Vtrigger is set slightly above the nominal output:

$$ V_{trigger} = V_{out} + \Delta V_{margin} $$

where ΔVmargin accounts for normal regulation tolerances. For negative regulators, the protection circuitry must account for the inverted polarity, often requiring P-channel devices or careful level-shifting of control signals.

Thermal Considerations

Protection circuits must account for thermal runaway in bipolar designs, where increased junction temperature raises leakage currents, further increasing dissipation. Modern IC regulators integrate temperature sensors that enforce a derating curve:

$$ I_{allowed} = I_{max} \cdot \left(1 - \frac{T_j - T_{amb}}{T_{j(max)} - T_{amb}}\right) $$

with Tj as junction temperature and Tamb as ambient temperature. This necessitates careful PCB layout to ensure accurate thermal sensing.

Practical Implementation

In the LM337 adjustable negative regulator, the internal protection network includes:

For mission-critical applications, external TVS diodes (e.g., SMAJ series) provide additional transient suppression, with breakdown voltages selected per:

$$ V_{BR} \geq 1.3 \cdot |V_{in(max)}| $$

where the 30% margin accounts for temperature coefficients and aging effects.

Negative Voltage Regulator Protection Mechanisms Schematic diagram showing protection circuits for a negative voltage regulator, including current-sensing, crowbar, and thermal protection elements. VIN VOUT Current Sense R_sense Comparator V_trigger Foldback SCR Crowbar Trigger TVS V_BR Thermal T_j I_max
Diagram Description: The section describes multiple protection circuits (current-sensing, crowbar, thermal) with spatial relationships between components and triggering thresholds that would be clearer visually.

4. Use in Audio Amplifiers

4.1 Use in Audio Amplifiers

Negative voltage regulators play a critical role in audio amplifier designs, particularly in dual-rail power supplies that require symmetric positive and negative voltage rails. High-fidelity audio circuits, such as operational amplifier (op-amp) stages and Class-AB amplifiers, depend on stable negative voltage rails to minimize distortion and ensure proper biasing of complementary output transistors.

Biasing and Signal Integrity

In Class-AB amplifiers, the output stage consists of complementary NPN and PNP transistors. A negative voltage regulator (e.g., LM337 or 79xx series) ensures the PNP transistor operates in its active region without crossover distortion. The regulator maintains a constant negative rail voltage, preventing thermal drift and load-induced fluctuations that degrade Total Harmonic Distortion (THD) performance.

$$ V_{out} = -V_{ref} \left(1 + \frac{R_2}{R_1}\right) + I_{adj}R_2 $$

Where Vref is the reference voltage (typically -1.25V for LM337), R1 and R2 set the output voltage, and Iadj is the adjustment pin current (≈50µA).

Noise Suppression Techniques

Audio amplifiers are sensitive to power supply noise, which manifests as audible hum or hiss. Negative regulators mitigate this by:

Case Study: Op-amp Rail Splitting

In a ±15V op-amp supply, a 7915 negative regulator paired with a 7815 positive regulator creates a balanced dual-rail system. Asymmetry in rail voltages introduces DC offset, distorting the output waveform. The regulator’s line regulation (typically 0.01%/V) ensures symmetry even with varying input voltages.

Dual-Rail Power Supply with 7815 and 7915 +15V -15V

Thermal Considerations

Negative regulators dissipate power as:

$$ P_{diss} = (V_{in} - V_{out}) \times I_{load} $$

In high-current audio stages (e.g., >500mA), heat sinks or switching preregulators (e.g., Buck-Boost converters) are necessary to maintain junction temperatures below 125°C for reliable operation.

Dual-Rail Power Supply Circuit for Audio Amplifiers Schematic diagram of a dual-rail power supply using 7815 and 7915 voltage regulators, with input/output capacitors and ground connections. 7815 Vin + +15V 7915 Vin - -15V GND 10µF 10µF 0.1µF 0.1µF Load
Diagram Description: The diagram would physically show the dual-rail power supply circuit with 7815 and 7915 regulators, including connections and voltage labels.

4.2 Role in Bipolar Power Supplies

Negative voltage regulators are essential in bipolar power supplies, where symmetrical positive and negative rails are required. These regulators maintain a stable negative output voltage despite variations in input voltage or load conditions, ensuring balanced operation in analog circuits such as operational amplifiers, data converters, and audio systems.

Symmetrical Voltage Regulation

In bipolar supplies, the negative regulator complements the positive regulator to achieve dual-rail operation. The relationship between input and output voltages is governed by:

$$ V_{out} = -V_{ref} \left(1 + \frac{R_2}{R_1}\right) + I_{adj}R_2 $$

where Vref is the reference voltage (typically -1.25 V for LM337), R1 and R2 are feedback resistors, and Iadj is the adjustment pin current. The negative regulator mirrors the positive regulator's behavior but inverts the output polarity.

Stability Considerations

Bipolar supplies demand matched transient response and load regulation between positive and negative regulators. Key stability factors include:

Practical Implementation

A typical bipolar supply using LM317 (positive) and LM337 (negative) requires:

LM317 LM337 Common Ground

The ground reference becomes critical in such configurations. Any impedance in the ground path introduces errors, necessitating a star grounding topology for precision applications.

Noise and Ripple Rejection

Negative regulators exhibit comparable PSRR (Power Supply Rejection Ratio) to their positive counterparts. For the LM337:

$$ \text{PSRR} = 60\,\text{dB} \quad \text{at}\, 120\,\text{Hz} $$

However, in bipolar configurations, differential noise becomes a concern. Cross-coupling through shared ground paths can degrade system performance, requiring:

Current Balancing

Asymmetric loading between rails causes DC offset errors. The current imbalance ΔI relates to offset voltage Vos through:

$$ V_{os} = \Delta I \times (Z_{gnd} + Z_{reg}) $$

where Zgnd is ground impedance and Zreg is regulator output impedance. Active current balancing techniques using sense resistors and error amplifiers may be employed in critical applications.

Bipolar Power Supply with LM317 and LM337 A schematic diagram of a bipolar power supply using LM317 (positive regulator) and LM337 (negative regulator) with symmetrical components and shared ground. Star Ground (GND) LM317 Vin+ Adj Vout+ R1 R2 C1 LM337 Vin- Adj Vout- R1 R2 C2 +Vin +Vout -Vin -Vout
Diagram Description: The section describes a bipolar power supply configuration with symmetrical components and grounding, which is inherently spatial and benefits from visual representation of the dual-rail setup.

4.3 Industrial and Laboratory Equipment

Stability Requirements in Precision Systems

Negative voltage regulators in industrial and laboratory settings must maintain ultra-low output noise and high power supply rejection ratio (PSRR) to avoid interference with sensitive analog instrumentation. For instance, operational amplifiers and data acquisition systems often require bipolar supplies (±15V or ±5V) with ripple rejection exceeding 80 dB at 100 Hz. The relationship between PSRR and output noise can be modeled as:

$$ \text{PSRR}(f) = 20 \log_{10} \left( \frac{\Delta V_{\text{in}}}{\Delta V_{\text{out}}} \right) $$

where f is the noise frequency, and ΔVin/ΔVout represents the input-to-output noise attenuation.

Thermal Management in High-Current Applications

Industrial loads such as motor controllers or RF amplifiers demand regulators like the LM337 with current handling beyond 1.5A. The junction-to-ambient thermal resistance (θJA) must be minimized to prevent thermal shutdown. For a TO-220 package dissipating 10W:

$$ T_j = T_a + (P_d \times \theta_{JA}) $$

where Tj is junction temperature, Ta is ambient temperature, and Pd is power dissipation. Active cooling or heatsinks are often mandatory for θJA values below 40°C/W.

Case Study: Mass Spectrometer Power Supply

A -1250V bias supply for ion detectors exemplifies custom regulator designs. Here, a Zener-referenced pass transistor (e.g., 2N6515) with optocoupled feedback ensures stability. Key parameters include:

Fault Protection Mechanisms

Industrial regulators integrate foldback current limiting and safe operating area (SOA) protection. For a -12V/3A supply, the SOA boundary is defined by:

$$ V_{\text{DS}} \times I_D \leq P_{\text{max}}(T_j) $$

where VDS is drain-source voltage and ID is drain current. This prevents catastrophic failure during load transients.

EMI Compliance Challenges

Switching negative regulators (e.g., LTspice-simulated buck-boost topologies) must suppress conducted emissions below CISPR 22 Class B limits. A π-filter with X7R ceramics achieves >40 dB attenuation at 1 MHz:

L1 C1 C2

5. Output Voltage Instability

5.1 Output Voltage Instability

Output voltage instability in negative voltage regulators arises from multiple factors, including load transients, thermal drift, and feedback loop dynamics. Unlike positive regulators, negative regulators must handle inverted control signals, which can introduce phase shifts and exacerbate instability.

Feedback Loop Stability

The stability of a negative voltage regulator is governed by its feedback loop, where the error amplifier compares a fraction of the output voltage to a reference. The loop gain L(s) is given by:

$$ L(s) = A_{OL}(s) \cdot \beta(s) $$

where AOL(s) is the open-loop gain of the error amplifier and β(s) is the feedback factor. For stability, the phase margin must exceed 45° at the crossover frequency where |L(s)| = 1.

Thermal and Load Effects

Thermal gradients across the regulator IC can induce voltage drift due to temperature-dependent variations in the bandgap reference and pass transistor characteristics. Load transients, particularly in switching regulators, cause abrupt changes in output current, leading to undershoot or overshoot if the control loop cannot respond quickly enough.

$$ \Delta V_{out} = \frac{\Delta I_{load}}{C_{out}} \cdot t_{response} $$

where ΔIload is the load step, Cout is the output capacitance, and tresponse is the regulator's settling time.

Compensation Techniques

To mitigate instability, compensation networks are employed. A dominant pole at the error amplifier output ensures high DC gain while rolling off at higher frequencies. A zero is introduced to counteract phase lag:

$$ f_{pole} = \frac{1}{2\pi R_{comp}C_{comp}} $$ $$ f_{zero} = \frac{1}{2\pi R_{esr}C_{out}} $$

where Rcomp and Ccomp are the compensation components, and Resr is the equivalent series resistance of the output capacitor.

Practical Considerations

Case Study: LM337 Instability

The LM337, a common adjustable negative regulator, exhibits oscillation when the output capacitance exceeds 10µF without proper ESR. The datasheet mandates a minimum ESR of 0.1Ω–0.5Ω to ensure stability. This constraint arises from the non-inverting feedback architecture, which is more sensitive to capacitive loads than positive regulators.

Negative Voltage Regulator Feedback Loop and Compensation Block diagram of a negative voltage regulator feedback loop with compensation components and an inset Bode plot showing phase margin. Error Amp A_OL(s) Output Stage Feedback β(s) Rcomp Ccomp Cout ΔI_load ΔV_out Gain Phase Frequency Magnitude/Phase f_pole f_zero
Diagram Description: The section discusses feedback loop stability, compensation techniques, and transient responses, which are highly visual concepts involving signal flow and time-domain behavior.

5.2 Excessive Heat Generation

Negative voltage regulators, like their positive counterparts, dissipate power as heat when regulating higher input voltages to lower output levels. The primary mechanism of heat generation stems from the voltage drop across the pass transistor and the associated current flow. The power dissipation Pdiss is given by:

$$ P_{diss} = (V_{in} - V_{out}) \cdot I_{load} + V_{in} \cdot I_{q} $$

where Vin is the input voltage, Vout the regulated output voltage, Iload the load current, and Iq the quiescent current of the regulator itself. The first term represents the dominant power loss due to the voltage differential, while the second accounts for internal circuit inefficiencies.

Thermal Resistance and Junction Temperature

The regulator's ability to dissipate heat is constrained by its thermal resistance θJA (junction-to-ambient). The junction temperature TJ is calculated as:

$$ T_J = T_A + (P_{diss} \cdot \theta_{JA}) $$

where TA is the ambient temperature. Exceeding the maximum junction temperature (typically 125–150°C for silicon devices) triggers thermal shutdown or irreversible damage. For example, an LM337 with θJA = 50°C/W dissipating 5W in a 25°C ambient reaches:

$$ T_J = 25°C + (5W \cdot 50°C/W) = 275°C $$

This far exceeds safe limits, necessitating heat sinks or improved thermal management.

Mitigation Strategies

$$ \theta_{total} = \theta_{JC} + \theta_{CS} + \theta_{SA} $$

where θJC (junction-to-case), θCS (case-to-sink), and θSA (sink-to-ambient) are additive.

Real-World Implications

In aerospace applications, where ambient temperatures vary drastically, designers must account for worst-case TA and derate components accordingly. For instance, a -15V regulator powering an RF amplifier in a satellite must ensure TJ remains within bounds even during solar heating phases.

5.3 Noise and Ripple Problems

Sources of Noise in Negative Voltage Regulators

Negative voltage regulators, like their positive counterparts, are susceptible to noise and ripple originating from both internal and external sources. Internally, thermal noise in the pass transistor, shot noise from current flow across semiconductor junctions, and flicker noise (1/f noise) in the error amplifier contribute to output voltage fluctuations. Externally, conducted noise from the input supply and radiated electromagnetic interference (EMI) can couple into the regulator's feedback network.

$$ v_{n,thermal} = \sqrt{4kTR\Delta f} $$

where k is Boltzmann's constant, T is temperature in Kelvin, R is the resistance, and Δf is the bandwidth.

Ripple Rejection and Power Supply Rejection Ratio (PSRR)

The ability of a regulator to attenuate input ripple is quantified by its Power Supply Rejection Ratio (PSRR), defined as:

$$ \text{PSRR} = 20\log_{10}\left(\frac{V_{ripple,in}}{V_{ripple,out}}\right) $$

For negative regulators such as the LM337, PSRR typically exceeds 60 dB at low frequencies (below 100 Hz) but degrades at higher frequencies due to the limited bandwidth of the internal error amplifier. The PSRR roll-off follows a first-order response:

$$ \text{PSRR}(f) = \text{PSRR}_{DC} \cdot \frac{1}{\sqrt{1 + (f/f_c)^2}} $$

where fc is the corner frequency (typically 1-10 kHz for linear regulators).

Minimizing Output Ripple

Three primary methods exist for reducing output ripple in negative voltage regulator circuits:

Case Study: Noise in Adjustable Negative Regulators

Adjustable negative regulators like the LM337 introduce additional noise through the feedback resistor network. The noise gain of the circuit amplifies the regulator's internal noise by:

$$ NG = 1 + \frac{R_1}{R_2} $$

where R1 and R2 set the output voltage. Bypassing R2 with a capacitor (C_{adj) creates a low-pass filter for the feedback path, reducing high-frequency noise. The optimal value balances stability and noise rejection:

$$ C_{adj} = \frac{1}{2\pi f_c R_2} $$

where fc should be at least one decade below the regulator's unity-gain bandwidth.

PSRR vs Frequency and Ripple Reduction Techniques A combined Bode plot and schematic diagram showing PSRR vs frequency characteristics and ripple reduction techniques in a negative voltage regulator circuit. PSRR vs Frequency Frequency (Hz) PSRR (dB) 10 100 1k 10k PSRR(f) fc Ripple Reduction Circuit Vripple(in) Cin L IC Vripple(out) Cout R1 R2 Cadj Star Ground
Diagram Description: The section discusses ripple rejection and noise sources with mathematical relationships that would benefit from visual representation of frequency-domain behavior and filtering techniques.

6. Recommended Datasheets and Manuals

6.1 Recommended Datasheets and Manuals

6.2 Advanced Topics and Research Papers

6.3 Online Resources and Communities