High Input Voltage Adjustable 3-Terminal Linear Regulator # Technical Documentation: LR8N8G High-Voltage Adjustable Regulator
Manufacturer : SUPERTEX
Document Version : 1.0
Last Updated : October 2023
---
## 1. Application Scenarios
### 1.1 Typical Use Cases
The LR8N8G is a high-voltage, low-current, adjustable linear regulator designed for applications requiring stable voltage from high input sources. Key use cases include:
- High-Voltage Bias Supplies : Generating stable bias voltages (e.g., 50–100 V) for vacuum fluorescent displays (VFDs), photomultiplier tubes, or piezoelectric actuators from rectified AC mains or DC sources up to 450 V.
- Low-Current Sensor Interfaces : Powering isolated sensors, such as in industrial monitoring, where the sensor is referenced to a high common-mode voltage.
- Auxiliary Power in Switch-Mode Power Supplies (SMPS) : Providing a low-noise, regulated auxiliary rail for control ICs or feedback circuits in offline SMPS, where the input is derived from the bulk DC bus.
- Prototyping and Bench Supplies : Serving as a simple, adjustable high-voltage regulator for laboratory setups, eliminating the need for complex custom circuits.
### 1.2 Industry Applications
- Industrial Automation : Used in PLC I/O modules, motor drive control circuits, and high-voltage instrumentation.
- Medical Equipment : Provides bias voltages in imaging systems or diagnostic devices where low noise and stability are critical.
- Consumer Electronics : Found in niche applications like VFD-based audio equipment or legacy display systems.
- Telecommunications : Powers low-current circuits in line cards or base station equipment where high-voltage rails are present.
- Renewable Energy Systems : Regulates voltage from high-voltage DC links in solar inverters or battery management systems.
### 1.3 Practical Advantages and Limitations
Advantages:
- Wide Input Range : Operates from 12 V to 450 V DC, making it suitable for direct off-line or high-voltage DC applications.
- Adjustable Output : Output voltage adjustable from 1.20 V to 440 V via two external resistors, offering design flexibility.
- Low Quiescent Current : Typically 50 µA, minimizing power loss in standby or low-load conditions.
- Built-in Protection : Includes current limiting, thermal shutdown, and safe operating area (SOA) protection, enhancing reliability.
- Simple Implementation : Requires only two external resistors and input/output capacitors for basic operation, reducing component count.
Limitations:
- Low Output Current : Maximum output current is 10 mA, restricting use to very low-power circuits.
- Power Dissipation : In high input-output differentials, power dissipation must be carefully managed due to the TO-252 (DPAK) package’s thermal limits.
- Efficiency : As a linear regulator, efficiency is poor when the input-output voltage differential is large; not suitable for high-current or high-differential applications.
- External Components Required : Needs external capacitors for stability and noise reduction, and resistors for voltage adjustment, increasing board space.
---
## 2. Design Considerations
### 2.1 Common Design Pitfalls and Solutions
- Pitfall 1: Thermal Overload
*Issue*: High input-output differential at moderate currents can exceed the package’s power dissipation capability, triggering thermal shutdown or failure.
*Solution*: Calculate maximum power dissipation \(P_D = (V_{IN} - V_{OUT}) \times I_{OUT}\). Ensure \(P_D\) remains within the package limits (e.g., 1.5 W for DPAK with adequate heatsinking). Use a heatsink or reduce the input voltage if necessary.
- Pitfall 2: Output Instability
*Issue*: Insufficient output capacitance or
