Adjustable/2.5V, 2% Tolerance Shunt Regulator# Technical Datasheet: KA431SLMF2TF Programmable Precision Reference
Manufacturer : FAIRCHILD (ON Semiconductor)
Component Type : Adjustable Precision Shunt Regulator (Programmable Zener)
Package : SOT-23-3 (Marking: 431)
---
## 1. Application Scenarios
### Typical Use Cases
The KA431SLMF2TF is a three-terminal adjustable shunt regulator, commonly employed as a precision voltage reference and error amplifier in closed-loop control systems. Its primary function is to maintain a stable reference voltage between its cathode and anode terminals, which can be programmed via two external resistors.
Primary applications include:
- Voltage Regulation in Switch-Mode Power Supplies (SMPS): Serves as the error amplifier in the feedback loop of AC/DC adapters, LED drivers, and DC/DC converters. It compares a sample of the output voltage against its internal 2.5V reference and adjusts the duty cycle of the PWM controller via an optocoupler.
- Series Pass Regulator: Used in linear regulator circuits to provide a stable, low-noise output voltage that is adjustable and more precise than a simple Zener diode.
- Voltage Monitoring: Functions as a precision undervoltage or overvoltage lockout (UVLO/OVLO) circuit. When the voltage at the REF pin crosses the 2.5V threshold, the cathode-anode impedance switches, triggering a logic signal.
- Constant Current Sinks/Sources: By fixing the reference voltage across a sense resistor, it can regulate current for applications like battery charging or LED driving.
### Industry Applications
- Consumer Electronics: Power supplies for TVs, set-top boxes, routers, and audio equipment.
- Computer & IT: Auxiliary power rails on motherboards, peripheral power modules.
- Industrial Control: PLCs, sensor interfaces, and actuator drivers requiring stable voltage rails.
- Automotive (Aftermarket/Non-Critical): Infotainment systems and lighting controls (note: not typically AEC-Q100 qualified).
- Telecommunications: Power modules for network switches and optical line terminals.
### Practical Advantages and Limitations
Advantages:
- High Precision: Typical reference voltage tolerance of ±0.4% (B-grade) at 25°C ensures accurate regulation.
- Low Dynamic Output Impedance (~0.2Ω): Provides good load regulation and transient response.
- Wide Operating Current Range (1mA to 100mA): Flexible for various power levels.
- Low Temperature Drift: Typically 50 ppm/°C, ensuring stability across operating temperatures.
- Cost-Effective & Ubiquitous: Industry-standard part with multiple second sources, simplifying design-in and procurement.
Limitations:
- Requires External Resistor Divider: The reference voltage must be set externally, adding components and potential drift sources.
- Minimum Cathode Current (Ika min): Requires a minimum bias current (typically 1mA) to maintain regulation. Operation below this current leads to inaccurate reference voltage.
- Stability Considerations: Requires careful compensation of the feedback loop, particularly when driving capacitive loads (like an optocoupler's LED). An improper phase margin can cause oscillations.
- Power Dissipation: As a shunt regulator, excess current is dissipated as heat. Efficiency can be poor in high-current, high-dropout applications compared to series regulators.
---
## 2. Design Considerations
### Common Design Pitfalls and Solutions
1. Pitfall: Oscillations in the Feedback Loop.
* Cause: Insufficient phase margin due to the combined capacitance at the REF pin (from PCB traces, resistor parasitics) and the optocoupler's LED capacitance.
* Solution: Add a compensation capacitor (typically
