Adjustable/2.5V, 2% Tolerance Shunt Regulator# Technical Documentation: KA431SLMFTF Programmable Shunt Regulator
Manufacturer : FAIRCHILD (ON Semiconductor)
Component : KA431SLMFTF (SOT-23 Package)
Document Version : 1.0
Date : October 26, 2023
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## 1. Application Scenarios
### 1.1 Typical Use Cases
The KA431SLMFTF is a three-terminal adjustable precision shunt regulator, commonly employed as a voltage reference and error amplifier in regulation circuits. Its primary function is to maintain a stable reference voltage (typically 2.5V) between its cathode and anode, with the third terminal (reference) allowing adjustment via external resistors.
Primary Applications Include:
- Voltage Regulation in Switch-Mode Power Supplies (SMPS): Serves as the error amplifier in feedback loops, comparing a sample of the output voltage against the internal 2.5V reference to control the PWM controller.
- Series Pass Regulator Control: Used to drive the base of a pass transistor in linear regulator circuits, providing precise output voltage control.
- Voltage Monitoring and Over-Voltage Protection (OVP): Configurable as a voltage detector to trigger a crowbar circuit or shutdown mechanism when a threshold is exceeded.
- Constant Current Sources/Sinks: When combined with a sense resistor, it can regulate current by maintaining a fixed voltage drop across the resistor.
- ADC/DAC Reference Voltage Generation: Provides a stable, low-cost reference for precision analog circuits, though temperature stability must be considered for high-accuracy applications.
### 1.2 Industry Applications
- Consumer Electronics: Widely used in AC/DC adapters, LED TV power boards, and charger circuits due to its low cost and reliability.
- Computing: Found in desktop PC ATX power supplies, laptop adapters, and server PSUs for secondary-side regulation.
- Industrial Controls: Employed in 24V industrial power supplies, motor drives, and PLC power modules where robust voltage regulation is required.
- Automotive: Used in non-critical DC-DC converters and interior lighting controls (note: this specific SOT-23 package may not be AEC-Q100 qualified; verify manufacturer documentation for automotive-grade variants).
- Telecommunications: Provides reference and regulation in DC-DC converters for network equipment.
### 1.3 Practical Advantages and Limitations
Advantages:
- High Precision: Typical reference voltage tolerance of ±0.4% (B-grade) ensures accurate regulation.
- Wide Operating Current Range: Can operate from 1 mA to 100 mA cathode current, offering design flexibility.
- Low Dynamic Output Impedance: Typically 0.2Ω, which improves line and load regulation.
- Low Cost and High Availability: Industry-standard 431 footprint makes it a cost-effective choice for volume production.
- Temperature Compensated: Internal bandgap reference provides stable operation over a commercial temperature range (0°C to 70°C).
Limitations:
- Temperature Drift: While compensated, the reference voltage still drifts (typically 50 ppm/°C). Not suitable for ultra-high precision references without additional compensation.
- Noise Performance: The internal reference generates typical broadband noise of ~50 µVrms; for low-noise applications, external filtering is required.
- Stability Concerns: Requires careful compensation of the feedback loop when used in high-gain configurations to avoid oscillation.
- Power Dissipation: In shunt configurations, excess current is dissipated as heat. The SOT-23 package has a limited thermal capability (approx. 350 mW), constraining its use in high-current shunt applications.
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## 2. Design Considerations
### 2.1 Common Design Pitfalls and Solutions
Pitfall 1: Oscillation in Feedback Loops
