The **TL431K-AB3-R** is a precision programmable shunt voltage reference. It acts like a Zener diode but with a significant advantage: its breakdown voltage can be set to any value between **2.5V and 36V** using two external resistors. --- ### 1. Key Technical Specifications | Parameter | Specification | | :--- | :--- | | **Reference Voltage ($V_{ref}$)** | 2.495V (Typical) | | **Output Voltage Range** | $V_{ref}$ to 36V | | **Sink Current Capability** | 1.0mA to 100mA | | **Tolerance** | Usually 0.5% or 1% (K version often denotes higher precision) | | **Package Type** | SOT-89 (Common for AB3 suffix) | | **Operating Temperature** | -40°C to +125°C | --- ### 2. Pin Configuration The device typically has three pins. In the **SOT-89** package, the layout is usually: 1. **REF (Reference):** The input that monitors the output voltage. 2. **ANODE:** Connected to the common/ground. 3. **CATHODE:** The output terminal that "shunts" current to regulate voltage. --- ### 3. Internal Block Diagram Logic The TL431 functions as an operational amplifier coupled with a high-current transistor: * **Comparator:** It compares the voltage at the **REF** pin to an internal **2.5V bandgap reference**. * **Output Driver:** If the REF pin exceeds 2.5V, the internal transistor turns on more, sinking current from the Cathode to the Anode. * **Regulation:** This behavior allows it to maintain a stable voltage in feedback loops. --- ### 4. Common Applications * **Shunt Regulator:** Creating a stable voltage rail from a higher, unregulated source. * **Switch Mode Power Supplies (SMPS):** Used on the secondary side of isolated power supplies (often paired with an **Optocoupler**) to provide voltage feedback. * **Voltage Monitoring:** Acting as a trigger for over-voltage or under-voltage protection circuits. * **Current Limiting:** Used in constant current source designs. --- ### 5. Calculation Formula To set the output voltage ($V_{out}$), the following formula is used (ignoring the small reference input current): ```math V_{out} = V_{ref} \times (1 + \frac{R1}{R2}) ``` * **R1:** Resistor between Cathode and Ref. * **R2:** Resistor between Ref and Anode. ---