## Overview of TL431K-AF5-R The **TL431K-AF5-R** is a precision programmable shunt regulator diode. It acts like a Zener diode but with a significant advantage: its breakdown voltage can be adjusted (programmed) using two external resistors. ### 1. Key Technical Specifications | Parameter | Specification | | :--- | :--- | | **Reference Voltage ($V_{ref}$)** | 2.495V (Typical) | | **Output Voltage Range** | $V_{ref}$ to 36V | | **Output Sink Current** | 1.0mA to 100mA | | **Dynamic Impedance** | 0.22 $\Omega$ (Typical) | | **Package Type** | SOT-25 / SOT-23-5 | | **Operating Temperature** | -40°C to +125°C | --- ### 2. Pin Configuration (SOT-25/AF5) The "AF5" suffix typically denotes a 5-pin SOT-25 package. Unlike the standard 3-pin TO-92 version, this package provides specific pinouts for better thermal management or PCB routing. * **Ref (Reference):** Input pin to set the output voltage. * **Anode:** Common ground pin. * **Cathode:** The "output" pin that sinks current. * **NC:** No connection (often present in 5-pin variants). --- ### 3. Core Functional Features * **Programmable Output:** By using a voltage divider connected to the **Ref** pin, the user can set any voltage between 2.5V and 36V. * **Temperature Stability:** The "K" version often implies a specific grade or improved temperature coefficient, ensuring the voltage remains stable across wide thermal ranges. * **Sharp Turn-on:** It has a very steep turn-on characteristic compared to standard Zener diodes, making it ideal for precision regulation. --- ### 4. Common Applications 1. **Switch Mode Power Supplies (SMPS):** Used in the feedback loop (often paired with an optocoupler) to regulate the output voltage. 2. **Voltage Monitoring:** Acts as a comparator for over-voltage or under-voltage detection. 3. **Constant Current Source:** Can be configured to provide a stable current for LEDs or sensor circuits. 4. **Zener Diode Replacement:** Used where a specific or non-standard Zener voltage is required. --- ### 5. Typical Application Circuit (Formula) To calculate the output voltage ($V_{out}$), use the following formula: ```math V_{out} = V_{ref} \times (1 + \frac{R1}{R2}) ``` *Where $V_{ref}$ is 2.495V.*