The **TL431G-AB3-R** is a precision programmable shunt regulator diode. It is widely used in power management circuits as a voltage reference because of its high thermal stability and adjustable output voltage. --- ## 1. Technical Specifications The table below summarizes the core electrical characteristics of the device: | Parameter | Specification | | :--- | :--- | | **Reference Voltage ($V_{ref}$)** | 2.495 V (Typical) | | **Output Voltage Range** | $V_{ref}$ to 36 V | | **Cathode Current Range** | 1.0 mA to 100 mA | | **Output Impedance** | 0.2 $\Omega$ (Typical) | | **Operating Temperature** | -40°C to +125°C | | **Package Type** | SOT-89 (Standard for "AB3" suffix) | --- ## 2. Key Features and Pin Configuration The "G" in the part number usually indicates a "Green" (Halogen-free) product, and "R" refers to Tape & Reel packaging. ### Pinout (SOT-89) 1. **Reference (REF):** Used to set the output voltage via a resistor divider. 2. **Anode (A):** Usually connected to the common ground. 3. **Cathode (K):** The output terminal; connected to the load and the feedback network. --- ## 3. Operating Principle The TL431 acts like a Zener diode, but with a customizable breakdown voltage. - It compares the voltage at the **REF** pin to its internal 2.495V bandgap reference. - If the REF voltage rises above 2.495V, the device increases its cathode current (shunts more current to ground). - This behavior allows it to be used as an error amplifier in feedback loops. ### Basic Voltage Formula To set a specific output voltage ($V_{out}$), use two resistors ($R_1$ and $R_2$): ```text Vout = Vref * (1 + R1 / R2) ``` *(Where R1 is between Cathode and Ref, and R2 is between Ref and Anode)* --- ## 4. Common Applications * **Switch Mode Power Supplies (SMPS):** Used on the secondary side as a feedback controller (often paired with an optocoupler). * **Voltage Monitoring:** Used in over-voltage or under-voltage detection circuits. * **Constant Current Sources:** Used to maintain a steady current through a load. * **Linear Regulators:** Serving as the reference for high-current pass transistors.