## Technical Overview: TL431G-AF5-R The **TL431G-AF5-R** is a precision programmable shunt voltage reference. It is widely regarded as an "adjustable Zener diode" because it maintains a stable reference voltage that can be programmed using two external resistors. ### 1. Key Technical Specifications | Parameter | Specification | | :--- | :--- | | **Reference Voltage ($V_{ref}$)** | 2.495 V (Typical) | | **Output Voltage Range** | $V_{ref}$ to 36 V | | **Output Sink Current** | 1.0 mA to 100 mA | | **Dynamic Impedance** | 0.22 $\Omega$ (Typical) | | **Package Type** | SOT-25 / SOT-23-5 | | **Operating Temperature** | -40°C to +125°C | | **Tolerance** | Usually 0.5% or 1% (Suffix dependent) | --- ### 2. Pin Configuration (SOT-25/AF5) The "AF5" designation typically refers to the 5-pin SOT-25 package. Unlike the standard 3-pin TO-92 version, this package provides specific pinouts for surface mount applications: 1. **NC**: No Connection. 2. **Anode**: Common ground pin. 3. **NC**: No Connection. 4. **Reference**: Input pin to set the output voltage. 5. **Cathode**: The output pin (connected to the load/supply via a resistor). --- ### 3. Functional Description The device behaves like a high-gain operational amplifier driving an NPN transistor. * **Voltage Regulation**: When the voltage at the **Reference** pin exceeds 2.495V, the device increases current flow through the **Cathode** to ground (**Anode**). * **Programming**: By using a voltage divider from the Cathode to the Reference pin, the output voltage ($V_{out}$) is calculated as: $$V_{out} = V_{ref} \times (1 + \frac{R1}{R2})$$ --- ### 4. Common Applications * **Linear Regulators**: Used as the reference element for adjustable power supplies. * **Isolated Feedback**: Often paired with an **Optocoupler** (like the PC817) in Switch Mode Power Supplies (SMPS) to provide feedback from the secondary side to the primary side. * **Voltage Monitoring**: Acts as a comparator for over-voltage or under-voltage detection. * **Current Limiting**: Can be configured to sense current across a shunt resistor. --- ### 5. Part Number Breakdown (UTC - Unisonic Technologies) * **TL431**: Base part number (Programmable Precision Reference). * **G**: Halogen-free and Lead-free (Green). * **AF5**: SOT-25 package type. * **R**: Tape and Reel packaging.