The **TL431-AF5-R** is a precision programmable shunt voltage regulator. It is often described as an "adjustable Zener diode" because it maintains a stable reference voltage based on an external resistor divider. --- ### 1. Key Technical Specifications | Parameter | Description / Value | | :--- | :--- | | **Reference Voltage ($V_{ref}$)** | 2.495 V (Typical) | | **Output Voltage Range** | $V_{ref}$ to 36 V | | **Output Current Range** | 1.0 mA to 100 mA | | **Package Type** | SOT-23-5 (indicated by "AF5") | | **Tolerance** | Usually ±0.5% or ±1% (Grade dependent) | | **Operating Temp Range** | -40°C to +125°C | --- ### 2. Pin Configuration (SOT-23-5) The **AF5** suffix specifies the 5-pin SOT-23 package. While it has 5 pins, only 3 are functional for the circuit operation: 1. **REF**: Reference input. Connects to the center of a voltage divider to set the output voltage. 2. **Anode**: Common ground pin. 3. **Cathode**: The "output" pin connected to the positive supply via a pull-up resistor. 4. **NC**: No connection (usually). 5. **NC**: No connection (usually). --- ### 3. Functional Block Diagram Logic The internal structure consists of an operational amplifier, a reference voltage source, and a transistor. * **Error Amplifier:** Compares the voltage at the **REF** pin with the internal 2.495V bandgap reference. * **Output Transistor:** If the voltage at the REF pin exceeds 2.495V, the transistor turns on more, sinking more current through the **Cathode**, which lowers the voltage. --- ### 4. Application: Voltage Selection Formula To program the output voltage ($V_{out}$), two resistors ($R_1$ and $R_2$) are used. $R_1$ is connected between the Cathode and REF, while $R_2$ is connected between REF and Anode. ```text Vout = Vref * (1 + R1/R2) ``` *(Where Vref is approximately 2.495V)* --- ### 5. Common Uses * **Switch Mode Power Supplies (SMPS):** Used in the feedback loop (often paired with an Optocoupler) to provide isolated voltage regulation. * **Voltage Monitoring:** Acting as a comparator to trigger an alarm if a battery voltage exceeds a threshold. * **Precision Constant Current Source:** By maintaining a constant voltage across a fixed resistor.