The LMV431AIMFX/NOPB is an adjustable precision shunt regulator from Texas Instruments in SOT-23-3, with 1.24V reference voltage and 0.5% initial accuracy (A-grade). It is the low-voltage version of the classic TL431 (2.495V reference), optimized for 1.24V feedback in modern low-voltage DC-DC converters. The device operates as a programmable Zener diode: when the REF pin exceeds the 1.24V internal reference, the cathode conducts current, pulling the cathode voltage down. In a typical application: a resistor divider from the output voltage connects to REF; the cathode drives an optocoupler LED for isolated feedback; the anode connects to the primary-side ground. When VOUT rises above the setpoint, REF exceeds 1.24V, the cathode sinks more current through the optocoupler LED, the PWM controller reduces duty cycle, and VOUT decreases. The operating cathode current range is 60uA to 15mA. The dynamic impedance is 0.3 ohm (the cathode voltage changes only 0.3V per 1A change in cathode current). The output voltage is set by: VOUT = 1.24V x (1 + R1/R2) + IREF x R1, where IREF is typically 7nA (negligible). The -AIMFX suffix specifies SOT-23-3, A-grade (0.5%), industrial temperature. The /NOPB suffix specifies lead-free. The LMV431 is the de facto standard for isolated flyback feedback loops in AC-DC adapters and chargers.