Adjustable shunt reference in TO-92
The Texas Instruments LM431CIZ/NOPB is a three-terminal adjustable shunt voltage reference in a through-hole TO-92-3 package. It is the classic 431 topology — a precision reference that sets its output voltage with two external resistors, covering 2.5 V to 37 V. This makes it suitable for power-supply feedback loops, adjustable regulators, and A/D converter references where the drift budget is tight.
100 mA sink current and 1 mA minimum cathode current
The device sinks up to 100 mA continuous output current — enough to drive the optocoupler LED in a flyback supply or to bias a string of comparators. The minimum cathode current for regulation is 1 mA, which sets the lower bound on the bias resistor from the supply rail. Because it is a shunt reference, the LM431CIZ/NOPB regulates by sinking current from the cathode to the anode. This topology works equally well for positive-rail references and for floating or negative-rail applications where a series reference would not fit.
For a BOM that needs a surface-mount alternative, the TL431IDBVR is a functional peer — same adjustable shunt topology and 100 mA output, but in an SOT-23 package with a wider ±2.2% tolerance band. The LM431CIZ/NOPB holds tighter accuracy and the through-hole footprint that legacy and prototyping boards require.