Adjustable shunt reference — the TL431 workhorse in a through-hole package
The 1% tolerance grade (AC suffix) gives tighter output accuracy than the standard 2% grade, useful when the reference directly sets a regulator output or an ADC threshold.
The 100 mA output current is the maximum cathode current the device can sink while maintaining regulation — enough to drive an optocoupler LED in a feedback loop or bias a small load, but not a power stage directly. The 600 µA minimum cathode current is the keep-alive current needed to stay in regulation; below that the reference may drop out, so the external resistor from cathode to supply must be sized to deliver at least 600 µA at the lowest input voltage.
For dual-sourcing resilience, the TL431 family has broad second-source coverage across multiple manufacturers — any TO-92-3 TL431 with matching tolerance grade is a drop-in substitute, though the AC grade and package variant should be verified against the BOM.