Why the ±0.5% grade matters for your reference rail
The TL431BQPK is a B-grade adjustable shunt voltage reference from TI's TL431 family, specified with a ±0.5% initial tolerance on the 2.495 V internal reference. That half-percent band is the difference between a supply rail that holds a 12-bit ADC's INL budget and one that drifts the output by several LSBs. For a 5 V output divider, ±0.5% translates to ±25 mV worst-case — tight enough for most precision analog and mixed-signal rails without needing a trim pot.
100 mA cathode current — what it buys you
The 100 mA continuous cathode current rating determines how much load the shunt can drive while maintaining regulation. In a typical optocoupler feedback loop for an isolated power supply, the TL431BQPK biases the LED with a few milliamps; the 100 mA headroom means it can also sink the bias current of a secondary-side error amplifier or a small auxiliary load without dropping out of regulation.
The base product number TL431 spans multiple tolerance grades and packages, so the BQPK suffix locks in the ±0.5% SOT-89-3 variant.