Precision shunt reference for the 2.5 V rail
The TL431BCPK is an adjustable shunt voltage reference from the industry-standard TL431 family, offering a ±0.5% initial tolerance — the tightest bin among the standard TL431 grades. It delivers a programmable output from the 2.495 V internal reference up to 36 V, set by two external resistors. With a 100 mA cathode current rating and a 600 µA minimum cathode current for regulation, it suits both low-power bias circuits and higher-current shunt regulators in AC-DC secondary-side feedback loops.
±0.5% tolerance — why the B-grade matters
The BPK suffix denotes the ±0.5% tolerance bin, compared to the TL431ACPK at 1%. For a 5 V output rail, that 0.5% vs 1% difference translates to 25 mV vs 50 mV initial accuracy — enough to avoid a separate trim pot in a precision supply or ADC reference.
SOT-89-3 package and thermal handling
The SOT-89-3 (TO-243AA) package has a large collector tab on the centre pin — the cathode connection — that conducts heat into the PCB copper.