2.5 V shunt reference in a three-pin SOT-23
The Texas Instruments LM4040CEM3-2.5/NOPB is a precision shunt voltage reference that delivers a fixed 2.5 V output with a ±0.5% initial tolerance. It operates as a shunt (two-terminal Zener-like) reference, so it needs only a single series resistor from the supply rail to set the bias current — no external compensation or bypass cap required for stability. The SOT-23-3 package makes it a direct drop-in for tight layouts where board area is the constraint, not the reference accuracy.
The ±0.5% initial accuracy covers the full distribution at 25°C — enough headroom for a 12-bit ADC reference or a comparator threshold without trimming. If your system needs tighter drift, you step up to the LM4040A or LM4050 series with lower TC grades; for most industrial control loops and automotive sensor interfaces, this grade holds the line.
Shunt operation — bias current and output drive
As a shunt reference, the LM4040CEM3-2.5/NOPB sinks current from the load through the cathode pin. The minimum cathode current is 68 µA — below that the reference drops out of regulation. Maximum continuous output current is 15 mA, which sets the upper bound on the load you can drive directly. The series resistor value is chosen so that (Vin - 2.5 V) / R delivers a bias current between 68 µA and 15 mA across all load and temperature conditions. For a typical 5 V rail and a 1 mA load, a 2.2 kΩ resistor lands the bias in the sweet spot.
The base product number LM4040 spans multiple voltage options and tolerance grades, all in the same SOT-23-3 footprint, so if a tighter tolerance or a different output voltage is needed later, the board layout stays the same.
The SOT-23-3 package is standard across the LM4040 family, so if the BOM calls for a different voltage or tolerance, the footprint is already validated.