2.5 V fixed shunt reference — what the ±1.5% tolerance buys the design
The LM385BYMX-2.5/NOPB from Texas Instruments is a 2.5 V fixed-output shunt voltage reference in an 8-SOIC surface-mount package. The ±1.5% initial tolerance means the output sits between 2.4625 V and 2.5375 V at 25 °C — tight enough for a 12-bit ADC reference without external trimming, but not a 16-bit precision rail. The shunt topology lets it operate from a single resistor, which simplifies the BOM but forces the designer to size the bias resistor for the full load current range.
Biasing the shunt — 20 µA cathode current sets the resistor
The minimum cathode current is 20 µA — the bias resistor must deliver at least that plus the load current to keep the reference in regulation. Maximum output current is 20 mA, so the resistor value is chosen to supply the total current at the minimum supply voltage. The 120 µVrms noise (10 Hz to 10 kHz) and 50 ppm/°C temperature coefficient are the specs that matter when this reference feeds a precision ADC or a sensor bridge — the noise floor limits the effective resolution, and the drift shifts the output over temperature.