Slow or inaccurate responses can also occur when the reference pin is not provided enough overdrive voltage. This is the amount of voltage that is higher than the internal virtual reference. The internal virtual reference voltage will be within the range of 2.5 V ±(0.5%, 1.0% or 1.5%) depending on which version is being used. The more overdrive voltage provided, the faster the TL431 will respond.
For applications where TL431 is being used as a comparator, it is best to set the trip point to greater than the positive expected error (i.e. +1.0% for the A version). For fast response, setting the trip point to >10% of the internal VREF should suffice.
For minimal voltage drop or difference from Vin to the ref pin, it is recommended to use an input resistor <10kΩ to provide Iref.