For ATL431LI-Q1 to properly be used as a comparator, the logic output must be readable by the receiving logic device. This is accomplished by knowing the input high and low level threshold voltage levels, typically denoted by VIH and VIL.
As seen in Figure 24, the output low level voltage of the ATL431LI-Q1 in open-loop/comparator mode is approximately 2 V, which is typically sufficient for 5 V supplied logic. However, this does not work for 3.3 V and 1.8 V supplied logic. To accommodate this, a resistive divider can be tied to the output to attenuate the output voltage to a voltage legible to the receiving low voltage logic device.
The output high voltage of the ATL431 is equal to VSUP due to ATL431LI-Q1 being open-collector. If VSUP is much higher than the maximum input voltage tolerance of the receiving logic, the output must be attenuated to accommodate the reliability of the outgoing logic.
When using a resistive divider on the output, make sure the sum of the resistive divider (R1 and R2 in Figure 23) is much greater than RSUP to not interfere with the ability of the ATL431LI-Q1 to pull close to VSUP when turning off.