In the AD8561 datasheet, page 8, a hysteresis circuit is shown:
The capacitor CF is added to introduce a pole into the feedback network.
The calculation for this pole is:
Is this formula correct? In the formula, I would use R1 instead of R2.
I think the AD8561 is referring to the pole when considering the transfer function from Vref to the output.
The hysteresis width function (R1/ (R1+R2)) has a zero at the fp noted in the AD8561 datasheet beyond which hysteresis will increase.
Here are the hysteresis zero and pole locations (not to be confused with the feedback pole the datasheet refers to):
zero_hysteresis = 1/ (2 * pi() * R2 * CF)
pole_hysteresis = 1/ (2 * pi() * Req * CF)
where:Req = R1 || R2 = R1 * R2 / (R1 + R2)
So, I think you'd agree that fp is the frequency where hysteresis would start to increase.