TPS57040-Q1: TPS57040-Q1 Feed forward capacitor

Generally I place the feed forward zero just above the crossover frequency.  That way you get some phase boost without changing the crossover frequency. In that case:

Cff = 1 / ( 2 * pi * Rtop * (Fco + delta F))

Where Rtop is the upper feed back resistor and delta F is the amount above the crossover frequency where the zero is placed.  I usually make that around 5 kHz, depending on the original Fco.

For maximum phase boost you can place the feed forward zero and pole symmetrically about the crossover frequency.  That approach will increase Fco.  Usually you have to calculate the value iteratively, so I prefer the easier approach above.

I don't know if there is a strict limit.  But from a practical standpoint with the zero frequency of 50 kHz or greater and Rtop 10 kohm or higher, then Cff should be around 330 pF or lower.  I have successfully used that value, but in general if you want to use a feed forward capacitor, I would recommend a large value resistor divider and a small value feed forward capacitor.