I'm reading SBVA042.
In section 2.1 Start-Up Issue. The start-up time is divided into tref and tcff.
At t1 , the PG pin is asserted (pulled up to VOUT) when VFB reaches the reference voltage because the internal power-good comparator monitors the VFB voltage and VFB increases above the PG threshold (0.9Vref). However, it takes tCFF time for VOUT to reach the regulated voltage. tCFF is the feedforward capacitor charging time.
But I think even during tref(before PG pin asserted), the cff is already working and it needs time to be charged.
I don't understand why in figure 9b, Vout is increasing proportional to time before t1, but increased slower after t1.