In discontinuous conduction mode (DCM) mode of operation, the MAX17557 cannot sink a significant amount of current when VOUT is pulled up by an external source. The controller will see that the voltage at FB is greater than the regulated value (i.e. FB>0.8V) and stop switching. The MAX17557 will wait for FB to fall to a level where switching is needed to support VOUT. PGOOD has an overvoltage threshold and will pull low when FB is greater the 0.88V.