TPS63710: what limits the min output voltage except duty limitation?

Hi 

from TPS63710 datasheet, it shows the min output voltage is higher than -5.5V.

from |Vout|<0.7*Vin, it means the max duty is 0.7V

if only duty is the limitation factor, if output voltage is -8V, input voltage is 12V

the duty is 8/12=0.67 < 0.7

i think -8V should work, but it is lower than -5.5V

so what's other factor to limit output voltage range?

thanks.