LT3501 : 6 VDC to 5 VDC step down Switching  Regulator

Hi 

I have used LT3501  for 6VDC to 5VDC  , 1 A   conversion . I am getting a drop in 1.5V  and  maximum output voltage of 4.5VDC.

Is there any possibility to improve  input/ out put ratio,  to reach  5VDC?

The schematics used is 

Can any body help on this.

Thanks,

TY JOSE