Siyu,
I think the toggle flip-flop in the block diagram on page one of the spec should have let you know that the max you will get with this device is 50% duty.
Go to page 43 and 44 of the attached and select a more suitable part.
Peter,
i am enclosing the Power Management Guide for 2011. The section you will be interested in starts on page 43 and actual devices listed on page 46, With the minimum information you have given me I would expect that you will want something like…
