Question for LTC3309A application

I have used LTC3309A from 5Vin to 3.3Vout, Frequency:2MHz(With RT connecting to Vin for Default value), and Vout  setting for 3.3V; Sync/Mode is tied to Vin for Burst mode.

Question/Problem: I find the output voltage is dropped with the load is rised, ez: output current 0A, vout:3.3V; ouput current 1A, vout:3.25V; output current 3A, vout: 3.18V; output current 6A,

vout:3.1V.  This problem will cause my FPGA core voltage not steady and may make my FPGA broken down. Would someone help me to handle this case. Thanks.