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LTC3779 and DC2456: how to change the feedback to current loop

I want to use the LTC3779 and modifiy its demo board DC2456 to change the feed backloop to current loop. The original voltage feedback resistor is not used. Instead, a low resistor for current sensing is added between the load and the ground. In this way, the voltage drop across the resistor for current sensing feedback to the feedback pin (pin10). But the output curve is triangular wave with very low frequency of 12Hz.

Could you please explain why? When I use voltage feedback, its ok. But when I just change it to current feedback, it's wrong.