[LTC1760] Can't start charging battery

I am currently using a board with an LTC1760 to charge smart batteries. I'm having a problem with the buck fonctionnality. The schematic of my board is exactly the same as the schematic given on the  datasheet (typycal application) :

Communication with smart batteries works. By visualizing the registers I see that by inserting a battery the system is ready to charge. The LTC1760 starts Buck operation but something goes wrong.  If I visualize the voltage at the output of Rsens that's what I have :

Vin = 19V


It looks like the LTC1760 try every 250ms to restart buck. It force the QTG to stay open.....

If I look on the Vset pin the voltage is on the limit of the 0,8V reference voltage:


Any Idea ?

thank you in advance

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