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LTC3775 FAIL

Hi,
I'm using the LTC3775 and I'm a little confusing because the exposed pad is connected to PGND. In the past I have used other controllers, like LTC3851 and LTC7149, and I have always seen the exposed pad connected to the signal ground. Why in the LTC3775 there is this solution? I'm asking because I had some failure with damage in the LTC3775 where, when I rise the load, after few seconds, the LTC3775 shut down. When I tried to restart the circuit I found that the SW pin is in short with the SGND or the BG pin is in short with the PGND. Is there someone that can help?