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LM5116: 求助,同步整流降压电路无输出,疑似上电就热关断

Part Number: LM5116

求助,用LM5116参考文档设计了一片同步整流降压电路,Vin12V,Vout5V,IoutMX6A
目前问题是没有输出,然后根据手册自查了EN,VIN,VCC,UVLO的电压,均是手册里的正常电压
但是SS电压却为低

手册里提到ss为低还有可能触发了热关断
但是实际情况是,上电一瞬间HO有短暂的方波,然后消失,但芯片温度肯定没有170°

现在上电后,上管断开,下管一直导通

附上原理图和pcb


求助大家,请指教一二,感激不尽