I get this question frequently so I will post the question and then answer myself:
"I made a mistake and did not notice that the LT3080 datasheet says for the DD PACKAGE that "EXPOSED PAD (PIN 9) IS OUT, MUST BE SOLDERED TO PCB". I made my pcbs with the exposed pad soldered to GROUND instead of OUT but the circuit still works - Do I need to fix the pcb?" I show the LT3080 datasheet Pin Configuration for the DD PACKAGE in the image below:
The bottom of the LT3080 die is supposed to be a resistive version of the OUT pin. I say “supposed to be” because in reality there is a thin layer of unavoidable but inadvertent oxide between the die silicon…
The bottom of the LT3080 die is supposed to be a resistive version of the OUT pin. I say “supposed to be” because in reality there is a thin layer of unavoidable but inadvertent oxide between the die silicon and the exposed pad metal. That is why the customer can solder the exposed pad to GND and their circuit will work. Even if the exposed pad ends up electrically connected to the die bottom, there is still tens of ohms of resistance between the die bottom and the die top where the real OUT connection is made. In other words, if your customer gets a 3080 with a solid electrical connection between the exposed pad and the die bottom and solders that ic into the customer’s circuit with the exposed pad = GND mistake, the result will be a roughly 50 Ohm short from GND to OUT in the customer’s system – not a dead short. It is important for the customer to fix their design for a proper exposed pad connection. The OUT exposed pad can float if it is not necessary for thermal performance.
thanks for your detailed explanation!