I have a board with a LT3042 set for 2.5V and a LT3081 set for 5V. Both are powered by a 7V supply. And both are not working... The output voltage is something like Vin - Vdropout. Vset is approximately Vout. Vout follows Vin-vdopout if I change Vin.
I have successfully used the LT3042 for some other design.
The board is a 4 layer PCB with good design practices. Checked 2 times pinout, chip orientation, schematic and layout. I have also desoldered the Rset to check values. Thermal imaging don't show any hot spot. For the LT3081, TEMP pin indicates 23°C.
I could have made a mistake for the LT3042, with the PGFB pin tied to ground. According to my comprehension of the datasheet, this is correct ("do not drive PG more than 0.3V below GND"), but they is no example with PGFB connected to GND in the app notes. Is it mandatory to connect the pin to Vin if not used ? Could it be left floating ?
I have no idea for the LT3081. The PCB is quite complex and I have usually no problem with the LDO regulators, they just... works.
Any idea will be well appreciated :)