Hello, I am back with another obscure issue.
What would cause the LT8490 to get hot during float charging only? This was my initial question. But I found out that LT8490 switches back to VIN supply which is high voltage despite EXTVCC being available. Now my question is: why would the LT8490 switch to VIN for its power when EXTVCC is available during float stage?
I did an experiment to prove this was the case: connecting an external power supply to EXTVCC who displays current output. I put it at the most efficient voltage for the LT8490 which is 7V = EXTVCC. On the board normally it gets 24V, but I cut that line and put 7, also tried 12V and 24. (The results are the same; EXTVCC is forfeited during float.)
At 7V EXTVCC, The chip used about 20 to 30mA from EXTVCC until it entered float. In float, amps into EXTVCC became zero but switching is still active and outputting about 2Amps to charge the battery. It is still operating the switches, with zero EXTVCC amp draw, which means it is indeed drawing power from VIN during float stage which is 45 to 50V solar. This makes the chip hot.
Some other details:
50V solar input, 44V absorption out, 41V float, these are rounded numbers.
30A max. Was doing about 15A out during bulk, then absorption down to 2A, then float engaged and the chip stopped using EXTVCC and got hot while still doing 2A float. I promptly shut it off because I am uncomfortable with that heat. It was room temp up to this point during a nicely sustained 15A charging.
I put my thumb on the LT8490 up until it floats and inside about 2 seconds its hot to touch. It is instant when float stage begins. I also see it get hot precisely when I see my EXTVCC amps go from 30mA to zero.
I will be reading the papers again but does anyone have an educated guess as to why it is doing this? Is this normal? It's really bad for me.