TCA9554: TCA9554 junction temp

Hello Nanfang,

The "Maximum junction temperature" is 100C, which is stated in the "Absolute Maximum Ratings" section of the datasheet.  Unfortunately, we specify the max "Operating free-air temperature" in the "Recommended Operating Conditions" section of the datasheet.  This is based on a JEDEC standard which specifies how the device is tested, the PCB size, layer count, and form factor is also defined and it also specifies the conditions it is tested under.  This causes confusion from the customer because they generally have a totally different environment, PCB, and test condition and they generally measure the temperature of the device at the case to extrapolate what the junction temperature is because that thermal resistance is well defined.  Assuming they aren't dissipating a lot of power then the thermal resistance between the case to junction is small which means the temperature delta will be small.   In the JEDEC testing, which defines our theta_JA, if the power dissipation is very small (which is should be unless the designers drive a lot of heavy loads) then the temp delta from Ta to Tj is small.  So, that means you can approximate the max operating junction temperature as the max "Operating free-air temperature".   

Does this make sense? 

-Francis Houde