I'd like to measure the temperature of PIN29 or the other terminals and make an estimate of TJ.
Could you tell me θJC?
To find the junction temperature, find the hot spot on the top surface. This will be the case temperature. Either a thermocouple or thermal imaging camera can be used. For most applications, the junction temperature is only 1 or 2 degrees hotter than the case temperature.
For a bit more margin, the temperature rise going from case to junction can be calculated by multiplying the power loss of the chip by the ThetaJC and adding this result to the measured case temperature. See below.
TJunction = Pchip * ThetaJC + TCase (for the sake of margin, assume all of the heat leaves through the case)
Pchip = Vin * Iintvcc, if 5V is not tied to EXTVCC
Pchip = 5V * Iintvcc, if 5V is tied to EXTVCC (power losses are much lower with 5V on EXTVCC)
Iintvcc ~= Fsw * (QT + QB) where:
Fsw = switching frequency
QT, QB = gate charge of the top and bottom MOSFETs
The equations for the above and further explanation are on pages 20 and 24 of the data sheet.
The value for ThetaJC for the LTC3850EUFD is not specified in the data sheet unfortunately. But per the thermal resistance table, the ThetaJC is 3.4 degrees C per Watt for the 28 lead 4mm X 5mm QFN package (UFD28) used by the part. The thermal resistance table can be found at this location:
Design Center -> Packaging, Quality, Symbols & Footprints -> Package Resources
After doing the math, the temperature rise from case to junction is only 1 or 2 degrees for most applications as mentioned above. Also, much of the heat will exit the part through the bottom GND pad and the PCB instead of through the case. Therefore, 1 or 2 degrees is already on the high end. The case temperature will be a good indicator of the junction temperature.
Thank you for your answer
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