My customer is using PSIJT and PSIJB parameters to calculate the junction temp of a TLV73318PDQNR in their design. They are using thermistors epoxied to the IC case and on the ground directly under the IC.
During operation their data showed the ground temp at 73.8C and the top of the die at 88.2C with 210 mA of current. Power dissipation of 0.315W (3.3Vin to 1.8Vout @ 210mA).
Using PSIJT the die temp is calculated as: 88.2C + (5.6C/W x.315W) = 89.964C (ambient of 23.3C).
Using PSIJB the junction temp is calculated as : 73.8C + (163.9C/W x .315W) = 125.4 C.
1.) Why is there such a difference in their calculations?
2.) Are these calculations and/or their setup valid?
3.) The PSIJB parameter for the TLV733P in the DQN package is significantly higher than the PSIJB parameter for the SOT-23 package (163.9C/W vs 54.8C/W). This is counter intuitive as the DQN package has an exposed pad on the bottom which should conduct heat better. Are these values correct?
4.) Does the calculation using PSIJB assume a perfect heat sink with no other heat sources? The customer board has many other heat sources which contribute to the board temp. Does this impact the calculations?
