LM73 self-heating

Hi Leonardo,

There are several parameters to take into account in order to find the LM73 self-heating.
To determine the T_J of the LM73, you need to know the thermal resistance of the package (R_qJA), the device power dissipation (Q) and the ambient temperature (TA). In the picture example attached, the heat flows from the junction (T_J) to outside ambient (TA), it shows the independent thermal resistances that make up R_qJA. The thermal resistance of the PCB depends on how many layers that you are going to use.

1. Find Q, the maximum power dissipated by the device:

             a. Let assume that you are using Vs(max) = 3.0V.

             b. The maximum current in the datasheet is 320 uA.

             c. To calculate the maximum power that the die is allowed to dissipate is Q = Vs(max) * I_ps = 3.0*320 uA = 0.96 mW

2. Then Delta T, the temperature difference from the junction to the ambient

             a. The LM73 thermal resistance assuming you have a 2-layer board and no airflow is R_qJA   = 186°C/W

             b. Delta T = (Q)( R_qJA) = (0.96mW)(186°C/W) = 0.179°C

             c. The junction temperature can be calculated:  T_J  = delta T + TA = 0.179°C + 25°C = 25.179°C

Q is the power dissipated by the device.

T_J is the junction temperature in the device.

TC is the temperature at the case.

TA is the temperature at ambient.

TPCBis the temperature at PCB.

R_qJA is the device’s absolute thermal resistance from junction to case.

R_qJC is the device's absolute thermal resistance from junction to case

R_qCPCB is the case absolute thermal resistance from case to PCB.

R_qPCBA is the thermal resistance from PCB to ambient.

I hope this will answer your question. Please let me know if you have more question.

Aaron Heng