SNVS485I June 2007 – September 2018 LM2735
PRODUCTION DATA.
Now the internal power dissipation is known, and the junction temperature is attempted to be kept at or below 125°C. The next step is to calculate the value for RθJA and/or RψJC. This is actually very simple to accomplish, and necessary if marginality is a possibility in regards to thermals or determining what package option is correct.
The LM2735 has a thermal shutdown comparator. When the silicon reaches a temperature of 160°C, the device shuts down until the temperature reduces to 150°C. Knowing this, the RθJA or the RψJC of a specific application can be calculated. Because the junction-to-top case thermal impedance is much lower than the thermal impedance of junction to ambient air, the error in calculating RψJC is lower than for RθJA . However, a small thermocouple will need to be attached onto the top case of the LM2735 to obtain the RψJC value.
Knowing the temperature of the silicon when the device shuts down allows three of the four variables to be known. Once the thermal impedance is calculated, work backwards with the junction temperature set to 125°C to determine what maximum ambient air temperature keeps the silicon below the 125°C temperature.