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LMR33620: Strange ICThetaJA Effective

Part Number: LMR33620


I made a Webench-Simulation, the ICThetaJA Effective is given with 6°C/W (Effective IC Junction-to-Ambient Thermal Resistance).

In the datasheet the value RθJA Junction-to-ambient thermal resistance is given with 42.9°C/W. I dont think that this value can be reduced such

like this, also with a perfect layout design (I dont know what kind of layout Webench use for this desing). I hope according JESD 51-7.


  • Hello,

    Can you post a screen shot?

    IT seems the webench is doing some calculation for the given power conditions and anticipated IC rise...this would be dependent on power condition.

    I presume, if you select a higher output power, say, 5V @ 2A, you will approach 30-40degC/W for VQFN package.

  • Hi,

    here you can see the screenshot:

  • If you use for the Simulation following Type (SOIC8) you get the 6°C/W

    br Daniel

  • The theta jA  effective parameter would be done per the power loss of the test.

    A more appreciable curve to use (in my opinion), would be figure 8-7 and figure 8-8 in LMR33620-Q1 D/S. This is based on experimental data. This gives you theta jA per available copper area and max ambient your system can support at full output power.

    It looks like theta jA saturates around 45degC/W, which is a little surprising to me per my earlier assessement....this die may not have been designed as well as other dies I am used to working with that utilized VQFN package technology.

    If your application is stressed for thermals and you can allow for a little bigger solution size with this device I would recommend the leaded orderable in the LMR33620 D/S (no Q1).

    Alternatively, the newest generation of this device has even better thermals and comes in WSON for best of both worlds for small size and thermals...LM63625/LM63635.

    Let me know what you think. Hopefully I did not make this more confusing!!! Slight smile

  • Good Morning,

    nop, its all OK what you say but 6°C/W it still wrong from my side.

    I made the simulation to verify our temperture behavior of a power supply already developed and designed with this chip in package SOIC8.

    To avoid making our Lab. Eng. several temperature tests in the climate chamber again and again, I created a short simulation with Webench.

    Yeah, with the other packages the values are right I have seen, but anyway with the SOIC 8= wrong.

    Anyway I made my own calculations/simulation now with the right values...