LM337-N: θjc of LM337KCS and LM337T/NOPB

Hi Ryusuke, 

Great question, yes, the difference in thermals can be a collection of things. Die size is definitely a factor that plays a role in thermals, as well as differences in packaging. Package size and layout also affect thermals greatly. Two devices can have the same die but if they are packaged differently, they will have different thermals.

Here you are comparing two different packages so the thermals will not be equivalent. 

Thank you,
Josh 

Hi Ryusuke, 

Unfortunately LM337KCS is an old legacy part and we do not have the Ψjt or Ψjb information for this device. However, based on the two being the two devices being in the same package type and after reviewing the thermal metrics we do have listed in each datasheet I wanted to provide some comparison. 

for Ψjb I would expect LM337KCS to have a VERY similar Ψjb as LM337T/NOPB. This is because the primary method to offload heat is through the board and they both have very similar RθJA values (24.8C/W vs 22.9C/W respectively), so knowing that LM337T/NOPB has Ψjb=4.1C/W I'd estimate that LM337KCS  would range 4.1C/W≤Ψjb≤4.6C/W. 

Ψjt is harder to provide a tight estimate because there is a larger difference between the two devices' RθJC(top), but Ψjt should be lower for LM337KCS because its RθJC(top) is significantly lower than LM337T/NOPB while the two RθJA are very similar. I would estimate that LM337KCS would range 1C/W≤Ψjt≤2.1C/W