Datasheet show Theta-Ja small than Theta-Jc question

[Mitchell_shen]

Base on thermal resistance Theta ja= Theta jc+ Theta ca

So basically Theta Ja should be big than Theta Jc.

But we had find some of datasheet show Theta Jc big then Theta Ja.

Could you explain why these Theta Ja is small then Theta Jc?

[MallikaK_22 · Accepted solution]

Hi,

Apologies for the delay in response.

The Jc value mentioned in the datasheet can be the theta Jc top. In some cases on QFN, it can be more than Ja.

For QFN-68 & QFN-88, please refer to the table below:

	FBGA	LT88	LT68
Ja	35	15	16.2
Jc	12	5.92	3.96

Regards,

Mallika 

[Mitchell_shen]

Hi Mallika,

Could you explain more "In some cases" Jc > Ja?

And why your table different with HX3 datasheet?

	FBGA	LT88	LT68
Ja	35	15(15.7)	16.2
Jc	12	5.92(18.9)	3.96(23.8)

Is means datasheet information incorrect?

Thanks,

Mitchell

[MallikaK_22]

Hi Mitchell,

The theta jc value in the datasheet which is greater than the theta ja value is the theta jc (top) which is typical for QFN since the heat dissipation at top case of package is higher than the heat dissipation at the bottom where there is an exposed paddle that helps with the heat flow.

Regards,

Mallika