Datasheet show Theta-Ja small than Theta-Jc question

Tip / Sign in to post questions, reply, level up, and achieve exciting badges. Know more

cross mob
Mitchell_shen
Level 4
Level 4
Distributor - Zenitron(GC)
100 sign-ins 5 solutions authored First solution authored

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?

Mitchell_shen_0-1626422172312.jpegMitchell_shen_1-1626422200190.jpeg

 

0 Likes
1 Solution
MallikaK_22
Moderator
Moderator
Moderator
50 likes received 750 replies posted 250 solutions authored

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 

View solution in original post

0 Likes
3 Replies
MallikaK_22
Moderator
Moderator
Moderator
50 likes received 750 replies posted 250 solutions authored

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 

0 Likes
Mitchell_shen
Level 4
Level 4
Distributor - Zenitron(GC)
100 sign-ins 5 solutions authored First solution authored

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

0 Likes
MallikaK_22
Moderator
Moderator
Moderator
50 likes received 750 replies posted 250 solutions authored

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

0 Likes