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USB hosts hubs transceivers

Mitchell_shen
Level 4
5 solutions authored First solution authored 50 questions asked
Level 4

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

 

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1 Solution
MallikaK_22
Moderator
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Moderator

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 

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MallikaK_22
Moderator
Moderator 500 replies posted 250 replies posted 250 sign-ins
Moderator

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 

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Mitchell_shen
Level 4
5 solutions authored First solution authored 50 questions asked
Level 4

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

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MallikaK_22
Moderator
Moderator 500 replies posted 250 replies posted 250 sign-ins
Moderator

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

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