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Do not understand how to use Theta values for the i.mx6 dual lite

Question asked by tom abcd on Aug 3, 2017
Latest reply on Aug 15, 2017 by tom abcd

I am looking at the thermal resistance numbers in the table below I do not understand how to apply them .

I ran a test where I just let the processor run with a cpu usage number of 25% (number from Top app) for a while in my office environment of  22

c. 

 

Looking at Table 7 it list various thermal items.

Junction to Ambient = 23 C/W,         Junction to Case = 6 C/W    Junction to Package top = 2 C/W...

So in my test of the internal temp read out of the CPU = 60 and top of BGA = 44 (using a thermal laser device)

1. Is the top of the bga package consider the Case? 

2. Since I see a delta for about 15 C between internal and top does that means the processor is doing over 2 watts????

this is based on the Junction to Case = 6 C/W so  15 / 6 = 2.5

    This seems unlikely since I am only loading the cpu at 25 % with a simple app that is reading the serial port and scrolling the received HEX data on the screen.  So I am guessing, I am misunderstanding how to use these numbers.

 

What seem more likely is the Junction to Ambient for the 4 layer  of  23 C/W is more likely is what is happening so this would be less than a watt 15 / 23 = 0.65 .    So if that is true then how do I figure out the numbers for my heat sink.  

 

So if the goal is to keep the processor under 85C for say a 2 watt load then what temperature does the top of BGA need to be?  If the 23C/Watt rule is what really applies then I have to keep the top  85 - (23 x2) = 39C which means there is no way to use the processor in a 50C environment.

 

 

FYI - another test I did was to put a heat sink on the BGA and ran it in my office again.  The heat sink did not make a noticeable difference.

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