AnsweredAssumed Answered

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



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.