How to estimate the chip temperature? - Page 1
Forum section: electronics / projects

author: zapta
author_rank: Super Contributor
timestamp: February 16, 2014, 11:34:18 pm
content: I am using this chip which has an internal LDOATA6631-TAQYhttp://www.atmel.com/Images/doc9165.pdfWith an ambient temperature of 19C the external temperature of the package is 34C (measured with an E4).1. Is it sufficient information to estimate the internal silicon temperature?2. Is it sufficient information to predict the silicon temperature with an ambient temperature of let's say 50C?3. Is it sufficient information to estimate the max ambient temperature that will still be within spec?(for 2, 3 assuming no change in power dissipation).

author: dannyf
author_rank: Super Contributor
timestamp: February 17, 2014, 01:26:23 am
content: Yes, yes, and yes, assuming a typical datasheet for the device.

author: zapta
author_rank: Super Contributor
timestamp: February 17, 2014, 01:29:50 am
content: Quote from: dannyf on February 17, 2014, 01:26:23 amGreat!Can you explain to me how? (a datasheet is included in the original post).

author: Andreas
author_rank: Super Contributor
timestamp: February 17, 2014, 06:02:08 am
content: See page 15:RTHJA = 80K/W

author: Dago
author_rank: Frequent Contributor
timestamp: February 17, 2014, 06:08:58 am
content: This should help (it has the equation needed):http://en.wikipedia.org/wiki/Junction_temperature

author: zapta
author_rank: Super Contributor
timestamp: February 17, 2014, 08:55:51 am
content: Quote from: Dago on February 17, 2014, 06:08:58 amBut this requires also the power. Does it means than that the answer for 1, 2, and 3, is no since the power is not given?(I can probably figure out the power by measuring the input and output currents).

author: peter.mitchell
author_rank: Super Contributor
timestamp: February 17, 2014, 10:30:37 am
content: I don't believe they provide enough information; they are missing R?JC(junction-case) and/or R?CA(case-ambient/air).The following only loosely applies in potential method, not in exact values. Values used are fromthisdocument and your post.The document shows a R?JCof 39K/W and a R?JAof 120K/W, that implies R?CAis approximately 81K/W.^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^It isn't recommended you do the above, or use any ?XAvalues, ambient measurement conditions can vary greatly based on cases, closeness of components, how the device is attached to the board, how the measurement is being done ect.Bearing this in mind, we continue;Your post suggests your C-A is 15K [(307.15K-292.15K)34oC - 19oC]; 81/15=5.4, reciprocal of 5.4 = 0.185185..., so you are supposedly dissipating 0.185185WSubstitute 0.185185 in to R?JAof 120K/W and you get a J-A of 22.22K.22.22K+292.15K=314.37K22.22oC+19oC=41.22oC

author: dannyf
author_rank: Super Contributor
timestamp: February 17, 2014, 11:59:07 am
content: You need two things, power dissipation and thermal resistance in general, for the calculation.In your case, you need thermal resistance, junction to air, and thermal resistance, case to air, both typical, for the calculation.For SOIC8, I would say Rth.ja=80c/w (about right), and Rth.jc=40c/w (a little bit too high). So Pd = (34-19)/40=0.4w.Junction temperature then must be Tj=19+0.4*80=49c.2. the junction temperature would then be 50+0.4*80=80c.3. Tj max is typically 125 - 150c. So the max ambient would be 150-0.4*80=120c.Fairly basic.

author: zapta
author_rank: Super Contributor
timestamp: February 18, 2014, 04:05:40 pm
content: Thanks Peter and Danny. I think I understand it now.