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In this datasheet - Link there is no value of maximum power dissipation or junction to ambient thermal resistance.

In this case, how to find the value of it?

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2 Answers 2

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See section 7.

The storage temperature is listed as -65 to 150 C. This might mean that Tjmax is 150 C. But there is no guarantee that it will function correctly at that temp.

See note 7-4.

The maximum ambient temperature is listed as 70 C for commercial and 85 C for industrial. So even though you cannot work out the thermal resistance to ambient, you DO know that the maximum ambient temperature is 70 or 85 C depending on which version you have. It seems like that should be enough information for you. They are guaranteeing that it will work in still air at that temperature. So you just have to keep the ambient temp below that limit.

The largest nominal power dissipation shown in table 7-1 is 380 mW. If we use that number we can possibly calculate the thermal resistance to ambient, assuming that Tjmax is 150 (possibly a bad assumption).

(150 - 85) / (0.38) = 171 K/W

That does not sound very plausible. So my conclusion is that the chip cannot function when it reaches 150 C. You just have to respect the ambient temperature limits. If necessary, you may need to add ventilation to keep the ambient temp within the limit.

It seems there is not enough information to design a heatsink solution.

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  • \$\begingroup\$ Thank you for your answer! So, to arrive at a crude and somewhat decent estimation of the thermal resistance, if I am using a industrial part, shall I assume the Tjmax to be 85 and then calculate? \$\endgroup\$
    – Newbie
    May 17 at 3:44
  • \$\begingroup\$ Tjmax is not 85'C It depends on your Vdd, speed, load , capacitance and timing margin \$\endgroup\$ May 17 at 3:59
  • \$\begingroup\$ No. Unfortunately the only information given in this datasheet is that you must keep the ambient temperature below 70 C (if you purchase commercial parts) or 85 C (if you purchase industrial parts). Ambient temperature is the air temperature in the immediate vicinity of the chip. You don't do any calculations. You just keep the ambient temp below that. If you use a fan, you will have to do more thermal analysis at the system level. How much heat inside the enclosure, temp outside the enclosure, flow rate, etc. \$\endgroup\$
    – mkeith
    May 17 at 4:08
  • \$\begingroup\$ MK 380 mW is not the "worst case". It's just the biggest "nominal" value in that table with no loads and nominal voltage and room temp. \$\endgroup\$ May 17 at 4:39
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    \$\begingroup\$ @TonyStewartEE75 maybe I didn't understand what you mean by "environmental ambient" and "internal ambient." So maybe just a misunderstanding on my part do to a terminology issue. Apologies. \$\endgroup\$
    – mkeith
    May 17 at 16:23
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Web search results may vary. enter image description here

It depends greatly on PCB & heatsink design, convection, airflow or enclosed

https://www.datasheetarchive.com/pdf/download.php?id=1cf469beaf3b10d4ded6287136f7c147cfcab5&type=P&term=QFN-64%2520thermal%2520resistance

enter image description here https://www.ti.com/lit/an/spra953c/spra953c.pdf

Recommended Layout from your link

enter image description here

This ground plane with lots of micro vias improves both ELECTRICAL and THERMAL performance.

Do not overlook the thermal variables.

enter image description here

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