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