SN75ALS181: Thermal resistance (Theta-jc top)

Hi Ryusuke-san,

We do not have this value - however we can estimate it from the thermal impedance + system parameters. Information on where these equations come from can be found here: https://www.ti.com/lit/an/spra953c/spra953c.pdf?ts=1657038151809&ref_url=https%253A%252F%252Fwww.google.com%252F

I will note a couple things:

1. This calculation does not take into account effective theta-J-A as this value varies with system - we do give a base theta-J-A with this device - but the system could effect this and for the most accurate prediction this must be taken into account (more info in above link)

2. This approximation is pretty accurate for applications without heat-sinks - as those will change some information

3. psiJT varies with system parameters so this will get you an approximation but the actual value for your system may be different. 

So take the equations:

Tj = TA + (thetaJA * Power)

 Tj = TC + (psiJT * Power)

This leads to:

psiJT = (TA - TC + thetaJA*Power) / Power  - where TA is ambient temperature, TC is case temperature, thetaJA is thermal resistance (for this device its 80C/W in the N package and 76C/W for the NS package), and power is the power dissipated by the device during operation. 

I still strongly recommend reading the linked document as it goes much deeper in how the system will affect these values as that will have a large impact on these parameters.

Best,

Parker Dodson

Hi Ryusuke,

The absolute max rating of the junction temperature TJ is 150C - but as noted in the datasheet (note 4 on page 4 under abs max table) running  the device at this high of a junction temp could affect reliability. The best TJ to use is an estimated TJ based on the applications needs - so if you know the ambient temperature with the help of the datasheet you can estimate power consumption in that scenario and use the TJ equation from the theta-JA value to estimate this.  The reason you should look at a more accurate version of TJ for your system is due to the possibility of the variance in the psi-jt value (as this value is not constant).

Please let me know if you have any other questions!

Best,

Parker Dodson

Hi Ryusuke,

I don't see much issue with a TJ of 90C - especially with an ambient temperature of <= 70C. I don't think you need to worry about reliability at that point as you are 60C under the max junction temperature. 

Please let me know if you have any other questions!

Best,

Parker Dodson