SN74LV164A: PD and Tj

Hi Kuan,

There's an FAQ that gives the correct value to use for TJ(max) located here:

I know that some of our competitors list a very basic maximum power dissipation for their devices, however from what I have seen, they don't provide any information for the conditions under which that maximum power is calculated.  How do you know at what temperature they determined maximum power dissipation? If it says 400mW, can you do that at TA = max? or is that only at room temperature? What if you are cooling the device to -40C?

The values given in our datasheets remove ambiguity and provide much more information about the actual operation of the part. They allow for accuracy and flexibility in system design.

Maximum power dissipation is calculated using the ambient operating temperature (determined by the customer), the maximum junction temperature (150C), and the thermal resistance of the device (R_thetaJA).

For an example, if you are operating in an environment with maximum temperature of 50C using the PW package, the maximum power dissipation would be:

PW package R_thetaJA = 120.2 C/W

Pdiss = (TJMAX - TAMAX)/R_thetaJA = 0.8319 W ~= 831mW

This doesn't take into account the other limitations of the device -- obviously you cannot violate any of the other absolute maximum ratings.

This app note has some useful information for power consumption calculation: http://www.ti.com/lit/an/scaa035b/scaa035b.pdf