SN74LVTH574: SN74LVTH574 power dissipation capacitance

Roger Dixon

Part Number: SN74LVTH574

Is there some reason why there's no spec for the power dissipation capacitance for the SN74LVTH574? How is its supply current estimated?

Thanks,

Roger Dixon

over 4 years ago

0 Emrys Maier over 4 years ago

TI__Guru* 96591 points

Hi Roger,

I can't say for sure why this was left out of the datasheet, since the part was released over 20 years ago. It looks like the entire LVTH family of parts doesn't have Cpd values, but other bus-hold parts we have released do include Cpd, so I can't think of a good reason as to why it was left out. My guess would be that it was an oversight in the test plan since I also don't see characterization data for Cpd from any parts in the LVTH family.

I'm not sure if an estimate will help you, but I would expect the typical Cpd to be around 50pF with the outputs active and 2pF with the outputs disabled (based on similar devices / similar technology).

If you would like to perform a test on your end, Cpd is tested by running the device at a specific frequency then measuring the average supply current and back calculating the Cpd value. Some details are available here: http://www.ti.com/lit/an/scaa035b/scaa035b.pdf

0 Roger Dixon over 4 years ago in reply to Emrys Maier

Prodigy 185 points

Thanks so much Emrys for the quick reply. I noticed that the 2nd source, On Semi, also neglected to include the Cpd spec, so TI isn't alone.

Your 50 pF number seems like a good conservative guess using a 3.3V supply, but I'd think something closer to 20 pF max would apply when the outputs are disabled (just comparing to the LVC574 typical value of 11 pF). Those values should suffice for my 60 MHz application, but I see that the 50 pF value would likely restrict its usage to less than the rated 150 MHz clock rate based on power dissipation limits. The LVC574 specs that at 500 mW, but this is another missing item in the LVTH574 data sheet. If the designer of this series is still around please wag your finger at him/her for me. :)

Thanks!

Roger

0 Emrys Maier over 4 years ago in reply to Roger Dixon

TI__Guru* 96591 points

Hey Roger,

Sorry I didn't get back to you sooner - I don't spend as much time on the forums as I used to.

On the max power consumption, we've switched to using R_thetaJA, max operating temp, and max junction temp to calculate it.

For this device, if you are operating over the entire temp range (85°C max) in the PW package (83°C/W), then you can use calculate the max power dissipation as:

P_max = (T_J(max) - T_A(max)) / R_thetaJA = (150 - 85) / 83 = 783 mW

In case you wonder where I pulled 150C from:

0 Roger Dixon over 4 years ago in reply to Emrys Maier

Prodigy 185 points

Oh, no problem Emrys, you'd already effectively resolved the issue. I do hope that TI can eventually add the Cpd value into these old data sheets though since they're still quite useful parts.

As an interesting aside, the older LVC574A had a (very odd) dual slope to its thermal resistance, where for the PW package you mention it was 83 deg C/W below 60 deg C but 181.8 deg C/W above. It was odd in that it was the opposite of what you'd see with a metal heat sink, which improves with increasing temperature. Perhaps plastic conducts heat less well as the temperature increases, but then I'm at a loss as to why this isn't seen in the data sheets of parts in similar packages, e.g. the LVTH574. So your computation is certainly correct for a constant thermal resistance, but I have to wonder if it really is constant.

In any event MTBF issues would drive me to use much lower dissipation levels, which is why the Cpd value was primarily of interest.

Thanks again!

Roger

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SN74LVTH574: SN74LVTH574 power dissipation capacitance