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TPS7A4501-SP: TPS7A4501-SP

Part Number: TPS7A4501-SP

I have a hi-rel design using the TPS7A4501-SP and have more questions on the current limit, specifically figure 15.  

 

1. How was this graph created?  Was this pulsed or CW tested?  2A x 7V = 14W (corresponds to ~70C free air temp).  It seems like the junction temperature would be well above the thermal shutdown of 175C, if i assumed the TJA of 51 degC/W was used (HKU package)?  Can you please elaborate?

2. My application is for space, so "free-air" temperature isn't useful for me.  Do you have a graph vs junction (or case) temperature instead?

 

3. Do you have a suggested way to correlate Figure 15 "free-air" temperature to junction (or case) temperature?  I tried using the TJA and it shows junction temperatures above the thermal shutdown.

 

4. Since both packages have different TJA's, what package was used for Figure 15.

 

5. What temperature is used for the thermal shutdown of 175C listed in the datasheet?  "Free-air", junction, or case?

 

Thanks.  I'm sure I'm just mis-interpreting the graph.
Mike

  • Hi Mike,

    This is one of our older space-rated devices, so I will need a little time to look into what information is available. There are a few high-level things I can address now. 

    Generally, the typical graphs shown over temperature in our datasheets are collected using a temperature forcing system to control the ambient temperature while a thermocouple is connected to the heat sink of the device package or on the package as close to the DUT as possible if there is no heat sink.

    I am checking what information may be available about the test procedure. The graphs in the datasheet are the only ones we have on hand showing behavior across temp. It appears they were likely taken using a device with the U package type since that is the only option shown as available in an earlier version of the datasheet.

    TJA is useful as a comparison of different packages overall, but it is not very useful in terms of calculating junction temp. This is because most of a device's dissipated power is through the board. This Application Report contains a lot of useful information about how package thermal metrics are collected and should be used.

    The thermal shutdown is defined as the junction temp.

    Thanks,

    Sarah

  • Thanks this is helping.  It's interesting that you said "a thermocouple is connected to the heat sink of the device package".  This makes sense to me, but I think of this as case temperature, not "free-air".  The label of "free-air" on Figure 15 really confuses things for me.  If Figure 15 was meant to say "case temp" then I understand completely.  It still seems like the thermal shutdown would have kicked in, even with Figure 15 being case temp.   The graph stops at 125C, and if you assume it is case temp, then the junction was 1.7A x 7V * 10.3 degC/W (ThetaJC - U package) + 125C = 247C.  Maybe figure 15 was done with pulsed loads to avoid thermal shutdown?  

    I look forward to your additional inputs.

    Thanks,
    Mike

  • Hi Mike,

    I'm still looking into this and should hopefully have more feedback in 1-2 days. I'll update you here once I have more information.

    Thanks,

    Sarah

  • Thanks for the update.  

  • Hi Mike,

    Thanks for your patience with this. I believe your assessment is largely correct. The device would likely encounter thermal shutdown if operating at full load with a high voltage differential under ambient temperatures that are that high.

    We don't have documentation showing setup of the temperature measurements, so I can't say exactly where the measurement is placed, but it is not ambient temp. This makes sense intuitively as the temp of the DUT is ultimately what causes shifts in the electrical characteristics of the device and therefore what we are interested in showing.

    With that said, the device is not hitting thermal shutdown during the current limit test as TSD disables current flow entirely. As you suggested earlier, these tests are run for short times in order to avoid heating the part so that TSD does not impact the measurement. Even if the documentation does not exist for an exact answer, hopefully this at least clears up the conceptual confusion.

    Thanks,

    Sarah

  • Thanks for the help.  I guess I'll use Figure 15 as "Case Temp (U package)" and not "free-air".  This doesn't solve my issue, but at least now I understand everything.  Is it fair to say that I could expect an updated datasheet in the near future?  This was particularly confusing.  Thanks again.

  • Yes, I've noted this source of confusion in our tracking system for datasheet updates. 

    Thanks,

    Sarah