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UA78: About μA78 Operating virtual junction temperature

Tsukui Yusuke
Guru 21930 points
Part Number: UA78

Hi,

Does this LDO support -40℃?
The absolute maximum rating is -65℃.
However, the recommended operating temperature is 0℃.
Could you tell me this reason？





And Does this device get the following data?
Customers want to know the relationship between temperature characteristics and power consumption.



Best Regards,
Yusuke

  • 0
    TI__Genius 13085 points

    Hi Yusuke-san,

    Unfortunately, no the UA78 does not ensure operation below 0C, the -65C listed in the Abs Max table is the lowest temperature unpowered devices should be exposed to. This device was released more than 40 years ago so unfortunately we do not know why this device was only specified down to 0C. It could be because the specifications in the EC table get worse, or it's possible that a small portion of devices may fail to startup at such cold temperatures (this was more common with older ICs). I've communicated with a few companies who have been using this old device in some applications which go below 0C and none of them have reported any failures, however it is possible the device may not work as you require and you'd need to verify that it meets your application's needs.  

    If you need to go to a lower temperature then I'd suggest evaluating some other devices which are rated down to -40C? I'd suggest looking at UA78M, TLV760, TLV767 to see if they meet your needs. 

    Regarding the junction temperature question, the increase in the junction temperature (relative to the ambient temperature) is proportional to the amount of power being dissipated across the LDO. The equation for estimating the junction temperature is Tj=Ta+Rja*[(Vin-Vout)*Iout]

    Where:

    Tj= junction temp

    Ta= ambient temp

    Rja= junction to ambient thermal resistance (this is in the Thermal Information table of the datasheet)

    Vin= input voltage

    Vout= output voltage

    Iout= output current

    For more in-depth information on thermals you can check out some of our app notes, including: 

    An empirical analysis of the impact of board layout on LDO thermal performance

    Semiconductor and IC Package Thermal Metrics

  • 0 in reply to Kyle Van Renterghem
    Guru 21930 points

    Kyle-san,

    Thank you for your response and support.
    Customers need that information by October 14th.
    Please contact me again once you have some updates.
    We are sorry to rush you.

    We look forward to hearing back from you.

    Best regards,
    Yusuke

  • 0 in reply to Tsukui Yusuke
    TI__Genius 13085 points

    Hi Yusuke-san,

    We do not have any documentation on why this old part was only specified down to 0C and due to its age there are no simulations that can be run to find out, so unfortunately we cannot provide any guidance about operating this device outside the datasheet specifications. If the customer requires -40C operation, they will need to either evaluate it in their application or select a different part. I'm sorry we cannot be more helpful on this matter.