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LM317 Operating Virtual Junction Temperature

John Garrett
Expert 4220 points
Other Parts Discussed in Thread: LM317M

Need a replacement to the On Semi NCV317 part, but in a D-pack part.  TI's LM317MQKTPR, looks like it may work but the data sheet is confusing.  The Absolute maximum ratings table has an entry for the maximum Operating Virtual Junction Temperature.  What is the virtual junction temperature? 

 

Also, the data sheet says it is internally protected from over temperature, but can't find an entry in the tables that indicate whether it limits temperature to the maximum recommended junction temperature of 125C, or the maximum virtual junction temperature of 150C.  Looking to use the part near 150C, so need to know if this part will work there.

 

  • 0
    TI__Guru**** 164976 points

    Hello John,

    Virtual junction temperature is the temperature of the integrated circuit die (silicon).

    Over temperature circuits generally engage at roughly 160-170C, however the actual temperature for most devices is unspecified but is greater than 125C.

    150C operation greatly reduces the expected lifetime of the device.
    The "DPPM / MTBF / FIT Rate" section of the product web page has an  excel spreadsheet calculator that can be used to see the effect of elevated temperature.
    The answer to will it work at 150C is probably.

    I suggest increasing the heat sink capability to lower the device temperature.

    Regards,
    Ron Michallick

     

  • 0 in reply to Ron Michallick
    TI__Guru**** 164976 points

    John,

    The NCV317 is a 1.5A output current device and the LM317M has a 500mA output current.
    The "M" means medium output current.

     

  • 0
    TI__Mastermind 22610 points

    : “… someone technical at TI to explain to me what the virtual junction temperature is? “

     

    “The definition of junction temperature is the temperature of a semiconductor junction. The definition of virtual-junction temperature is the temperature of the virtual junction. The definition of virtual junction is the theoretical point or region in a simplified model of the thermal and electrical behavior of a semiconductor device at or in which all the power dissipation within the device is assumed to occur. In virtually all cases in encapsulated devices, we are talking about the temperature of the virtual junction.”

    http://www-k.ext.ti.com/srvs/cgi-bin/webcgi.exe?Company={5761bcd8-11f5-4e08-84e0-8167176a4ed9},kb=analog,case=802,new

     

    The JEDEC definition is similar to the above, but includes this:

    “The term ’virtual-junction temperature’ is used interchangeably with the term ’junction temperature’. “