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UA78L: Thermal performance checking : UA78L05, LM317L and LM337L

user021864
Intellectual 2145 points
Part Number: UA78L
Other Parts Discussed in Thread: LM317L, LM337L,

I want to ask about how to calculate the thermal performance of these three devices.

My operating condition is shown below: 

(1) UA78L05, from 15V to 5V at 0.1A under 105 degree of ambient.

(2) LM317L from 18V to 15V at 0.1A under 105 degree of ambient.

(3) LM337L from -18V to -15V at 0.1A under 105 degree of ambient.

please let me know how to calculate thermal performance and if this operating condition is available or not.

Thank you.

  • 0
    TI__Genius 14245 points

    Hi,

    I'm not sure which packages you are looking at for each of these devices and this is going to depend on thermal resistance which is dependent on the package chosen for each device and the amount of metal on the board connected to the device (for devices with power pads the amount of metal connected to the power pad is the dominant factor).

    To determine the junction temperature of the die you can use the equation Tj=Ta+Power*Rja where:

    • Ta=ambient temp
    • Power=(Vin-Vout)*Iout
    • Rja=junction to ambient thermal resistance from the datasheet. 

    Note that the thermal resistance assumes a high-k, 4 layer board that adheres to the JEDEC standard (this is what most power devices show in their datasheets), if your board has more or less metal connected to the device then the thermal resistance will vary. 

    -Kyle

  • 0
    TI__Intellectual 2145 points
    Kyle,
    Thanks for your reply.
    sorry for missing the package information.

    here is the chosen package:

    UA78L => SOT-89
    LM317L => PK
    LM337L => SOIC

    if I just use two layer board, will the thermal performance become better or worse?
  • 0 in reply to Kyle Van Renterghem
    TI__Intellectual 2145 points
    Kyle,
    Thanks for your reply.
    sorry for missing the package information.

    here is the chosen package:

    UA78L => SOT-89
    LM317L => PK
    LM337L => SOIC

    if I just use two layer board, will the thermal performance become better or worse?
  • 0 in reply to user021864
    TI__Genius 14245 points

    In general a two layer board would perform worse since theoretically you'd have less metal connected to the device through which to dissipate heat. 

    In the scenarios presented here is the rise in junction temperature:

    UA78L: 54.7°C rise, so if Ta=105°C then Tj=159.7°C
    LM317L: 15.45°C rise, so if Ta=105°C then Tj=120.45°C
    LM337L: 33.39°C rise, so if Ta=105°C then Tj=138.39°C

    Note that all of these devices are rated to  operate up to 125°C so care must be taken to either provide additional thermal relief (thermal relief is extremely layout dependent) or to keep the ambient temperature low enough so that the junction temperature does not exceed 125°C. 

  • 0 in reply to Kyle Van Renterghem
    TI__Intellectual 2145 points
    Thanks for the help to find out the thermal performance