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TLV1117: how to understand junction-to-ambient thermal resistance?

Bill Xu
Prodigy 50 points
Part Number: TLV1117

Dear,

     I use TLV1117-33CDCYR to produce a 3.3V power supply from a regulated 12V for cost down and simple circuit. the excepted output current will be less than 200mA.

    The page 4 of datasheet of TLV1117 said, TLV1117-33 inpur voltage in range 4.7V~15V and maximum output is 800mA. for my application, the consumed power by TLV1117 will be (12-3.3)*0.2=1.74W. the page 5 said the junction-to-ambient thermal resistance of TLV1117-33CDCYR is 104.3C/W. then the junction temperature will be rise 1.74*104.3=181.5C. this will causeTLV1117 be damaged.

    I then study the datasheet, the best performance related thermal resistance is 27.5 according datasheet. for a 15V input,5V and 800mA output, the chip will be consume (15-5)*0.8=8W, then the junction will be 27.5*8=220C. the chip will be damaged. 

    How to deal this issue? Do I have misunderstanding the datasheet? Does Ti's datasheet has errors? thanks for your explain and helpful.

Bill

   

  • 0
    Prodigy 50 points
    one concerns again. review thermal performance again. it is strange for DCY package that RJA large than RJC. but for all other packages such as KUV,KCS,KTT that RjA<RJC. it meets engineer common sense. but for DCY, it is no meet engineer common sense. I think TI's datasheet has errors information for RJA of DCY package.
  • 0 in reply to Bill Xu
    TI__Genius 10490 points
    Hi Bill,

    Rja compared to Rjc will vary depending on the package. The DCY (SOT-223) and other similar packages like the DBV (SOT-23) can have very poor thermal characteristics and Rja > Rjc due to their small package size and lack of a thermal pad. This is not a datasheet error as other LDOs in these packages will have similar thermal characteristics.

    Your understanding of the junction temperature in relation to power dissipation and Rja is correct. If the expected power dissipation for your application is large, Rja can be reduced by increasing the amount of copper on the PCB to dissipate the heat generated by the device. Internal ground planes and thermal vias around the device will help further reduce Rja. Incorporating these layout techniques into the PCB can reduce Rja by around 30% or 40%.

    Thanks,
    Gerard