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LM1117 / Thermal Concerns

Amanda Ross95
Intellectual 2740 points
Other Parts Discussed in Thread: LM1117

Hello,

I have a customer who is using the LM1117 in his design. He has 12Vin and 5V@500mA output. He is using the TO-252 package and would like to make sure that he is not going to have any thermal issues.

What is the smallest he can make his heat sink? Based off the equations in the datasheet, I'm getting a junction to ambient thermal resistance of like 30 C/W assuming his ambient temperature is 23.9 degrees Celsius. This makes for a pretty large heat sink area on the copper. Could you double check me?

Thanks so much for your time,

-Amanda

  • 0
    TI__Genius 12630 points
    Hello Amanda,

    You are correct. At 12Vin and 5Vout @ 500mA, the maximum allowable thermal resistance is around 30 C/W. If they drop the input voltage to say 8V, the maximum allowable thermal resistance would be around 67 C/W which would allow for a heat sink that is 0.3 in^2 top side or 0.6 in^2 bottom side or 0.125 in^2 top and 0.125 in^2 bottom. If they use input voltage of say 9V, they could use a 1in^2 heat sink on top side. It could work with 12Vin but the heat sink would need to be large. I am not exactly sure how large. I can put in a thermal model request to try to see how large the heat sink would need to be at 12Vin if you would like but it could take a week or so. Let me know if this is worth trying.
  • 0 in reply to Michael_DeSando
    TI__Intellectual 2740 points

    Hey Michael,

    Thanks so much for your quick response. So by heat sink you mean just the copper space around the IC right? Making the ground trace, input and outraces aas sizeable as possible? Or is the sink meant to be for a certain leg of the IC like GND?

  • 0 in reply to Michael_DeSando
    TI__Intellectual 2740 points

    If possible he would like to go through with the thermal model request. What information do you need on our end in order to process?