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LM1085IT-5.0(TO-220) package junction-to-ambient thermal resistance from graphs

Anoop Joshi
Expert 1125 points

Hi,

I am planning to use the part LM1085IT-5.0 for generating 5V from a 14V supply, and the max load current is expected to be ~0.1A. I was specifically interested in the junction-to-ambient thermal resistance spec, and I'm planning to solder the tab of the TO-220 package to a metal area on the PCB.

In the specsheet, figure 21 is a graph named heat sink thermal resistance vs area. My doubt is, can I consider the quantity given on the Y axis of this graph, ie, "PCB heat sink thermal resistance(degC/W)" as the final junction-to-ambient thermal resistance or theta j-A? For instance, going by the graph, if I solder the tab of the T-220 package to metal filled area of area 5 sq cm, can you please tell if I can expect a theta j-A of ~25degC/W?

This aspect is important for my application as I will be doing some temperature testing on the board, and there are chances of the ambient temperature reaching ~80C. I'm more interested in using a linear regulator rather than a switching regulator for my application, so I wanted to confirm if this IC would be a good choice.

Thanks,

Anoop

  • 0
    Expert 1125 points

    Hi,


    I'm sorry, I misread the x -axis units. For ~25 degC/W, it is around 5000 sq mm, which is in fact quite a lot of area, and it doesn't appear practical to allocate such a big metal area in my application

    Thanks,

    Anoop

  • 0 in reply to Anoop Joshi
    TI__Mastermind 22610 points

    Getting to 25degC/W will be a challenge, but thermal simulations where a TO-220 is mounted to a PCB in the same fashion as a TO-263 implies that it is possible due to the larger tab area of the TO-220. Typical TO-263 under the same conditions will get from the low 30's to the low 40's degC/W

    The PCB requirements are per the JEDEC JESD51-7 specifications, at a minimum.

    Four layer board (aka '2s2p'): top (component) layer is 2oz copper, 2 inner layers are 1oz copper each, bottom layer is 2oz copper. The two inner planes are at least 75mm x 75 mm with >90% coverage. Thermal vias from the copper area under the tab to the first inner plane.

    All-in-all it might be easier to mount the TO-220 vertically, and use a classic heat-sink approach.

  • 0 in reply to Donald Jones
    Expert 1125 points

    Hi Donald,


    Thanks a lot for the reply, I'll keep these in mind


    Thanks,

    Anoop