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LM317M: faliure after running for long durations

ravi babu50
Prodigy 70 points
Part Number: LM317M
Other Parts Discussed in Thread: LM317-N,

Hi,

In one of our projects we are using LM317-N for converting 27V to 5V @ 50mA, in field after running for sevaral months the regualtor is faling(getting short).

Input : 27V

Output: 5V

Current: 50mA

power dissipation: 1.1W

Please review the layout attached, is the copper plane given for thermal dissipation(Vout) is enough?

  • plane in orange is the vout plane(copper thinkness:- 0.5 oz.).
  • components in red are the feed back circut.
  • yellow is the input capacitor
  • blue is the output cap.
  • 0
    TI__Mastermind 43630 points

    Hi Ravi,

    Can you measure how hot the LM317M is getting in your application? I think your layout has a thermal relief for solderability which is negatively impacting the thermal performance of the package. 

    Please refer to the following application note on how your layout impacts thermal performance. 

    I suspect the device is running well above the recommended operating TJ of 125C.  Can you confirm?

  • 0 in reply to JCHK
    Prodigy 70 points

    Hi John,

    Thanks for the quick reply,

    form the layout share can you suggest me on what could be the junction to ambient resistance(θJA) value, as it depends on copper area mentioned in Figure 41. RθJA vs 2-oz. Copper Area for TO-252( datasheet)

    layout detilas :-

    copper thinkness:- 0.5 oz

    soldering PAD area:- 6.7x6.7mm

    thermal relief:- 8mils

    Thanks in advance

  • 0 in reply to ravi babu50
    TI__Mastermind 43630 points

    HI Ravi,

    I have been working to bring this thermal widget online for all of our devices here is an example using the TLV1117 to give an idea as to how your copper pour impacts thermal performance (you will have to scroll down to see it):

    If you look at the EVM for the LM317M, you see the Copper pour is significantly larger. While this layout is clearly not practical in an actual system you can see from the above chart how increasing the pour helps a lot. 

    I hope this helps.