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LMR36015-Q1: The chip is heat then expected.

Jeff Chen71
Expert 1345 points
Part Number: LMR36015-Q1
Other Parts Discussed in Thread: LMR36015, LMR36006

Hi,

     Customer measurement chip temperature using FLUKE thermal camera, they found LMR36015-Q1 surface is hot then PCB board 20°C  Below is customer provide info.     

  • Ambient Temperature      : 25°C
  • Board Temperature         : 45°C
  • LMR36015 Temperature : 65°C
  • Pin : 24V * 37mA             : 0.89W
  • Pout : 3.3 * 230mA          : 0.759W
  • PLOSS : 0.89 – 0.759      :  0.13W

 

      

Question1: LMR36015-Q1 current rating  is 1.5A but customer board output current just 230mA, why the chip surface temperature higher then board 20°C.

On customer board LMR36015 is the most hot chip, the other chip didn’t have significant temperature increase.Using below formula to roughly calculate heat increase the LMR36015 should not so hot.

  TJ = ΨJB * PLOSS + TB = 23.5 * 0.178 + 45°C = 49 °C

Question2: If above formula is not right ,please help to answer how to roughly calculate the TT and TJ. Should I using ψJB or θJA

Question3:This chip do not have thermal pad, and on the d/s layout guide the PCB heat sink is put in PIN6(AGND), customer think the pad is so small . Is it effect improve the thermal resistor?

Why choosing PIN6 to transfer heat not PGND or other pin? Below picture is customer’s schematic and layout, they do not follow the layout guide pour ground plane on Pin6. Whther or not this cause the heat increase? 

 

 

 

      

    

     

     

 Thanks

     

     

 

     

 

  • 0
    TI__Mastermind 35210 points

    Hi,

    Here is a good resource for determining junction temperature for LMR36006/LMR36015.

    Thanks for sharing the component layer of your design.

    Are you able to share the layers underneath the board?LMR36006-Q1-thermal-case-study.pptx

  • 0 in reply to Marshall_Beck
    Expert 1345 points

    Hi Marshall,

         Below is layout screenshot,please help to check. if you could provide email address I can send allegro .brd file to you.

    I had a question about the LMR36006-Q1-thermal-case-study.pptx. Why  recalculate thermal resistor ,rather than using θJA of d/s.

    Because too many customer ask how to using thermal resistor to decide pcb pour copper size .

    Thanks again

  • 0 in reply to Jeff Chen71
    TI__Mastermind 35210 points

    Do you have the ability share a thermal image of the board and also the power stage?

    We find the specific thermal resistance of the board for the device-under-test.

    The copper area of the board, specifically, the copper area and continuity of ground in the component layer and layers directly underneath will dictate thermal performance. This is why you see a curve similar to this in most datasheets (below).

    Please contact me at m-beck1@ti.com

    If we are going to handle this over email, could you please close this thread by clicking the resolve button so my team knows I am working to resolve this with you directly over email?

    Thank you.