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LM53635-Q1: Thermal ANalysis Simulation on LM53635-Q1

SHREYAS S BAGI
Intellectual 400 points
Part Number: LM53635-Q1

Hi,

I using LM5365-Q1 regulator in my design. The design parameters as follows:

1. Iin (Current input) --- 470mA

2. Vin (Voltage input) ---- 24V

3. Iout(Current out) ---- 1.8mA

4. Vout (Voltage out) ---- 5V

5. Efficiency ---- 80%

6. Power Dissipation --- 2.25W.

I am doing thermal simulation for my board below are my Boundary conditions:

Ambient Temperature --> 25 degree C and Air velocity --> 0.1m/sec (19.685 ft/min). The system is In rack placement. My board is placed between two boards. The power dissipation of the adjacent boards are 3W. I am observing  Junction Temp. ---> 127 degreeC.  

1.The Junction Temp. is exceeding more than expected value. Did I wrongly consider any parameters ? 

2. I have considered Theta jc top (junction to case top) ----> 14.2 C/W. Should I consider Theta jc (top) or Theta jc (bottom) for my simulation ? 

  • 0
    TI__Expert 4555 points

    Hello Shreyas,

    You mentioned 'The Junction Temp. is exceeding more than the expected value'. Can you please let me know what junction temp you are seeing.

    Best Regards,

    Ankit Gupta

  • 0 in reply to Ankit Gupta95
    Intellectual 400 points

    Hi Ankit Gupta,

    As mentioned in the early query in bold Junction Temperature value (Theta J) is 127 degree Celsius for Theta jc = 14.2 C/W.  If I change Theta jc = 2.4 C/W then Junction Temperature is reduced. So, I am confused which Theta jc value to be considered ?  

  • 0 in reply to SHREYAS S BAGI
    Intellectual 400 points

    Hi Ankit Gupta,

    Did you get a chance to try out the solution for the above error? . It would be great if you help me with the current status update, since the thermal simulation activity is stopped. 

    Best Regards,

    Shreyas S Bagi

  • 0 in reply to SHREYAS S BAGI
    TI__Expert 4555 points

    Hello Shreyas,

    Sorry for the late reply.

    You need to consider Theta jc (bottom) for your simulation. Hope this helps!

    Please close the thread if you think your question is resolved.

    Best Regards,

    Ankit Gupta

    Application Engineer

  • 0 in reply to Ankit Gupta95
    Intellectual 400 points

    Hi Ankit Gupta,

    I want to know the reason for considering the Theta jc(bottom) instead of Theta jc(top) . 

    According to my understanding for the IC with thermal pad Theta jc(bottom) has to be considered and the IC without thermal pad Theta jc(top) to be considered.

    So we would like to reason behind considering Theta jc(bottom) for LM53635-Q1 which does not thermal pad.

    LM53625/35-Q1

  • 0 in reply to SHREYAS S BAGI
    TI__Expert 4555 points

    Hello Shreyas,

    Sorry my bad, I was under the impression that LM53635 has a thermal pad but its actually VQFN-hotrod package. The estimated junction temperature can be calculated with equation Tj = Ta (Rtheta J X Power Dissipation), where Rtheta-J for VQFN package average around 22 degrees/W. So you should not see this high junction temperature. 

    You said "My board is placed between two boards. The power dissipation of the adjacent boards are 3W" - Do you think your simulations are considering thermal coupling here due to other two boards?

    Best Regards,

    Ankit Gupta

  • 0 in reply to Ankit Gupta95
    TI__Expert 4555 points

    Hello Shreyas,

    It is been a while we have not heard back from you. I am closing the thread for now assuming the issue is resolved, if the issue persists please feel free to re-open the thread. Thanks!

    Best Regards,

    Ankit Gupta

    Application Engineer