• State TI Thinks Resolved
  • Locked Locked
  • Replies 5 replies
  • Answers 1 answer
  • Subscribers 64 subscribers
  • Views 273 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LP2951: The methods of evaluation of Tj

Zirui Su
Expert 5340 points
Part Number: LP2951
Regarding the heat dissipation of LP2951, most of the heat dissipation should be from the bottom copper foil. When we evaluate Tj, there are two methods.
1. Measure the temperature rise of the Top case, assuming that 40% (this is from TI reference data) power consumption is calculated by TOP heat dissipation, so calculate Rthja=Rthjc+Rthca, and then calculate Tj, the result is 173C, fail;
2. Test the Bot case (measure the temperature of the copper foil close to the Bottom case), calculate with the thermal resistance RJC, and have not tested this way yet
image.png
The following is the temperature data of our test Top case:
image.png
 I don't know which is more reasonable, or is there another most accurate measurement of Tj method?
  • 0
    TI__Mastermind 23361 points

    Hi Zirui, 

    We could not see the image you attached. Instead of directly copying the image to the editor, you have to use "insert image" button which can be found at the toolbar. 

    Regards, 
    Jason

  • 0 in reply to Jason Song
    TI__Expert 5340 points

    Sorry and here are the images, can you see it now:

  • 0
    TI__Guru 50025 points

    Hi Zirui,

    The method we use is as follows:

    1. Using an oven, carefully bring the ambient temperature up to the thermal shutdown temperature and record the temperature at which it trips. For this test the LDO should have negligible load so that there is no junction temperature rise due to power dissipation. 
    2. Reduce the ambient temperature to some temperature lower than the thermal shutdown temperature (it is not critical what temperature you choose - e.g. if thermal shutdown is T1 = 150C you may choose T2 = 120C or 130C). Now increase the load until the LDO starts to cycle in and out of thermal shutdown. Record the load current, input voltage, and output voltage at the point just before this starts happening. The power dissipated in the LDO is, to good approximation, PD = I_OUT * (VIN - VOUT).
    3. Theta_JA is then calculated as theta_JA = (T1 - T2)/PD.

    If there is something that is unclear about this, I will be glad to clarify.

    Best regards,

    Nick

  • 0 in reply to Nick Butts
    TI__Expert 5340 points

    I have some points to confirm with you.
    According to your test step, it can calculate the actual thermal resistance junction to ambient temperature. it's right?
    About the shutdown temperature,datasheet just mentioned it，but not definition，so the value is not stable value？
    1. Using an oven, carefully bring the ambient temperature up to the thermal shutdown temperature and record the temperature at which it trips. For this test the LDO should have negligible load so that there is no junction temperature rise due to power dissipation. 
    ---find internal shutdown temperature  T1
    2. Reduce the ambient temperature to some temperature lower than the thermal shutdown temperature (it is not critical what temperature you choose - e.g. if thermal shutdown is T1 = 150C you may choose T2 = 120C or 130C). Now increase the load until the LDO starts to cycle in and out of thermal shutdown. Record the load current, input voltage, and output voltage at the point just before this starts happening. The power dissipated in the LDO is, to good approximation, PD = I_OUT * (VIN - VOUT). 
    ---actual thermal resistance Rthja=（T1-T2）/PD

  • 0 in reply to Zirui Su
    TI__Guru 50025 points

    Hi Zirui, 

    Zirui Su said:
    According to your test step, it can calculate the actual thermal resistance junction to ambient temperature. it's right?

    That is correct. Normally the datasheet values are obtained using computational fluid dynamics (CFD) simulations, but if we need to do it manually this is how we do it.

    Zirui Su said:
    About the shutdown temperature,datasheet just mentioned it，but not definition，so the value is not stable value？

    For a given part the shutdown temperature is stable and will not vary by a significant amount between individual components. Thermal shutdown temperature for LP2951 has been measured in the lab in the past, measured at 184C. You should still measure the shutdown temperature yourself for the purpose of this Rthja measurement. You can read about the question about thermal shutdown temperature here.

    Zirui Su said:
    1. Using an oven, carefully bring the ambient temperature up to the thermal shutdown temperature and record the temperature at which it trips. For this test the LDO should have negligible load so that there is no junction temperature rise due to power dissipation. 
    ---find internal shutdown temperature  T1

    Correct.

    Zirui Su said:
    2. Reduce the ambient temperature to some temperature lower than the thermal shutdown temperature (it is not critical what temperature you choose - e.g. if thermal shutdown is T1 = 150C you may choose T2 = 120C or 130C). Now increase the load until the LDO starts to cycle in and out of thermal shutdown. Record the load current, input voltage, and output voltage at the point just before this starts happening. The power dissipated in the LDO is, to good approximation, PD = I_OUT * (VIN - VOUT). 
    ---actual thermal resistance Rthja=（T1-T2）/PD

    Correct.

    Best regards,

    Nick