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Original question:

LMX2820: LMX2820: Internal Temp sensor ReadBack

LMX2820: Die temperature using TEMPSENSE readback

Ben Ooi
Prodigy 10 points
Part Number: LMX2820

Hi,

Does the die temperature readback correspond to the junction temperature? I am reading back 100degC using the formula provided in datasheet: 0.85*rb_TEMP_SENSE-415 at 25degC ambient. This means at ambient temperature of 55degC, the readback would be 130degC (did not try as i don't want to damage the chip). As max junction temperature in the datasheet is specified at 150degC, thus giving me a 20degC margin. Is my understanding correct? 

Thank you 

  • 0
    TI__Guru** 107030 points

    Hi Ben,

    Please use below formula, datasheet formula is not accurate, we will update the datasheet later.

    Die temperature ≈ (A × rb_TEMP_SENS) – B + Err

    A = 0.572

    B = 273

    rb_TEMP_SENS = R76[10:0] in decimal

    Err = temperature senor measurement accuracy = ± 5 degC

    Right, you may assume die temperature is close to junction temperature.

  • 0 in reply to Noel Fung
    Prodigy 10 points

    Hi Noel,

    This is a surprise. We have done the measurements and the die temperature using 0.85*rb_TEMP_SENSE-415 gave 120.5degC at 55degC ambient on a systems board that had the FPGA temperature close to 100degC. Now with this new formula, 0.572*rb_TEMP_SENSE-273, the temperature is actually 87.36degC, which is really too low to be true at a 55degC ambient. My apologies for repeating this question but can you help confirm the accuracy of the formula? Thank you.

  • 0 in reply to Ben Ooi
    TI__Guru** 107030 points

    Hi Ben,

    I believe the power dissipation in the FPGA is different from LMX2820. Furthermore, thermal resistant of these devices are also different. I am not convinced that we can benchmark the die temperature in this way. 

  • 0 in reply to Noel Fung
    Prodigy 10 points

    Hi Noel,

    Never did I ask about benchmarking the FPGA with the LMX2820 in my previous post. You have selectively read the post. What I have pointed out is that there is a 33.14degC delta between using the equation specified in datasheet and your equation! This is huge and was merely seeking confirmation on this. This is why I wrote "Can you help confirm the accuracy of the formula?". Perhaps I did a wrong calculation. Perhaps there is something wrong with my readback that you may have caught on. This is what I would have expected in the reply and not crudely stating the obvious how an FPGA is dissimilar to the LMX2820. Anyhow, I will assume your equation is correct. Thank you for the reply.