Temperature Sensing FAQ

Part Number: LMT70

Questions

General

Q1: Which devices are covered in this Forum? 

Device configuration

Q2: Which ADC should I use for the LMT70? Can I use the internal ADC of the MSP430? 

Q3: Why are the first temperature readings of the LM95235 after power-up incorrect?

Q4: How do I calculate the junction temperature of the LM73?

Best practices

Q5: What are best practices for measuring skin temperature? 


Answers

Q1: Which devices are covered in this Forum?

Answer: This forum covers TI's Analog, Digital, and IR temperature sensors. This FAQ contains questions on Analog and Digital temperature sensors. IR temperature sensors are covered in the IR temperature sensing FAQ


Q2: Which ADC should I use for the LMT70? Can I use the internal ADC of the MSP430?

Yes, it is psosible to use the internal MSP430 ADC for this purpose. Note that the LMT70 is a high precision temperature sensor, and using the general purpose ADC of the MSP430 will limit the accuracy of the temperature measurement. For optimal measurement accuracy, it is recommended to pair the LMT70 with an external ADC such as the ADS1113.


Q3: Why are the first temperature readings of the LM95235 after power-up incorrect?

The LM95235 starts converting during its power-up process. Depending on the system configuration, it is possible for the LM95235 to start converting before the external diodes are ready. This can lead to corrupt temperature readings during the first conversions. In this scenario, it is recommended to discard the conversion results of at least the first two conversions. Note that the LM95235 contains a digital filter to suppress any erroneous spikes for remote temperature readings only. When the digital filter is enabled, it may take several conversion cycles for the reading to converge with the actual temperature as shown in the diagram below.


Q4: How do I calculate the junction temperature of the LM73?

There are several parameters to take into account in order to find the LM73 self-heating. To determine the TJ of the LM73, you need to know the thermal resistance of the package (RθJA), the device power dissipation (Q) and the ambient temperature (TA). In the picture example below, the heat flows from the junction (TJ) to outside ambient (TA), it shows the independent thermal resistances that make up RθJA. The thermal resistance of the PCB depends on how many layers that you are going to use.

  1. Find Q, the maximum power dissipated by the device:
    1. Let’s assume that Vs (max)= 3.0V.
    2. The maximum current consumption according to the datasheet is 320 uA.
    3. To calculate the maximum power that the die is allowed to dissipate is Q = Vs(max) * Ips = 3.0*320 uA = 0.96 mW
  2. Then find ΔT, the temperature difference from the junction temperature and the ambient temperature
    1. With a 2-layer board and no airflow, the LM73 thermal resistance is RθJA = 186°C/W
    2. ∆T=(Q)(RθJA)=(0.96mW)(186°C/W)=0.179°C

  3. The junction temperature can then be calculated: TJ=∆T+ TA=0.179+25℃ = 25.179℃.

  • Q is the power dissipated by the device.
  • TJ is the junction temperature in the device.
  • TC is the temperature at the case.
  • TA is the temperature at ambient.
  • TPCB is the temperature at PCB.
  • RθJA is the device’s absolute thermal resistance from junction to ambient.
  • RθJC is the device’s absolute thermal resistance from junction to case.
  • RθCPCB is the case absolute thermal resistance from case to PCB.
  • RθPCBA is the thermal resistance from PCB to ambient.

Q5: What are best practices for measuring skin temperature?

Please refer to the following material for information:


Best regards,
Ben Kasemsadeh
Applications Engineer
Silicon Valley Analog Group 
Texas Instruments