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TMP117: Circuit

Rajkumar S
Prodigy 60 points
Part Number: TMP117
Other Parts Discussed in Thread: LMT70A, SN74AUP1G04, LMT70

Hi

we are in the process of developing a breakout board for testing our use case probe model, incorporating both LMT70A and TMP117 sensors. Our objective is to achieve an accuracy of 0.1°C, ensuring compatibility with various MCUs such as Arduino, ESP32, and STM32. I have attached the schematic diagrams for both sensors. Your feedback on these designs would be highly appreciated.

**LMT70A:**    https://drive.google.com/file/d/1j2MVZp0KV-FrCBKdv6_HK51-misFT5Ha/view?usp=sharing 

Equipped with two LMT70A analog temperature sensors – one for body temperature and one for ambient temperature. Both sensors are connected to two separate ADC channels, each featuring a 12-bit built-in ADC channel in the MCU. The temperature measurement range spans from 20°C to 42°C. The operating voltage for the system is 3.3V. Could you please review the schematic to confirm its accuracy? Additionally, with this schematic, is an accuracy of 0.1 achievable or not?

**TMP117:**   https://drive.google.com/file/d/1m_Z95TNhb_sLXOfgleLUPCUVH2t3JZa1/view?usp=sharing

Equipped with two TMP117 digital temperature sensors – one for body temperature and one for ambient temperature. Both sensors are connected to two separate I2C channels in one MCU. The temperature measurement range spans from 20°C to 42°C. The operating voltage for the system is 3.3V. Could you please review the schematic to confirm its accuracy? Additionally, with this schematic, is an accuracy of 0.1 achievable or not?

Is the U3 and U4 (SN74AUP1G04) component needed in the alert pin? Also, want to know the purpose of the alert pin?

Your expertise and insights will be invaluable in refining our designs.

Thank you,
Best Regards

  • 0
    TI__Expert 3525 points

    Hello,

    The LMT70 schematic looks good.

    For the TMP117 schematic, are you using U3 and U4 to have LED D2 light up when an ALERT is triggered? That is OK. They are not needed though. The simpler way to do it would be to take the ALERT outputs and put them into 2 separate input pins to the MCU, so in the case of an ALERT the MCU can make a quick decision instead of the relying on a user seeing an LED. But otherwise its OK as is.

    Both schematics look good for temp accuracy. To best achieve 0.1C accuracy, following best layout practices is essential. Please follow these recommendations for both temp sensors in the linked app note: Temperature sensors: PCB guidelines for surface mount devices. And for the TMP117 in particular, please follow the directions on page 36 of the TMP117 datasheet.

    Regards

    -Alex Thompson

  • 0 in reply to Alex Thompson
    Prodigy 60 points

    Dear Alex Thompson, 

    Thank you for your response,good to know. Noted your points. 

    I had some question, 

    1. Given the size constraints with the metal tube (3mm outer size, 10mm length, and 2.5mm internal diameter), for our digital clinical electronic thermometer probe model, we are considering a tiny package sensor with a flex PCB (2x4mm) and tiny insulated wires. However, soldering the 6 pins of TMP117 is challenging. Can you recommend an alternative solution, or is it possible to reduce the pins to 3 or 4 while still using tiny wires for temperature capture?

    2. The TMP117 layout sheet specifies placing capacitors near the sensor (within 5mm) and keeping pull-up resistors at least 10mm away. Due to size considerations, is it acceptable to keep all pull-up resistors 100mm apart from the board? Will this affect the sensor's accuracy?

    3. For our small PCB dimensions (2mm width, 0.15mm thin), what internal circuit pin line size should we follow for pin connections (+, SDA, SCL, -)? 

    4. What thermal epoxy, with high thermal conductivity and low electrical conductivity, do you recommend?

    5. In the future, if we plan to integrate two TMP117 sensors on one I2C, what considerations should be taken into account in the circuit, and could you mark and show this on our existing circuit diagram for clarity?

    6. The datasheet mentions that TMP117 is a factory-calibrated sensor. After assembly with the metal tube and epoxy, is it necessary to perform calibration again?

    7.Just want to clarify my understanding, as many companies manufacture breakout boards for the TMP117 sensor, such as Adafruit, BlueDot, etc. In this context, I have a question about the recommended pull-up resistor values. Some companies use 4.7k ohms, 5.6k ohms, 5.1k ohms, and even 10k ohms. I would like to understand the purpose of the pull-up resistor and whether its value affects the accuracy and performance of the sensor. Additionally, does the pull-up resistor value change based on the input voltage, considering our input voltage is either 3V or 3.3V or 5V?

    Thanks

    Regards

    Rajkumar

  • 0 in reply to Rajkumar S
    TI__Expert 3525 points

    Hi Rajkumar,

    Let me answer your questions one at a time.

    1.) You dont have to use the ALERT pin, so that could be left floating if you dont need to use the functionality. The ADD0 pin can be shorted to V+, GND, SCL, or SDA. So since ADD0 is next to V+, that could be easy to solder. Since size is a concern, I would recommend the YBG package option if you havent chosen that yet.

    2.) That should be OK, keeping the resistors away is to prevent the heater radiating off of them as current runs through from affecting the TMP117 temp readings

    3.) Are you asking about trace width of the traces connected to the TMP117? If that's the case, I would go with whatever is practical, then test it out. This is a very low current device so very wide traces are not necessary.

    4.) What are you planning to use the thermal epoxy for? You should not need to apply thermal epoxy for the TMP117 for it to work correctly to sense the ambient air temp, so I dont have one to recommend.

    5.) If using 2 TMP117s, just make sure they are on different I2C addresses. So on your current schematic for example, just make sure U5 ADD0 pin connects to V+, and U6 ADD0 connects to GND.

    6.) No additional calibration is necessary

    7.) The pull-up resistor is necessary to pull-up an open drain output or I/O pin so that the pin can drive high or low. The key thing to understand is you need to pick the right resistor value to minimize the RC time constant on the rising and falling edges of the SDA/SCL signals, and provide enough current to communicate properly. It is not important for temp accuracy. If you connect the pull-up voltage to the TMP117 V+, the current going through that pull-up will increase, so you will need to be mindful that it does not exceed the 3mA limit in the VOL spec (if the current increases past this limit, we cant guarantee the device will pull low correctly). However, those listed values are all large enough that it should be fine across the voltages you listed.

    Regards

    -Alex Thompson

  • 0 in reply to Alex Thompson
    TI__Expert 3525 points

    Rajkumar,

    You might also benefit from looking at this at this reference design with 2 TMP117s on a flex PCB;

    https://www.ti.com/tool/TIDA-060034

    Regards

    -Alex Thompson