LMT70: How to deal with the accuracy

Hi Kevin,

Thank you for reaching out to us! The fastest thermal path contribution is its leads (4 large balls). In TIDA-00824 reference design, there are a lot element that helps boost the temperature response faster such as board layout, thermal via, thermal epoxy, and the metal can.

1. In figure 9, we created a small rectangular plane underneath the bottom layer connected to T_ON and VDD since we are not using T_ON pin for switching. We can directly connect both pins to VDD, and two square planes connect to GND and Vout.

2. In figure 10, there is a metal can underneath the watch, so the heat transfer is going through the metal can, and heat the LMT70 board.

If you want to take this offline by sending me the design, I would be happy to assist you.

Aaron

Hello, Aaron

Thanks a lot for your response.
I am copying the PCB layout as in the reference design.
Could you please let me know exactly what's the thermal expoxy material? what's the metal can material? what's the size of the metal can? do I need to dip the whole remote PCB inside the thermal expoxy, or just the pad size of the PCB need to touch to the thermal expoxy?

Thanks again for your helps.

regards,
Kevin.

Hi Kevin,

Unfortunately, I couldn't disclose the exact thermal expoy material that we used for the reference design, but if you are replicating the reference design. I would suggest to use the Duralco 128 or Duralco 132. Please see attachment for information.

12 132 132IP.pdf

13 128 134 135 133.pdf

Please keep in mind that the larger thermal mass the slower is heating, so applies a small thin layer of thermal epoxy underneath the tiny PCB enough to hold the board steady in place.

If you need more information of the thermal epoxy, please visit the link below.

I believe the material of the metal can is Kovar, and please see attached file for the dimension. Basically, we used the hollow metal can from the TO-CAN package, and assembled into the wearable watch.

Hope this helps. Please let me know if you have further question.

Aaron

Hello, Aaron

May I have your email and phone number so that I could talk to you offline for design review?

Thanks.

Kevin

Hi Kevin,

Please send me your question to my email aaron.heng@ti.com

Aaron

Hi, Aaron

1. I am using 2.1V DC as the power supply for LMT70. Does the level of power supply affect the sensor response time? What's the power level do you recommend? The reason I am using 2.1V is that it is same as CC2541.
2. Does the copper shape on the bottom layer need to be bare, or keeping the solder mask on the copper?
3. Is the metal can the bigger the better? The diameter of my metal can is about 10mm.

regards,
Kevin

Hi Kevin,

Sorry I missed your posted. The 2.1V power supply should be fine, and the LMT70 temperature accuracy specs is gauranteed across supply voltage 2.0V to 5.5V. As long as the supply votlage is within power supply specs, there should be any affect of the thermal response time.

It is better to open the rectangular shape expose to ambient temperature, but keep in mind that copper is corrosion material when it is exposure to atmosphere, sweat, water, and moisture causing the copper oxidize and bright surface. This is why always recommended using the stainless steel materia. The thermal conductivity properties for the soldermask is 0.245 W/m°C. You might want to think about this when you are planning to use the copper.

It depends on what you are looking for. The bigger shape is the better thermal mass, but it will slow down your thermal response.

Hope this helps. Let me know if you have further question.

Aaron

Hello, Aaron

Thanks for your response.
The metal can is stainless steel. Are you saying the layout on the PCB should also be stainless steel?
The application of the product is monitoring the human body temperature.
1. do you recommend to use a bigger rectangular shape on the PCB?
2. what's the metal can size do you recommend?

Regards,
Kevin

Kevin,
Aaron is on vacation - he will reply to you when he returns. Probably tomorrow.

Take care,