The response time of a temperature sensor can be a critical feature needed for applications such as smart watches, medical equipment, or gas meters. The benefits from a faster response time can range from allowing a system to make quicker adjustments to cooling, providing a more detailed range of data for a user’s sleep pattern in a smart watch, or alerting medical staff to sudden changes in a patient’s condition. The TMP118 comes in an ultra-small, ultra-thin PicoStar™ package, and the low thermal mass allows for competitive response times. This response time can be further improved through the implementation of a Flexible Printed Circuit (FPC) board.
Figure 1 shows rigid and flex PCB response time data.
Thermal mass is a significant factor to consider when designing a system for speed and responsiveness. Not only does the IC need to have a small footprint, but for even greater response improvement, the circuit board needs to contain minimal thermal mass. Some possible ways to go about this are designing the sensor on a thinner FR4-based board with minimal layers; however, to maximize the capability of the TMP118, it is strongly recommended to consider an FPC board.
To better explain this, testing was conducted using a 62mil FR4-based prototype board and the TMP118EVM FPC sensor board (5.1mil) was conducted to collect data comparing the response time of each board when submerged into an oil bath running at 75°C. The TMP118 sensors on each board read an ambient temperature of nearly 25°C prior to submersion. In the testing for the Rigid (FR4-based) PCB, an average rise time of 1.4 seconds was observed. Meanwhile, the flex PCB was observed with an average rise time of 0.16 seconds, a nearly 87% improvement over the FR4-based board.
The significant improvement in the response time with the FPC Board is attributed to the low thermal mass, allowing the TMP118 sensor to quickly absorb the temperature of the environment (or surface) compared to a rigid board which takes longer due to the FR4 material absorbing some of the ambient heat in the environment, slowing the response time. The calculated thermal mass of the FPC board is approximately 0.769J/°C compared to 1.76J/°C for the Rigid board, over two times the mass of the FPC board. Both calculations included the thermal mass of the TMP118 device (0.14mJ/°C). The mass of these boards was found using the constants and equations in Table 1 and Equation 1.
Board Dimensions (mm) | ||
---|---|---|
Rigid PCB Board | 12.7 (l) × 12.7 (w) × 0.76 (h/thickness) | |
FPC Board | 177.32 (l) × 2.5 (w) × 0.3 (h) at connector; 0.13 (h) | |
Board Constants | ||
Density | Rigid PCB Board | FPC Board |
Copper: 8.96g/cm3 | ||
FR4: 1.85g/cm3 Solder Resist: 1.4g/cm3 | Polyimide: 1.43g/cm3 | |
Heat Capacity | Copper: 0.385J/g°C | |
FR4: 0.85J/g°C Solder Resist: 0.9J/g°C | Polyimide: 1.1J/g°C |
The combination of a circuit board with low thermal mass and the TMP118 sensor can provide a meaningful and easy-to-implement answer to systems which require ultra-fast response times while maintaining accuracy.
All trademarks are the property of their respective owners.
TI PROVIDES TECHNICAL AND RELIABILITY DATA (INCLUDING DATASHEETS), DESIGN RESOURCES (INCLUDING REFERENCE DESIGNS), APPLICATION OR OTHER DESIGN ADVICE, WEB TOOLS, SAFETY INFORMATION, AND OTHER RESOURCES “AS IS” AND WITH ALL FAULTS, AND DISCLAIMS ALL WARRANTIES, EXPRESS AND IMPLIED, INCLUDING WITHOUT LIMITATION ANY IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE OR NON-INFRINGEMENT OF THIRD PARTY INTELLECTUAL PROPERTY RIGHTS.
These resources are intended for skilled developers designing with TI products. You are solely responsible for (1) selecting the appropriate TI products for your application, (2) designing, validating and testing your application, and (3) ensuring your application meets applicable standards, and any other safety, security, or other requirements. These resources are subject to change without notice. TI grants you permission to use these resources only for development of an application that uses the TI products described in the resource. Other reproduction and display of these resources is prohibited. No license is granted to any other TI intellectual property right or to any third party intellectual property right. TI disclaims responsibility for, and you will fully indemnify TI and its representatives against, any claims, damages, costs, losses, and liabilities arising out of your use of these resources.
TI’s products are provided subject to TI’s Terms of Sale (www.ti.com/legal/termsofsale.html) or other applicable terms available either on ti.com or provided in conjunction with such TI products. TI’s provision of these resources does not expand or otherwise alter TI’s applicable warranties or warranty disclaimers for TI products.
Mailing Address: Texas Instruments, Post Office Box 655303, Dallas, Texas 75265
Copyright © 2024, Texas Instruments Incorporated