AS62xx Thermal Design Guideline for Wearables

Introduction

The AS62xx Product Family is a digital temperature sensor family designed for applications requiring small form factors, ultra-low power consumption, and high accuracy. Wearable devices are an ideal application for this sensor. Designing a housing for effective skin and environmental temperature measurement in wearables can be challenging. This document provides design guidelines to assist in product development.

Ordering Information

Sensing Skin Temperature

To accurately measure a user's skin temperature with the AS62xx sensor, a good thermal connection between the sensor and the skin is essential. This can be achieved by incorporating a conductive pin within the device housing. This pin should be made of a thermally conductive material, such as metal. If the housing is metallic, the pin must be thermally isolated from the housing to prevent the housing's thermal capacity from affecting the temperature measurement.

The AS62xx temperature sensor is thermally connected to this pin using thermal adhesive or thermal paste and electrically connected via a flex PCB, as illustrated in Figure 1.

Figure 1: Flex Mounted Sensor Description: A diagram showing a flex-mounted sensor. The components are arranged as follows: a housing is positioned above a contact pin. Thermal adhesive is between the contact pin and the AS62xx sensor. The AS62xx sensor is connected electrically to a flex PCB. The contact pin is designed to interface with the skin.

This flex PCB design offers greater flexibility in sensor placement within the product design but may increase costs due to the more expensive flex PCB material.

Alternatively, if mounting the sensor directly on the mainboard is preferred, a thermal connection can be established using either a contact spring or a thermal pad. For the contact spring method (Figure 2), the sensor's exposed pad (or GND pad) must be connected to the contact spring, either via a circuit path (for top-side mounting) or a thermal via (for bottom-side mounting). This approach allows for a cost-effective design that can accommodate a significant distance between the PCB and the housing.

Figure 2: PCB Mounted Sensor (Contact Spring) Description: A diagram illustrating a PCB-mounted sensor using a contact spring. A housing is above a contact pin. Below the contact pin is a contact spring. The AS62xx sensor is mounted on a PCB, connected via thermal vias to a copper layer. The contact spring is positioned to bridge the gap between the contact pin and the AS62xx sensor on the PCB.

Connecting the sensor via a thermal pad (Figure 3) is generally the easiest and cleanest method for establishing a thermal connection between the sensor and the exterior of the wearable. This involves placing a thermal pad, sized to match the sensor, between the device and the contact pin. A key factor for fast and accurate temperature readings with this method is the use of a thermal pad with high thermal conductivity. However, this method typically allows only a small distance between the device and the contact pin.

Figure 3: PCB Mounted Sensor (Thermal Pad) Description: A diagram showing a PCB-mounted sensor utilizing a thermal pad. A housing is above a contact pin. Below the contact pin is a thermal pad. The AS62xx sensor is mounted on a PCB, connected via thermal vias to a copper layer. The thermal pad is positioned between the contact pin and the AS62xx sensor on the PCB.

Temperature Response Time

To further enhance the sensor's response time, it is recommended to minimize the thermal capacity of the PCB. A crucial step is to ensure the area surrounding the sensor is free from copper planes. Additionally, a PCB cutout, as depicted in Figure 4, should be implemented.

Figure 4: PCB Cutout Description: A diagram illustrating a PCB cutout recommendation for improved sensor response time. It shows a PCB with the AS62xx sensor placed on it, surrounded by a 'PCB cutout area' indicated by dashed lines, suggesting that copper planes should be removed from this area.

Sensing Environmental Temperature

Measuring environmental temperature is more complex, as the housing's temperature is significantly influenced by the user's skin temperature. To mitigate this, it is advisable to use two sensors: one dedicated to sensing skin temperature and another for sensing environmental temperature. Developing a thermal model of the device and implementing a temperature prediction algorithm will yield the most accurate results for determining environmental parameters.

Figure 5: Environmental Sensing Description: A diagram depicting environmental sensing in a wearable device. A wearable device is shown with two temperature sensors. One sensor is labeled 'Skin Temperature Sensor' and is in contact with the 'Skin'. The other sensor is labeled 'Environmental Temperature Sensor' and is positioned to measure ambient temperature. A sun symbol indicates the environment. Thermal resistance (Rth) is shown between the two sensors, highlighting the need for isolation.

A higher thermal resistance between the environmental sensor and the skin sensor leads to more precise results. This can be achieved by using materials with low thermal conductivity for the wearable device's housing and by ensuring good isolation between the sensors and the housing.

Revision Information

Changes from previous version to current revision v4-00:

• Updated Ordering Information table with AS6221.
• Page and figure numbers for the previous version may differ from page and figure numbers in the current revision.
• Correction of typographical errors is not explicitly mentioned.

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