Protecting the Health of a Fitness Tracker

Not too long ago, my only fitness tracker was a pedometer in my pocket to measure my daily step count. Things have since changed very quickly. I now have a smartwatch on my wrist to track my daily activities, including steps completed, distance covered, calories burned, heart rate, and breathing pattern. I also receive alerts for messages, take calls, listen to my favorite podcast, and check the weather via my smartwatch. These are all in addition to seeing the accurate time of the day. Due to all these innovative features, wearing a smartwatch and using it as a fitness tracker is the trend for the health-conscious population worldwide. While these wearables help people remain fit, extra care needs to be taken by the manufacturers to protect these wearables from electrical overstress (EOS) and electrostatic discharge (ESD) generated from the body of the person wearing these devices.

ESD Protection of Fitness Tracker Ports and Interfaces

When an object with a higher charge comes in contact with an object with a lower charge, the charges flow from one object to the other. This transfer of charges (electrons) may lead to an ESD event reaching up to thousands of volts. When a human body comes in contact with an electronic device, static electricity is discharged, potentially leading to a catastrophic ESD event. Fitness trackers have ports and interfaces like USBs for battery charging, antennas for communication, a touch panel for visual display, and several side buttons. There are also multiple sensors in any fitness tracker to monitor the activities and health of the person wearing the tracker. Please refer to Figure 1 for a schematic of the potential interfaces of a fitness tracker. Having many interfaces, sensors and data ports, fitness trackers are particularly prone to damage from multiple ESD events created by human contact. These events can damage the fitness tracker’s intricate circuitry, which may also lead to serious injury to the person wearing it. Hence, ESD protection is not just important for fitness trackers; it is a must.

Figure 1. Basic block diagram of fitness tracker interfaces

There are at least three crucial things to remember when protecting the trackers with transient voltage suppressor (TVS) diodes.

1. To protect the sensitive integrated circuitry of fitness trackers from electrical overstress (EOS) and ESD events, the TVS diode needs to meet the ESD discharge standards specified in IEC 61000-4-2 test standard, which is at least ±8kV for contact and ±15kV for air. In most cases, it is preferred to select TVS diodes with even higher specifications, such as ±30kV for air and ±30kV for contact, for better protection against multiple ESD/EOS events.

2. Several communication protocols are used in a fitness tracker to transfer data between a smartphone, tablet or other fitness trackers. These data transfers are done via wireless connections such as Bluetooth, near-field communication (NFC), wired connections such as USB 2.0 (D+/D-), or low-power Wi-Fi interfaces. Since these are high-speed differential data lines, TVS diodes need to protect the circuit during transient events by maintaining a low line-to-line capacitance so that the strength of the signals does not degrade. As these differential lines transmit data while maintaining a high speed, having the TVS diode with ultra-low line-to-line capacitance is essential to ensure signal integrity.

3. The last point to remember is that the size of the TVS diode should be minimal to fit into the tiny package of the fitness tracker. A TVS diode in a 0201 (0.024" x 0.012") package is a suitable solution for the fitness tracker. In a metric unit, this package is called 0603 (0.6mm x 0.3mm).

Bluetooth & NFC Antennas

Antennas used in fitness trackers are pivotal in collecting and exchanging data to and from the device while attached to the human body. Bluetooth supports a maximum data rate of 1Mbps to 2Mbps and typically uses a 2.4GHz transmission signal for communication. A NFC antenna, on the other hand, operates at a frequency of 13.56MHz. The Bluetooth or NFC antenna is usually connected to the controller IC via contact points on the fitness tracker. ESD strikes can happen via those contact points. Therefore, TVS diodes must be placed between the antenna and the controller. (Please refer to Figure 2 for a reference diagram.) A bidirectional ESD protection device with an ultra-low junction capacitance is required to preserve the signal integrity of the antennas at that high-frequency range. The peak-to-peak voltage of these signals should also remain stable.

Semtech's RClamp®2451ZA is designed to protect antenna interfaces with an exceptionally low line-to-line capacitance of 0.35pF (Figure 3), ensuring signal integrity and no harmonic distortion in the RF signal. The device exhibits minimal effect on the transmission line impedance and shallow insertion loss characteristics, 1.2dB loss at 10GHz frequency (Figure 4), which makes it a compelling ESD protection device for fitness tracker antennas. It protects one bidirectional high-speed data line operating at 24V. RClamp2451ZA can withstand over 1000 ESD strikes as per IEC 61000-4-2 (ESD) level 4 with a specification of ±18kV (Air) and ±14kV (Contact). It is built in a tiny 0201 (0.6x0.3x0.25mm) package.

Figure 2. ESD protection of an antenna

Figure 3. Capacitance vs. Reverse voltage of RClamp2451ZA

Figure 4. Insertion loss (S21) of RClamp2451ZA

Touch Screen Display

The most common display on fitness trackers is a capacitive touch screen display. It uses a series of electrodes in the touch sensor. The electrode is a piece of conductive material. The touch controller monitors the electric field generated around the electrodes and provides that information to the processor. Static electricity can easily pass through the touch screen and the components beneath it when a user touches the screen with a finger. A dangerous ESD event can surpass the circuit's maximum allowable forward voltage and damage the touch controller IC's internal circuitry. Accordingly, the capacitive touch screen display must be protected. Please refer to Figure 5 below.

Figure 5. ESD protection of a touch screen display

Semtech's RClamp3391ZC is designed to protect the delicate electronics of touch controllers from damage due to ESD and EOS events. It protects one bidirectional high-speed data line operating at 3.3V. It features excellent ESD protection characteristics such as ultra-low typical capacitance of 0.17pF at VR=0V, f=1MHz (Figure 6), and typical dynamic resistance of only 0.18Ω ((Figure 7). RClamp3391ZC has a high ESD withstand voltage of ±8kV contact and ±15kV air per IEC 61000-4-2. RClamp3391ZC is also available in an industry-standard 0201 DFN package (0.6x 0.3 x 0.25 mm).

Figure 6. Capacitance vs. reverse voltage of RClamp3391ZC

Figure 7. TLP characteristics of RClamp3391ZC

Protecting Sensors

Fitness trackers depend on sensors to measure how the human body operates. For example, a heart rate monitor measures the heart beats per minute. A GPS tracks the distance covered by running or biking, a gyroscope detects the motion of a person if running or walking, a skin temperature sensor detects slight changes in body temperature, and a proximity sensor wakes up the display when needed. These are only a few examples. In reality, there can be more than a dozen sensors in a basic fitness tracker today. More sensors are incorporated in high-end fitness trackers to provide more data to the user. Without the proper ESD protection, as shown in Figure 8, the sensor circuits can be damaged by human interaction.

Semtech's RClamp3371ZC is an ultra-low capacitance ESD protection device suitable to protect the sensors in a fitness tracker, as shown in Figure 8. It features superior ESD protection characteristics highlighted by extremely low typical junction capacitance of 0.23pF (Figure 9) and dynamic resistance of only 0.14Ω. It protects one data line with a minimum breakdown voltage of 5.5V. The maximum clamping voltage is 7V. RClamp3371ZC provides the transient protection as per the specifications in IEC 61000-4-2 (ESD) at ±17kV (Air) and ±10kV (Contact). RClamp3371ZC is also available in a small 0201 DFN package with a nominal dimension of 0.6 x 0.3 x 0.25 mm.

Figure 8. ESD protection of sensor circuits

Figure 9. Capacitance vs.reverse voltage of RClamp3371ZC

USB Type-C Interface (VBUS and D+/D-)

A large number of fitness trackers incorporate USB Type-C ports for charging. A USB Type-C connector can deliver up to 100W of power, 5A of current and 20V of voltage utilizing USB power delivery (USB PD). If USB PD is not required for any application, the VBUS can support 5V at 3A, making it 15W deliverable power. Figure 10 details the USB Type-C pin configuration with ESD protection.

Figure 10. ESD protection of USB Type-C port

Most contemporary fitness trackers use 5V at 3A at the VBUS pin for charging the battery. The security of the VBUS pin requires a unidirectional ESD protection device with a fast response time and low clamping voltage to protect the VBUS pin. Generally, low capacitance is not a consideration for the VBUS line as it is usually not sensitive to additional capacitance. Satisfying all of these criteria, Semtech's µClamp®1291ZA is ideally suited to safeguard the VBUS pin. µClamp1291ZA has a working voltage of 12V. It provides transient protection as per the specification in IEC 61000-4-2 (ESD) at ±20kV (Air) and ±15kV (Contact). µClamp1291ZA offers a dynamic resistance of 0.29Ω (Figure 11) and a peak pulse current of 10A (tp=8/20µs). The ESD Clamping (+8kV Contact) of µClamp1291ZA is shown in Figure 12. µClamp1291ZA comes in an ultra-small DFN 0201 2-lead package.

Figure 11. TLP characteristic of µClamp1291ZA

Figure 12. ESD Clamping (+8kV Contact) of µClamp1291ZA

D+/D- lines in a USB Type-C port are used for USB 2.0 interfaces. D+ and D- pins carry a differential 480Mbps data signal, and the voltages on these differential lines can reach 5V under normal operating conditions. D+/D- lines can be protected using Semtech's RClamp2261ZA while maintaining signal integrity (Figure 2) as it only has 0.35pF (Typical) capacitance (Figure 13). The RClamp2261ZA has a working voltage of 22V. The device offers a typical clamping voltage of 10.5V (IPP=18A, tp=8/20µs) (Figure 14) and a minimum breakdown voltage of 24V. It provides transient protection as per the specification in IEC 61000-4-2 (ESD) at ±30kV (Air) and ±25kV (Contact). This bidirectional TVS diode comes in an ultra-small DFN 0201 2-lead package.

Figure 13. TLP characteristics of RClamp2261ZA

Figure 14. ESD Clamping (+8kV Contact) of RClamp2261ZA

Side Buttons

There are at least two side buttons in every fitness tracking device. These side buttons may suffer from ESD strikes when they come in immediate contact with a user (Figure 15). Most of these side buttons are DC switches working at an operating voltage of 5V or less. Ultra-low capacitance is not a compulsory requirement to protect the side buttons. Semtech's µClamp5031ZA features outstanding ESD protection characteristics highlighted by a meager dynamic resistance of 0.008Ω (Figure 16). It has an operating voltage of 5V and is rated for a maximum EOS peak pulse current of 7.5A (tp=8/20µs). The typical clamping voltage at a peak current of 4A is 7.5V. µClamp5031ZA provides transient protection as per the specification in IEC 61000-4-2 (ESD) at ±30kV (Air) and ±30kV (Contact). This bidirectional TVS diode is available in a tiny 0201 DFN package (0.6x 0.3 x 0.25 mm) as well.

Figure 15. ESD protection of side keys

Figure 16. TLP characteristic of µClamp5031ZA

Semtech offers various ESD protection diodes for intelligent fitness trackers, as shown in Table 1.

Table 1: Wide range of applicable Semtech parts for protecting fitness trackers

The acceptance and use of fitness trackers are on the rise worldwide. According to Fortune Business Insights, the global fitness tracker market is projected to grow from $36.34 billion in 2020 to $114.36 billion in 2028 at a CAGR of 15.4%. With the enormous popularity of fitness trackers, it is critical to adequately protect each interface, data and communication port from all ESD and EOS threats. Semtech's wide range of reliable ESD protection products solves design challenges and shields many of the world's most intelligent fitness tracker brands. Contact Semtech while designing and developing the next safe and reliable next-generation fitness tracker.

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