Longer playing time in True Wireless Stereo earbuds: the role of an integrated IR proximity sensor
By Dave Moon
A new youthful generation of consumers has transformed a usage habit that had been established for decades. Once, headphones or earphones were an accessory that a user would wear for as long as they were listening, and remove when they had finished listening. The introduction of True Wireless (TWS) earbuds has changed this habit: now, users wear their earbuds all the time, even when not listening to them – just as people wear a watch all the time, not only when they want to tell the time. TWS earbuds are so comfortable, convenient and unobtrusive that users feel no need to remove them.
So popular are these devices becoming that industry analysts expect the market to enjoy compound annual growth of 27% through to 2023. At that point, TWS earbuds are expected to overtake all other types of wireless and wired earphones and headphones in sales volumes.
With such fast growth projected, earbud manufacturers are bound to face intense competition. Consumer product choices will be affected by important parameters such as audio quality, comfort and reliability.
An additional and crucial factor will be battery life to sustain prolonged periods of use between charges. One way of minimizing power use is to ensure the earbud automatically stops playback when removed from the ear and powers-up again when inserted in the ear.
This calls for short-range proximity sensing. In mobile phones, an infrared (IR) proximity sensing module detects when the phone is held to the user’s face during a voice call, so that the display can be switched off. The following article describes how this technology is being adapted to fit the smaller space in a TWS earbud, while reliably detecting when the earbud is in or out of the ear.
IR Proximity Detection: principle of operation
The basic operation of an IR proximity sensor is shown in figure 1.
It consists of two main components:
- a source of invisible IR radiation which emits modulated pulses of light. Ideally, the optical power of the emission should be concentrated in a narrow wavelength band
- a photodiode (light sensor) which has its peak sensitivity at a wavelength matching the peak intensity of the emitter.
By tightly controlling the wavelength at which the system operates, and by modulating the pulses, the sensor system can be made immune to noise, mostly consisting of interference from external sources of IR energy, such as sunlight, as well as internal reflections – crosstalk – from the module’s cover and other parts of its optical system stack. When the emitted IR light hits a target within range, it is reflected to the photodiode, which converts the measured IR energy to a digital value which rises in proportion as the target moves closer.
In TWS earbuds, the proximity sensor will normally be configured to trigger a detection signal when an object, in this case, the user’s ear opening, is within 3 mm, and a release signal when the nearest object is more than 10-mm away. Reliable proximity detection requires an adequate signal-to-noise ratio (SNR). To determine the SNR, manufacturers calculate a ratio of the difference between the detect and release threshold counts divided by the baseline jitter value when there is no object in range:
(average detect count value) – (average release count value)
(Jitter count value)
Typically, SNR is considered acceptable when this ratio is >4.
TWS Earbuds: Why Every mW Counts
The proximity sensor can help to reduce the rate at which energy is drawn from the earbud’s small battery by detecting when the earbud has been removed from the ear. But the sensor itself consumes power: most of the sensor’s energy use is attributable to the IR emitter. Fortunately, earbud designers can deploy one of two techniques for limiting the sensor’s power use. The first is by controlling proximity cycle timing. In the TMD2635, an integrated proximity sensor module from ams, duty cycle configuration is easily controlled (see figure 2).
The number of times the emitter is pulsed (PPULSE) and the duration of the active drive current (PPULSE_LEN) of each pulse can be adjusted. Power consumption is directly proportional to the number of pulses and pulse length. The total time of one proximity measurement (PRATE) can be lengthened or shortened, and this provides the main way to control the duty cycle. The system designer can also introduce a wait time (PWTIME) between proximity measurement cycles.
The second method to control the duty cycle is through signals generated at the application software level. Here, the host processor can be programmed to cycle the sensor’s active/inactive states in either a polled or interrupt-driven fashion. The polling method gives the host MCU precise control over system timing. Here, the proximity sensor is normally in a quiescent, low-power state. Periodically, the host microcontroller issues a command to wake up, take a proximity measurement, and then return to the quiescent state. In this polling mode, the designer will configure the optimal duty cycle which uses the least possible power while providing acceptable latency – that is, the delay between the user inserting/removing the earbud, and the sensor detecting the event.
In the interrupt-driven method, the MCU wakes up the sensor, reads its previous sample, then lets it run freely. When the next data event occurs, the sensor signals an interrupt to the host, and then automatically goes to sleep. The advantage of this interrupt-driven method is that the designer can choose which types of event generate an interrupt signal. This allows the system to offload many tasks from the host firmware to the sensor. Since the CPU in the host is power-hungry, offloading saves power. So for example, when the TMD2635 asserts its Sleep After Interrupt feature, it automatically deactivates its internal oscillator, entering a lower power state.
The TMD2635’s programmable threshold feature is particularly useful for triggering interrupts when proximity data events fall outside the pre-set range between the high and low count thresholds. This is because it can be set to trigger only after the count repeatedly falls outside the threshold window a number of times. This and other interrupt filtering features are implemented in the TMD2635’s hardware, reducing the burden on the host processor.
It should be noted that timing in interrupt-driven mode is less deterministic than when polling. Event-driven duty cycles will vary with changes in the host processor’s response time, and with changes in the number of proximity events. This variability makes accurate power calculation difficult unless simplifying assumptions are made. Bench characterization is normally the best way to determine power consumption under dynamic operating conditions.
In the interrupt-driven mode, the sensor spends most of its time in a free-running Idle mode, drawing typically 30 µA of average current. This consumes more power than polling, where the sensor is normally in Sleep mode drawing just 0.7 µA.
In proximity detection systems based on the TMD2635, the module’s emitter, a low-power Vertical-Cavity Surface-Emitting Laser (VCSEL) confers a further advantage. Most IR proximity sensors have an LED emitter, but a VCSEL offers higher electrical-to-optical conversion efficiency – generally ten times more efficient than an LED’s. In addition, because its beam is very narrow, with a viewing angle of just 1° to 5°, all of the emitter’s optical energy can be directed towards the target. The result is markedly lower total power consumption than in an equivalent LED-based sensor system, as well as lower interference from crosstalk and a higher SNR.
Incorporating VCSEL technology for the first time, the latest IR proximity sensor modules produce a substantial improvement in power use compared to earlier devices. Proximity sensor manufacturers are also adapting their product designs to fit in the tight space inside a TWS earbud while maintaining high levels of optical performance.
Figure 3 shows how little space a proximity sensor can occupy inside a TWS earbud reference design developed by ams. The TMD2635 used in this design has a package size of 1 mm x 2 mm x 0.5 mm (see figure 4).
The biggest difficulty in making such a small device is the optical design: ensuring that the emitted and reflected light beams have a clear path to and from the target, while restricting the effect of crosstalk on the photodiode’s measurements. In the TMD2635, ams achieves this through a combination of component miniaturization, precise assembly, and a high-performance optical stack (see figure 5).
The apertures above the emitter and photodiode are covered by a polycarbonate material which is highly transparent to IR light. The aperture can be either circular (1.5-mm diameter) or oval (1 mm x 2 mm), giving the designer greater flexibility in the positioning of the sensor inside the earbud’s housing.
The combination in the TMD2635 module of configurable power management techniques, a highly efficient VCSEL emitter, and an optical assembly, now provides designers with a way to more easily integrate proximity sensing within the very small space inside an earbud, while providing reliable detection of the earbud’s position in or out of the ear. The high optical efficiency of the module’s laser emitter and its low Sleep Mode current help maintain average power consumption at very low levels, helping earbud manufacturers to offer extended runtime between charges, even from a battery as small as 25 mAh.
Source: True Wireless Earbuds: Market Growth, Market Shares, Technologies Used, Components Used. SAR Insight & Consulting, SensiAn Research Limited. Analyst: Peter Cooney. Published Q4 2018.
About the Author
Dave Moon is a Senior Product Marketing Manager for the Advanced Optical Solutions group at ams AG. He has over 25 years’ experience working in the semiconductor industry and has held various applications, systems, and product definition positions at Texas Instruments, Agere Systems, Lucent Microelectronics, and AT&T Bell Labs. Dave received a Bachelor’s of Electrical Engineering from the University of Delaware and a Master’s of Science in Electrical Engineering from The Johns Hopkins University.
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