Can You Hear Me Now? (Wireless Charging Solutions)

A hearing aid is generally a small wearable electronic device that amplifies sounds to assist people with hearing loss. Hearing aid technology has continuously improved over the last 20-30 years. For example, compared to the relatively inexpensive older analog circuit type of hearing aid, a more sophisticated and newer type of digital hearing aid can be programmed to amplify some frequencies more than others. Furthermore, a digital hearing aid can be adjusted to the wearer’s individual unique hearing needs, tuned to certain listening environments, and can also be programmed to focus on sounds coming from a specific direction. These features make hearing aids much more sophisticated than simple sound amplification.

Approximately 15% of American adults (~37.5 million) aged 18 and over report some trouble with hearing (Source: NIDCD). Historically, the overall U.S. hearing aid sales have grown by an average of 3% to 4% per year, and in 2014, sales of hearing aids in the U.S. topped 3 million (source: NIH). The two most popular models are the behind-the-ear (BTE) style hearing aid, and the receiver in-the-canal and receiver-in-the-ear (RIC/RITE) type hearing aid.

For a BTE or RIC/RITE type hearing aid, the most common powering solution today involves using a non-rechargeable small Zn-Air primary battery (0.9V – 1.25V). This battery chemistry has extremely high volumetric energy density, resulting in a long run time and a small form factor. However, Zn-Air batteries cannot be recharged, forcing the user to change out the battery every 7 to 10 days. Frequent changes of a very small battery in a very small enclosure are particularly problematic for the manual dexterity-challenged, and elderly retirees.

In contrast, a Li-Ion battery offers reasonable run time, plus it can also be recharged, thus it does not need to be replaced frequently.  However, there is currently no single-IC battery charging solution available on the market. Typical hearing aid electronics run directly from a single-cell Zn-Air battery, and the Li-Ion output voltage is roughly three times higher than this. Therefore, a Li-Ion based solution requires both a battery charger and a step-down regulator to deliver the correct voltage to power the hearing aid ASIC (application specific integrated circuit) chip. This multiple-IC approach is relatively large and generates switching noise/EMI which can be problematic for the sensitive audio circuitry.

A rechargeable NiMH powered solution offers the best of both worlds. The NiMH cell has nearly identical voltage output to the Zn-Air cells (therefore no additional step-down regulator is necessary), is rechargeable and available in form factors that are the same as those of the standard Zn-Air battery, allowing for a small overall hearing aid unit, making it a very compelling choice.

Why is it then, that a wireless charger is needed? The answer is clear; recharging a battery removes the need for frequently swapping out battery cells; as already mentioned this is very beneficial for the manual dexterity challenged. Furthermore, its more convenient even for those that are still good with their hands. Wireless charging is just that – charging without wires – no connectors required. Combining a wireless charging method with a NiMH battery cell can provide a robust and convenient charging solution. This allows the hearing aid to be sealed and waterproof, thereby reducing the need to open the unit while also protecting it at the same time, thereby increasing reliability and longevity.

Table 1 lists some of the advantages and disadvantages of each of the 3 battery types already mentioned.

Table 1. Comparison of Batteries for Hearing Aids

Wireless Power Transfer (WPT)

An inductive WPT system (as shown in Figure 1) consists of transmitter electronics, a transmit coil, a receive coil, and receiver electronics. Received power depends on many factors: transmit power, coupling between the transmit (Tx) Coil & receive (Rx) coils (distance, alignment, physical properties, ferrites), nearby unrelated metal, as well as component tolerances. In a wireless power transfer system, power is transmitted using an alternating magnetic field. An AC current in the transmit coil generates a magnetic field. When the receive coil is placed in this field, an AC current is induced in the receive coil. The AC current induced at the receive coil is a function of the applied AC current at the transmitter and the coupling between the transmit and receive coils. The power transmission range across the air gap can be improved using resonance by connecting a resonant capacitor to the receiver coil to create an LC tank tuned to the same frequency as the transmit coil AC current frequency.

Historically, to build a WPT charging system required a complex solution: a battery charger, switching buck regulator and WPT circuitry. This complex solution tends to be large and difficult to design.

New Wireless Power Receiver & NiMH Charger

A wireless power receiver and charger solution which solves the issues outlined requires the following attributes:

•                Wireless charging – removes the need for frequently swapping of the battery, and allows for a sealed, waterproof, more robust hearing aid.

•                Monolithic solution – small integrated receiver and WPT circuitry all in 1 IC

•                Temperature compensated charging -  allows for safe charging of the NiMH battery

•                Zn-Air battery detection - the hearing aid may function with either NiMH or Zn-Air batteries. A rechargeable NiMH cell is for normal use, but in an emergency when the user may have forgotten to charge the NiMH cell, a non-rechargeable Zn-Air battery cell can be safely inserted and the LTC4123 will ensure that it does not charge it (therefore not damaging it).

•                Reverse polarity detection – stops charging in case batteries inserted backward

•                Charge status indication – so the user knows when to recharge the battery

•                Charging safety timer – safety and protection for the battery

•                Hot/Cold detection – pauses charging if the battery temperature goes to extremes

•                Overall tiny solution size

To address these specific needs, Linear Technology now part of Analog Devices Inc. introduced the LTC4123. It is a 30mW wireless receiver with a constant-current/constant-voltage linear charger for NiMH batteries, such as Varta’s Power One ACCU Plus Series. An external resonant LC tank connected to the LTC4123 enables the IC to receive power wirelessly from an alternating magnetic field generated by a transmit coil.  Integrated power management circuitry converts the coupled AC current into the DC current required to charge the battery. Wireless charging with the LTC4123 allows for a completely sealed product and eliminates the need to constantly replace Zn-Air primary batteries.

However, for products that demand the flexibility of operating from multiple battery chemistries, the Zn-Air detection feature in LTC4123 allows the same application circuit to work interchangeably with both rechargeable NiMH batteries and primary Zn-Air batteries. Both battery types can directly power a hearing aid ASIC without the need for additional voltage conversion. By contrast, a 3.7V Li-Ion battery requires a step-down regulator in addition to wireless battery charging functionality to power the ASIC.

The LTC4123 rectifies AC power from the receive coil and can also accept a 2.2V to 5V input to power a full-featured constant-current/constant-voltage battery charger. Features of the charger include programmable charge current up to 25mA, a temperature compensated single-cell 1.5V battery charge voltage with ±1% accuracy, charge status indication and an onboard safety charge termination timer. The temperature-compensated charge voltage protects the NiMH battery and prevents overcharging. The LTC4123 prevents charging when batteries are inserted with reverse polarity and pauses charging if the temperature becomes too hot or too cold.

The LTC4123 is housed in a highly compact, low profile (0.75mm) 6-lead 2mm x 2mm DFN package. The device is guaranteed for operation from –20°C to 85°C in the E-grade version.

Wireless Power Transfer Using the LTC4123

An inductive wireless power system consists of transmitter electronics, transmit coil, receiver electronics and a receive coil. The LTC4123 forms the basis for the receiver electronics in such a system. The receive coil can be integrated into the receiver electronics printed circuit board (PCB). An external resonant LC tank connected to the ACIN pin allows the device to receive power wirelessly from an alternating magnetic field generated by a transmit coil.  An LTC6990 TimerBlox Voltage Controlled Silicon Oscillator can be used as a transmitter as shown in the LTC4123 datasheet. See Figure 2 for a typical application schematic. See Figures 3, 4 and 5 for receiver/charger and transmitter demo boards illustrating the tiny solution size. 

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Figure 2. LTC4123 Typical Application Schematic (4123 TA03)

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Figure 3. LTC4123’s Compact and Simple Charger/Receiver Circuitry

(6mm Diameter Solution Size)

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Figures 4. Compact and Simple Transmit Circuitry and Transmit Coil

(front and backside of transmitter demo board)

Architecture Advantages

The LTC4123 solution has the following architecture advantages compared to a Li-Ion + step-down regulator based multi-chip approach:

1. Single-cell NiMH rechargeable batteries are a drop-in replacement for the standard hearing aid application Zinc-Air (Zn-Air) primary cells

2. NiMH may not provide as much runtime as a Li-Ion solution for a given battery size, however its runtime is adequate for the application.

3. No need for an additional step-down voltage regulation stage, thus reducing solution size, complexity and cost without concern for EMI/EMC switching frequency noise interfering with the audio quality.

4. Very simple one-chip wireless charging solution for NiMH battery chemistries.

Conclusion

The LTC4123-based rechargeable NiMH powered solution offers hearing aid designers excellent features and ease of implementation.  The NiMH cell has nearly identical voltage output to the Zn-Air cells, is rechargeable and has form factors that are virtually the same as those of the standard Zn-Air battery. The LTC4123’s unique feature set adds wireless rechargeability and extensive protection features to a hearing aid or other wearable devices with very little change to the system. Lastly, manual dexterity is not a pre-requisite for changing out a battery!

Linear Technology

www.linear.com