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Renesas wireless charger for wearables resists water and dust

Steve Rogerson
September 27, 2016

A contactless wireless charging system for low-power applications, such as hearing aids and other wearable devices that require resistance to water and dust, has been announced by Japanese electronics giant Renesas.
The two chips should let system manufactures easily add wireless charging functionality to low-power applications such as wearable devices and hearing aids.
The system comprises a power receiver IC (RAA457100) and a power transmitter IC (RAA458100). Each chip includes all functions needed for wireless charging on a single chip. Renesas will also offer an evaluation kit to help manufacturers with their wireless charging designs.
Wireless charging technology eliminates the need to change batteries or connect a power cable. This suits applications such as smartphones and wearable devices. Wireless charging technology is of particular interest in low-power applications such as hearing aids that require small form factors as well as resistance to water and dust.
However, existing wireless charging technologies have been considered unsuitable for charging systems employing compact lithium-ion (Li-ion) secondary batteries for these low-power applications associated with the difficulties in; achieving smaller form factors with the antenna size mandated by the current standards; and heat dispersion due to large charging currents that are required to charge low-power applications.
With these wireless charging chips, Renesas has combined its wireless charging expertise with its microcontrollers to enable all key system components in a small space for easier design implementation.
The power receiver IC incorporates a synchronous rectifier circuit that rectifies the AC power used to excite the antenna coil of the power receiver into direct current, and a charging control circuit for charging a Li-ion secondary battery. A 12bit A-D converter monitors the battery voltage and current during charging, and these data are supplied to the power transmitter to maintain the optimal charging power level. Protection features for the Li-ion secondary battery and a DC-DC regulator are also incorporated on a single chip.
Low-power applications such as wearable devices and hearing aids generally use compact batteries with small capacities, making efficiency crucial. The integrated DC-DC converter itself achieves efficiency of 85 per cent when the system is operating at a low load level of around 1mA, which enables longer battery life.
In general, contactless power transmission involves applying a 125kHz alternating current to the antenna coil of the power transmitter to excite the antenna coil of the power receiver and generate AC power. The power transmitter IC drives a bridge circuit and controls the alternating current to obtain the transmission power value required by the power receiver.
The power transmitter IC integrates a bridge circuit overcurrent protection function and a two-line external overheating protection. System manufacturers can modify parameter values by programming the read registers with external eeprom data via an I2C interface to meet their application needs. In addition, they can further customise it by connecting an external microcontroller.
The μPA2690T1R power mosfet from Renesas is recommended for configuring the bridge circuit. When it is combined with the RAA458100, the system manufacturer can choose between half-bridge and full-bridge circuit configurations to match the power level of the power transmitter.
Samples will be available in November.