Contactless charging for healthcare and wearable devices

Each IC 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 convenience has wide appeal for a variety of 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 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 the new wireless charging solution, Renesas has combined its wireless charging system expertise with its proven MCUs to enable all key system components in a small space for easier design implementation.

Key features of the RAA457100 Power Receiver IC:

1) Enabling more compact devices by integrating all functions needed for wireless charging in a 3.22x2.77mm package
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 12-bit A/D converter is used to monitor the battery voltage and current during charging, and this data is 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.

2) On-chip DC/DC converter with industry-leading efficiency of 85% for extended operation on battery power
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 industry-leading high efficiency of 85% when the system is operating at a low load level of around 1mA, which enables longer battery life.

Key features of the RAA458100 Power Transmitter IC:

3) Integration of all circuits needed for wireless power transmission and support for single 5V power supply voltage from mobile batteries
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. Renesas’ newly developed 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 function. 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 the solution to their requirements by connecting an external MCU.

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 of the wireless charging system solution will be available in November 2016.