Contactless wireless charging brings convenience to wearables

To charge low power applications, such as hearing aids and other wearable devices that require resistance to water and dust, Renesas has developed the RAA457100 power receiver IC and a power transmitter IC (the RAA458100). Each includes all the functions needed for wireless charging on a single chip. An evaluation kit is also available.

Existing wireless charging technologies have been considered unsuitable for charging systems that use compact Li-ion secondary batteries for low-power applications, hampered 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.

The RAA457100 integrates all functions needed for wireless charging in a 3.22 x 2.77mm 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 DC, and a charging control circuit for charging a Li-ion secondary battery. A 12bit ADC monitors the battery voltage and current during charging. This data is supplied to the power transmitter to maintain the optimal charging power level. Protection features for the secondary battery and a DC/DC regulator are also incorporated.

An on-chip DC/DC converter boasts industry-leading efficiency of 85 per cent when the system is operating at a low load level of around 1mA, which enables longer battery life.

The RAA458100 drives a bridge circuit and controls the AC to obtain the transmission power value required by the power receiver. The power transmitter IC integrates a bridge circuit over-current protection function and a two-line external over-heating protection function. System manufacturers can modify parameter values by programming the read registers with external EEPROM data via an I²C interface. They can further customise, by connecting an external MCU.

The uPA2690T1R power MOSFET is recommended to configure the bridge circuit. When 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 2016.

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