Ryan Roderick
System Engineering Manager
Published: February 19, 2020

Electronic products today are transitioning from wired to wireless connections. The trend started with low-voltage devices, such as cell phones and tablets, to higher-voltage devices, such as PCs and power tools. This trend continues to grow with higher voltage and power applications. Advancements made in battery technology and inductive power transfers, wireless power, are also driving this change. Battery advancements have yielded products that hold more capacity at a fraction of the weight of traditional lead acid batteries.

Mobile applications are susceptible to mechanical fatigue from charging the battery while the device is idle. Wireless power technology has made significant advancements in efficiency and the amount of power that is transferred, allowing the transmitting device to charge the mobile device’s battery while also powering the mobile device.

This winning combination (Figure 1) illustrates the system block diagram for a 6V to 16V system. The system is powered from the battery when the mobile device is not docked to the wireless transmitter. The RAJ240045 is a device that has a microcontroller and battery front-end functionalities. The RAJ240045 is the brain of the system and supports 3- to 4-cell Lithium-ion batteries.

As a functional block, the RAJ240045 is programmed to monitor each cell of the battery pack, testing for cell voltage, pack current and temperature extremes. If any of the extreme conditions are met, the device controls the power FETs that connect the battery to the system, ensuring that the battery and the system are in a safe state. The device has very precise current measurements (18 bits) and a feature that detects impulse discharge current, causing the device to increase the sampling rate of the pack current. These features enable accurate fuel gauging. The MCU controls and monitors the rest of the signal chain through I2C communications, ADC measurements and GPIOs.

A bi-directional buck-boost controller is connected to the RAJ240045. When the system connects to the wireless receiver (W_RX), current flows from W_RX to the battery. The battery regulates the voltage and clamps the current to the battery input via the RAJ240045. The input and output currents are monitored by the MCU as a system and redundancy safety checks. The ISL81401 has safety comparisons for current, voltage and temperature. The Power Good pin can be used as an interrupt for quick status reports.

The ISL81401 is a bi-directional buck-boost regulator. When the wireless transmitter (W_TX) is not connected to the system, the battery powers the system. The current flows from the battery to the system. The regulation node is between the wireless charging output and the input of the ISL81401. Other part considerations for the functional block are the ISL9238 and ISL9241. These parts have charger algorithms digitally encoded for signal chain simplification.

The buck-boost controller is connected to a wireless receiver (P221-R) and the system. The P9221-R is paired with the P9242-R (wireless transmitter), which delivers up to 15W with 87% efficiency of isolated power to the mobile device. The P9221-R connects to the RAJ240045 (MCU) via I2C and interrupt pins. The MCU is used to enable and terminate communication with the transmitter. The MCU also monitors the output voltage, the load current and the temperature to determine the health of the system. The receiver output voltage is resistively programmed to either 9V or 12V. These voltages are good for 3- to 4-cell battery systems.

The wireless chipset has a foreign object detection (FOD) feature that detects metal objects or any device that alters the magnetic field transmitted to the receiver. Periodically, the receiver and transmitter communicate to determine the amount of power transmitted versus received. The communication is performed inductively by modulating the transmitting current and voltage. If there is an unexplained power loss, a flag is set. The chipset has alignment registers that indicate the amount the receiving device is off in the x or y direction from the transmitters true center. The chipset passes WPC1.2 3 industry standards.

A p-channel FET is needed between the regulation node and the output of the P9221-R to prevent abnormal operation when the receiver is disabled, or the transmitter (P9242-R) is not connected. The PMOS is controlled by the RAJ240045.

This solution is for powering a system through a power transfer or by battery. The system assumes all components are located physically next to each other. Other systems are constructed with electronics localized to the battery and the charger. Both solutions are dependent on the end application.

With this new approach, Renesas is enabling partners and designers to accelerate their designs and deliver comprehensive solutions with our complementary portfolio.

Figure 1. Wireless Charging for Portable Devices block diagram

Visit the winning combinations webpage to learn more.

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