True wireless charging audio experience

As more and more mobile phones eliminate audio jacks, true wireless stereo (TWS) earbuds are becoming more and more popular. By wearing tiny earbuds, consumers can conveniently listen to music and take calls without the tangle of wired headphones. Charging these tiny earbuds directly is a difficult design challenge. Many manufacturers have developed battery cases that allow the earbuds to slide in and make contact with the charging pins. When the battery case can be wirelessly charged through any Qi-certified wireless charger or the back of any Wattshare™-enabled phone, consumers will get a truly wireless experience.

Due to the rave reviews from users for these earbud charging cases, many manufacturers of hearing aids and other small battery-powered devices have followed suit and designed battery cases with wireless charging capabilities for their products. The P9222-R is an ultra-compact, efficient wireless power receiver that can deliver up to 5W of power. Unique features such as ping detection make it ideal for low-power applications such as battery case charging.

Advanced Ping Detection at 100% Charge

The P9222-R Ping Detect feature on the PETB pin is critical to ensure that the receiver surface does not get hot at 100% battery charge. The PETB pin is open drain and needs to be pulled to the system 1.8V. As shown in the figure below, it indicates whether the receiving device is placed on the wireless transmitter. Once the P9222-R is placed on the transmitter, the PDETB pin is pulled low and then pulled high to the system 1.8V when removed from the transmitter (Tx).

CH1: Transmitter coil voltage, CH2: P9222-R enable pin status, CH3: P9222-R PETB pin status, CH4: P9222-R VOUT voltage

Battery charging case manufacturers who previously attempted to implement this functionality using the ENABLE pin on other wireless power receivers had difficulty knowing if the receiver was still on the transmitter board or had been removed from the transmitter board, which also caused confusion for the end user. When a wireless power receiver is disabled, it completely shuts down and stops all communication with the system application processor (AP).

With the P9222-R, the application processor can continuously monitor the PETB pin state even when the part is disabled and know if the receiver is still on the transmitter board or has been removed from it. Based on this information, the AP can send an end power transfer packet to the transmitter and disable the P9222-R when the battery reaches 100% charge state. If the PETB pin is continuously low (indicating that the P9222-R is on Tx) and the battery charge drops below a certain level due to system use, the AP can enable the P9222-R and the Rx will start responding to digital pings and return to the power transfer phase to replenish the battery.

In addition, the charging case has less mass than a smartphone, which can dissipate the heat generated during power transmission. If the earbud case is left on the transmitter pad overnight or for a long time, and the transmitter is not powered off when the earbud battery is at 100%, the case starts to heat up and can reach 40°C. A lot of energy is also wasted. Using the ping detection function, the AP can end the wireless power transmission phase when the battery reaches 100%. This will reduce the case temperature by nearly 10°C and improve the long-term reliability of the product.

Lowering UVLO rising threshold

Typical wireless power receivers, including the P9222-R, do not draw input power from the system voltage and are completely shut down when not placed over a transmitter. The wireless power receiver is awakened by the transmitter ping energy and begins communicating with the transmitter. After successful communication, the receiver enters the power transfer phase and turns on the output voltage. If the receiver is far from the center of the transmitter coil, the transmitter ping energy will not be enough to wake up the receiver because the voltage on the receiver's internal regulator will be below the undervoltage lockout voltage (UVLO) voltage. The low UVLO rising threshold allows the receiver to start up even with a weak digital ping signal from the transmitter over an extended area.

The P9222-R’s 2.5V UVLO rising threshold is 10–20% lower than other available products, including previous generation receivers from Renesas. Because of this, the P9222-R’s effective charging area is increased by approximately 20-30% compared to other products. A higher effective charging area is important for a better end-user experience.

Easily configure design parameters

The P9222-R features an integrated Arm microcontroller and firmware that provide great flexibility in customizing operating parameters for custom applications. The default values ​​for the P9222-R operating parameters, such as output voltage, FOD parameters, and current limit, are set in the firmware that is programmed into the internal One-Time Programmable (OTP) memory. Depending on the end application, the P9222-R operating parameters can be configured by writing to the internal SRAM registers through the I2C interface or by loading the user configuration generated by the P9222-R Windows GUI into an external EEPROM.

All these advantages of the P9222-R add great value to your next wireless charging enabled battery case design. The product is supported by EVK reference designs and extensive online materials to shorten time to market.