Gopi Akkinepally
Wireless Power Marketing Manager
Published: October 1, 2019

True wireless stereo (TWS) earbuds are getting very popular as more and more mobile phones are removing the audio jack. By wearing tiny earbuds, consumers can conveniently listen to music, take phone calls, and avoid the tangle of wired headphones. It is a difficult design challenge to charge these tiny earbuds directly. Many manufacturers have developed battery cases the earbuds slide into and make contact with charging pins. Consumers will have a truly wireless experience when the battery cases can be charged wirelessly on any Qi-certified wireless charger or from the back of any Wattshare™-enabled phone.

Because of the rave user reviews of these charging cases for earbuds, many manufacturers of hearing aids and other small battery-powered devices are following suit and designing battery cases with wireless charging for their products. The P9222-R is an ultra-compact, highly efficient wireless power receiver that can deliver up to 5W of power. Unique features, such as ping detect, make it ideal for low-power applications such as battery case charging.

Advanced Ping Detect at 100% Charged State

The P9222-R Ping detect feature on the PETB pin is critical to make sure that the receiver surface does not get hot at 100% battery charge state. The PETB pin is an open-drain that needs to be pulled to system 1.8V. As shown in the figure below, it indicates whether the receiver device is placed on a wireless transmitter or not. PDETB pin gets pulled to LOW as soon as the P9222-R is placed on the transmitter and then pulled HIGH to system 1.8V when it is 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 tried to implement this feature using the ENABLE pin on other wireless power receivers had great difficulties knowing whether the receiver was still on the transmitter pad or removed from it and it also caused confusion for end-users. When a wireless power receiver is disabled, it completely turns off and stops all communication with the system application processor (AP).

With the P9222-R, even if the part is disabled, the application processor can continuously monitor the PETB pin status and know whether the receiver is still on a transmitter pad or has been removed from it. Based on this information, the AP can send an end-of-power transfer packet to the transmitter and disable the P9222-R when the battery reaches the 100% charged state. If the PETB pin is continuously low (indicating the P9222-R is on the Tx) and the battery charge goes below a certain level due to system usage, the AP can enable P9222-R and the Rx will start responding to the digital ping and go back into the power transfer phase to top off the battery.

Also, compared to smartphones, charging cases have a smaller mass to dissipate the heat generated during power transfer. If the earbud case is left on the transmitter pad overnight or for a long time and the transmitter doesn’t shut off power when the earbuds battery is at 100%, the case starts to get warm and can reach 40 °C. It will also be wasting a lot of energy. With the ping detect feature, the AP can end the wireless power transfer phase when the battery reaches 100%. This will lower the case temperature by almost 10 °C and increase the product’s long-term reliability.

Lower UVLO Raising Threshold

A typical wireless power receiver, including the P9222-R, does not take input power from the system voltage and is completely turned off when not placed on a transmitter. Wireless power receivers wake up with the transmitter ping energy and start communicating with the transmitter. After successful communication, the receiver enters the power transfer phase and turns on the output voltage. If the receiver is placed away from the center of the transmitter coil, the transmitter ping energy will not be sufficient to wake up the receiver because the voltage on the receiver’s internal regulators will be less than the under-voltage lockout voltage (UVLO) voltage. A low UVLO raising threshold allows the receiver to start up even with weaker digital ping signals from a transmitter over an extended area.

The 2.5V UVLO rising threshold of the P9222-R is 10–20% lower than other available products, including Renesas' previous generation of receivers. Because of this, the P9222-R has roughly 20-30% more active charging area compared to these other offerings. The higher active charging area is very important for a better end-user experience.

Easy Configuration of Design Parameters

The P9222-R has an integrated Arm microcontroller and firmware provides great flexibility to customize operating parameters for custom applications. Default values of the P9222-R operating parameters such as output voltage, FOD parameters, and current limit are set in the firmware programmed into the internal one-time programmable (OTP) memory. Based on the end application, the P9222-R operating parameters can be configured by either writing to internal SRAM registers via the I2C interface, or by loading the user configuration generated by the P9222-R Windows GUI into an external EEPROM.

All of these P9222-R benefits add great value to your next wireless charging-enabled battery case design. The product is supported by an EVK reference design and extensive online collateral to reduce time to market.

To get more details on the new P9222-R wireless power receiver, visit For more information on Renesas' complete portfolio of wireless power receiver and transmitter solutions, visit

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