Gopi Akkinepally
Wireless Power Marketing Manager
Published: September 9, 2020

Many premium smartphones now include WattShare™ (TRx) power sharing technology which allows a wireless power receiver device to become a transmitter and wirelessly charge another mobile device such as a phone, earbuds charging case, or watch. The P9415-R is the newest 15W wireless power receiver with WattShare mode. It features up to 5W of transmit power capability in WattShare mode when deployed in applications such as smartphones and power banks. There are endless possibilities with WattShare technologies to keep. Watch the WattShare™ Technology at Work – Easily Share Power with Other Qi-Compatible Phones video to see how WattShare technology on the latest smartphones can save the day.

Below are some hardware and software considerations for implementing WattShare technology with P9415-R in your next wireless power design:

Hardware Considerations

To implement the WattShare (TRx) mode using the P9415-R, as well as the standard 15W Rx mode components, the device needs load switches, a regulator and demodulator filter.

Load switch connection in Receiver (Rx) mode

When the P9415-R is placed on a wireless power transmitter such as P9243-GB, after successful negotiation the iP9415-R output LDO turns on and the VOUT pin goes to 12V or default voltage programmed in multi-time programmable (MTP) memory. The applications processor (AP) in the device turns on the load switch that connects the P9415-R VOUT node to the battery charger to charge the battery.


Load switch connection in WattShare (TRx) mode

When the user turns WattShare mode on in their device with P9415-R integrated, the AP in the device turns on the load switch which connects the regulator output to the P9415-R VOUT and turns on the TRx mode using I2C commands. The regulator provides the power that can be transferred to charge another receiver placed on the coil connected to the P9415-R.


Regulator input voltage in WattShare (TRx) mode

The P9415-R uses the same L-C tank in both Rx mode and TRx mode. The maximum power delivered in TRx mode is dependent on the input voltage on VOUT pin, coil characteristics such as AC resistance, and the friendly metal around the coil. Because the coil used for power transfer in TRx mode has high AC resistance compared to the transmitter coil used in a typical charging pad, the input voltage on the VOUT pin in TRx mode must be around 7V to 9V to deliver 5W of power to the receiver. So typically, devices use a boost regulator that boosts the battery voltage.

Demodulator filter

When a Qi-certified Rx is placed on the coil connected to P9415-R in TRx mode, the Rx sends WPC protocol packets to the P9415-R. Using the demodulation (DEMOD) filter connected to the resonant tank and internal demodulator circuitry, P9415-R decodes the WPC protocol packets sent. Based on the packet information, the P9415-R enters the power transfer phase, modulates the transmitted power by adjusting the operating frequency or duty cycle, and detects a foreign object between Tx and Rx.



System Software Considerations

AP needs to interface with the P9415-R using I2C commands to implement WattShare mode. Below are some of the system-level software modes the AP needs to implement to ensure a good user experience.

Power saving

After AP enables TRx mode, the P9415-R starts sending digital pings and waits for the user to place the receiver device for charging. If the user accidentally enables TRx mode and doesn’t place a receiver device on the coil connected to P9415-R, the device battery continues to drain. Or in TRx mode, if the user removes the receiver device from the coil after the charge completion, the P9415-R exits the power transfer phase and starts sending the digital ping and continues to drain the device battery. AP needs to implement proper power saving timers to avoid battery drain in these situations.

Thermal throttling

Sometimes when the coil coupling between the receiver device (Rx coil) and the coil connected to P9415-R (TRx coil) is bad, the TRx coil and the device surface might get hot. AP can continuously monitor the device temperature and reduce the amount of power transferred to the receiver device by adjusting the input current limit.

Tx conflict fault protection mode

When the P9415-R is in TRx mode, users might accidentally place the P9415-R on another WPC-certified transmitter. The pings from the WPC transmitter will generate an AC voltage in the resonant tank connected to the P9415-R. The P9415-R detects this voltage, sends an interrupt to the AP, and exits TRx mode. The AP must disable the external power supply connected to the VOUT pin of the P9415-R to avoid back powering the regulator device.

For more information on P9415-R including product features and documentation, please visit the P9415-R product page. For additional information on power sharing with WattShare technology, please visit the WattShare™ Wireless Charging Technology page.

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