Using NXQ1TXH5 in pairs

[damien_genouill]

Hello NXP Team,

I'm working on a product working as a pair of devices. Each device is charged using one wireless chargind board based on NXQ1TXH5 (aka "charger board"), the design is very close to the reference design (except FOD that is disabled).

Both charger boards are powered with the same USB port (the cable has a Y shape).

Each charger works fine individually, however when I use both chargers at the same time (both charger boards LED indicating an on-going power transfer) and then remove one from a receiver device, its LED stays on as if it was still attached to the device. The LED only goes off when I remove the second charger from the second device. 

Since there is no other link except power (USB 5V & ground) from one charger board to the other, it looks to me like the charger that is not attached to the receiver interprets the messages sent from the other charger that is still attached to the second device as if it was for him.

Looking at the 5V line, I can see around 250mV of noise that I believe matches the communications of the other charger (frequency of the larger pulses is around 2 to 4 kHz):

Do you have any idea of what could be done to have two NXQ1TXH5 working together on the same power supply?

Thank you.

Damien

[TomasVaverka]

Hello Damien,

We would recommend to use a better power supply: a voltage source is supposed to keep the voltage constant regardless of the current that is drawn. If you have a 250mV ripple on a 5V supply, then this is 5% which is not a good voltage source.

With the NXQ1TXH5DB1401 demoboard connected to a USB adapter, there are Qi communication signals in the supply current, but not on the supply voltage:

I hope it helps!

Best regards,

Tomas

PS: If this answer helps to solve your question, please mark it as "Correct" or “Helpful”. Thank you.

[damien_genouill]

Hello Tomas,

Thank you for taking some time to study my case.

The power is definitely good enough, however the cable serial resistance & decoupling quality might not be. 

I'll run some more tests to confirm.

In the meantime, could you please explain to me the details of how the IC decides to stop a tranfer, and what are the roles of ASEN and VSEN pins in detection/transfer?

Thank you very much,

Damien

[TomasVaverka]

Hello Damien,

Yes, using a proper cable is also very important as mentioned in the AN1175.

The NXQ1TXH5 stops charging when it receives a “End Power Transfer” packet from a receiver.

The voltage on the VSEN input is a measure for the current through the coil. It is used for the calculation of the FOD power.

The voltage on ASEN pins is used for decoding the ASK signals from the receiver.

Best regards,

Tomas