Design Considerations

This version of QI is designed to pump out 5 watts.

To ensure that this maximum is not exceeded;

• the selected Li-Ion battery charger chosen supports current limiting.
• the output power supply's specification match

Since a Li-Ion Cell voltage ranges from 3.7 to 4.2 volts, output voltages above this will require a step up power supply.

Similarly for voltages below this will require a step down power supply.

To minimise the part count I have selected;

• BQ51013T Qi Receiver
• BQ24075T Li-Ion Battery Charger with dynamic power path Management (DPPM)
• LMR62014XMFE/NOPBLMR62014XMFE/NOPB A 1.4W Step Up Simple Switcher (up to a maximum of 20V) or LMZ10503 Simple Switcher  (for 0.8-5.0V)

The BQ24075T is a clever part. Besides being able to charge a Li-Ion battery;

• it has its own Dynamic Power Path Management (DPPM) that removes the need for an additional MOSFET, TPS2115APWTPS2115APW or similar power multiplexter.
• it provides current limiting control to limit the load upon its input power supply
• it's also got two indicator leds to indicate Charging and Power Good.

Here is a photo of the TI EVMs connected and working to prove the concept.

This particular configuration uses the LMZ10503 to produce 2.5V but this can be adjusted up to 5V to power the beagleboard or other device as required.

The blue battery (a 18650 Li-Ion cell with 2200mAh) looks like an AA cell but is much larger in size.

To fit into the case I will need to obtain a flat format battery like those used for digital cameras and phones.

Power comes out from the Simple Switcher outputs - These are not connected just for the photo.

Initial testing shows it that works a treat with seamless power switching.

• Without wireless power and with battery connected - the battery is powering the Simple Switcher and the output voltage from the Simple Switcher is correct is correct.
• With wireless power and battery connected - the wireless power is powering the Simple Switcher and (charging the battery as required) and the output voltage from the Simple Switcher is correct.