BQ500212A: Wireless Tx with BQ500212A and small coil

Hello

The Wurth 760308101208 is a small 10mm coil, this is high risk.  This coil will have problems at start up due to low digital ping voltage.

1.) I know that I need to change the capacitors C27-C30 to get a frequency ~150kHz-- Bill J - C27-C30 should be tuned to 100kHz will the TX coil L value

2.) Does the FOD threshold need modifications? -- Bill J -- If you see an FOD fault then increase threshold

3.) Is there any other functionality that might need changes with the smaller coil? -- Bill J -- this should be OK

4.) Does this circuit seem adapted for the small coil overall? -- Bill J - if the coil L value is kept the same design should scale.

5.) I noticed some resistor values (in the part connected to usb) change between these two ref. designs, what do these depend on? -- Bill J - OK to use sluuap8, without network.  Pull pin 45 to 3.3V.

• sluuap8
• slvu928c

Hello Bill, thank you for your response.

- I tuned the capacitors to obtain 100kHz with the TX coil

- Based my design on sluuap8, an wired pin 45 to 3V3_VCC

I now have some questions regarding my layout/PCB design. Here is my current schematic (adapted sluuap8):

My PCB layout is as follows.

Top layer:

It contains all the components, most of the traces, and a large part of AGND. FOD resistors placed on the border for easier tuning. 

Inner layer 1:

This layer is mainly AGND, components that I could not connect to the top layer AGND are connected to this layer through vias in the pads.

Inner layer 2:

It contains the remaining traces that I was not able to route on the top layer. Does not have a ground plane.

VIN follows the drawing in red, 3V3_VCC in blue, COMM+/- in purple and the DPWM-1A/B in yellow.

Bottom layer:

It contains the GND plane, components that need it are connected through vias. The GND plane is connected to AGND with a single 0Ohm resistor.

My questions are:
- Is it okay to have AGND on two layers, and linked with suture vias?

- Should the VIN, 3V3_VCC and DPWM traces be shorter, or can it work fine this way?

- All the traces have a width of 10mil. Does any trace (or all) need to be thicker?

- Does the layout seem okay, or am I missing something obvious?

PS. I know that the Qi standard requires specific types of coils, and tuning capacitors to be 400nF. However I do not need it to be Qi certified and I'm okay with lower efficiency due to the smaller coils I would like to use.