Datasheet

bq500212AEVM-550 Assembly Drawings and Layout

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6.2.2.7 Foreign Object Detection

The bq500212A EVM incorporated Foreign Object Detection (FOD) call in WPC 1.1. Power loss is

calculated by comparing the power sent to the receiver (RX) with the power the RX reported receiving,

less know power loss. The transmitter determines the power sent to the RX by measuring input power and

calculating internal losses. The RX measures the power it received and also calculates losses. The RX

sends this information to the driver (TX) in a digital word, message packet. Unaccounted for power loss is

presumed to be a foreign object on the charging pad. Should this lost power exceed the threshold set by

R24, a FOD fault is set and power transfer is stopped.

Three key measurements for the TX FOD calculation:

• Input Power – Product of input voltage and current. Input voltage is measured at Pin 46 though R10

and R11. Input current is measured using sense resistor R32 and current sense amp U7. Both

measurements need to be very accurate.

• Power Loss in Transmitter – This is an internal calculation based on the operating point of the

transmitter. The calculation is adjusted using FOD_Cal resistor, R99. This calculation changes with

external component changes in the power path such as MOSFETs, resonate capacitors, and TX coil.

Recalculation of R99 and R24 will be required.

• Receiver Reported Power – The receiver calculates and reports power it receives in the message

packet “Received Power Packet (0X04)”.

The FOD threshold on the EVM is set to 350 mW, R24 is set to 48.7 kΩ. Increasing R24 increases the

threshold and reduces the sensitivity to foreign objects.

This loss threshold is determined after making a measurement of transmitter performance using a FOD

Calibration Receiver similar to unit manufactured by Avid

Technology. Contact Texas Instruments for the

Foreign Object Detection Calibration Procedure for bq500212A.

6.2.2.8 WPC Certification

The bq500212AEVM-550 was tested and certified to WPC version 1.1.1, July 2013.

7 bq500212AEVM-550 Assembly Drawings and Layout

Figure 6 through Figure 11 show the design of the bq500212AEVM PCB. The EVM has been designed

using a 4-layer, 2-oz, copper-clad circuit board 13.2 cm × 7.24 cm with all components in a 4.0-cm x 5.0-

cm active area on the top side and all active traces to the top and bottom layers to allow the user to easily

view, probe, and evaluate the bq500212A control IC in a practical application. Moving components to

both sides of the PCB or using additional internal layers offers additional size reduction for space-

constrained systems. Gerber files are available for download from the EVM product folder.

A 4-layer PCB design is recommended to provide a good low-noise ground plane for all circuits. A 2-layer

PCB presents a high risk of poor performance. Grounding between the bq500212A GND pin 47, 36, and

32 and filter capacitor returns C19, C1, C5, and C3 should be a good low-impedance path.

Coil Grounding – A ground plane area under the coil is recommended to reduce noise coupling into the

receiver. The ground plane for the EVM is slightly larger than the coil footprint and grounded at one point

back to the circuit area.

Note: The clear plastic cover thickness (0.93 in or 2.4 mm) is the z-gap thickness for the transmitter.

bq500212A bqTESLA Wireless Power TX EVM SLVU928A–September 2013–Revised October 2013

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