BQ500412 Datasheet, PDF (10/28 Pages) Texas Instruments – Low System Cost, Wireless Power Controller for WPC TX A6

bq500412
SLUSBO2A – NOVEMBER 2013 – REVISED DECEMBER 2013
www.ti.com
Communication
Communication within the WPC is from the receiver to the transmitter, where the receiver tells the transmitter to
send power and how much. In order to regulate, the receiver must communicate with the transmitter whether to
increase or decrease frequency. The receiver monitors the rectifier output and using Amplitude Modulation (AM),
sends packets of information to the transmitter. A packet is comprised of a preamble, a header, the actual
message and a checksum, as defined by the WPC standard.
The receiver sends a packet by modulating an impedance network. This AM signal reflects back as a change in
the voltage amplitude on the transmitter coil. The signal is demodulated and decoded by the transmitter side
electronics and the frequency of its coil drive output is adjusted to close the regulation loop. The bq500412
features internal digital demodulation circuitry.
The modulated impedance network on the receiver can either be resistive or capacitive. Figure 1 shows the
resistive modulation approach, where a resistor is periodically added to the load and also shows the resulting
change in resonant curve which causes the amplitude change in the transmitter voltage indicated by the two
operating points at the same frequency. Figure 2 shows the capacitive modulation approach, where a capacitor
is periodically added to the load and also shows the resulting amplitude change in the transmitter voltage.
Rectifier
Receiver Coil
Receiver
Capacitor
Modulation
Resitor
Amax
A(0)
Operating state at logic “0”
Comm
A(1)
Operating state at logic “1”
Fsw
F, kHz
a)
b)
Figure 1. Receiver Resistive Modulation Circuit
Rectifier
Receiver Coil
Receiver
Capacitor
Modulation
Capacitors
Amax
A(0)
Operating state at logic “ 0”
Comm
A(1)
Operating state at logic “ 1”
Fsw
Fo(1) < Fo(0)
F, kHz
a)
b)
Figure 2. Receiver Capacitive Modulation Circuit
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