4.11.2 Wireless power interface
Wireless power interface is the area in which power transfer takes place. It consists of two parts: the transmitter
(primary) power interface and the receiver (secondary) power interface. The main component of the interfaces is
the transmitting/receiving coil. The user should avoid placing any magnetically active objects that are not required
for the power transfer on or near the wireless power interface to avoid possible damage or injury (see
Section 4.9: Foreign object detection (FOD)).
First, the device performs an analog ping. This is done to detect the appearance of magnetically susceptible
material on Tx coil. Detection of a valid receiver on the primary wireless power interface is performed by a digital
ping. Digital ping is a burst of power generated by the transmitter; its parameters are defined by the Qi
specification. The target of this burst is to generate enough power for the receiver to establish communication
between the transmitter and the receiver.
The Qi specification defines a specific way that the receiver is expected to answer the digital ping. The reason is
that only a proper answer can inform the transmitter that a valid power receiver was placed on the wireless power
interface.
4.12 Bidirectional communication
The amount of power transmitted is (from a control standpoint) fully dependent on the operating conditions of the
power transmitter (i. e. its bridge voltage, operating frequency, and duty cycle).
Since there is no direct electrical feedback from the load (system output) to the power transmitter, the power
receiver must establish communication with the power transmitter to provide this feedback instead.
To match load requirements and prevent excessive power transmission, the power receiver communicates to the
power transmitter the required power level using amplitude shift keying (ASK). This is done by the receiver
modulating the amount of power that it draws from the power signal. The transmitter detects this as a modulation
of the voltage, current, and/or phase on the transmitting coil and adjusts its operating conditions accordingly.
The power transmitter can communicate to the receiver as well, this time using frequency shift keying (FSK). This
is done by the power transmitter directly modulating the power signal. Communication in this direction enables
proprietary communication and is required for power transfer in the Extended Power Profile or STSC profile.
In STWBC2-HP firmware, the ASK demodulation and FSK modulation is implemented in compliance with the Qi
specification. The processing of ASK packets and possible FSK reply are done autonomously. Any proprietary
communication above the scope of Authentication or STSC that are implemented in FW needs to be implemented
in the custom FW and ST support team needs to be contacted for such support.
4.12.1 ASK communication
A state (either High or Low) is characterized by the amplitude being constant (with a certain variation Δ) for at
least 150 µs.
If the power receiver and the power transmitter coils are properly aligned, then for all appropriate loads at least
one of the following three conditions shall apply.
• The difference of the amplitude of the transmitter coil current in the high and low state is at least 15 mA.
• The difference of the amplitude of the transmitter coil voltage in the high and low state is at least 200 mV.
• The difference of the transmitter coil current in the high and low state is at least 15 mA. The transmitter coil
current is measured at instants in time that correspond to one quarter of the cycle of the control signal
driving the half-bridge inverter.
UM3286
Device description and operation
UM3286 - Rev 1
page 14/84
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