STWBC2-HP datasheet(17/96 Pages) STMICROELECTRONICS

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2.1.6

Quality factor driver

The purpose of the circuit is to drive the resonant LC network used in the wireless power transfer in order to

transmit a steady oscillation by which to measure its quality factor (Q factor).

When a Qi receiver is coupled, the Q factor of the network would decrease. Moreover, a foreign object

inadvertently and dangerously placed in the coupling region would absorb power from the transmitter causing

the Q factor to fall by even orders of magnitude.

The measurement is done by evaluating the amplitude and the period of the waveform so obtained on the ringing

node. The Q factor is first evaluated on the transmitter in absence of receiver during the final assembly test

procedure. The measurement needs not to be absolute but it is recorded in the device NVM as a calibration

point. The same measurement is repeated during normal functioning before starting the power transfer process.

By comparison with the stored value, the application can reveal the presence of a receiver or detect an undesired

foreign object.

Figure 22. QF driver

The analog driver is a square current generator with symmetrical source/sink programmable current strength.

During power transfer, the QF_DRV pin must be shorted to GND to avoid damage from the high voltage ringing

node. In this case the total internal/external QF_DRV pin resistance to GND has to be as low as to guarantee a

maximum voltage on the QF_DRV pin of ±300 mV.

The QF_drv signal is driven by the QF controller digital machine (see Section 2.2.3 ).

Table 12. QF driver operating parameters

Topic

Value

Current generator

250 µA to 4 mA, 16 steps

QF_DRV swing

20 mVpp min. (low Q)

1Vpp typ.

2Vpp max.

Ringing node voltage

20 mVpp min. (low Q)

1Vpp typ. (high Q)

STWBC2-HP

Analog AFE functions

DS13825 - Rev 1.0

page 17/96

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