STWBC2-HP datasheet(37/96 Pages) STMICROELECTRONICS

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2.2.3.4

Programming

The QF controller builds an oscillating loop through the LC tank of the transmitter. To do so, the controller drives

the analog current generator polarity synchronously with a polarity feedback of the LC tank. The polarity feedback

is typically made with a voltage zero-cross detector probing the common node of the L and the C. When the QF

controller runs a normal case, the resulting oscillating frequency is equal to the self-resonant frequency of the LC

tank.

2.2.3.4.1

Oscillation width

The QF controller limits the oscillating period to a minimum and maximum value. This allows the locking of the

oscillation.

•

The minimum period is set in the QF_BLANKING_TIME registers. The blanking value corresponds to the

minimum time between each toggling of the current generator. Hence, the programmed value corresponds

to the minimum half period of the oscillation.

•

The maximum period is set in the QF_TIMEOUT registers. The time-out value corresponds to the

maximum time between each toggling of the current generator. Hence, the programmed value corresponds

to the maximum half period of the oscillation. When the time-out time is reached, the current generator

polarity is forced to toggle. If the LC tank polarity comparator toggles between the blanking and the time-out

times, the QF controller toggles the current generator polarity.

If the zcd_det signal coming from the zero-cross comparator toggles between the blanking and the time-out times,

the QF controller toggles the current generator polarity QF_drv

2.2.3.4.2

Cycle time oscillation

The QF controller tracks the time taken between each toggling of the current generator polarity. It sums the

time taken when polarity is low and the time taken when polarity is high. The results are available in the

QF_CYCLE_TIME registers. The cycle_time value corresponds directly to the period of oscillation.

The timing of the QF controller is expressed in clock cycles of the selected source operation.

The drive current is tunable in the QF_ISET register.

2.2.3.4.3

Programmable current level

The drive current is programmable through the QF_ISET register. Depending on losses of the tank, the user has

to set the correct current level to make the LC network oscillate around 1 Vpp.

2.2.3.4.4

Enable sequence

The QF controller requires the following configuration sequences:

Selection frequency operation

Program the blanking and time-out times. Typically the blanking time is 0.75 times the typical half period of

the LC tank and time-out is 1.25 times the typical half period of the LC tank.

Set the current generator drive current as needed for the tank.

In the QF_CTRL control register, clear the SHORT bit, choose the POLARITY needed and set the ENABLE

bit (this can be done in a single access). The POLARITY bit inverts the polarity of the current generator to

adapt to an inverted configuration of the tank.

Wait for the oscillation to lock (1 ms to 10 ms).

Get the cycle time measured by the QF controller from the corresponding register.

2.2.3.4.5

Disable sequence

The disabling of QF controller consists in writing the QF_CTRL control register with the ENABLE bit cleared.

The current generator injects current through a capacitor connected to the LC common node. During power

transfer, this LC node rings up to 200 Vpp.

To avoid damaging the QF current generator due to this high voltage, the QF driver pin of the STWBC2-HP

must be shorted to GND during power transfer phase.

The on-chip NMOS short switch is enabled by setting the bit SHORT in the QF_CTRL control register.

STWBC2-HP

Digital AFE functions

DS13825 - Rev 1.0

page 37/96

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