5.5

Protections

Over-voltage protection

The STWLC68 integrates different Over-Voltage Protection circuits to protect itself, the load connected to its

output rail and the external components from damage due to overheating and/or exceeding AMR condition.

Under normal operating conditions the voltage at the output of the synchronous rectifier is slightly higher than the

output one thanks to the communication with the transmitter. A sudden change in the coupling factor between

transmitting and receiving coils, for example due to abrupt reciprocal repositioning of the coils, easily leads to

unpredicatable voltage peaks at the AC input terminals: the TX-RX regulation loop is not fast enough to prevent

such an event and additional precautions must be taken.

Both the VRECT and VOUT outputs are constantly monitored. The VRECT rail has three concurrent OVP

mechanism: two fixed, analog ones based on a HW comparator (OVPH and Fixed-OVPS) and a digital ones (Adj-

condition occurs, the protection circuit immediately shorts both AC1 and AC2 pins to ground. This clamping action

is released as soon as the voltage at the V5V0 pin falls down below its power-good level (4V typ.). Normally (it

actually depends on the output load) the transmitter reacts to this protection by cutting-off the power transfer.

Both OVPS (Fixed and Adjustable) protections activate a different mechanism compared to OVPH: the IEXT

pin goes low and an external resistor starts drawing current from VRECT (see block diagram). The digital OVP

protection (Adj-OVPS) has an adjustable threshold and can be taylored for the application. In most of cases both

soft OVP protections do not interrupt the ongoing power transfer, but a significant energy could be dissipated in

the external resistor and a careful design is required. Optionally, the STWLC68 can be programmed to issue an

End-of-Power-Transfer (EPT) packet to the transmitter on any OVP event.

Over-temperature protection

The STWLC68 is equipped with three over-temperature detection circuits based on different sources:

1) Synchronous rectifier temperature sensor

2) Main linear regulator temperature sensor

3) Coil (external NTC) temperature sensor

The signals coming from the two internal temperature sensors and the NTC pin (analog input) are conditioned

and routed to the multi-channel ADC. The temperature of both the rectifier and the linear regulator can be directly

read via dedicated registers. The external sensor, typically placed very close to the coil to prevent over-heating,

is often a low-sided NTC of a resistor divider whose center tap is connected to the NTC pin, while the high-side

resistor is connected to the V1V8 pin.

Reading the temperature of the external sensor requires some calculation from the host, since it depends on the

NTC characterisitcs (beta, nominal value), the high-side resistor and the input range of the NTC pin (0 V - 1.5 V).

The internal sensors are also monitored and compared to a fixed 125°C threshold (10°C hysteresis) to trigger a

HOVP event (AC1 and AC2 shorted to ground).

In addition, the STWLC68 allows the user to define for each source a threshold with associated set of actions.

The selectable actions are any combination of:

•

EPT packet generation

•

Output disconnection

•

INT pin status change

For example, a chip over-temperature alert could be sent to the host system by setting two temperature

thresholds (e.g. 100°C for the rectifier and 90°C for the linear regulator) and linking the INT pin to the related

interrupts.

Over-current protection

As mentioned in the description of the main linear regulator, the current through the pass transitor is sensed and

limited to a fixed 1.5 A threshold. An internal interrupt is generated in such a case and specific actions could

be linked to it. In any case, a severe over-load condition quikly leads to an output voltage drop: if VOUT drops

below a fixed 1.25 V threshold (e.g. in case of output short-circuit), the linear regulator is disabled an an interrupt

is generated. A programmable threshold allows the user to define an early over-load alert interrupt and a set of

associated actions.

STWLC68

Protections

DS13131 - Rev 3

page 11/32