Coil thermal protection

Maximizing the power transfer is often desirable, but not always possible in all operating conditions: applications

in which the wireless power receiver has poor power dissipation capability and/or the power loss in the receiving

coil is relevant (e.g. very tiny and slim coils with relatively high DC-resistance) may require some thermal

protection. This feature is easily implemented with STWLC68 thanks to its NTC pin. A channel of the internal ADC

is routed to the NTC pin, allowing the user to read the voltage across an external NTC thermistor.

Defining a protection threshold is clearly possible by setting the internal registers of STWLC68. The voltage at the

NTC pin is sampled, converted and compared to the threshold every millisecond: if an over-temperature occurs

and the corresponding interrupt is enabled, the INT pin goes low. Additionally, one of the following actions (or

both) may take place according to the selected option.

1.

Main LDO linear regulator output disconnection;

2.

End-of-Power-Transfer packet transmission

The value of the NTC thermistor and the high-side resistor (R6 in Figure 15) are not critical. An example for R6

and R7 is shown in Table 7: with these parts the 0.6 V threshold at the NTC pin is crossed at about 65°C.

6.3

PCB routing guidelines

1.

Auxiliary 1V8 and 5V0 LDO filtering capacitors should be placed as close as possible to the STWLC68.

Connection traces should be short and placed in top layer. Capacitors ground can be connected directly into

GND plane.

2.

3.

Power traces (AC1, AC2, VRECT, VOUT) should be kept wide enough to sustain high current. Duplicating

these traces in inner layers is advisable wherever possible.

4.

AC1 and AC2 tracks should be routed closely to minimize the area of the resulting loop antenna.

5.

Thermal performance and grounding should be always optimized by preserving bottom layer (usually

assigned to ground) integrity.

6.4

References

1.

The Qi Wireless Power Transfer System; Power Class 0 Specification; Version 1.2.2

2.

PMA Inductive Wireless Power Transfer Receiver Specification - System Release 1

STWLC68

PCB routing guidelines

DS13131 - Rev 3

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