6.2

External components selection

RX series resonant circuit components

The design of the receiving series resonant circuit, namely the receiving coil (Ls), the resonant capacitor (Cs)

and the detection capacitor (Cd), is out of the scope of this document. In principle the resonant capacitor and

the detection capacitor are easily calculated via simple equations involving standardized (Section 6.4 [1],[2]) or

customized resonance frequencies. The receiving coil is certainly the bulkiest and most critical component and

it will be assumed mechanically and electrically defined for the target application. A coil showing a high quality

factor is synonym of good power transfer performance.

Being essential part of the series resonant circuit, both Cs and Cd should show excellent quality factor, relatively

high RMS current capability and superior capacitance stability in the frequency range of interest. Thanks to their

inner structure, Multi-Layer Ceramic capacitors (MLCCs) are inherently good devices in terms of RMS current

capability and quality factor. Capacitance tolerance and stability strongly depend on the dielectric type: NP0, that

shows good characteristics, is unfortunately not suitable for compact applications, since typical values in the order

of hundreds of nano-Farads are not available in small packages.

Therefore, other dielectric types (like X5R, X7R and similar), are used to achieve higher capacitance per volume

at the cost of lower accuracy and undesired dependencies (e.g. DC-biasing, temperature, etc.). In practice, the

most critical Cs usually consists of few smaller, low-profile and X5R/X7R dielectric-type capacitors in parallel. The

parallel connection also helps in increasing the RMS current capability and in mitigating the effect of capacitance

tolerance due to production spread. The voltage rating for these capacitors is usually maximized to take into

account the voltage developed in proximity of resonance: 50V-rated capacitors are generally a good choice.

ASK modulation capacitors

The capacitors at the COMM1/COMM2 pins are connected to the AC1-AC2 terminals through controlled switches

(ASK modulator): the de-tuning effect of closing these switches results in an amplitude modulation detected by

the transmitter and also visible at the rectified voltage. Positive or negative modulation may occur, depending on

the operating frequency and other factors. The ASK modulation index clearly depends on the capacitance value

of these capacitors, whose value has to be adjusted in case of heavy negative modulation at VRECT (that is

generally undesirable). The same considerations made above for the resonant capacitors is also applicable here,

where capacitance tolerance is less critical: X5R dielectric-type are a good choice and an initial value of 47 nF is

typically doing the job. CLAMP1 and CLAMP2 pins are basically a replica of COMM1 and COMM2: their function

is providing a deeper ASK modulation under particular operating conditions and their activity can be controlled

runtime.

VRECT over-voltage clamping resistor

The voltage at the VRECT pin is primarily dictated by the transmitter, whose operating point is linked to the

feedback information received via ASK modulation. Unexpected conditions, however, may increase the VRECT

voltage to dangerous values (proximity to AMR levels). A sudden change in relative alignment between the

transmitting and receiving coils, for example, could result in a dramatic change in coupling factor and, in turn, in a

fast-rising voltage. Since the reaction of the transmitter is relatively slow, the STWLC68 protects itself by closing

the switch internally connected to the IEXT pin. This switch is externally connected to VRECT via a resistor

place in it during the clamping action, rather than inside the chip. Special resistors (surge resistors) capable of

withstanding higher energy pulses are recommended.

ESD protection diodes

Since the receiving coil is an easy entry point for ESD (relatively large area with remarkable capacitive coupling),

a good application design should consider protections for the most exposed pins: AC1 and AC2. Uni-directional

Transient Voltage Suppression (TVS) diodes at both pins is the right choice. ESDs have essentially a common-

mode nature and, although the receiving coil has low DC-resistance, its AC impedance may appear quite high to

fast voltage spikes: independent clamping at AC1 and AC2 pins is thus madatory. The knee-voltage of the TVS

diodes should be selected considering the maximum VRECT voltage plus some margin to avoid non-negligible

leakage current at higher temperature, while their energy dissipation capability should be maximized considering

the size of the package.

STWLC68

External components selection

DS13131 - Rev 3

page 24/32