The modulation capacitors used in STEVAL-WLC38RX were chosen to ensure good communication performance
under all operating conditions. Using a custom board design may require changing the COMA and COMB
capacitors.
The COMA and COMB capacitors are used for power signal modulation. Although the modulation period is short,
the voltage drop it creates across the LDO must be considered, as this drop further increases overall losses of the
system. This is particularly important when the device is delivering a high load current.
The GUI allows the customer to enable a particular pair to be active during low output current operation, while
using the other and/or same during high current operation. The same can be set for low/high VRECT voltage
condition. Therefore, four separate capacitor combinations can be selected in total.
Figure 62. Rx ASK configuration
Table 12. Default STWLC38 ASK configuration
Setting
Low Current (LC) Threshold Normal Current (NC)
Low Voltage (LV)
COMA enabled
Low Current to Normal Current:
200 mA
Normal Current to Low Current:
120 mA
COMA disabled
COMB enabled COMB enabled
Threshold 8V 8V
High Voltage (HV)
COMA enabled COMA disabled
COMB enabled COMB enabled
The following screenshot shows an example of RX Vrect (yellow), RX COMA (blue), RX COMB (red) and TX
COIL sensing (orange). In the picture you can see how the modulation capacitors affect the Vrect voltage and coil
sense voltage of a coupled transmitter - used for ASK demodulation.
The modulation effect shall be considered in regards to the overall power loss, as the LDO voltage drop (Vrect -
Vout) multiplied by the output current adds an addition power loss into the system.
Changing the modulation capacitor value may have an impact on the TX coil voltage. This voltage is used for ASK
demodulation circuit. The impact depends on various variables such as coil, gap, voltage, transfered power,
misalignment and another parameters.
UM3154
External component design
UM3154 - Rev 2
page 56/81
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