2
Configuration guidelines
2.1
Changing VOUT voltage: constraints
The power LDO supports setting VOUT from 3.5 to 12.5 V; but, selecting an appropriate VOUT value is more
complex and involves other aspects in the system, like:
1.
OVP and margin for modulation
(especially when using VOUT higher than 9 V): the first line OVP protec-
tion is the pre-clamp with fixed trigger at 13.5 V on VRECT node. During modulation (packet data sent from
Rx to Tx), the voltage on VRECT rises on the basis of conditions like Tx/Rx coil parameters, loading current,
VOUT voltage and so on. The VOUT setting must be always low enough to maintain VRECT during modula-
tion under the pre-clamp level. The safe VOUT voltage for the
STEVAL-ISB042V1
evaluation board is 10 V.
The user should not set a higher value unless previously verified (via an oscilloscope) that the VRECT mod-
ulation has enough margin with respect to the pre-clamp threshold.
2.
Tx coil voltage and Tx/Rx coil ratio
: the whole system can be compared to a transformer where the coil
ratio defines the transformation ratio. The transmitter circuits and the Tx coil are designed to operate within
the expected optimal range in which the Rx coil and VOUT voltage should fit. If the configured VOUT voltage
is too high or too low, it shifts the whole system out of the optimal range. The right VOUT voltage for the
STEVAL-ISB042V1 evaluation board is roughly 4 to 5.5 V with 5 W transmitters and 8 to 10 V with 15 W
transmitters. Using a different output voltage may require a different Rx coil and input resonant circuit capac-
itors.
2.2
Input current limit
The power LDO is able to limit the output current. This limitation starts softly reducing the VOUT voltage even
before reaching the limit.
2.3
Minimal load
All wireless systems are designed to transfer power. If power is not being transferred, it becomes hard to maintain
Rx-to-Tx communication.
STWLC33
is equipped with a dummyload circuit that increases the load by consuming the power when no output
load is present. Due to heat dissipation the dummy consumption is limited to tens of milliamps.
Even if this should be enough to maintain communication with most transmitters, it is recommended to always
apply at least 100 mA.
UM2289
Configuration guidelines
UM2289
-
Rev 2
page 11/26