To maintain rectifier stability and ASK communication reliability , it is recommended to provide a certain minimum
load to the rectifier. STWLC38 is equipped with Iload ballast function which draws sufficient current from the
rectifier when the output load current is too small.
The rectifier current consists of the Iload ballast current, the output load current and the IC consumption current.
The Iload ballast current is automatically adjusted to maintain the minimal total load current of the rectifier, as set
in the RX ILOAD BALLAST register.
Naturally when the sum of the output load current and IC consumption exceeds the RX ILOAD BALLAST
threshold value there is no additional current sinked by ILOAD BALLAST block and it is disabled and will be
automatically re-enabled when the total current would drop below the threshold.
5.5
Chip under voltage lockout
STWLC38 is equipped with an under voltage lockout, which triggers when the VAA voltage drops below 2 V,
which results in the device being powered down. Normal operation is resumed as soon as the VAA voltage rises
above 2.1 V.
5.6 Chip enable
The device can be disabled (forced into reset) by pulling the chip enable signal high. When the chip enable signal
is released and left to be pulled down to ground, the device resumes normal operation. If the STWLC38 is in reset
mode, then AC1 and AC2 are connected to GND to protect the device.
5.7 Power up sequence
When the STWLC38 receiver is placed onto a power transmitter, and the patch and configuration files are loaded
into the device, it will initiate communication with the transmitter. After completing all the necessary steps
described in the Qi specification, power transfer is established.
If Patch and/or Configuration files are not loaded, the device will not initialize the communication, therefore power
transfer will not be established. Please refer to Section 4.6 Programming the device for device programming
instructions. The communication will also not be initialized if either the Patch or Configuration or both files are
corrupted.
During the initial phase of the power transfer STWLC38 operates in ARC mode, ST’s proprietary mode, which
makes the power-up sequence smoother and more reliable. For more information, see Section 5.12 Adaptive
rectifier configuration (ARC) mode .
Once the VRECT voltage rises high enough, the internal power management system starts operating. The digital
core is powered up and ready to control the internal circuitry. Default device settings are used until the digital core
is woken up after which the firmware loads settings saved in the Cofiguration file saved in NVM. The device is
now ready to operate.
5.8
LDOs
The device is equipped with two internal low drop-out regulators (LDOs) – a 2.5 V and a 1.1 V one, with the later
deriving its power from the former.
The 1.1 V LDO powers the digital part of the IC, while the 2.5 V LDO powers the analog part of the IC but can
also be used to power external low-power circuitry (such as LEDs). The maximum current externally drawn from
this LDO shall not exceed 20 mA.
• External LDO capacitors should be placed as close to the IC as possible.
• Connect a 1 μF filtering capacitor between the VDD pin and ground.
• Connect a 4.7 μF filtering capacitor between the VAA pin and ground.
5.9
Protections overview
STEVAL-WLC38RX board uses both hardware and software protections to ensure safe operation of both the
device and the board. The purpose of those protections is to avoid damage caused by unexpected operating
conditions – over-voltage and/or over-current. Temperature is monitored as well – the device is equipped with
both internal temperature sensor and pins for external NTC connection.
The majority of software protections can be configured in a GUI and/or by a host controller.
Each software protection can also be configured to disable the device's output and force the receiver to send an
EPT (end power transfer) packet when triggered. If the transmitter receives this packet, it should terminate power
transfer immediately.
UM3154
Chip under voltage lockout
UM3154 - Rev 2
page 16/81
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