Power supplies AN4938
8/48 DocID029918 Rev 1
• When the VDD33USB pin is available, this pin can be used to supply the internal
transceiver. In this case, VDD33USB pin should receive a voltage ranging from 3.0 to
3.6
V. If VDD50USB is also available, it must be connected to VDD33USB. As an
example, when the device is powered at 1.8
V, an independent 3.3 V power supply can
be applied to VDD33USB.
When VDD33USB is connected to a separate power supply, it is independent from V
DD
and V
DDA
. It must be the last supply applied and the first supply switched OFF. The
following conditions must be respected (see Figure 2):
– During the power-on and power-down phases (V
DD
< V
DD
minimum value),
V
DD33USB
should always be lower than V
DD
.
–V
DD33USB
rising and falling time specifications must be respected (refer to table
power-up/power-down operating conditions (regulator ON) and table power-
up/power-down operating conditions (regulator OFF) provided in the device
datasheets).
– In operating mode, V
DD33USB
can be either lower or higher than V
DD
:
If a USB interface is used (USB OTG_HS/OTG_FS), the associated GPIOs
powered by V
DD33USB
operate between V
DD33USB_MIN
and V
DD33USB_MAX
(see
Figure 2).
V
DD33USB
supplies both USB OTG_HS and USB OTG_FS transceivers. If only
one USB transceiver is used in the application, the GPIOs associated to the other
USB transceiver are still supplied by V
DD33USB
.
If no USB interface is used (USB OTG_HS/OTG_FS), the associated GPIOs
powered by V
DD33USB
operate in V
DD
range (between V
DD_MIN
and V
DD_MAX
).
In the above two configurations, an external capacitor must be connected to
VDD33USB.
• When neither VDD33USB nor VDD50USB is available, VDD pins are use to supply
USB transceivers and must be in the range of 3.0 to 3.6
V for the transceiver to operate
correctly.
Figure 1. VDD33USB connected to V
DD
power supply
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