To avoid damage to the CP2102N device, a resistor divider MUST used in all power supply configurations when the VBUS pin is connected to a 5V USB VBUS supply. This is true whether the device is self-powered (i.e. VREGIN and VDD supplied from an external power supply) or the device is bus powered (i.e. VREGIN supplied from 5V USB VBUS and VDD is used as the 3.3 V internal regulator output.
There are two electrical specifications for this device that come into play in these situations. The first is the absolute maximum voltage requirement for all GPIO pins (including VBUS), given as VIO - 2.5 V in Table 3.10 on page 15 of the CP2102N datasheet, Rev. 1.2:
The second is the input high voltage for GPIO pins (including VBUS), which must be met for VBUS to accurately sense the device connection to USB. This spec is given in Table 3.7 on page 13 of the CP2102N datasheet, Rev. 1.2:
On devices with no VIO pin, then VIO = VDD for the purposes of the above specifications. The voltage divider requirement is necessary for self powered configurations to avoid exceeding the absolute maximum VBUS voltage specification in the case where the device is not powered but connected to USB. The voltage divider is necessary for bus-powered configuration in which the internal regulator is used in order to avoid damage to the device when VBUS = VREGIN = 5V but the regulator output (VDD) has not yet come up to 3.3 V.
The following diagrams show the correct use of the voltage divider with the recommended resistor values of 22.1 kOhm and 47.5 kOhm between USB VBUS supply and GND.
Self Powered mode:
Bus powered mode:
This information has been updated in the device datasheet and is reflected in the CP2102N datasheet, revision 1.2.