I am using the CP2102N and I have an external 3.3 V regulator that is used to supply 3.3 V to VREGIN and VDD. I have the VBUS pin connected to the USB 5 V. Do I need a voltage divider for VBUS to reduce the input voltage? Alternately, is connecting the VBUS pin to the 3.3 V regulator output a supported configuration, and if so, can the required voltage divider on VBUS be omitted?
The power configuration described above is that of a self-powered configuration, and requires a voltage divider on the VBUS pin. This power configuration, with the voltage divider, is shown in Figure 2.6 on page 6 of the CP2102N datasheet. Please note that any power configuration (self or bus-powered) in which the USB VBUS signal is connected to the CP2102N VBUS pin requires a voltage divider (as shown in Figure 2.6) on VBUS to avoid a possible overvoltage condition on VBUS (see https://www.silabs.com/community/interface/knowledge-base.entry.html/2018/03/31/cp2102n_requiresav-NFly).
As for the connecting of the VBUS pin to the same 3.3V supply connected to VDD and VREGIN, this configuration should be acceptable as long as the regulator on the CP2102N is bypassed (i.e. VDD = VREGIN = externally supplied 3.3 V). If the device is self powered with the internal regulator used to supply VDD (for instance if the USB 5 V VBUS signal is connected to VREGIN), the delay in the rise time of VDD may cause a violation of the maximum voltage rating on the VBUS pin, which is not supported. In these cases, the VBUS voltage divider should be used.