I added the USB port on my new board with the CP2102 USB/serial converter 20pin
The power supply connection is the same as fig2.3 of CP2102N datatsheet rev1.1
VREGIN and VDD both connected to 3.3V. (Schematic added)
When power supply is off (no 3.3V supply) but with USB cable connected to PC I have some voltage on my 3.3V supply line.
Which is the correct connection schema for CP2102N that isolate 5V and 3.3V?
I would like to turn off my device even if the USB cable is connected and 5V are present
The VBUS pin is rated for VIO + 2.5V. When the power supply is not connected, VIO is effectively zero, and you are exceeding this rating. This has the potential to damage the part, as well as backpower VDD through the ESD diodes on VBUS. I recommend a resistor divider on the VBUS pin for self powered devices that can be connected to the USB bus when not powered.
@accacca_1 See Figure 2.6 USB Self-Powered Connection on page 6 of CP2102N Datasheet rev. 1.1
Thanks to all for reply
Yes I saw the Figure 2.6 but
VBUS must be > VIO -0.6 > 2.7V (VIO=VDD in 20pin device) for bus detection.
VBUS mst be below 2.5V while device is not powered.
Both conditions cannot be meet with the resistor divide.
The CP2102N is great for its small size and requires very few external components. However, I would not use the component in production without respecting the specification.
Among Silabs USB / serial devices is there one more suitable for use in self-powered peripherals that can be turned off?
The CP2102 (without the "N"), can tolerate 5.8V on VBUS even when powered off.
Ok Joe the CP2102 can be an alternative.
One more question on CP2102N:
If I use the bus powered configuration with isolated serial lines, I think this solve the VBUS voltage level problem. Can I drain few mA from Vdd pin (3.3 regulator output now) for opto interface ?
Yes, the voltage regulator (CP2102, or CP2102N) can provide up to 100mA, and it is capable of powering other devices on the board.