CP2102: diode before REGIN/VBUS inputs?

[dornovelk]

While diving into the schematic for the CP2102 USB/UART today I realized I made a mistake on my PCB design and didn't connect the VBUS pin coming off of the USB connector directly to the REGIN/VBUS pins on the CP2102 as instructed on the datasheet. Instead, I accidentally connected both of these pins to the VBUS line after it went through a protection schottky diode (Per the datasheet, REGIN & VBUS both expect 5V, and VBUS does indicate that it should be connected directly to the VBUS net coming off USB). Looking at the datasheet for the diode, forward voltage drop at my worst case current is 470mV. For context, after the diode, the 5V rail is connected to the 5V input pin of a voltage regulator.

I haven't received the parts yet, but would this be a showstopper and require revision? Or could I get away with the design as is for testing purposes? (this is just a prototype).

Thanks!

[janoc]

That could well be a showstopper because USB VBUS is defined to be 5V +-5%, so it could be anywhere from 4.75 to 5.25V. If you are at the bottom end of that range, add voltage drop of the cable and with the extra voltage drop of the diode you could be easily below the 4V minimum input voltage for the REGIN pin. In that case all bets are going to be off. That's going to be a very marginal design. Do you really need that diode there?

[dornovelk]

That could well be a showstopper because USB VBUS is defined to be 5V +-5%, so it could be anywhere from 4.75 to 5.25V. If you are at the bottom end of that range, add voltage drop of the cable and with the extra voltage drop of the diode you could be easily below the 4V minimum input voltage for the REGIN pin. In that case all bets are going to be off. That's going to be a very marginal design. Do you really need that diode there?

Thanks for your response! I was leaning towards removing the diode from the circuit for now and just letting USB VBUS go directly to the input pin of the AP2114H-3.3 & directly to the CP2102 pins (per datasheet requirement).