FT323R advanced connection problem

[1icarus1]

ft323r

MODERATOR ACTION: topic moved from microcontroller section to here Fragrance


Hallo,

I would like to use the FT232R it in 2 different ways:

1st: My circuit will supply the FT232R and an external device with 5V / 75mA and will comunicate with this via the USB connector.
2nd: Using the same connector I would like to remove the external device and connect a PC.

The problem is that once my circuit supplies the connector and the FT232R with 5V how could I connect a PC? The PC gives it's own 5V supply, so supplying the connector and the FT232R with my 5V regulator and simultaneusly connect a PC is a very critical connection.

Does anybody can help me on this issue?

Thanks.

 

[pico]

ft232r problem

If you want to isolate the 2 supply, you can use diode connect in sery with each supply rail.

 

[1icarus1]

ft323r usb

In this way how an external device could be supplied from my circuit board via the usb connector?????????

 

[Davood Amerion]

Use 1 series shottky diode at VCC line.
connect its anode to your circuit, and cathode to USB connector.

 

[1icarus1]

So what will be if the PC supply voltage will fall at 4,5V, which means 0,2V lower than the schottky cathode voltage????? It seems very critival.

ANY OTHER IDEAS?????

 

[Davood Amerion]

in this case, simply you can use current limit circuit

 

[1icarus1]

You know:

1. current limit circuits have drop out voltage and the output of this circuit is called to supply the external usb sensor when it is connected.
2. also the external circuit (sensor ) needs 75mA at 5V.

The first thing I decided to do is to use a small resistor in series but the problem is the dropout voltage. After this I tried an 140mA polyfuse (at 120 C) but there is the same problem.

ANY OTHER IDEAS????????????????

 

[kender]

ANY OTHER IDEAS????????????????

At the risk of being obvious: how about a separate power supply connector for the sensor? :|