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Topic: Understanding the Uno's USB Isolation Circuit (Read 2 times) previous topic - next topic

retrolefty

Quote
So 6.6v is the cut off point for Vin to power the device.


Not really a cut off point, as the board will still be powered by whatever the on-board +5vdc regulator is putting out even if Vin is below the 'input regulation drop-out' spec of the regulator. So Vin is never cut in or cut out, just USB is cut in or cut out if Vin is below or above 6.6vdc respectively. The FET circuit switching action is SPST not SPDT.

Lefty

UNTEngineer



It detects the presence of Vin and switches power supplies: U1A is a comparator. It takes Vin, divides it by 2 and compares it to 3.3v. If Vin / 2 is greater than 3.3v (aka Vin > 6.6v), U1A outputs 1 and turns off T1, which isolates USBVcc from U2. So U2 is powered by +5v (which hopefully is powered by Vin at this point).

If Vin / 2 is less than 3.3v (aka Vin < 6.6v), U1A outputs 0 and turns on T1, which switches in USBVcc.

So 6.6v is the cut off point for Vin to power the device.

They could have done a better job around U1A.


But what if my Vin is a regulated +5V? And why are they comparing the Vin with 3.3V?
Working on a Smart Home Management System. Visit my blog: http://tae09.blogspot.com/ to find out more, and to keep track with the project.

retrolefty




It detects the presence of Vin and switches power supplies: U1A is a comparator. It takes Vin, divides it by 2 and compares it to 3.3v. If Vin / 2 is greater than 3.3v (aka Vin > 6.6v), U1A outputs 1 and turns off T1, which isolates USBVcc from U2. So U2 is powered by +5v (which hopefully is powered by Vin at this point).

If Vin / 2 is less than 3.3v (aka Vin < 6.6v), U1A outputs 0 and turns on T1, which switches in USBVcc.

So 6.6v is the cut off point for Vin to power the device.

They could have done a better job around U1A.


But what if my Vin is a regulated +5V? And why are they comparing the Vin with 3.3V?


  Vin is an 'official' shield pin name for the 7-12vdc input for external DC power that which then only feeds the on-board +5vdc voltage regulator and the resistor divider feeding the comparator opamp. Vin is the same as the voltage coming from the external DC power connector except for a series polarity protection diode added between the connector and the Vin pin. A regulated +5vdc should not be wired to the Vin pin because that would be too low a voltage to feed to the on-board +5vdc output voltage regulator. If you have an external regulated +5vdc power supply you would wire it to the board via the shield 5V pin and ground pins (but you then have the non-isloation situation with USB power we already talked about). The 3.3vdc is used as a reference switching voltage for the comparator just because it's available and allows them to use equal size resistors for the voltage divider.

Lefty

dhenry

Quote
But what if my Vin is a regulated +5V?


That has been answered.

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And why are they comparing the Vin with 3.3V?


You have to ask the Arduino people for sure. The voltage regulator they used isn't an ldo and its drop out voltage ranges from 0.7v (low/no load) - 2v (rated load), so I guess that they thought a 1.6v headroom is adequate.

UNTEngineer

#9
Dec 02, 2012, 05:27 am Last Edit: Dec 03, 2012, 11:03 pm by UNTEngineer Reason: 1

 Vin is an 'official' shield pin name for the 7-12vdc input for external DC power that which then only feeds the on-board +5vdc voltage regulator and the resistor divider feeding the comparator opamp. Vin is the same as the voltage coming from the external DC power connector except for a series polarity protection diode added between the connector and the Vin pin.


Aha!  $) Ok, Vin is the unregulated power supply, so I should place the mosfet between the Raw DC in and the regulator.  I know the unregulated voltage before my 5V reg is about 22V. Im thinking that is too high for the mosfet they use. I'll just use a beefier mosfet than the one they use, such as an FDN360P.

Is there any advantage to using a comparator, or can I just go with a set of mosfets to determine using the USB VCC vs the 5V?

Also as a side question, should I use reverse diodes on the D+/D- USB signal lines?
Working on a Smart Home Management System. Visit my blog: http://tae09.blogspot.com/ to find out more, and to keep track with the project.

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