QuickStart 3.3V as a power source?

EMHmark7

Hi,


If it is USB powered, or anyway powered with a source before the voltage regulator,
can I use the Quickstart 3.3V pin to power something (in my case a XBee 50mA module that it 3.3V too)?
If so, what Amps max (because maybe it is passing through the PCB, and the power regulator limit which I don't know, etc.)?

Thanks,
Marc

LoopyByteloose

Well, the general limit for USB power is +5 volts at 500ma.

And then the QuickStart has a Propeller that has a maximum power limit (to avoid stress) of 300ma.

There is also the USB to serial chip that requires power (from the 5 volts) and provides the Propeller with +3.3v. And don't forget the EEPROM. So there is NOT much headroom.

Take a look at the specs for the FTDI 232RL, which is the USB to Serial chip. It provides the 3.3v output.

Try page 17, but I can't seem to find a figure.

Try top of page 11, and it seems that the FTDI 232RL provides 50ma of 3.3v Total, for the Propeller, the EEPROM, and whatever you might want to add.

(Wow, that is low!)

http://www.ftdichip.com/Support/Documents/DataSheets/ICs/DS_FT232R.pdf

WBA Consulting

The Quickstart has an onboard regulator to provide the 3.3V available at the header. See attached snippet from the schematic. The Quickstart user guide has details on using the "/USB_PWR_EN" pin properly also and it is available on the Quickstart Document page. As for the opposite way of thinking, you can power the Quickstart by supplying 3.3V into the 3.3V pin; I am doing that on a project right now.

From the user guide:
/USB_PWR_EN — USB power enable pin, inverted. Pulled to USB 5 V supply. Internally pulled
low after successful USB power negotiation. Drive low to force the USB power input to drive the
QuickStart power supply. Drive high with 6 VDC to disable power sourcing from the USB
connector. Rev B and later boards also internally pull low /USB_PWR_EN low upon detecting a
USB charger. Rev A boards must have /USB_PWR_EN externally driven low to power from a USB
charger.

LoopyByteloose

Well, the AP1117-3.3 is supposed to be able to provide 1 amp. But from the USB, there may only be 500ma, so it will likely brown out from that source before it gets to 1 amp.

But if you use Vin at +5v, it seems that the AP1117-3.3 would be able to deliver up to 1 amp distributed to the parts in place, and whatever is added.

The FTDI chip apparently will be happy just as long as it is getting somewhere between 5 and 3.3 volt DC.

That drive High to 6VDC is a bit disconcerting. Not sure about the Vin being safe for over 5 VDC. I guess the FTDI chip is being powered with 3.3V downstream from the regulator, so all is safe. I still don't under how the 3V3OUT pin wiring in the schmatic.

I need to read more about the QuickStart. I just have never bought one or used one. I still have quite a few ProtoBoards.

LoopyByteloose

From what I can see, Vin does not have to be restricted to +5 volts DC. It might go much higher.. maybe to 9 VDC.

That Mosfet Q2 is there to shut out the +5VDC from the USB port. Is that correct.

So, with power from a USB port, you migh have only 500ma total at best, but with another Vin source you can get up to 1 amp total.

Total means that you have to include the Propeller, the EEProm the FTDI chip, and the on-board LEDs in your power consumption budget.

Since how you use the Propeller varies and might go as high at 300 ma, my 'best guess' is that you easily have 100ma available for external device power from the 3.3v powered only by the USB ports +5, and it could be much more.