Another 3.3V power supply + USB thread

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luisonoff

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Hello,
Just finishing my PCB layout for Teensy 3.1.
I have read every existent thread on this topic but did not find the answer.


My case:
- Regulated 3.3V external power connected to 3.3V Teensy pin.
- Vin pin unconnected.
- Vusb-Vin trace NOT cutted


What I want to do:
- Work when powering only by 3.3V pin -> DONE
- Work when powering only by USB -> DONE
- Work when powering from 3.3V and USB simulatenously -> Is it safe with my setup?


3.3V and USB at the same time in detail:
5V USB goes to Vin (trace not cutted), then it goes to regulator and converts to 3.3V. There it finds my external 3.3V. Voltages will not be identical, so current will flow from one place to the other I guess, but they will be quite similar, so small current -> SAFE?

Should I add a diode so current cannot flow from the Teensy to my external power supply? But, what happens in the case current is flowing from external power to Teensy's regulator?


Thank you for your time!
 
luisonoff said:
Hello,
...
- Vusb-Vin trace NOT cut
….
- Work when powering from 3.3V and USB simulatenously -> Is it safe with my setup?


3.3V and USB at the same time in detail:
5V USB goes to Vin (trace not cutted), then it goes to regulator and converts to 3.3V. There it finds my external 3.3V. Voltages will not be identical, so current will flow from one place to the other I guess, but they will be quite similar, so small current -> SAFE?
Click to expand...

This has been covered several times. No, it is usually not safe to have two competing power supplies / regulators jockeying to maintain 3.3V. Don't do it. See the previous threads re: how to supply 3.3V from multiple sources.

My implementations feature multiple 5V sources (USB, external, FTDI cable, etc.), a Schottky diode for each power source, and then the downstream regulator decides which power source to draw from. Common ground pulls them all together.
 
I asked a similar question some months ago, and somebody pointed me to this power multiplexer from pololu.com: http://www.pololu.com/product/2594. I've bought one, but I haven't yet set it up. I was planning to add a 5v boost regulator as the secondary input and create a small micro USB cable that would plug into the Teensy and connect to the VOUT, D-, D+, ID, and GROUND pins output from this regulator. This of course loses some power as it first boosts the power up from 3.3v (or 3.7v in my case since I would be using a li-po battery) to 5v and the Teensy reduces it down to 3.3v.
 
That's an awesome product. Too bad that the chip at the heart of it all (fpf1320) is not available in a format that is usable by hamfisted folk like me.
 
Constantin said:
That's an awesome product. Too bad that the chip at the heart of it all (fpf1320) is not available in a format that is usable by hamfisted folk like me.
Click to expand...
Well pololu says that is one of the reasons they made the breakout board:

However, the FPF1320 is only available in a tiny 1 mm × 1.5 mm BGA (ball grid array) package, making it difficult for a student or hobbyist to use. To address that issue, this board mounts the chip onto a compact carrier with 0.1″ pin spacing that makes it easy to use with standard solderless breadboards, and it also serves as a breakout for a USB Micro-B connector that can be used to supply one of the input power rails.
 
MichaelMeissner said:
Well pololu says that is one of the reasons they made the breakout board
Click to expand...

I thought the pictures showing the chip next to some grains of rice was pretty illustrative. The grains of rice were bigger. A more reasonable form factor (including a more integrated design also) is the TPS2105 from Ti. It's a pity though that a SOT23 chip could cost $3 in single quantities! For $2, the OP can buy an 'ideal' 3.3V switcher from Digikey. Trouble is, switching between an external 3.3V source and Vout33 would only be possible if there were a separate breakout pin for vout33, which there isn't.

The costs of these switchers is still a bit outrageous IMO but for the right applications (operating from batteries, for example) the price may be worth the extra 0.3V of potential you gain by not using Schottky diodes for the same purpose. Interestingly, Intersil even makes a DIP version that costs about $2 as well. Nifty chip, a similar solution I cooked up used a dual comparator and a p-channel MOSfet instead. The idea being that the comparator notices the external power failing and then turns on the battery, but it also turns off the p-channel MOSFET when the battery power goes too low. So the only time that the MOSFET should be conducting is when the external power falls below 4.2V but the battery voltage is above 3.6V. Have yet to test it!
 
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Thank you all for your answers. I have read all your references and links.
New simpler question:

-Is there any way of powering Teensy 3.1 from two sources WITHOUT having to cut the Vin-Vusb pad?
 
You can't do it by connecting two pins, but you can have something that takes 2 inputs and produces an output, and feed that to the Teensy, by making a USB micro cable. I haven't wired it up yet, but in theory, you would have one input, that you use a voltage regulator to produce 5v, and feed it into the Ground and VinA pins on the Pololu board. You connect a USB cable to the mini-USB port (Ground and VinB). On the other side of the chip, you take the 5 pins (Vout, D-, D+, Id, Ground) and solder them to a male USB micro cable that attaches to the Teensy.

Here is a rough parts list:

As I said, I haven't actually done this yet, but I'm just starting to get to the point in my soldering ability that I might attempt it shortly. I would probably wire in a switch to disable power and provide alternate 5v/ground outputs for use in powering things like neopixels.

If you aren't concerned about needing to program the Teensy, you could either hook 3.7-6v to the VIN pin, or 3.3v to the 3.3v pin. However, for me a big draw of using a USB cable would be I wouldn't have to unplug the Teensy for programming, but I could test it out with the power source I wanted to use.
 
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Funny story, I often used the USB and VIN (teensy 2.0++). for an unrelated reason I decided to tie my power supply transformer case to earth ground.... about a week after I tried the same procedure except as soon as I pluged in the usb I got a BSOD. blew motherboard USB port, USB hub and 300$ worth of electronics that the teensy was mounted too. It was a bad day. Since then I removed the Earth Gnd on the power supply, tie everything to earth with high voltage ceramic caps, do not use bench power supply to power Teensy while on USB and I bought one of these with a nice price tag:http://www.bb-elec.com/Products/USB-Connectivity/USB-2-0-Hubs/Industrial-Grade-USB-Hubs.aspx. My powers to smoke electronics are second to none!
 
Thank you again for your answers.

Still I believe that I should be able to power teensy using 3.3V pin and USB power at the same time without risk if I put a diode from the 3.3V power source to the Teensy, so current cannot flow from the computer to the power source. I believe it will not flow from the power source to the PC because USB power is always higher, 5V.

I will try it with an spare teensy 3, unless somebody really thinks that my computer could damage. Again, take into account that USB voltage will always be higher.

Anyway, I left this matter for next revision of my PCB. Actual one I will just cut the Vin-Vusb pad and forget about powering from USB.
Thank you again!
 
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