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t3andy
11-03-2012, 02:03 AM
T3 Power and protection questions

I just cut and paste the two emails I sent to Paul before this forum board got started.
I thought it would be helpful to post this Q/A on the forum board for other users.
Here is Paul's replies from two emails. Hope you don't mind Paul. ;)



#1. Current draw of Teensy 3 with nothing connected except USB?
Someplace I read ~ 27 mA.


Yes.


#2. If an output pin gets "direct" shorted to ground, you will probably lose the pin on the K20 ?
If one pin is an output [High] and another pin is an output [Low] and both pins are shorted together -- you probably will lose some pins?


Maybe. It's certainly not good for the hardware to do this. But often the stress does not actually result in permanent damage.


#3. What series limiting resistor would you recommend to prevent all the current going to ground on a direct short and still have
some usable output drive current?
3.3 v / 5 ma. = 660 ohms? (Using K20 9 ma. max.) <--- my guess.


Yes, 9 ma is the recommended maximum.



#4. Does the Teensy 3 has only one input VIN regulator on the T3 module?


Yes.



#5. On the Teensy 3, is there input reverse polarity protection on the VIN input ? (with the on-board VIN bridge pads "cut")


Yes, there's a schottky diode between VIN and the regulator input.



#6. Will you destroy the Teensy 3 with polarity reversal on Teensy 3 VIN and ground?


It should be fine.



#7. The 3.3 VDC out has a max. 100 ma. Does the K20 idle current of 27 ma. "eat into" (subtracts from) this max current output?
The max. power current for "all" K20 I/O pins is 100 ma. by Freescale specs. Question 4 is related to this question.

Yes. The regulator is specified for 120 mA.



#8. What will happen, if you have the VIN jumper in, and power the Teensy 3 with USB power and VIN power at the same time?


The Teensy 3.0 would probably survive, but this would short the USB power to your power supply. One will likely drive current into the other, which isn't good. If they're both very close to 5.0 volts, hopefully it won't be a terribly destructive event.

djsz
11-03-2012, 12:41 PM
On point 8 above, would the risk be eliminated if you cut the VUSB and Vin pads to separate them and soldered a Schottky diode across those pads? This is similar to the recommendation on Paul's web page for running with external power on Teensy 2.0.

It also seems like the Teensy 3 has a Schottky diode already on board to protect against reverse polarity, so the additional diode that Paul recommended on the positive battery lead is unnecessary. Is that correct?

Paul
11-03-2012, 01:11 PM
On point 8 above, would the risk be eliminated if you cut the VUSB and Vin pads to separate them and soldered a Schottky diode across those pads?

Adding that diode would prevent power applied from VIN from attempting to flow back into your computer. However, power from your computer would drive VIN though the diode, so it would attempt to drive any power supply connected to VIN.

If the power connected to VIN is from a power supply with a low dropout regulator, most of those regulator chips are ok with the output being driven from another source (they just stop providing current).

But if the power is from something like a battery, the USB power would attempt to drive current into the battery. Uncontrolled charging for rechargeable batteries is not a good idea, and of course it's really bad to drive current back into normal batteries.

djsz
11-03-2012, 01:53 PM
Right. In your documentation for teensy 2 you specify that one should connect a diode between the battery and vin to prevent the computer from pushing current successfully to the battery. Does adding that diode still serve as a mechanism for allowing USB and battery to be connected simultaneously without harm to either power source? Or does T3 have that diode in place?

t3andy
11-03-2012, 06:06 PM
Since I don't have a schematic of the Teensy 3 then this is what I think the power diagram of the Teensy 3?
Hope this helps in this discussion.

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