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meccup
12-24-2012, 01:03 PM
Hi all,

while I'm waiting for delivering of the teensy 3.0, I'd like to find out how can I use T3 with external power. I read some topics on this forum related to this but I didn't find the answer.

I'm going to power T3 with single Li-Ion battery (18650). As per T3 datasheet I know that I should supply 1.7 - 3.6 volts to work properly. In theory, 18650 batteries have 3.6v output, but in practice it can go up to 4.1 volts. Therefore I have some questions, please be kind to help me.

1. Can 4.1 volts destroy T3?
2. Does T3 have onboard voltage regulator? If so, which one is it? Can I use it to reduce voltage of the 18650 battery?
3. How voltage of the Teensy USB connector is converted to 3.3v to supply MCU?

Thank you.

Jp3141
12-24-2012, 04:06 PM
The schematic is here:
http://forum.pjrc.com/threads/486-Teensy-3-0-Schematic-Available?p=10283#post10283

Actually a Li-Ion could go to over 4.2 V. You cannot apply that directly the the processor's supply (VDD), but the VIN pin on the T3 board drives the processor's VREGIN through a protection diode. VREGIN is the supply to the processor's internal LDO (low dropout regulator) which then generates the 3.3 V that runs VDD. VREGIN can go to 5.5 V.

So, you can just connect the 18650 to the existing module VIN pin -- the processor will get a voltage about 0.4 V lower than that (because of the (schottky) diode), but will run happily until its VDD voltage falls to about 1.7 V -- e.g. 2.1 V on VIN. Note that these low voltages (e.g. 1.7 V on VDD), the processor will work fine, but the I/O will be weak and may not drive loads such as LEDs etc. Also, the USB programming MCU needs ~ 2.5 V to work, but I don't think it gets used unless an external USB is connected, so that's OK.

To summarize -- VIN supplies a protection diode (0.4 V) which supplies VREGIN of the processor (2.x .. 5.5 V). VREGIN supplies an LDO which generates VDD (1.7 .. 3.3 V).

You could bypass the diode(s) and drive VREGIN (see on the sch.) directly from the 18650, This keeps the processor's supply higher, but then you have no protection against reverse battery etc.

Be careful working with 18650's -- they can give over 100 A when shorted !

Secondly, you should not let the Li-Ion discharge below about 2.5 V or it will be damaged -- you really need circuits to disconnect the load when it becomes so low. I don't (yet) know how to use the low power modes of the T3's processor.

Paul -- The T3's processor has some very flexible low power modes -- will these work on the Teensy 3.0 ? Does the MINI54TAN get set to its power down mode when the T3 has booted ?

meccup
12-25-2012, 06:51 AM
Jp3141, thank you for your comprehensive answer.