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Thread: Running Teensy LC on external 3.3V

  1. #1
    Junior Member
    Join Date
    Apr 2013

    Running Teensy LC on external 3.3V

    Hi all,
    I realize this may seem like a slight variation on a question that has been asked before (many times), but I don't think it is (I have read all threads I could find).

    Anyway, we have an upcoming project that will include a Teensy LC, a number of industrial components (all running on 24V) and several 3.3V devices. I'd rather avoid having a 5V power supply just for the Teensy, so I'd like to run it from the same 3.3V module (powered from a big 24V supply) as the rest (RS-232 module, I2C port expanders, etc.). I will cut the trace between VUSB and VIN to allow a sporadic USB connection.

    The Teensy LC's pinout card says VIN should be between 3.7V and 5.5V, but I believe that is to avoid having the MKL26's internal 3.3V regulator drop out, because, if I read the schematic correctly, the MKL26 itself (and pretty much everything else on the Teensy) is powered by the output of this regulator (VREG33OUT). The input to this regulator is VREGIN, which normally is tied to both VUSB and VIN. The MKL26's datasheet says on page 37 that if VREGIN falls below 3.6V, the internal regulator circuitry goes into pass-through mode.

    So, as long as I have clean 3.3V on VIN I should be good, or am I missing something? As far as I can tell, all I loose is the ability to have pin 17 on 5V (if VIN were 5V) instead of 3.3V.
    I have done a little testing running a few random sketches, and all seems to be fine. Could there be any other pitfalls?


  2. #2
    Senior Member
    Join Date
    Apr 2013
    The better solution if you have truly stable 3.3V supply is to connect to the 3.3V pin. Normally that would be an output but you can supply it as well. Just don't let it spike or the LC is toast.

    Running 3.3V to Vin what voltage do you see on GPIO pins when high? Would have expected about 3.1 volt due to the lower supply voltage? You may also find offsets in analog reads as well.

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