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Thread: VIN up to 7.2V after VUSB-VIN cut on Teensy 3.2/3.5, but not on LC/3.0/3.1/3.6?

  1. #1
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    VIN up to 7.2V after VUSB-VIN cut on Teensy 3.2/3.5, but not on LC/3.0/3.1/3.6?

    Paul's comment #4 in this thread:

    https://forum.pjrc.com/threads/54054...stand-6-4V-Vin

    ...leads me to believe that it might be possible to run some, but not all, Teensy boards on a 2S LiFePO4 battery (5V-7.2V) without needing an additional voltage regulator. If I'm reading that comment and the linked schematics correctly (which I might well not be), it seems like it might be safe to run a Teensy 3.2 or 3.5 directly from that voltage on VIN, but not a Teensy LC, 3.0, 3.1, or 3.6.

    Is that correct, or would an additional voltage regulator be required for a Teensy 3.2 or 3.5 powered by a 2S LiFePO4 battery?

    FWIW, if it makes a difference, I'm planning on around 30mA of current draw from the 3.3V pin.

    Thanks!

  2. #2
    Senior Member PaulStoffregen's Avatar
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    It should work on Teensy 3.2 and 3.5, but you ought to monitor the voltage regulator's temperature under the heaviest load. Neither board has a dedicated heatsink, so all the heat has to be dissipated by coupling through a couple vias to the copper layer of the PCB.

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    Quote Originally Posted by PaulStoffregen View Post
    It should work on Teensy 3.2 and 3.5, but you ought to monitor the voltage regulator's temperature under the heaviest load. Neither board has a dedicated heatsink, so all the heat has to be dissipated by coupling through a couple vias to the copper layer of the PCB.
    Thanks. I see that there's thermal protection on the voltage regulator at 160 degrees Celsius, but I don't see any thermal monitoring circuitry. Is it more a matter of putting full load on it for a few minutes and sticking my finger on it periodically to see if it's getting hot? (Or a temperature probe/some kind of grease?)

    Thanks again.

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    The regulator has internal temp protection. I think Paul is saying if it gets really really hot under use it might just decide to shut down on you at random points. 160C is too hot to touch so you will know if it is getting into the overheat zone.

    The internal CPU temp sensor probably won't help either since it is not coupled thermally to the regulator.

    You can estimate the regulator temp by its thermal coefficient times the power dissipated by the linear regulator (Vin - Vreg) * current = Pheat * Rja = degC rise over ambient

  5. #5
    Senior Member PaulStoffregen's Avatar
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    Quote Originally Posted by clawsoon View Post
    Is it more a matter of putting full load on it for a few minutes and sticking my finger on it periodically to see if it's getting hot?
    Yes. Or you could use a digital thermometer or thermal camera if you have one. But I'd go with a quick & simple touch test.

  6. #6
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    Thanks for the clarifications.

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