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Thread: Driving Teensy with 3.7V with a USB flash drive connected?

  1. #1

    Driving Teensy with 3.7V with a USB flash drive connected?

    Teensy 4.1 drops the 5V supply voltage down to 3.3V, so I understand it can be driven from a 3.7V LiPo battery without any issues. However, if I connect a USB flash drive to the USB host port, I assume that the USB drive will also be supplied with 3.7V.

    I am trying to find info about whether this is considered a reasonable risk to take. From what I can find, most USB flash drives internally use 3.3V logic. But I can't find information about whether they drop the usual USB supply voltage from 5V to 3.3V using a pair of resistors (which would then drop 3.7V down to 3.7V * 3.3V/5V = 2.442V), or whether they use a voltage regulator.

    The best advice I could find is "Most USB flash drives should work fine on 3.7V, but some may simply not work."

    I can test this with specific drives, but I need a sense of how commonly drives will work vs. fail. Does anybody have a sense for this?

    If I'm supplying Teensy with power from a 3.7V battery, is there any simple way to step up the voltage to 5V for the USB host port?

  2. #2
    Senior Member+ MichaelMeissner's Avatar
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    Quote Originally Posted by lukexyz View Post
    Teensy 4.1 drops the 5V supply voltage down to 3.3V, so I understand it can be driven from a 3.7V LiPo battery without any issues. However, if I connect a USB flash drive to the USB host port, I assume that the USB drive will also be supplied with 3.7V.

    I am trying to find info about whether this is considered a reasonable risk to take. From what I can find, most USB flash drives internally use 3.3V logic. But I can't find information about whether they drop the usual USB supply voltage from 5V to 3.3V using a pair of resistors (which would then drop 3.7V down to 3.7V * 3.3V/5V = 2.442V), or whether they use a voltage regulator.

    The best advice I could find is "Most USB flash drives should work fine on 3.7V, but some may simply not work."

    I can test this with specific drives, but I need a sense of how commonly drives will work vs. fail. Does anybody have a sense for this?

    If I'm supplying Teensy with power from a 3.7V battery, is there any simple way to step up the voltage to 5V for the USB host port?
    There are step-up voltage regulators that boost the voltage from a lower voltage to a higher voltage. There are step-up/step-down regulators that can either boost or lower the input voltage to a fixed output voltage. And there are step-down regulators that aren't appropriate in this case.

    While I've only dabbled with them, I like this Pololu step-up/step-down regulator. It normally is fixed to 3.3v, but with a wire, you can make it produce 5 volt. What I like for battery usage is it has a fixed cutoff at 3v to prevent draining the battery. From the charts, it looks like it can produce about an amp of power from 3.7 volt lithium batteries:


    There are step-up regulators, but I'm not aware of any that have a fixed input regulator.

  3. #3
    Senior Member wwatson's Avatar
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    Quote Originally Posted by lukexyz View Post
    Teensy 4.1 drops the 5V supply voltage down to 3.3V, so I understand it can be driven from a 3.7V LiPo battery without any issues. However, if I connect a USB flash drive to the USB host port, I assume that the USB drive will also be supplied with 3.7V.

    I am trying to find info about whether this is considered a reasonable risk to take. From what I can find, most USB flash drives internally use 3.3V logic. But I can't find information about whether they drop the usual USB supply voltage from 5V to 3.3V using a pair of resistors (which would then drop 3.7V down to 3.7V * 3.3V/5V = 2.442V), or whether they use a voltage regulator.

    The best advice I could find is "Most USB flash drives should work fine on 3.7V, but some may simply not work."

    I can test this with specific drives, but I need a sense of how commonly drives will work vs. fail. Does anybody have a sense for this?

    If I'm supplying Teensy with power from a 3.7V battery, is there any simple way to step up the voltage to 5V for the USB host port?
    The USB Host port on the T3.6/T4.0/T4.1 always is at 5V. The voltage regulation is done on the device that is plugged into the USB Host port at 500mv when the the Teensy is plugged into a PC. You will still need a 5 volt supply for the USB Host port to operate USB host devices at a minimum of 500 mv. This also depends on the number of USB devices you have plugged into the Teensy USB Host port. Hubs, Flash drives etc...

    The Teensy internals can work at 3.7v but as far as I know the USB Host port cannot.

    Check this:
    https://www.pjrc.com/store/teensy41.html#tech

  4. #4
    @wwatson: thanks for the electrical info. Sounds like I should supply the Teensy with a boosted 5V supply anytime I'm planning to use USB devices.

    @MichaelMeissner: Thanks, the Pololu board is an excellent recommendation. I was trying to avoid having to use a full-sized boost converter, since they are too bulky for my application, but this board is tiny.

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