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Thread: Actual current limitation of Teensy 3.2 and rules of thumb about typical/max current

  1. #1
    Junior Member
    Join Date
    Jul 2020

    Actual current limitation of Teensy 3.2 and rules of thumb about typical/max current

    I have a design where I need to power about 60 I2C hall effect sensors. Their datasheet says a typical draw of 2mA, but a max draw of 5mA. I called their tech support and they couldn't give a clear answer on when that 5mA happens, though I suspect it is when it is either actively reading or when the I2C transmission is happening.

    The best case scenario is that the draw is 5mA * 1 active sensor + 2mA * 59 waiting sensors = 123 mA at any given time.

    The worst case scenario is that the draw is 5mA * 60 = 300 mA at any given time.

    Hence, you see the problem. The Teensy 3.2's 3.3V output is listed in the pinout document as 250mA max. I looked at the regulator IC and it can do 500mA. I looked at the main CPU and it is max 185mA. I looked at the bootloader CPU and it is max 120mA.

    Now, I could add another 3.3V regulator on my carrier board, but was wondering whether I can get by with just the Teensy's regulated 3.3V output.

    Any thought from those with more experience than me? Am I safe trusting that those sensors won't ever hit their max current simultaneously?

  2. #2
    Senior Member
    Join Date
    Nov 2015
    Cold hollow VT
    I believe there is about 250mA available from the 3.3v regulated by the Teensy 3.2. Your calculations seem correct. I2C talks to only one (1) sensor at any time. Decoupling capacitors sprinkled among the sensors may soften the load by supplying the transient power.

    You could possibly (I don't know what your sensors are) run the sensors from the 5v USB power which defaults to 500mA typically and not have to worry about over burdening the Teensy 3.3v regulator.

  3. #3
    Junior Member
    Join Date
    Jul 2020
    These sensors are pretty sweet. They are the Allegro A1454 and only run on 3.3V, unfortunately. One of the awesome things about these chips is that you can either use two external pins with various analog voltages from voltage dividers to get 15 different I2C addresses, or you can program an internal EEPROM to set up to 127 different addresses. For a super-long linear sensor like I am building, this feels a lot easier than other I2C hall effect sensors with only 1 or 2 address and the need to use an I2C muxer to get up to 60, or to use a bunch of analog out hall effect sensors and several ADC ICs to handle 60 sensors.

    My design does have a decoupling capacitor for each IC very close to the power/ground pins of each.

    I think I am going to build my board such that there is a jumper that allows me to easily swap out the Teensy 3.2's 3.3V power for an external regulator, and give it a whirl. Once I have one of them built, it is pretty easy to run the board through the paces and see the actual max current draw over time during testing.

    Thanks for the feedback. It gives me a bit more assurance that my calculations of likely current draw were reasonable.

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