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Thread: Teensy LC: power converter question

  1. #1
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    Teensy LC: power converter question

    Hello,

    coming from Arduino, this is my first project with a Teensy. I understand that the Teensy LC likes 3.3V on all pins, so all components connected must work at that voltage. The primary power source will be batteries (@24V, needed for stepper motors), a step down converter brings 5V to VIN on the Teensy, with the power connection to USB cut apart.

    The Teensy can provide up to 100mA in total for other components and consumes <50mA for itself (got that from a chart somewhere here in the forum). I plan on connecting:
    • OLED SSD1306, consumes ~20mA
    • i2c encoder, power consumption unknown
    • 4x TMC2130 stepper motor driver with SPI connected, power consumption unknown
    • two potentiometers, a joystick (basically two potentiometers and a switch), a pushbutton
    • optocoupler 4N27


    I can't really predict the total needed current from all components. So, a additional step down converter with an output of 3.3V might be needed to power at least the OLED and/or the stepper drivers logic, bypassing the Teensy.
    My question: can I do that? =) The step down converter might not match the 3.3V level coming from the Teensy exactly, will that be a problem?

    Thanks in advance,
    Philip

  2. #2
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    Stepper motor drivers tend to need a lot of current, so powering them from the LC does not sound good.

    If you power the LC via VIN with VIN-USB cut, you should be able to supply 3.3V at VIN (instead of 5V), so no need for 2 different DC converters.

    That said, if for whatever reason you want to power the LC with 5V via VIN, you can still power the stepper driver (and other external stuff) via a separate 3.3V DC converter without issues, provided the GNDs are all connected together.

  3. #3
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    Thanks for you answer!
    Stepper motor drivers tend to need a lot of current, so powering them from the LC does not sound good.
    Ok, I think I was not clear about that. I was talking about the stepper driver logic part being powered by the LC. The motor powering part on the drivers are directly fed by the batteries @24V.

    If you power the LC via VIN with VIN-USB cut, you should be able to supply 3.3V at VIN (instead of 5V), so no need for 2 different DC converters.
    The pinout says "3.7 to 5.5V" on VIN, so I guessed 5V would be "save". But if 3.3V is ok, it would make things definitely easyer-. Though I have a bit of a problem to understand that. Isn't there a step-down converter somewhere on the LC where the 3.3V on VIN have to go through? Aren't these converters always consuming some voltage?

    ... you can still power the stepper driver (and other external stuff) via a separate 3.3V DC converter without issues, provided the GNDs are all connected together.
    I guess that answers my original question.

  4. #4
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    The pinout says "3.7 to 5.5V" on VIN, so I guessed 5V would be "save". But if 3.3V is ok, it would make things definitely easyer-. Though I have a bit of a problem to understand that. Isn't there a step-down converter somewhere on the LC where the 3.3V on VIN have to go through? Aren't these converters always consuming some voltage?
    In general, yes, you are absolutely correct: regulators usually drop between 0.2 and 0.7V.
    But from experience, Teensies seem to have some sort of regulator bypass when VIn drops below say 3.5V, so the Vdrop seem not to be an issue if you apply 3.3V to Vin.
    That said, you could power the LC directly from the "3.3V" pin (I know, it's considered an output, not a DC input; and yet...) and bypass the regulator entirely. Of course you are in charge to provide a clean, stabilized 3.3V at that point.

  5. #5
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    Just to be on the safe side, I'll add this: while having external modules powered by 3.3V is (in general) safe with the LC (provided GNDs are connected together), having external modules powered by 5V is not (in general) safe: LC can't tolerate 5V on its I/O pins (3.3V only), and modules powered by 5V tend to run their I/O logic at that same 5V voltage (unless explicitely stated), thus potentially burning the LC.
    Last edited by XFer; 04-10-2019 at 09:00 PM.

  6. #6
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    Thank you for your advice! There seems to be no need for any 5V lines anymore, so I'll stick to 3.3V entirely.
    Do you have any recommendations for a reliable regulator? I was looking at the TSR 1-2433 with an output accuracy of ±2 %. From your experience, is that ok?

  7. #7
    Senior Member PaulStoffregen's Avatar
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    Multiple people have reported problems with those Traco TSR power supplies. The application info on page 3 shows a capacitor needed "For input voltage higher than 32 VDC". In fact, it turns out the Traco products sometimes behave very badly without that capacitor, at any input voltage. I personally tested one that was sent to me, so I have seen with my own eyes (and oscilloscope) how badly these can behave without the capacitor, especially if the power source is a battery running low. If you do use this part, you *MUST* add that capacitor at the input. It's definitely not optional as the datasheet seems to say.

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    Okay, thanks for the heads up. What would you recommend instead? I wasn‘t planning on going higher than 24V on the input. 32V just seemed save..

  9. #9
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    I like MP1584-based regulators.
    https://www.monolithicpower.com/en/mp1584.html

    Cheap, stable, energy efficient (personally measured: 88% with 12V in 3.3V/0.6A out), OK up to 2A out without heatsink.
    Lots of cheap ready-to-work boards available, example:
    https://www.amazon.de/Einstellbar-Co...gateway&sr=8-1

  10. #10
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    Oh and by the way: welcome on board on the Teensy side!
    I come from Arduino, too.
    Teensies are little marvels, true gems! Cannot thank Paul enough for his incredible work.

  11. #11
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    Thanks, just ordered one!

    And thank you for the welcome! For quite some time I've been looking for a project where the Nanos that are usually my go-to choice wouldn't be sufficient anymore. Coming from a rather designy-ish oriented Arduino world, it took some time and grubbing through the website to grasp the ideas to the Teensy project. So far, it feels really structured and focussed-- For me, kind of a step up from "just make it work" projects to "make it work very nicely" =)

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