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Thread: Teensy Series Vin (MAX)?

  1. #1
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    Teensy Series Vin (MAX)?

    I am starting a design that needs to run on a very unregulated power supply. Arcade systems for those that are interested. Voltage can vary quite a bit, so it would be good if the microcontroller could survive 5.4v or more. Since PJRC does not provide this directly, is it correct to use the Vin (MAX) of the onboard voltage regulator instead? If so, am I reading this right to say:

    Teensy 3.2/3.5/3.6, LP38691 regulator, Vin 10.0v MAX

    TeensyLC, integrated regulator, Vin 6.0v MAX

    Teensy 4.0/4.1, TLV75733P regulator, Vin 5.5v MAX

    This would be very unfortunate for the 4.x, but surprisingly robust for the 3.x. (So, of course 3.x chips are in short supply right now)

  2. #2
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    The pinout cards generally state 6.0V or 5.5V max but theoretically you should never feed more than that into VIN as it would mean feeding this voltage straight to the USB connector, which can become a hazard if you forget about this and plug it to any USB host while powered by external voltage.

    Also even if the regulators themselves might have a higher maximum input voltage, you'd have to look at the voltage rating of the capacitors & fuse around the Teensy voltage input.

    IMHO placing a 7805 (or whatever LDO of your choice, actually) before VIN would be a much more robust solution and you'd be good to up to 30+V without additional heat sinking as the current consumption on Teensies is quite low.

  3. #3
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    I want to say that is good advice, but doesn't line regulation on a 7805 not kick in until 7 volts input? I've never actually tried to use a linear voltage regulator that far below its rated input.

    Good point for the on-Teensy passives. 6v is a fairly normal rating for capacitors, so maybe I'll be fine there. Lots of other stuff in an arcade machine will fry before the 5v rail goes over 6v. I know this is heresy, but I really wish they would move to modern regulated power supplies.

    I *am* debating putting a SPDT switch that physically separates the USB and external Vin. It would mean cutting a trace and a wonky extra pin for VUSB on a production design, which would not be fun.

    It may not matter much. The design requires 5v tolerance on the RX/TX lines and *lots* of I/O. I have 4 Teensy 3.5's in stock, then it looks like I'll be out for a year.

    Thank you for your advice.

  4. #4
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    Lots of other stuff in an arcade machine will fry before the 5v rail goes over 6v.
    But then if your 5V rail won't actually reach 6V that easily, cutting the VBUS trace and letting the Teensy's regulator deal with it doesn't sound that bad, I've done this with T3.2s fed by 6V in the past, I suppose the T3.5 which has the same regulator would also take that well.

    Indeed the 78xx dropout is quite large, I think I somehow got stuck with the idea that you'd need to supply much more than 5.4V to the Teensy. If your rail will be around that mark perhaps something like a MCP1755S-5002 with VInMax=16V VOut=5V and VDrop=300mV @ IL=300mA might be more suitable, or even a LP38691 should you need a bit more current (500mA). This setup would spare you the trace cut on the Teensy board at the expense of a fairly cheap LDO.

  5. #5
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    The unique thing about T_4.1 is that USB Host will pass VIN to the 'device' if used. Also some onboard component (perhaps the USB power switch ?) gets that voltage and is not spec'd for higher voltage over 5.5V, so there is a reason for the PJRC Guidance to limit VIN to 5.5V - even if one part is known to be otherwise capable. Also the LDO may handle higher voltage - but the PCB isn't expected to handle the higher heat given higher voltage from the passing current. Discussed somewhere on the T_4.x beta threads thousands of posts if not elsewhere.

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