LoPy with 3.3V direct powering fails to start. (SOLVED)



  • UPDATE : Sept 2017
    Lopy powered with 3.25V from 5V pin is not able to start.

    I'm powering LoPy module with 3.3V (500mA and 1000 mA) on 3V3 pin, last firmware release.

    LoPy try to start (green light turns on for a while) but no blinking blue light and no wifi.

    Voltage on 3V3 pin is dropped by LoPy to 2.4V.

    Tried the same with another LoPy, voltage dropped to 1.8V.

    Tested five lopy modules, all of them behave the same way.

    If powered with 5.5V no problems.

    3.3V DC/DC switching regulator of LoPy module seems unable to start causing a voltage drop.

    My question to Pycom's guys is : is there a workaround to this issue ?

    Has anyone else in this forum met a similar issue while powering trough 3V3 pin ?

    Thanks,

    Cocoa



  • @jcaron

    Isn't this the opposite of what what stated here:

    Not completely the opposite, it's still more efficient to use a regulated 3.3V supply (low voltage forces the onboard regulator into bypass ECO mode)

    But it needs to be updated to reflect that 3.3V should be applied to the 5V/VIN pin.

    Is the Pysense/Pytrack sending power to the module via 5V

    Yes, all the various shields implementing deep sleep control the *py board's 5V rail



  • @jmarcelino @robert-hh

    Isn't this the opposite of what what stated here:
    https://forum.pycom.io/topic/1022/root-causes-of-high-deep-sleep-current

    Workaround:
    Power the board via the 3V3 pin using a stable 3V3 supply that is able to deliver at least 400mA. This lowers the deep-sleep current to around 2.5mA.

    ?

    Also, it's unclear to me what the setup on the Pysense/Pytrack actually is? Is the Pysense/Pytrack sending power to the module via 5V, and then receiving 3V3 back from it (which I suppose only goes to the 3V3_Pymodule pin)? Someone I had thought the module was powered by the Pysense/Pytrack via the 3V3 pin, but that's probably incorrect.



  • @jmarcelino @robert-hh @ssmith

    Thanks for help to solve this issue.



  • @robert-hh
    Yes I noticed that when I pasted the text here, already let them know :-) Thanks.



  • @jmarcelino To reduce confusion, they should change the range for Vin to 3.3V to 5V. I just verified that the LoPy indeed starts with Vin even as low as 3V. Maybe less. I Did not go below that.



  • @robert-hh
    I e-mailed the Pycom team this morning with the findings and they've already changed the usage notes to reflect this new information

    https://docs.pycom.io/chapter/datasheets/notes/

    When powering via VIN:
    The input voltage must be between 3.4V and 5.5V.
    Please DO NOT power the board via the 3.3V pin as this may damage the device. ONLY use the VIN pin for powering Pycom Devices.

    It's official. Thanks everyone for helping out.



  • @ssmith
    The LM3281 data sheet suggests this will be 16 μA.

    "7.3.3 Low IQ
    An ECOnomy (ECO) mode of operation draws 16 μA (typ.) quiescent current, permitting the LM3281 to be used in “always-on” applications. This low IQ is achieved over the entire input supply range of 5.5 V to 2.6 V, irrespective of whether LM3281 is operating in regulation (ECO Mode or Analog Bypass mode) or in full dropout (full bypass)."

    Thus substantially less - insignificant really - compared to the minimum current boards operate at (given the issues around deepsleep)



  • @cocoa Not a great answer but if you have a hot air gun you can remove the power supply chip from the LoPy. How much power are you wasting by putting 3.3V into the 5V input? Seems like that shouldn't add much.



  • @jmarcelino @ssmith @robert-hh

    Thanks to everyone of you , now is more clear what's happened.

    What indeed isn't so clear is why Pycom's guy @Fred stated , 5 month ago, to use pin 3V3 to power LoPy and mantain a deep sleep current around 2.5 mA.

    See https://forum.pycom.io/topic/1022/root-causes-of-high-deep-sleep-current Root Cause 1 : Workaround.

    Now there is this situation :
    if I want to have a low current in deep sleep state I've to supply LoPy trough pin 3V3, but using this pin, as shown today, is not a good practice and could fry LoPy module.

    Any idea ?



  • @jmarcelino Thanks for asking. @daniel, do you listen?



  • Official answer from TI's Simple Switcher group: It is a bad idea to power from the 3.3V pin (if directly on the regulator output as it seems to be)

    "Yes it is a bad idea to power any regulator from it's output. There is a parasitic diode from input to the switch pin that will be turned on in this configuration and can cause damage.
    The easiest way to power from two supplies would be to use a simple diode-OR connection.
    However, this will cause a voltage drop in your supply rail due too the diode drop. You might also look at some of TI's "ORing" controllers such as the LM5050 or similar."

    So always use the Vin/5V pin to power the board (even with 3.3V)



  • Well I guess I've been lucky.

    As @robert-hh says it is inherently safer to power it from the 5V/Vin pin with 3.3v, though there is a small voltage drop and presumable higher current consumption.

    In the meantime I'll reach out to TI themselves and hopefully get a more conclusive answer.



  • @robert-hh said in LoPy with 3.3V direct powering fails to start.:

    @cocoa As @ssmith and @jmarcelino pointed out, the regulator is a LM3281. The LoPy spec says: Input power 3.3-5.5V https://docs.pycom.io/chapter/datasheets/downloads/lopy-specsheet.pdf. The data sheet of the regulator says: Vin 3-5.5 V. In bypass mode, which kicks in between 3.3 and 3.4 V, the dropout is below 60mV at 600 mA. So it is absolutely safe and intended by the design of the regulator to use it that way.
    In my view, that is definitely more safe than connecting the regulator output at the 3.3V Pin with another power source. Connecting two outputs is never a good idea.

    Exactly. If you want to drive it through the 3.3V pin then there needs to be a diode on the output of the onboard supply so your not back feeding it. You will burn up boards doing this. Maybe something changed from the earlier version??? I definately burned up 2 LoPys by doing this though.



  • @jmarcelino

    Hi,

    tried with another module (a new one) , now LoPy starts when powered with 3.3V on 3V3 pin but it is over heating.

    Connected only with power supply, nothing else connected.
    5V pin is floating.

    Reached 65.5 Celsius degree in five minutes with air temp of 24 degree, disconnected for fear of burning it.

    Powered at 3.3V 500mA.

    More and more strange.



  • @cocoa As @ssmith and @jmarcelino pointed out, the regulator is a LM3281. The LoPy spec says: Input power 3.3-5.5V https://docs.pycom.io/chapter/datasheets/downloads/lopy-specsheet.pdf. The data sheet of the regulator says: Vin 3-5.5 V. In bypass mode, which kicks in between 3.3 and 3.4 V, the dropout is below 60mV at 600 mA. So it is absolutely safe and intended by the design of the regulator to use it that way.
    In my view, that is definitely more safe than connecting the regulator output at the 3.3V Pin with another power source. Connecting two outputs is never a good idea.





  • @robert-hh

    Hi Robert,
    thanks for the answer.

    Do you know where is documented that is possible to power LoPy with 3.3V using 5V pin ?

    I can do a try, but if it is not certified as a good practice by Pycom I cannot use this workaround in a definitive design.

    Have you a LoPy Dc/Dc converter schematic , it could be very useful to understand the regulator behavior.



  • @cocoa
    I don't understand this, none of my LoPys have problems running from from the 3.3v pin. Even if I limit the supply to 200mA it's fine. Don't notice anything getting hot either. My first LoPy still runs off a breadboard powered by a LD1117D33 LDO regulator from USB 5V into the 3.3v pin.

    Do you have anything else connected? Is the 5V pin floating?
    Is the LoPy mounted on the expansion board/something else?

    What kind of power supply are you using?



  • @cocoa Did you try 3.3v at the 5V pin? That's what I was talking about.
    I tried that with my Lopy and it would start even down to 3V at the 5V pin.



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