Designing battery powered circuit w/ microcontroller - want to extend battery life - confused

Discussion in 'General Electronics Chat' started by Mahonroy, May 13, 2016.

  1. Mahonroy

    Thread Starter Member

    Oct 21, 2014
    190
    5
    Hey guys,
    I am in the process of designing a battery powered circuit and had a couple questions regarding the power supply (more specifically extending battery life to as long as possible).

    Quick overview of design:

    • Powered from 2x CR123 batteries (3.0V, 1500mah per battery).
    • Optionally powered by a 12V dc adaptor wall wart
    • Parallax Propeller microcontroller, when in standby it will draw roughly 10uA
    • Various sensors
    • Nordic nRF24L01+ transceiver
    Basic operation is the microcontroller will be asleep for a couple minutes at a time, at which point it will wake up, take sensor readings, transmit data using nRF24L01+ transceiver, then go back to sleep.

    My plan was to use the batteries in series to create 6.0V, then buck this down to 3.3V as efficiently as possible. This will also be convenient for use with the optional 12V dc power adaptor.

    I was considering using one of these chips(they vary in cost, efficiency, and quiescent current):
    LTC3103
    LT1934
    LT3990
    LTC1174
    LM2596

    I am slightly confused regarding the "Quiescent Current" and "Shutdown" modes on buck voltage regulators:
    Originally I thought that when my microcontroller shuts down and removes power from sensors and transceiver, that the circuit will be draining roughly 10uA, and the voltage regulator will be draining its "Quiescent Current", which ranges from like 1uA to 150uA depending on the regulator. I am finding out that this is only if the regulator is in "Shutdown" mode, otherwise it will be draining a lot more current. But if I am to wire a GPIO pin from the microcontroller to the Shutdown pin of a regulator, then power will be removed from the microcontroller.... how is this supposed to work?
    Should I be using an external timer chip to control the Shutdown mode of the regulator and just have the microcontroller lose power completely for several minutes at a time?

    I am a bit confused how I should go about this, any help or advice is greatly appreciated, thanks!
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,023
    3,236
    Is it possible that the system will work directly from the 3V battery and you then wouldn't have to use a converter?

    Otherwise using an external timer chip to shut everything down may be the simplest approach.
     
  3. Mahonroy

    Thread Starter Member

    Oct 21, 2014
    190
    5
    I was considering that, the only problem is that after looking at the discharge curve (voltage vs. time) for these batteries, they drop down to 2.5V to 2.8V pretty quickly, and that is right on the edge of brown-out for the microcontroller.

    I was also considering running the batteries in parallel @ 3.0V, then using a step-up/boost regulator to bring it to 3.3V, but after doing some research, it seems buck is slightly more efficient than boost?

    Would all I need to do is have the external timer chip bring the "Shutdown" to ground, and everything connected to the voltage regulator will just shutdown and lose power? Will this indirectly use more power considering the power consumption of the microcontroller booting back up, or should it be very similar?
     
  4. hp1729

    Well-Known Member

    Nov 23, 2015
    1,957
    219
    I don't see a "Standby" power on the Propellor. So it is either on or off. If you turn power off is it smart enough to wake itself up? Or do not remove power and put it into sleep mode. I think that gets you the 10 uA rating. You still have power to the sensors unless you control that power separately before going into sleep mode.
     
  5. Mahonroy

    Thread Starter Member

    Oct 21, 2014
    190
    5
    Correct, there is not a standby function... but what you can do is turn off all of the cogs, reduce the clock speed, and just run on a single wait instruction, and this essentially is a standby type of a function which gets you the 10 uA.
     
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