Can a regulator's "power good" signal be used for low battery disconnect?

Discussion in 'General Electronics Chat' started by cham, Jun 8, 2015.

  1. cham

    Thread Starter New Member

    Jun 5, 2015
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    I am using a 3.6V lithium ion coin cell battery (LIR2450) that I am regulating to 3.3V. I want a low battery disconnect feature so the circuit won't be operational if the regulated output voltage goes below 3.3V.

    The MCP1825 regulator (http://ww1.microchip.com/downloads/en/devicedoc/22056b.pdf) has a "power good" signal (PWRGD) that is an open-drain output used to indicate when the LDO output voltage is within 92% (typically) of its nominal regulation value. I don't think it is intended to be used as a low battery indicator (it looks like it's meant to create a delay until the voltage is stable), but I was wondering if it could be used for the purpose of disconnecting the battery when it's low.

    I'm not sure the best way to do this...there is a shutdown input (SHDN) on the regulator but it affects the power good signal, so that's confusing. Does it make sense to use a mosfet with the power good signal turning it on? That's also confusing because if the mosfet disconnects the battery from the regulator, then the power good signal would be floating...any suggestions on how I could accomplish low battery disconnect using the power good signal?
     
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Well... I seriously doubt you can use a simple signal such as that to indicate the battery is drained. Here's why:

    The voltage on the battery when the signal first goes active is in part due to your circuits load. Turn that load off and the voltage will rise, turning off the "power is bad" signal.

    At best it will chatter on and off for a while till the battery is dead dead drained.

    Furthermore, you need something to stay on to remember the power is bad, but you want to turn everything off, right? Perhaps a CMOS flip flop powered right off the battery could do this but it is starting to sound like more trouble than it is worth. Such a flip flop could prevent the chatter, but then there is the question of how to reset the flip flop to get the thing to turn back on even given a new fresh battery.
     
  3. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Once the system is up, the power good signal could be tied to the shutdown input to latch off the regulator. The problem is that there is no way to clear the latch afterward, and prevent it from acting curing startup. So the chip has most of the right pieces, but not right enough for what you want. There are a zillion LDO regulators (National/TI, Unitrode/TI, Linear Tech, Maxim, Analog Devices...), but if the battery is the only system power then you have a control logic problem. How do you reset the system after it has latched off, and what powers the control logic when the battery is latched off? It takes energy to hold a latch.

    ak
     
  4. ian field

    Distinguished Member

    Oct 27, 2012
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    Maxim do various off the shelf voltage threshold sensing chips, They probably do power management devices for lithium cells.
     
  5. cham

    Thread Starter New Member

    Jun 5, 2015
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    There are lithium ion power management ICs that are supposed to shut off the output when the battery is drained to below 2.75V (for safety reasons), so how would that work? An example of that is STNS01 (http://www.st.com/st-web-ui/static/active/en/resource/technical/document/datasheet/DM00085223.pdf) It looks like it's using a comparator. I found an LDO that has a low battery detection comparator: (http://datasheets.maximintegrated.com/en/ds/MAX882-MAX884.pdf), what you get at the output pin is a logic signal, so how is that different than using "power good" as a logic signal? Are they assuming you have a separate battery to use as a supply to a mosfet or something? There's absolutely no way to do this with one battery?

    Another question: if I used a 3.3v buck/boost converter, would I not have this problem? If the battery was too low for it to boost, would it output 0V?
     
    Last edited: Jun 8, 2015
  6. cham

    Thread Starter New Member

    Jun 5, 2015
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