Portable Device Power Saver Circuit

Discussion in 'Analog & Mixed-Signal Design' started by Zfensler, Oct 6, 2016.

  1. Zfensler

    Thread Starter New Member

    May 28, 2014
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    I am trying to find or design a power saver circuit for a portable device. I figured this would be a fairly common circuit, maybe I am just missing the keywords to search. Basically I need to shut all power if a signal is not pulled back to ground after 5 minutes. This signal is an active low, "device active" signal. I originally tried 555 as a delay off driving a P channel mosfet whose source is the Vout and drain is Vbat, but had no luck. Here are some parameters:

    Vbat: battery voltage - 4.5 volts
    Vout: Can go as low a 3.5 volts, this powers multiple modules/chips.
    "device active" signal: 3.3V when not active, GND/LOW when active. Active signal on average is about 2-4 minutes. If the signal is not active for 5 minutes, shut all power down.

    I've seen circuits using microcontrollers, but I figured there had to be an easier and cheaper way. Appreciate any suggestions.
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    uC is the cheapest way as I see it.
    if you can program .i.e.
     
  3. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    A 7555 timer configured as a re-triggerable monostable is suitable and will work down to 2V.

    Do you have a schematic for your first attempt?
     
    Last edited: Oct 6, 2016
  4. Zfensler

    Thread Starter New Member

    May 28, 2014
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    Something like this?
     
  5. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    How much current do you need to switch?
    How accurate does the '5min' need to be?
    Do you have to use a high-side switch or would a low-side switch also be ok?
     
  6. Zfensler

    Thread Starter New Member

    May 28, 2014
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    1A, not accurate at all just longer than 5 minutes and a low side is totollay fine
     
  7. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Using a 555, even a CMOS version, could be problematic for a 5 min period. The problem is that the very low cap charging current via the high value resistor is comparable to the cap leakage current. If you don't want to use a micro then another option would be a CD4060-based timer.
     
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