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Low-leakage high-side switch?

Discussion in 'General Electronics Discussion' started by rdtsc, Mar 19, 2014.

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  1. rdtsc


    Mar 19, 2014
    Hello, I'm working on a battery-powered alarm project, where the idle current consumption must be as low as possible so that the device can operate for up to a year. Once a sensor trips, I'd like it to turn "on" the rest of the circuit.

    Simulation works fine in LTspice using an Enhancement-Mode P-Channel MOSFET. Lets say we have a 7v battery, to the Source of the PMOS and through a 1M pullup to Gate. The Drain connects to the load and ground. 500mA (1.5A peak) will be sunk. The sensor tripping pulls the gate to ground, thus turning on the rest of the circuit. CMOS and TTL logic-level signal voltages are available.

    What I'd like to know is, what is the "leakage" current of a PMOS used in this way? For example, the datasheet for a logic-level FDS6575 specifies the "Ioss" or "Zero gate voltage drain current" as a maximum of 1uA. That would be awful for this application (10x more than the sensor currently), so I'd much rather see something in the lines of single nano-amps or less.

    If a PMOS isn't the best solution for something like this, I'd appreciate any other ideas.

  2. BobK


    Jan 5, 2010
    Well, with a 1000mAH battery it would only last 114 years, so I can see why you are upset. :)

    Seriously, 1uA is likely to be far less than the self-discharge rate of your battery.

  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    The first thing is to read the datasheet and take the worst (or if that is unavailable) the typical value.

    If that's not good enough, I'd recommend you measure it. You'll need a way to measure nA because the actual device is almost guaranteed to be better than the worst case specs. And you will likely find that the next device you pick up has a different leakage.

    At currents this low, you may have to start thinking about leakage across your board, although unless it's really dirty this isn't usually a factor until the currents are 1 or 2 orders of magnitude lower.

    But as stated earlier, self discharge of the battery may be several orders of magnitude higher.
  4. gorgon


    Jun 6, 2011
    I wonder about the capacity of the battery you use in this project, and how long the active period is defined to last.
    I suppose you know that the battery needs to keep a rest charge large enough to tolerate your alarm condition load long enough. 500mA in this equation is a large load.

    An idle load of 5uA will only need around 44mAh for a year, so I wonder how long you need to activate the alarm condition under full power after that. If you need 30 minutes min active time, the restcharge must be >250mAh or more if the load is higher with peaks. And on top of that you also need a safety margin, for variations of the battery.
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