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Increasing Current Capacity of Battery Operated Devices Using Load Switches

Bob Kressin
Prodigy 30 points
Other Parts Discussed in Thread: TPS22912C, TPS22910A, TLV3691

I have a device that is powered by two, AA batteries connected in series.  This provides a 3VDC supply rail with 2000mAh of capacity.  However, the battery life is too short, and we're looking for a simple, cost-effective method of adding capacity.

One simple way of increasing capacity is to hook another bank of two, AA batteries connected in series, and in parallel with the first bank.  This effectively provides a 3VDC rail with 4000mAh of capacity.  However, it's well known that any "imbalance" in the circuit in terms of discharge rate, internal resistance, etc. can cause serious issues.

We are searching for the simplest and cheapest circuit that allows us to deplete one bank of 2xAA batteries, then switch over to the second bank.  There are many ways to solve this problem, but all of them seem to either involve a pricey PMIC, two load switches + firmware on our MCU + ADCs, or even load switches plus a fuel gauge.  Other methods using load switches may cause bad "bouncing" during the cross-over from battery bank #1 to battery bank #2.

Has anyone found a solution to this type of problem?  Thanks in advance for your assistance and advice.

- Bob

  • 0
    TI__Expert 5275 points

    Hi Bob,

     

    I think using load switches will help to keep the price and leakage current low.

    You could use an active low device such as the TPS22910A on the primary bank (supervisor on this bank + has priority over the other bank), and an active high device such as TPS22912C on the secondary bank.   The slower rise time of the TPS22912C could allow the output voltage to droop when switching between banks, but this can be reduced with additional capacitance.

    In order to control when the switch over happens, you will want some type of supervisor circuit with hysteresis:

    The TPS3838J25 is a nano-power supervisor circuit with a 2.25V threshold option.  However, the 30-40mV hysteresis may not be enough to prevent bouncing.

    The TPS3806J20 would allow you to set the hysteresis in addition to the threshold; however, the supply current would be a few uA.

    The other option for a supervisor circuit would be to use a comparator with a hysteresis.  The TLV3691 nano-power comparator could be used as described in this TI Reference Design:

    How to Set Hysteresis on a Comparator with Four Resistors

     

    Let me know if you think one of these approaches will work and we can dig in deeper.

    Additional Logic may be needed to account for the case where one bank is not present.

    Also, if you are wanting both switches to shut off after both banks have been depleted, another supervisor circuit may be needed on the secondary pack.

     

    Best Regards,

    Adam Hoover

    Load Switch Applications Engineer

  • 0 in reply to Adam Hoover
    Prodigy 30 points

    Hi Adam -

    Thanks for the fast reply.

    Of course, the more "things" I hang off my battery, the more leakage is going to effect battery life.  The TLV3691 is a good alternative for the managing the hysteresis problem that I'm worried about.  Please give me a day or so to work up a block diagram, and I'll post it for comments.

    - Bob

  • 0 in reply to Bob Kressin
    TI__Expert 5275 points

    Hey Bob,

     

    Just checking-in, have you found suitable solution?  I would be happy to review your block diagram and/or schematic and provide feedback.  If you would like to keep it confidential then you are welcome to email it to me.

     

    Regards,

    Adam