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Original question:

TPS22933: Searching for a power mux + LDO in one package with >300mA output current

TPS22933: TPS22933 and three large coin cells

Roland Portman
Prodigy 50 points
Part Number: TPS22933

Can I use the TPS22933 to select between three large coin cells?  

I am concerned the auto-selecting multiplexer will thrash if the three coin cells have nearly the same voltage.

Once the auto-selecting multiplexer selects a channel, will it have enough hysterisis to stay on that channel until one of the others are 150mV higher?

Thank you,

Roland

  • 0
    Prodigy 50 points
    I am particularly concerned with the 62.5mA statement in the TPS22933AEVM SLVU589 User Guide:
    3.5.1.2 3-Input Multiplexer with LDO Unused
    This example shows the TPS22933A used solely as a 3-input multiplexer with the LDO left unused. The
    CAP pin is used as the output of the multiplexer. Since the CAP is simply the highest of the three inputs,
    the CAP pin will be as high as the input supply (minus the drop across the input switch). In this use in
    particular the user needs to keep in mind that the voltage picker has a 150mV (typical) switchover point.
    With a 2.4Ω RON, the maximum current would be limited by not only the power dissipation, but also by the
    switchover voltage. For 150mV and 2.4 Ω, a current of 62.5mA may result in a switch between supplies of
    equal voltage.
  • 0 in reply to Roland Portman
    TI__Genius 10855 points
    Hi Roland,

    Thanks for reaching out on E2E!

    You are correct, if the three inputs are the same voltage (ie from battery), the device will start switching between the batteries once the hysteresis threshold is crossed. Depending on the output load conditions, the load battery voltage could dip slightly, which could cause the device to switch to a different battery.

    Can you provide additional details about the application? We might be able to find a different solution here depending on the system conditions.

    Thanks,
    Arthur Huang
  • 0 in reply to Alpha H
    Prodigy 50 points

    The primary issues are height and capacity.     We need at least 1.5Ah to meet the 3 year life requirement, but are limited on height.

    We had been considering using the Tadiran TLH-5934  1/16D  1.5Ah   but it is too tall to fit onto the plug-in card and clear the adjacent chassis wall.

    I thought about using two or more similar but shorter batteries in parallel.  But paralling batteries can be tricky.  We saw the TPS22933 and thought it may be ideal.

    The batteries we are now considering is the Tadiran TLH-5934  1/10D  0.9Ah   It has an operating temperature range of -55C to +125C

    If we use three of them with the TPS22933 we will meet the capacity / life  requirement and be well below the height maximum.

    The TLH-5934 can supply continuously 10mA and 50m bursts. 

    Using the TPS22933, Worse-Case, we are only going to pull 55uA.  15uA is given to the active channel of the TPS22933 the balance of 40uA to the microprocessor.

    I doubt the chosen battery voltage will sag under a 55uA load when selected by the TPS22933.  

    Using the TPS22933, when the computer is off the chosen battery will be sourcing 55uA.   When the computer is on, the drain drops to 17uA.

    The other two batteries on the TPS22933 will have a drain of 2uA (from the W.C. quiescent current spec of theTPS22933).

    My question stands, once a channel has been selected is there enough hysterisis (150mV ?) to keep the auto-switching circuitry from seeking another channel with similary voltage?

    If you have a recommendation for a different approach, I would gladly welcome it.

    Roland

  • 0 in reply to Roland Portman
    TI__Genius 10855 points
    Hi Roland,

    Thanks for the additional information.

    I think the TPS22933 will still be the best solution for 3:1 power muxing. You could always use a 2:1 power mux, which would require two components but it would circumvent this issue. I think the easiest way to test this behavior would be to use an evaluation board and test the configuration. As you mentioned, since the batteries could be draining / sourcing current in configurations that are depending on your loading and operating conditions, the results could vary slightly from the calculations.


    Thanks,
    Arthur Huang