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Help required with low battery hysteresis

Discussion in 'Hobby Electronics' started by Andrew Gregory, Nov 5, 2005.

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  1. Hi,

    I'm working on a small step-up switch-mode power supply based on the
    MAX1797 that will use two AAs. The circuit was quite simple, until I got
    to the low battery hysteresis. I found an example circuit with hysteresis:

    <http://www.maxim-ic.com/appnotes.cfm/appnote_number/960>

    the differences being it uses a Lion battery with a shutdown voltage of
    2.9V and switch-on of 3.3V, while for my AAs I was planning on a shutdown
    of 1.7V and switch-on of 2.1V.

    Quick summary of the relevant MAX1797 pins:

    LBI is the Low Battery Input, the threshold is 0.85V.
    LBO is the Low Battery Output, open drain. Sinks current to ground while
    LBI>0.85V.
    SHDN shuts down the IC when high (disconnects the battery from the output,
    LBI/LBO continue operating).

    The example circuit uses a mosfet to switch a resistor in and out of
    circuit to adjust the LBI voltage divider, but doesn't otherwise identify
    the component to use.

    My knowledge of transistors is virtually nil - I have no idea how to go
    about selecting a suitable transistor, or even if there is an alternative
    method of handling the hysteresis.

    I'll note that I already have the IC, inductor, caps and some resistors,
    all SMT, so I'd like any extra components to be SMT too.

    I understand the low voltages involved make things a little tricky. If it
    helps (I'm pretty sure it won't), there's no need to recover after a
    shutdown - it can stay shutdown until the battery is disconnected and
    reconnected.

    Any help, hints, advice, etc on how I should go about implementing this
    low battery hysteresis would be greatly appreciated.

    Thanks,
     
  2. Any "logic level" FET should do the trick here.
    I'd just go with the solution presented, at first glance that seems to
    be the easiest way to add the required hysteresis, just select the
    appropriate values for R1, R2, and R3 for your requirement.

    Dave :)
     
  3. Hi David,

    Thanks for your response.

    That implies I know enough to identify a "logic level" FET! :)
    Yes, figuring out the resistor values won't be hard, I'll use a
    spreadsheet for that. The problem is still selecting an appropriate FET.

    I've done some more research and found some terminology I don't
    understand. "Enhancement mode" and "Depletion mode". It appears the
    example circuit I mentioned
    <http://www.maxim-ic.com/appnotes.cfm/appnote_number/960> uses a
    "depletion mode" FET. I assume the difference is important? If so, I've
    had a really tough time finding even *one* depletion mode FET from RS or
    Farnell (DSE, Jaycar, Altronics don't do SMT MOSFETs - are there any other
    Australian suppliers of components?). Every MOSFET seems to be enhancement
    mode.

    In fact, I've only found one depletion mode MOSFET in my preferred SOT-23
    package size:

    <http://au.farnell.com/jsp/endecaSearch/partDetail.jsp?SKU=743641>

    Naturally, it's "out of stock" :-/ Would that be suitable in your opinion?

    RS also have exactly one (that I've found, anyway) depletion mode MOSFET,
    a BSP149, but it comes in an SOT-223 package. I'd give a link, but as I
    write this the RS site is down.

    Thanks.
     
  4. Simple, it's a FET that is advertised as being "logic level" or
    "digital"! :->
    Most FETs designed to operate at logic voltage levels (eg. 5V TTL, 3.3V
    etc) will be advertised as such. That is what you need here as the FET
    is just being used a switch. Eg. 5V on the gate and it switches on, 0V
    and it switches off.
    Yes, most are enhancement mode.
    Try here for an explanation:
    http://en.wikipedia.org/wiki/MOSFET
    You don't need a depletion mode FET, just a "logic level" or "digital"
    N-Channel one.
    Farnell have oodles of logic level FETs
    The very first device I found in the Farnell MOSFET section should do
    the job, the FDV301N $0.65ea (qty 5 min) SOT-23 package.

    Regards
    Dave :)
     
  5. Oops, sorry, that device has a switch on voltage (Vgs) of just under
    2.5V, you need 1.8V due to the open drain output and pull-up to VBatt.

    Try the Si1032R instead which is even cheaper and SOT-23 also (Farnell
    393-5486)

    Dave :)
     
  6. OK, I'll give that one a go.

    Thanks!
     
  7. I've now put it all together and it works great! Thanks for your help!
     
  8. No prob.
    Good work!

    Dave :)
     
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