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Low volt indicator for LDO regulator

Discussion in 'Electronic Design' started by Martin Evans, Oct 25, 2011.

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  1. Martin Evans

    Martin Evans Guest

    I an running a battery powered circuit that uses a 3.3V LDO regulator.
    The drop out voltage is 4.3. I would like to add a LED indicator to
    the circuit to show when the battery voltage has fallen below about
    4.5V (ie. LED goes on of off). Can anyone please offer a simple
    circuit to achieve this?

    Martin Evans
  2. Martin Evans

    Martin Evans Guest

    Yes, that is what I meant.

    Farnell has the LP2951 which has a low voltage error flag. The
    datasheet says it can be "pin strapped to 3, 3.3 or 5V", but I cannot
    find any further details there or online.

    Is anyone familliar with this procedure?

    Martin Evans
  3. Martin Evans

    Martin Evans Guest

  4. On Tue, 25 Oct 2011 01:19:15 GMT, the renowned
    BD4945G (plus a resistor & LED), provided 10V abs. max. input voltage
    is okay. Will switch at 4.5V +/-1%.

    Best regards,
    Spehro Pefhany
  5. Joerg

    Joerg Guest

    One of the cheapest methods is to use a reset chip such as this:

    They make a 4.5V version if you don't want a resistive divider. These
    come in open drain and push-pull versions. That would reduce the whole
    effort to one lone part of about 15 cents, so forgive me if I don't draw
    a schematic :)

    A reset chip has another advantage in that you can set a time delay. In
    other words you can keep it indicating "low" even if the batery just
    have a wee fainting spell of short duration because some load came on.
    All this feature requires is the investment into a 1-2 cent ceramic cap.

    As for drop-out, as Tim wrote, 1V isn't really LDO but marketeers
    sometimes call it that because it sounds cool. Be careful with real LDO
    regulators. Besides being iffy in stability with low ESR caps they often
    have some undocumented issues. Such as going berserk when the source
    impedance (your batery) becomes too high.
  6. Joerg

    Joerg Guest

    I will not use that series. In fact, I won't use LDOs in general if at
    all avoidable, and usually it's better to use a switcher anyhow. Had a
    very bad experience with the LM2931 which a client used against my
    advice. Sure, I got it stable. But it went berserk when the source
    impedance became too high and this was nowhere mentioned in the
    datasheet. In fact, it even surprised the manufacturer who did not have
    a solution short of a redesign of the IC.

    I'd rather use one that has a NPN or N-channel pass element and a charge
    pump. You can get even lower drop-out that way and it's stable. That
    would be an LDO I'd use but so far had to roll my own because commercial
    ones were either unavailable or prohibitively costly.
  7. Martin Evans

    Martin Evans Guest

    Not 15 cents, but I like the functionality:

    Would the above IC also work as an audio clipping indicator?

    Martin Evans
  8. What am I missing? - it looks okay to me (though the threshold for the
    LED turning off will be lower than 4.5V).

    Best regards,
    Spehro Pefhany
  9. Joerg

    Joerg Guest

    Looks like the 7665 is way too sluggish for that in terms of reaction
    speed. The NCP series chips could do that. However, keep in mind an
    intricacy of such chips: They do not have a separate supply voltage.
    Input is their supply. This means that at very low input amplitudes
    (well under 1V, usually) they "let go" because there is nothing left to
    coax the output FET into a conductive state. So you'd see a false alarm
    for very low audio levels unless you pre-bias the chip. For audio
    clipping indication I'd use a comparator.

  10. Okay, I'm kind of ignoring the numbers the OP used for dropout,
    because I think he added a margin to the actual dropout.

    Most of the LDOs I've used have dropout voltages of a few tens or
    hundreds of mV at moderate currents (battery power). Even old bipolar
    ones like the LP2950, so << 600mV.

    If he wanted to increase the ~4V LED "off" to 4.5V, a BE resistor
    would do it (>1V dropout, or "saturation" voltage)

    But if you postulate an old semi-low-dropout like the LM1117 (>1V
    saturation voltage), no it won't work properly as shown.

    Best regards,
    Spehro Pefhany
  11. ehsjr

    ehsjr Guest

    _Off_ perpetually, assuming the LDO in/out delta is always > ~.7

  12. ehsjr

    ehsjr Guest

    Well, I meant to reply to Bitrex, so I was in error. You just
    agreed with his statement, after already pointing out that there's
    a problem with the circuit. In any case, the on/off thing is like
    a typo, very easy to type (or think) off when you mean on or vice
    versa. You pointed out the important factor: the regulator chip
    in/out delta.

  13. ehsjr

    ehsjr Guest

    I don't like using battery power to indicate the battery is low. :-(
    I prefer auto power off and "negative indication", something like
    a led that goes out, or the gizmo being powered stops working.
    'Course that's not always a good idea - and that does make it an
    interesting problem. :)

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