Low volt indicator for LDO regulator

"Drop out" voltage refers to the difference in voltage between the supply and the regulated output voltage. Old-fashioned "high" drop out regulators like the LM78xx had drop out voltages on the order of 1.5 to

Even if you mean that your regulator drops out of regulation when your battery voltage is at 4.3V, that's still a 1V drop out voltage -- and that's not very low.

Look for an LDO that offers a "low voltage" pin. Or, make one yourself from a voltage reference, an op-amp, and a P-channel MOSFET or PNP transistor (it _is_ possible to tame the oscillations). When the transistor drive is pulled to ground -- you're out of regulation.

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

On a sunny day (Tue, 25 Oct 2011 01:19:15 GMT) it happened snipped-for-privacy@kentrend.com (Martin Evans) wrote in :

PIC, internal reference, some resistors, LED, decoupling cap.

Yes, why didn't I see that.

Mark

--- View using a fixed-pitch font:

-- JF

BD4945G (plus a resistor & LED), provided 10V abs. max. input voltage is okay. Will switch at 4.5V +/-1%.

Best regards, Spehro Pefhany

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.

you can set the output voltage, you cannot set the trip point of the error flag datasheet says it is fixed at ~5% below nominal output

and dropout is spec'ed as max 600mV@100mA

-Lasse

with a bit of tweaking with the output voltage you could just use another lp2951

if you can live with a slow rising error flag, a cap on it would do the same on a lp2951

the lp2951 is a real LDO, typ. 380mV@100mA and it is picky about the output cap

-Lasse

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.

Q: - Why it is impossible to have sex in the middle of the Red Square in Moscow ? A: - Because every bystander idiot would try to give his invaluable advice.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

Is that a lead acid 6V? If so then 4.3 is way too low. Generally you run those down to about 1.7V/cell minimum for a 5.1V shutdown with a 3- cell, anything less can cause damage.

Vlady, Did you post this jpg ----^^^^^ Didn't think it thru did you ?:-)

...Jim Thompson

Yes- and that is because the level of input voltage adequate to maintain regulation is load current dependent, whereas the regulated output voltage is not. Therefore using the output voltage to determine the trip point makes it automatically adaptive to its operating condition. This occurs at 5% below nominal output, and is stabilized- another smart idea from NS.

...Jim Thompson

Looks to me like that LED is going to be on perpetually...

Looks like that to me ;-) ...Jim Thompson

Not 15 cents, but I like the functionality:

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

Martin Evans

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