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I'm building a circuit powered by a small Li-Ion battery. The battery will be connected to a 3.3V LDO, which then powers the entire circuit.

Li-Ion batteries are damaged if they are allowed to drain below about 3V. There are chips to do this LVC cutoff.

But I figure the LDO will already have the pass transistor and voltage reference, so it seems wasteful to add another part. This seems to be the kind of thing that could be easily integrated.

Does anyone make an LDO with low-voltage cutoff? Parametric searches and Google seem to be failing me.

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I have never seen ready-made ICs with such a feature.

There are, however, ICs that have a shutdown (or enable) input that might be used for such a feature when you add an external comparator and/or reference.

You might also want to consider building your own LDO, using a PMOS (or pnp) pass transistor, a reference (LT1431, LM4121-ADJ, LM4051-ADJ or even TL431 come to mind), some discrete components and some sort of 2nd comparator for the under-voltage lock out.

Take care about the regulator's stability (ESR of output capacitor must neither be too small nor too large, cf. "Tunnel of Death"). These links are helpful:

Short and good Application Note by NSC

A paper that offers some really good theroetical background

An entire thesis by the same author as the paper, also a good read

Building your own LDO has the disadvantage that you don't get features like thermal shutdown or short-circuit protection without additional effort, but it offers a great deal of flexibility (and it's fun).

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  • \$\begingroup\$ This is for a real product, so rolling my own LDO isn't feasible. \$\endgroup\$ – markrages Feb 8 '11 at 8:28
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    \$\begingroup\$ O.K., then I suggest using an LDO with an enable pin. There are plenty... Hmmm. Should I feel bad because I've rolled my own LDO for a real product? Maybe not -- as long as I don't get field returns which have a root cause that can be attributed to this part of the board. \$\endgroup\$ – zebonaut Feb 8 '11 at 18:25
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    \$\begingroup\$ I don't want to roll my own LDO because an integrated one costs less than a quarter and (more importantly) 6 mm^2 real estate. \$\endgroup\$ – markrages Feb 11 '11 at 3:58
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One option is the LT1120, which has a comparator, reference and shutdown pin in one package. It's a simple enough matter of tying them together.

You can also get LDOs with a UVLO pin. The UVLO pin has a reference comparator. It could be something like UVLO falling below 1.25V or so triggers the shutdown of the regulator. Tie a voltage divider to the UVLO pin to tweak the shutdown point. Here's one example; it's for USB, but I'd expect you'll be able to find more suitable chips.

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  • \$\begingroup\$ LT1120 looks interesting, but the voltage range is wrong for a single Li-Ion cell. And the comparator output needs to be inverted to drive the shutdown pin, necessitating at least an external transistor. \$\endgroup\$ – markrages Feb 10 '11 at 7:40
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    \$\begingroup\$ So far the front-runner is an TC54VC2902 and TC1014-3.3 combo. This is a two part solution, $0.30 + $0.24 in 1k. (LT1120 is $2.55 in 1k.) \$\endgroup\$ – markrages Feb 10 '11 at 7:48
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This looks like an old question but there is a new (modern) answer: Microchip has introduced their MCP1791 regulator: 70 mA, 30 Vdc max input, surge input 48 Vdc (they don't say for how long).

This is not a true LDO in that the minimum input voltage is 6.0 Vdc for a 5 Vdc output but it has several features that are useful:

1) the output voltage turns OFF when the input voltage drops below about 4 Vdc.

2) The PG output goes LO (sinking) when the output voltage drops below about 90% of rated output.

Combine those two features and you have a voltage regulator that turns OFF cleanly when the input voltage is too low.

This part is rapidly becoming my "go-to" regulator for certain designs.

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