# LDO with low-voltage cutoff?

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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–  endolith Feb 9 '11 at 19:31

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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This is for a real product, so rolling my own LDO isn't feasible. –  markrages Feb 8 '11 at 8:28
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. –  zebonaut Feb 8 '11 at 18:25
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. –  markrages Feb 11 '11 at 3:58
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.) –  markrages Feb 10 '11 at 7:48