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In a battery powered application, it might be tempting to run a LDO in dropout mode as a LiPo battery falls from 4.2V to below 3.3V. Is this a normal thing to do, and would most regulators perform efficiently in dropout mode, or do you just have to select carefully for this quality based on the datasheet?

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  • \$\begingroup\$ I would let it ride the battery down, but I'd pick something closer to the maximum limit of the components so that there's less power dissipation with a full battery. \$\endgroup\$ Commented Jan 9, 2015 at 16:22
  • \$\begingroup\$ @IgnacioVazquez-Abrams many parts draw less current at lower voltages. So a lower-voltage LDO results in less power usage overall. (since current-in and current-out are about the same for a reasonable LDO.) \$\endgroup\$
    – markrages
    Commented Jan 9, 2015 at 22:40

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You need to carefully select this.

I looked at this about a year ago. Lots of linear regulators have high (about 500 μA) waste (ground pin) current when they are in dropout. This may or may not matter to your battery.

Linear Technology had a part that was nicely characterized for this. Like all LT parts, it was not the cheapest choice. Sorry I don't have the part number handy. I think Micrel also had one.

If I may rant, LDO selection tools are really annoying when you're trying to select for a certain characteristic like noise or current drain in dropout. Instead they only seem to be concerned with dropout voltage... I couldn't care less if it's 100 mV or 50 mV but the marketing department sees "low-dropout" in the name and assumes that's the only spec everyone wants to see.

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Most CMOS LDO regulators are fine, but some of the older bipolar types with lateral PNP pass transistors draw relatively huge current in dropout. So, yes, it's always good advice to carefully read the datasheet, and especially so in this case.

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