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I have a one of these:

Li-ion-3.7V-4000mAh

descriptions:

  • full charge voltage : 4.2V
  • cut off discharge voltage : 2.75V
  • capacity : 4000mAh
  • size : 18650

And I want to get 3.3V from it.

but the problem is I can't use regulators (such as AMS1117) because the drop out voltage of it is 1.1V and I should have a 3.3V + 1.1V = 4.4V at the input. And its impossible for my battery.

What should I do to have 3.3V until the battery discharges to 3.3V ?

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  • \$\begingroup\$ What current/range of current does your 3.3V load need? \$\endgroup\$
    – EM Fields
    Commented Aug 18, 2014 at 11:19
  • \$\begingroup\$ Less than 500mA. \$\endgroup\$
    – AHB
    Commented Aug 18, 2014 at 12:55

1 Answer 1

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Use a buck boost regulator - it would continue to produce 3.3 volts all the way down from probably over 5 volts to possibly 2.5 volts. Obviously you have to ensure that the Lithium battery doesn't sink too low or it will become damaged but that's another problem that is solved by using a comparator and a regulator shut-down circuit. Here's an idea: -

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Or....

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Or...

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Or maybe try using their search engine here

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  • \$\begingroup\$ Thank you. But none of those ICs are available in my location. \$\endgroup\$
    – AHB
    Commented Aug 18, 2014 at 9:58
  • \$\begingroup\$ Try texas instruments - they have a decent portfolio of switching regulators. \$\endgroup\$
    – Andy aka
    Commented Aug 18, 2014 at 9:59
  • \$\begingroup\$ Can I use a 3.3V zaner diode ? \$\endgroup\$
    – AHB
    Commented Aug 18, 2014 at 10:05
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    \$\begingroup\$ What would happen if you connected your battery (at 4.2 volts) to a zener diode of 3.3 volts - smoke! \$\endgroup\$
    – Andy aka
    Commented Aug 18, 2014 at 10:11
  • \$\begingroup\$ @BlueSky You have to say to Andy all things that you need. I know that what do you need then I'm going to say to Andy... \$\endgroup\$
    – Roh
    Commented Aug 18, 2014 at 17:08

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