I have a one of these:



  • 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 ?

  • \$\begingroup\$ What current/range of current does your 3.3V load need? \$\endgroup\$ – EM Fields Aug 18 '14 at 11:19
  • \$\begingroup\$ Less than 500mA. \$\endgroup\$ – AHB Aug 18 '14 at 12:55

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 maybe try using their search engine here

  • \$\begingroup\$ Thank you. But none of those ICs are available in my location. \$\endgroup\$ – AHB Aug 18 '14 at 9:58
  • \$\begingroup\$ Try texas instruments - they have a decent portfolio of switching regulators. \$\endgroup\$ – Andy aka Aug 18 '14 at 9:59
  • \$\begingroup\$ Can I use a 3.3V zaner diode ? \$\endgroup\$ – AHB Aug 18 '14 at 10:05
  • 2
    \$\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 Aug 18 '14 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 Aug 18 '14 at 17:08

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