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I have a Li-Po battery which has a discharge voltage range of (2.5-4.2)V and I want a regulator which is able to convert this voltage to 3.3V. I was told to use a buck boost converter. I have found one which is a buck boost converter and the other a buck converter. Reading the descriptions of both, I think both will work but I would like to ask for your opinions or if you have any other recommendations of components that'll work. Here below are the datasheets of each component:

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  • \$\begingroup\$ What you were told was good advice - why did you ignore it and think a buck regulator would do the job? \$\endgroup\$ – Andy aka Apr 22 at 15:42
  • \$\begingroup\$ @Andyaka im not ignoring it. i just wasnt sure and i thought it would work as well. thanks for the confirmation! \$\endgroup\$ – jellybean Apr 22 at 15:50
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The maximum output voltage of a buck converter is slightly lower than the input voltage. Therefore if you use the buck design, you won't get 3.3 V output when your battery voltage drops below 3.4 V or so.

This behavior isn't really explicitly mentioned in the datasheet (except where they call the chip a "step-down" converter), because it's such a fundamental limitation of buck converters that people using them are expected to know about it.

There is one section that does at least allude to the limitation:

enter image description here

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  • \$\begingroup\$ thank you very much! \$\endgroup\$ – jellybean Apr 22 at 15:53
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You will be unable to boost(step up) the voltage with only a buck(step down) converter when the voltage is lower than the intended output, excluding the conversion losses you will have to have an input voltage of >=3V3 for an output of 3V3 with a buck converter

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  • \$\begingroup\$ thank you very much! \$\endgroup\$ – jellybean Apr 22 at 15:53
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If you use only a buck, you can explore the option of re-specifying your 3.3V rail to be a lower voltage, say, 2.7V. Then choose a buck that has an LDO mode (100% duty with linear voltage regulation) that kicks in when the battery is close to the target voltage.

Otherwise, you need to use buck-boost.

And, you should not drain the battery all the way down to 2.5V, it will damage it. More here: https://batteryuniversity.com/learn/article/confusion_with_voltages

Buck-boost integrated with charge / discharge control is available as a single IC from the usual places (TI, Maxim, Analog Devices, etc.) Worth a look.

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  • \$\begingroup\$ thank you very much! just to be sure if i understand correctly, if my nominal cell voltage is of 3.7V then i should only drain it to 2.8-3V? \$\endgroup\$ – jellybean Apr 22 at 15:56
  • \$\begingroup\$ Check the spec of your specific battery - different electrode types have different end voltages. But, yes, an end discharge of about 2.8V is better than 2.5, even if your cell is rated for 2.5. It will prolong the life of the battery. \$\endgroup\$ – hacktastical Apr 22 at 15:58
  • \$\begingroup\$ @jellybean As just mentioned it is told in the specification of the battery, however I would for long life time of the battery you should never fully charge the battery or discharge it \$\endgroup\$ – Anton Ingemarson Apr 23 at 7:03

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