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I am designing power supply for IoT low power. The circuit is powered from 3.6V lithium battery. The system includes:

  • MCU: STM32L151, 1.65 V -> 3.6V
  • LTE Module: 3.4V -> 4V, Imax < 0.2 A
  • and sensors 3V - 3.6V

I'm wondering whether I should use direct power from the battery (3.6V) or integrate an additional power supply control IC.

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2 Answers 2

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That 3.6V from the lithium battery is only its nominal voltage. In reality the voltage varies from over 4 volts to less than 3 volts.

enter image description here Image source.

You'll need to use a regulator of some sort as mentioned in this earlier question.

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  • \$\begingroup\$ As your suggestion, I'll need to 2 regulators, first for 3.3V MCU and second for ~3.8V LTE \$\endgroup\$
    – Kay Juu
    Commented Feb 29 at 15:19
  • \$\begingroup\$ Take a good close look at the LTE module datasheet. It may run just fine on 3.3V. \$\endgroup\$
    – JRE
    Commented Feb 29 at 15:36
  • \$\begingroup\$ Unfortunately, I checked the module when running below 3.45V voltage, it didn't work \$\endgroup\$
    – Kay Juu
    Commented Feb 29 at 15:50
  • \$\begingroup\$ Then the answer from Andy aka is your best bet. \$\endgroup\$
    – JRE
    Commented Feb 29 at 15:51
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These two requirements can be made the same i.e. a 3.3 volt regulated supply: -

  • MCU: STM32L151, 1.65 V -> 3.6V
  • Sensors: 3V - 3.6V

For this requirement: -

  • LTE Module: 3.4V -> 4V, Imax < 0.2 A

I would consider using a buck-boost converter set to deliver close to 4 volts (maybe 3.8 volts as you said in a comment to another answer). This will allow the battery voltage to drop below 3 volts or be as high as 4.3 volts (or more).

Then, I'd use a linear regulator to drop down from 3.8 volts to 3.3 volts for the MCU and sensors. I'm presuming here that the MCU and sensors are not current hungry but, if they are maybe an additional buck converter might be necessary (although it's probably unlikely).

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  • \$\begingroup\$ Sorry for the lack of clarity in my post. In sleep mode, only the power to the MCU is enabled to save energy. In active mode, I will turn on the LTE power about 30 seconds to read sensor data and send it to the cloud, then turn it off. Therefore, I cannot use the 3.8V LTE power for converting to 3.3V for the MCU \$\endgroup\$
    – Kay Juu
    Commented Feb 29 at 16:10
  • \$\begingroup\$ You can still use what I describe but disengage sensors and LTE with MOSFETs instead. You can find a buck-boost that can be pushed into low power operation I suspect. \$\endgroup\$
    – Andy aka
    Commented Feb 29 at 16:14
  • \$\begingroup\$ As your solution, I have to always keep both 3.3V and 3.8V on. I think it would be better if I always keep the 3.3V on and only turn on the 3.8V when necessary \$\endgroup\$
    – Kay Juu
    Commented Feb 29 at 16:28
  • \$\begingroup\$ OK, I get you. Run the MCU from an LDO 3.0 volt regulator then, as soon as you need to power the 3.8 volts, swap the input of the 3.0 regulator to the 3.8 volt output using a MOSFET (for example) to ensure that sensors and MCU receive a 3.8 volts feed that will work even if the battery droops down to 2.8 volts. Is that what you mean @KayJuu \$\endgroup\$
    – Andy aka
    Commented Feb 29 at 17:29

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