Standard Simplest Battery Voltage Monitoring Circuit | 🧨 Engineering Schematic Solutions

Intro

If you have battery powered device here is the simplest schematic solution on discrete components to achieve low power consumption in sleep mode.

Issue Description

If you just connect voltage divider to the battery to measure voltage it will drain current always, even if the resistance is high that cause much more lower work time is save (sleep) mode. For example, when BAT=4V,I=4V/(100k+110k)=20.0 uA

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e.g. for ESP8684 MCU current in Deep-sleep mode is only 5 uA:

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That means any additional uA will affect on final device standby time significantly! 😮

Solution (Simulation)

You need positive power switch to be able to turn off and turn on (control) of powering of the voltage divider.

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As you can see current during measuring is higher by 40 uA, BUT in the save mode it's only 0.205uA (and depends on the used MOSFET):

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🟢 Enabled (MCU GPIO: 3.3V) -> I=60uA

🔴 Disabled (MCU GPIO: 0V) -> I=0.2uA

PCB Implementation (KiCad)

We don't need high power and want to save space on PCB, so just use P-FET + N-FET in one case (e.g. LBSS8402DW1T1G 50V-0.13A-5.6Ohm in SOT-363 package).

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Conclusions

🟢 This is a common (standard) solution of the battery voltage monitoring and it needs two pins: EN and ADC

🟢 Most likely you also have another power rail (such as 3V or 3.3V) so to save one pin you can use the same control pin to enable/disable power of the peripherals + battery voltage dividers

🟠 This schematic is not ideal because we still have small leakage current (less than 1 uA) and hard to measure output signal, because only small part of range is used. Instead of this chip can contain integrated OpAmp, that can stretch 3...4.2V to 0...3.3V to achieve best accuracy

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