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I want to measure Li-pol battery voltage with STM32 MCU. I power STM32 with 3V so I need a voltage divider for battery connection (Li-pol max voltage is 4.2V). Moreover, I'd like to add N-channel MOSFET to prevent battery discharging through the resistors.

There are a lot of similar topics here, but most of the decisions use extra BJT to drive a FET (Low Current Battery Monitor Detail Question) or a capacitor to automatically turn off the transistor (Selecting low power mosfet for battery monitor).

I'd like to use only one MOSFET in such a way: enter image description here

The only problem with this circuit (as I see) is the voltage at the source pin.

For example, if I use R1 = R2 = 10K then in open state there will be 2.1V at the source. MCU GPIO maximum voltage is 3V and 0.9V (3 - 2.1) will be not enough for FET to remain in open state.

But if I use R1 = 20K, R2 = 10K the maximum voltage at the source will be 1.4 V and 3V from GPIO should be enough (V gate threshold = 1.06V).

Are there any other problems with this circuit because of which it is so unpopular?

Spark
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  • No problem with adequate latency – Tony Stewart EE75 Jul 07 '18 at 14:32
  • What about a current mirror? You could use a small sensing resistor for the MCU. You could intentionally set the voltage range for the MCU. The mirrored current would drop as the Vbat drops. Google search for images MOSFET current mirror. This would increase parts list. I note that it may take 3 transistors to realize a mostly off setup controlled by the MCU pin. You may be able to realize a lower 'on' current this way too. – Jaden Jul 07 '18 at 16:28
  • @Jaden thank you! It's critical for this device to decrease parts list and the cost. – Spark Jul 07 '18 at 17:40
  • What is your intention? I presume you want to check the battery once an hour or some long interval to monitor it's level. – Jaden Jul 07 '18 at 17:56
  • @Jaden Maybe a little more often, I think the check period will be 10 minutes or less (depending on the user actions). – Spark Jul 07 '18 at 18:35

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