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I want to use the thermistor inside of a battery pack on the BQ25887 and on a GPIO pin on a microcontroller so that both the microcontroller and the charging IC can monitor the battery voltage. For reference, I'm using the ATMega32U4 with a 5V regulator.

If the charging IC is powered, then I want the thermistor to be used on the BQ25887. If it is not, then I want the thermistor to be used on the microcontroller. I'm trying to think of a way I can use two switches to switch between the charging IC and the MCU. I can't use a Optocoupler since that takes up too much space on my PCB. Here is what I have now, and here are the questions that I have.

  • Can NPN transistors be used as High Side switches like this?
  • If no current is flowing through the base, will the transistors truly be open? My concern is current backfeeding into either the LDO-5V line or BQ25887.

enter image description here

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It depends what voltage you want to deliver to the thermistor.

Remember you need to drive the base of the BJT about 0.6 or 0.7 V anniversary the emitter. So if you only want to power the thermistor with 2.5 or 3.3 V, then you have no problem.

But if you want to deliver the full 5 V to the thermistor, then you need to find a way to provide 5.6 V or so to the base. This can be done, for example using a boost converter circuit, even a very crude one.

But the more usual solution, when keeping the circuit simple is a priority, is to just use a PNP BJT instead of NPN, and invert the control logic.

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  • \$\begingroup\$ Wouldn't the thermistor always have a voltage lower then 5V across it in the circuit shown? \$\endgroup\$
    – Jay
    Aug 22, 2021 at 16:24
  • \$\begingroup\$ @Jay, it depends what is attached to the I/O1 and I/O2 pins. If you can apply, for example, 8 V there, then you can get more than 5 V at the thermistor (but then power would be coming from the I/01 or I/O2 pin, not from the BQ25887 or LDO-5V inputs). \$\endgroup\$
    – The Photon
    Aug 22, 2021 at 17:45

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