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Overdischarge Protection with N-MOSFET and MSP430F6635

Efstratios Petrou
Prodigy 140 points
Other Parts Discussed in Thread: MSP430F6635

Hi forum users,


I try to implement an easy way to protect an Li-ion cell from overdischarge.

The battery gauge TI BQ27410 sends every second the battery voltage to the MSP430F6635 and if the value 3.2V is reached, then the output of the MSP drives the gate to low and the device shuts down momentarily. But, instead of staying low, the Vgs rises again to 1V, the mosfet closes and the device restarts.

My goal: When 3.2V is reached, microcontroller switches off the mosfet and the whole system. If this happens, then through USB the MCU will start again and it will switch on the Mosfet again and a charging sequence will start over.

Thanks in advance for your ideas,

  • 0
    Guru 46710 points

    Efstratios Petrou said:
    The battery gauge TI BQ27410 sends every second the battery voltage to the MSP430F6635 and if the value 3.2V is reached, then the output of the MSP drives the gate to low and the device shuts down momentarily. But, instead of staying low, the Vgs rises again to 1V, the mosfet closes and the device restarts.

    When you (properly) use msp430 low power features, you protect battery against discharge not by disconnecting battery but by not discharging it.

    If for some unclear reasons you still want to use battery disconnect, I don't hink "-" terminal switch is right for device which is powered from same battery you disconnect. You shall be using high-side switch instead.

  • 0 in reply to Ilmars
    Prodigy 140 points

    Ilmars said:

    If for some unclear reasons you still want to use battery disconnect, I don't hink "-" terminal switch is right for device which is powered from same battery you disconnect. You shall be using high-side switch instead.

    I implement all these that you mentioned...Low power modes and high-side switches for every load of the system. The low side switch at battery is the worst-case solution of the scenario that the user will forget his device in a drawer for a really long time, maybe years. That's all!

    My question is...how does the mcu behave when such a case occurs...an mcu output drives low a n-mosfet, which shuts down the whole system and the mcu too? as you can see the gate voltage goes low but after some microseconds goes to 1Volt (Vgth) and the n-mosfet turns on again.

    Thanks in advance for your answer!!! 

  • 0 in reply to Efstratios Petrou
    Guru 46710 points

    Efstratios Petrou said:
    The low side switch at battery is the worst-case solution of the scenario that the user will forget his device in a drawer for a really long time, maybe years. That's all!

    Do you have specification of selected battery? Please share - what's capacity and self-discharge current.

    Efstratios Petrou said:
    My question is...how does the mcu behave when such a case occurs...

    When you disconnect "-"terminal of the battery, both VCC and GND rails of msp430 becomes virtually connected to "+" terminal of the battery, all pins of msp430 shall be considered connected to "+"terminal so gate of mosfet either. Why don't you simulate this using spice and see for yourself what happens? :)

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