Electrical – ON/OFF push button circuit with battery under voltage protection

battery-operatedlow-batterypower supply

I have put together the push button circuit explained in TI's SCEA048B application report with an under voltage protection using TI's LMS33460 chip. I ended up with the circuit attached.
The idea is to have the LMS33460 under voltage detector monitoring the +PACK line from the battery so it pulls down the regulator enable pin to power down the device in case the battery voltage falls bellow 3v, but I have several doubts:

  1. After doing the schematic I realized that I should use an under voltage detector that triggers at 3.4v instead of 3v. I think this because the 3.3v LDO regulator has a maximum dropout voltage of 250mV and the TM4C123GH6PM has a minimum operating voltage of 3.15v, so 3.15v + 0.25v = 3.4v.. under this battery voltage the microcontroller will not have enough voltage supply. But, since I am using a 3.7v lithium battery, this would only give 3.7-3.4 = 300mV of margin for the battery to discharge.. I don't know much about lithium batteries but that doesn't seem to be a lot… is that a problem?

  2. I haven't found the input resistance of the TLV70033's Enable pin in the datasheet, so how can I know the maximum pull down resistor (R33) to minimize consumption and still pull down the voltage to turn off the regulator. Anyway, this pull down resistor is not needed anymore after connecting the enable line to the flip-flop, since the flip-flop's output is not an open drain and it will command the line high or low without ambiguity, right?.

  3. Do you think this would work? : )

  4. Is there a better way for protecting the battery?


ON/OFF push button circuit with battery under voltage protection

Best Answer

That is what "D" input on SN74LVC1G175 is for. In application note it always transfers Vcc ("1") to the output. If you feed it the state of voltage detector you will essentially transfer battery state to the output (although you need inverted detection signal from the one of LMS33460)

As a side note, I don't see a reason for not using something like STM6601 which does everything you want and more in one tiny package.