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LVCO and low battery indicator

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bwilkins

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I'm trying to both protect a two cell LiPo pack and also indicate to a human when the pack is getting low. I chose to use a pair of voltage supervisors due to their low current consumption. One kills the system when the voltage drops to roughly 6.2V, simple enough. The other is used to switch from a green LED to a red LED when the voltage is getting near (0.5 V or so) to this cutoff. The outputs of these PMIC supervisors are active low.

I'm dividing down my battery voltage to match the actual voltage level to the internal reference voltage (3.08V) at my desired trigger point. When I hit this point, the base of an NPN controlling the current through my green LED is pulled low and turns it off as desired. My problem is getting my PNP controlled red LED to turn off when the voltage is above my trigger point. My initial thoughts were that at sufficiently high voltage, the supervisor IC would pull the base high and stop the current flow through the PNP and this the red LED. Turns out that since I divided down the voltage to be monitored, I can't get the output to be close enough to battery voltage to turn off the transistor. Anyone know the 'correct' way to fix my issue?

- Brent
 

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