I'm working on a small step-up switch-mode power supply based on the MAX1797 that will use two AAs. The circuit was quite simple, until I got to the low battery hysteresis. I found an example circuit with hysteresis:
the differences being it uses a Lion battery with a shutdown voltage of2.9V and switch-on of 3.3V, while for my AAs I was planning on a shutdown of 1.7V and switch-on of 2.1V.
Quick summary of the relevant MAX1797 pins:
LBI is the Low Battery Input, the threshold is 0.85V. LBO is the Low Battery Output, open drain. Sinks current to ground while LBI>0.85V. SHDN shuts down the IC when high (disconnects the battery from the output, LBI/LBO continue operating).
The example circuit uses a mosfet to switch a resistor in and out of circuit to adjust the LBI voltage divider, but doesn't otherwise identify the component to use.
My knowledge of transistors is virtually nil - I have no idea how to go about selecting a suitable transistor, or even if there is an alternative method of handling the hysteresis.
I'll note that I already have the IC, inductor, caps and some resistors, all SMT, so I'd like any extra components to be SMT too.
I understand the low voltages involved make things a little tricky. If it helps (I'm pretty sure it won't), there's no need to recover after a shutdown - it can stay shutdown until the battery is disconnected and reconnected.
Any help, hints, advice, etc on how I should go about implementing this low battery hysteresis would be greatly appreciated.