16 years ago
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 of
2.9V 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
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
Any help, hints, advice, etc on how I should go about implementing this
low battery hysteresis would be greatly appreciated.
-- Andrew Gregory