[HH] low battery indicator: reducing voltage divider current

[Tom Metro]

Greg London wrote:
> I was hoping to figure out a way to turn off the voltage divider, but
> so far I havent figured out a way that doesnt do it with less than 50
> uamps.

You mentioned you were going to try putting a MOSFET in series with the
voltage divider. So you're saying even with a MOSFET in the off state,
you're still getting too much leakage current through it? Or the
MOSFET's support circuitry is leaking?

This doesn't sound like it should be a hard problem to solve.


As you mostly only carte about current consumption during the off state,
you could always use a reed relay. :-)  It'll get you zero leakage
during the sleep cycle, but likely double your circuit's consumption
during the wake cycle.

Though even with something like a reed relay that can handle very high
cycle counts, I wouldn't be keen on a design that cycles it every few
seconds perpetually.


The other thought I had was to use a much higher impedance voltage
divider, which you'd then need to buffer through an op-amp. Then the
challenge becomes finding a super low current op-amp and the
complication of adding anther part.


Another possibility would be to leave the bottom leg of the voltage
divider open, and use the input impedance of the A-to-D for that. Here
again you could switch to much higher value resistors. The input
impedance is probably not very consistent from part to part, but likely
stable over the life of the part. The upper leg would need to be a
variable resistor to allow for calibration, and that would incorporate
compensating for the unit variances in impedance. (There is a
possibility that the input impedance may vary as the A-to-D takes a
sample, depending on its design and whether it has buffering.)

I like this last approach the best, but you'd have to do some
experiments, first on the bench, and then in some real world conditions,
to make sure it isn't thrown off by noise.

Drew will probably have some better suggestions.

 -Tom