Overdischarge protection for a li-ion powered circuit

[peu]

Good ideas, I found this ldo voltage reference: http://www.national.com/pf/LM/LM4121.html, it may do the trick.

One more question, do I need to use 2 pins? or could I compare this voltage reference to any internal value?

Thanks!

[milestag]

It looks like you would need 2 pins. GP0 is the A/D input and GP1 is the Vref.

But you can sometimes do multiple functions on a single pin.

In my circuit I have one pin performing 2 functions alternately as Digital output (driving an LCD) and Analog input (read battery voltage).

[Ron Marcus]

If you have a free on board A to D, it should be pretty easy. Get a silicon diode, say a 1N4148. These have a nominal voltage drop of .7 volts. The cathode goes to ground, and the anode is connected through a high value resistor, say 100K to B+. Connect the A/D input to the junction, and measure the voltage relative to the power supply voltage. As the battery drains, the number will rise, because the .7 volts becomes a larger portion of b+. Take samples at the target voltages, and tell the processor what to do at the preset trip points. You can even use an output pin from the processor as b+, to turn the circuit on or off for even more current savings. You may have to put a small bypass cap across the junction to ground as diodes can get noisy in forward bias. I'd try it without and see how it works first.

Ron

[Acetronics2]

It is not written in the specs ...

but the low to high voltage level change occurs for a quite precise voltage !!! ( TTL inputs ... of course).

a 1.5 v value, i.e.

so, a simple voltage divider is generally enough for these measurements

( a scope closer look with the RCTime function pin could enlight that ...)

Alain