(TL;DR version in blue)
Again, I am already using one of those ICs (MCP73833) for charging. It's just discharging that I'm trying to work with.
I have a few problems with the + feedback setup that I'd like some help with:
Now, about the positive feedback. I did a few calculations which led me to the optimal resistor values for the setup. 2 problems with that:
1. The voltage gap for the triggering is way too small (3.7V when on, and 4.2V when off)
2. The resistor values I get are too small, and too much current will go waste as a result.
So I did a bit of trial and error, and changed only one resistor value, but I had to sacrifice on triggering the 'on' state. I'll explain my concerns with the
circuit diagram:
-The AC source represents the fluctuating (charging and discharging) Li ion battery (it's biased to go from 3V to 5V).
-The battery and switch on the left represent the USB supply.
-I've added a diode to the Li ion supply Because when I plug in the charging chip to USB, I would also want to supply the load, and If I connect the +ve rails together without a diode, the 5V source would pump current through the Li-ion battery and damage it. I'll lose 0.7V though. There's a better way for sure, but I couldn't think of anything else that didn't involve bulky relays.
The +ve feedback has created a band where fluctuations are allowed. (When V
bat reaches 3.7V, the (+) input is pushed down by 0.08V, making sure the output stays LOW even if there is a small rise in voltage due to the load going off.
Here's the problem though:
As the schmitt trigger would, I have a threshold to overcome when the output starts out LOW as well. That happens to be ~4.5V, which my source would never reach.
So, lets say that it's operating happily at around 4V and I decide to switch off the device and switch it on again. When I switch it on, the output starts at LOW, positive feedback increases the voltage gap, and it checks if I have a voltage of 4.5V instead of the usual 3.6V/3.7V to turn on again, which I don't, so it stays off.
I don't need 2 thresholds, just one, but as far as I can see, the feedback imposes the 2nd threshold, which prevents me from switching off and on the device at will.
Also, there's no point in trying to bring the threshold down to 3.7V because then the band gets too narrow and the effect of hysteresis is pretty much negligible.
Anything I can do about this ?
EDIT:Sorry, the link was bad