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Undercharge prevention for a single LI-ion or Li-PO cellRechargeable Lithium Ion and Lithium Polymer cells have a ton of advantages over the old Ni-Cad cells, for a dozen reasons I won't go into here. But if you've worked with them you are no doubt aware that overcharge and over-discharge can lead to the rapid demise of a Lithium cell's ability to hold a new charge reliably. Well there are plenty of circuits dedicated to properly charging these cells, but there is always the looming worry of leaving your device on once placed in your project. You know it will happen sooner or later no matter how careful you are, and when it does a degraded (if not destroyed) cell is almost a certainty. What to do? Well here is a circuit I developed for the Elfin Technology's SOUNDMAN product, which has some components powered by a single LIPO cell. Basically, this circuit will allow power to flow normally until the cell voltage reaches about 3V, at which point it will become a virtual open circuit. How "open"? Well according the specifications of the key IC involved, its operational current will be only about 2uA in shutdown condition. Honestly, using a Fluke 87 digital multi meter, this writer could not even measure a full micro ampere in this state. To put this in perspective, Lithium-ion cells self-discharge about five percent in the first 24 hours and then lose 1 to 2 percent per month, according to this article on the famous "Battery University" website. For a small 300maH cell, it doesn't take much math to see that even 2uA really is near enough to a complete disconnection to make the difference irrelevant.
The heart of this circuit is a MicroChip MPS112-315. This is a chip normally used to force a reset signal to a micro controller when the voltage dips below a certain level. In this case that level point is about 3.08V, and there is some hysteresis involved. The voltage trigger levels are programmed into the IC, and you can buy several variations of the chip such as the MPS112-300, which triggers at 2.93V. My opinion though? Any Li-Ion cell at 3V has exhausted more than 95% of its capacity, and waiting to shutdown at lower voltages is not worth the risk to the cell. Some of the other voltage variants of this chip aren't easy to come by anyway.
One additional consideration. In the Elfin Technologies SOUNDMAN product in which this was included, additional voltage monitoring circuits were also incorporated. Remember, this circuit is great for protecting a cell from damage, but without some additional monitoring you still risk sudden shut down of your device at a very inconvenient moment. PCB Layout and prices
Not surprisingly, this is a pretty basic board. I typically do small projects like this as a single layer board with top copper only.
I have some of these blank PCBs available. They are "panelized" 4X as above, with silk screen labeling and solder mask on thin (0.82mm) board, to save you some trouble. You can cut them apart with a scissor. I also offer these as single fully assembled boards.
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So when the cell voltage dips below 3V, this MPS112 IC will drive its VOUT pin to 0V, which in turn is used to turn off a Fairchild FDC6331L "Load Switch" IC. This IC contains a convenient pair of CMOS FETS in a single package, and allows the control of loads up to 2.8A. Note that I've included the internal circuit diagrams of each of these ICs in the functional schematic. From this you can see how the concept could easily be expanded to situations where greater current capacity is required, simply by substituting your own higher capacity CMOS FETs for the Fairchild LoadSwitch IC. But for light duty applications, this circuit is pretty hard to beat.
For home DIY board making, you can copy this image for use as a top copper layout. Whatever size it scales to on your screen, it is still a 300dpi image, and should print to scale if copied (right click menu) and saved to your machine. Of course You'll also have to remember to "mirror" the image before printing, if you use a laser/toner method like PNP-Blue for fabrication. I've left all the board labels off this image, and "panelized" 4 of them together. The parts are simply the 3 items from the schematic. Both ICs are available from
