Lion 5V Power Supply...

[MrMacman2u]

Hello all I have a possibly brain puzzler for you all!

I've poked through these forums and wasn't able to find a question (or answer) that was close enough for me to really build off of so I decided to do something all forum goers dread seeing from a new member... Start a new thread.

I have a large quantity of Li-ion cells (mostly 18650's) hanging around not earning their keep (or storage space) and I have been thinking of ways to use them to make my life easier.

One way I thought of is to use them as a portable power/recharge source for the multitude of portable electronics I have that charge via USB.

So this is where my limited knowledge of power supplies is biting me in the rear.

The amperage draw should never exceed 1 amp and I would prefer to have something to buck down the 8.4 volts from 2 cells in series OR boost one cell up.

I would like the circuit to produce a "solid" 5 volts (+/- .5) under a variety of loads and finally, perhaps most importantly, provide some indication (even a hard shutdown will suffice) when the supply voltage drops below a certain point.

Now for the part that will make anyone still reading tell me to sod off... It's not critical, but I'd prefer to not need an IC, I like my discrete components.

Oh, I know I could use a simple regulator for the 2 cell arrangement, but that strikes me as wasteful to shunt the excess power as heat.

Hope I'm not being too much of a PITA!

Thanks in advance!

 

[Electronworks]

The best way to do this is using an IC. It will be far easier than wiring up a whole ton of discretes. For a dual Li+ stack, I would suggest a MAX730A. This will buck the voltage down to 5V with the minimum of fuss.

Your input voltage will be 5.4V to 8.4V and the 730A can cope with this. You will get an inherent current limit anyway from the buck nature of the dc/dc and more importantly, Maxim give out free samples... and it is in DIP package

Hope this helps

 

[MrMacman2u]

Fair enough I'll get that IC on order and start laying out the pcb and building the supporting hardware. I'll post the end circuit diagram and maybe a few pics when I get everything together

Oh yeah, any ideas on the low voltage notification and/or cut out? These are unprotected cells and I'd like to have some kind of cut out protection or notice that they are reaching a minimum allowable voltage in circuit.

I have found a few low voltage "alert" circuits but most of the are more complex than the space shuttle...

Thanks again!

 

[Electronworks]

You can do this one of 2 ways. While you are on the Maxim website, take a look at their comparators - MAX931. Hook this up to 2 resistors to set your trip threshold of your warning. Failing that, use one of their microprocessor supervisors, but here you are stuck with the voltage they give you (and chances are, the one you want they will not have in stock)