Thanks guys - I probably left quite a bit of background out of the first post but I'll add it in here so you can see what I'm looking for.
I want to use a single Li-Ion or Li-Poly cell to power a range of different circuits. Every circuit will have it's own unique power requirements but every circuit will also have USB connectivity. Because I'll already have a USB connector in any design that uses this, I'd like to use that connector to recharge the cell. So in terms of this specific circuit, I'm after the following logic:
* If the USB cable is disconnected, the cell provides power to drive the circuit
* If the USB cable is connected and the power requirements of the circuit are less than what is being drawn from the USB port, the remaining current is used to charge the cell (when required)
* If the USB cable is connected and the power requirements of the circuit exceed the USB supply, the cell augments the power provided by USB
From this, the output of this circuit should be power in the range ~2.7 - 5.5V (from discharged cell only through to full cell and then USB connected). I am comfortable with everything downstream (on the load side) of the power MUX.
In terms of the power MUX, my first preference is a simple diode OR gate design. The main issue with this is the voltage drop of the diodes - even if I use Schottky, at high currents there is still a significant (ie ~400mV+) voltage drop. At the low end of the cell I need to determine if this is acceptable. The other option I considered is a FET based solution - much lower voltage drop but more complex. Either way, I'm pretty happy with the options for the MUX (although I'm always open to advice).
So the key requirements for power management include:
* OVP, UVP and OCP (at a minimum) for the cell
* High accuracy capacity meter with uC interface
* Ability to charge from USB (ie ~4.5-5.5V)
* From a design perspective, small size is preferred over lower cost however no BGA components
My first take on this was to roll my own solution with a simple uC as a dedicated PMIC, interfacing to the cell/USB through a couple of DACs, a current sense and control FETs. I steered away from that as a solution because I don't believe I can manage it in a smaller area than dedicated PMIC devices, and I didn't want to spend the time/cost developing and debugging a proprietary system.
Second step was to look at dedicated PMIC from a range of manufacturers. I find it hard to believe that a single Li-Ion/Li-Po charged from USB is a novel/unique requirement, but I struggled to find any solution which could do protection, capacity monitoring and charge management in the one IC. Most manufacturers I looked at broke these three out into individual chips (although I could find charge + protection and capacity + protection).
So to take an example (and I haven't finalised a decision on these components, but they appear acceptable) - I could use bq2970 as the protection chip (
http://www.ti.com/product/Bq2970), bq78z100 as a gas gauge (
http://www.ti.com/product/bq78z100) and bq25896 as the charge controller (
http://www.ti.com/product/bq25896). The datasheets for each of these give example circuits, but all the examples are given in isolation (ie it doesn't show how to use all three at once).
So that gets me back to the original question - how would I integrate, for example, these three components on a single cell? I see a few options:
* All three directly connected to the cell in parallel. This appears to me like it would remove protection from both the monitor and charge circuits.
* All three connected in series (say protection -> monitor -> charge). This to me looks like the monitor circuit wouldn't be as accurate, because it wouldn't be tracking the power use of the protection circuit (which is admittedly small).
* A combination of series/parallel connections. This makes the most sense to me.
* Or there's a smarter way to do it - I'm hoping this is the case
So to answer your specific comments - ROBOT, that looks like a neat little chip and I will bookmark for later, but it doesn't look like it can monitor the remaining capacity of the cell?
Bob - thanks again, and I think I have answered most of those points, but to be sure:
1 - Two inputs, USB 5V and cell 2.7-4.3V (approx). In circuit will have common ground.
2 - Decided by power mux, but in accordance with above logic.
3 - Only single external source (USB 5V).
4 - Function of power mux, but I like the idea of zero drop.
5 - I'm comfortable with downstream of the mux - this will change for the different applications I have in mind. I should mention that this power won't be fed directly into anything other than another DC-DC convertor (ie a 1.8V LDO for low power circuit, etc).
6 - External supply problems is a really good point, I hadn't considered that. I'm planning a range of input currents (from ~150mA for low power USB up to a 3A fast charger) but I haven't looked at the case where it doesn't meet spec...
7 - I'll have ESD protection on the USB port, but again open to suggestions on additional protection. I'm not really planning to filter the USB power because everything downstream of the mux will run through another convertor - but again open to suggestions.
8 - I wasn't planning to log, but that's a good idea, I'll give it some thought.
Thanks once again for your help - hopefully this clarifies where I'm at and what I'm trying to achieve